Aircraft Comm Systems

7/30/2019 Aircraft Comm Systems

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-- - -2 Communication systems

IntroductionThere is a fundamen tal need for comm unicationbetween aircrew and ground controllers, among theaircrew and between aircrew and passengers. Externalcomm unication is achieved by m eans ofradio-telephone (RIT) ink while intern alcommunication (intercom or audio integrating system)is by wire as opposed t o wireless. Although intercom.is not a radio system, it is included in this chap terbecause of its intimate relationship with th e aircraftradio systems. Voice record ers and in-flightentertainment systems are also considered since theyare usually the responsibility of the aircraft radiotechnicianlengineer.The first items of radio equipme nt to appear onaircraft were low -frequency (1.f.) communicationssets in the W orld War 1 days of spark gap transmitters.Intercom was by means of a Gosport (speaking) tu be.By the 1930s the early keyed contin uou s wave (c.w.)(radio-telegraphy) was beginning to be replaced byR/T although 'key-bashing' had its place as long asaircraft carried radio operators. Early R/T as withinthe 1.f. and h.f. bands, the sets operating o n only oneor very few frequencies. With airfields widely spacedand low-powered transmission, there was littleinterference and so the need for many channels did

on v.h.f. frequencies is often fou nd ; unfo rtunate lyaeronautical commun ications satellites are not to befound (1979).The au dio integrating system (AIS) complexitydepends on the type of aircraft. A light aircraftsystem m ay provide tw o transm itlreceive channels fordual v.h.f. com ms and receive only for du al v.h.f.nav., A D F , DME and marker. Each receive channelhas a speaker-o ff-phone sw itch while the micr oph onecan be switched between v.h.f. comms 1 and v.h.f.comms 2. A mu lti-crew large airliner has very ma nymore facilities, as described later.

V.H.F. CommunicationsBasic PrinciplesAn aircraft v.h.f. comms transceiver is comprised ofeither a single or double conversion superhet receiverand an a.m. transmitter. A modern set provides 720channels at 25 kH z spacing betwee n 118MHz and135-975 MHz; until recently the spacing was 5 0 kHzgiving on ly 360 channels. The mode of operation issingle ch anne l simplex (s.c .s.), i.e. on e frequ ency andone antenna for both receiver and transmitter. Ifprovision for satellite communication is included inaccordance with ARINC 566 then in addition tonot arise. a.m. S.C.S.we will have f.m. double channel simplexThe situation has drastically changed since World (d.c.s.), i.e. different frequencies for transm it andWar 11; air traffic and facilities have increased -with the receive.consequent demand for ex tra channels which cann ot Com mun ication by v.h.f. is essentially 'line ofbe provided in the l.f., m.f. or h.f. band s. sight' by dire ct (space) wave. The rang e available canFortunately v.h.f. equipmen t has been successfully be approximated by 1.23 ( J h , + Jht)nm where h, isdeveloped fro m early beg innings in Wdrld War I1 the heig ht, in fe et, above sea level of the receiverfighter con trol, while h+ s the same for the transm itter. Thu s, withThe cur rent s ituation is the v.h.f. is used for the g o i n d stat ion at sea level , the approximateshort-range communication while h.f. is used for maxim um range for aircraft at 10 00 0 and 1000 f tlong-range. A large airliner, such as a Boeing 747, (30 000 and 3000 m) would be 123 and 40 nmcarries three v.h.f.s and dual h.f. In add itio n, in such respectively.aircraft, selective calling (Selcal) facilities areprovided by a dual installation s uch that a ground Installationstation can call aircraft either singly or in groups A single v.h.f. installation consists of three parts,without the need for constant monitoring by the namely control unit, transceiver and antenna. Increw. Provision for satellite comm unication (Satcorn) addition crew phones are connected to the v.h.f. via


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Fig. 2(court

.2 CN-2011 v.h.f. carnm./nav. e q u ip m e n tesy B e n d i ~ vionics Division)

selection switc hes in the AIS. Ligh t aircr aft v.h.f.susually have a pan el-m oun ted c omb ined transceiverand control un it, an example being the King K Y 196illustrated in Fig. 2.1. The current trend is forcombined COM/NA V/RNA V, Fig. 2.2 illustrates theBendix CN-201 I , a general av iation p ane l-mo un tedunit comprising tw o c om ms transceivers, tw o nav.receivers, glidepath receiver, ma rke r receiv er,

frequency con trol for internal circuits and d.m.e, andlast bu t no t least, audio selection switches. Suchequipm ent will he considered in Chapter 12 .Figure 2.3 show s one o f a tripl e v.h.f. comm sinstallation as might be fi tted t o a large passengertransport aircraft: VH F2 and V HF 3 are similar toVHFl but are supplied from a different 28 V d.c. busbar and feed different selection switches in the AIS.


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Mode Selector Control Provides selection of norma la.m., extended range a.m. or Sa tcom . If the Satcomantenna has switchable lobes such switching may beincluded in the mode switch, or could be separate.On-Off Switch Energizes master power relay intransceiver. The switch ma y be separate, incorpora tedin mode selector switch as an extr a switch position, organged with the volume or squelch co ntro l.Receiver Selectivity Switch Normal or sharpselectivity. When S atc om is selected sharp selectivityautomatically applies.

Receiver The receiver is a single. conversion superhet .The r.f. stage employs varactor diode tuning, utilizingthe tuning voltage from the stabilized masteroscillator (s.m.0.). Bo th th e r.f. amplifier and mixerare dual gate field-effect transistors (f.e.t.). The r,f.amplifier f.e.t. has the input signal applied to gate 1while the a.g.c. voltage is applied to gate 2 , Th emixer connec tions are: gate 1, signal; gate 2, s.rn.0.The difference frequency from the mixer, 11.4 MHz,is passed b y a crystal filte r, providing the desirednarrow bandpass, to the i.f. amplifiers. Tw o stages ofa.g.c.-controlled i.f. amplification are used; the first ofwhich is a linear integrated circu it.The dete ctor and squelch gate utilize transistors onBlock Diagram Operation (KY 196) an integrated circuit transistor array. A furth er arrayFigure 2.4 is a simplified block diagram of the King is used for the squelch-control circuitry. Noise atKY 196panel-mounted v.h.f. cornm. transceiver. 8 kHz from the d etector ou tput is sampled and usedThis equipment, intended for the general aviation to close th e squelch gate if its amplitude is asmarke t, is no t typical of in+ervice transceivers since from the receiver operatin g at full gain.frequency and display co ntrol is achieved wi th th e aid When a signal is received, the noise output from the

a microprocessor; h owev er within the lifetime of dete ctor dfxreases due t o the a.g.c. ac tion; as athis book such implem entation will become consequence the squelch gate opens allowing thecommonplace. aud io signal to pass. The squelch can be disabled by

Switchm

Microcomputer 8 0 4 8t t

Frequency display S' toneI I >

I Increment/Decrement

118.70 121.90Use StandbyT

Fig. 2.4 King KY 196 simplified block diagram

Mod. ---+ Tx Rx AI S

Anode Display Cathode -drive drive drive , _ _ _ _ _ _ _ _ - IIII

4- V.C.O. S.M.O.IIIIIII

IMultiplexcounter

i

:AClock

Displaydlrnmsr

Non-volatilememory

I

a IIsync.

f

II tPhasedet. ,,ef. Osc. anddivider

I Programmable divider II IL - - ---ep- -- -52- --- - - - 1MHz Code kHz Code

1


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R.F.Input Mixer

Tuning 1 R.F.Volts A.G.C. A.G.C.(s.m.0.)Carriersquelchdetector

NoisesquelchdetectorI , Sidetone

Squelchs udiooutFig. 2.5 King KY 196 simplified receiver block diagram

means of a switch incorp orated in the volumecontrol. When the received signal has excessive noiseon the carrier, the noise-operated squelch wou ld keepthe squelch gate closed were it not forcarrier-operated or backup squelch. As the carrierlevel increases, a poi nt is reached w here t he squelchgate is opened regardless o f the noise level.The mean de tecto r ou tp ut voltage is used todeterm ine t he i.f. a.g.c. voltage. As the i.f. a.g.c.voltage exceeds a set reference the r.f. a.g.c. voltagedecreases.The dete cted audio is fed via the squelch gate,low-pass filter, volume control and audio amplifier tothe rear panel conne ctor. A minimum of 100mWaudio power in to a 500 Ll load is provided.Thansrnitter The transm itter (Fig. 2.6) feeds 16W ofa.m. r.f. to the ante nna. Modulation is achieved bysuperimposing the amplified mic, audio o n thetransmitter chain supply . The carrier frequencycorresponds to the in-use display.Radio frequ ency is fed fro m the s.m.0. t o an r.f.amplifier. Th is in pu t drive is switched by thetransmit receive switching circuits, the drive beingeffectively shorted t o earth when the press totransmit (p.t .t .) but ton is no t depressed. The

transmitter chain comprises a pre-driver, driver andfinal stage all broad band tuned, operated in Class Cand with m odulated collectors . The a.m . r.f. is fed via low-pass filter, which atte nuates harmonics, to theantenn a. On receive the t.r. diode is forw ard biasedto feed the received signal from th e anten na thro ughthe low-pass filter to the receive r . f . amplifier.The modulator chain comprises microphonepre-am plifier, dio de limiting , an f.e.t . switching stage,integrated circuit modulator driver and two modulatotransistors connec ted in parallel. The pre-amp outp utis sufficient t o subsequ ently give at least 85 per centmodulation, the l imiter preventing the de pth ofmodulation exceeding 100 per cent. The mic. audioline is broke n by the f.e.t. switch du ring receive.Stabilized Master OscilEator The s.m.0. is aconventional phase locked lo op with the codes for theprogrammable divider being generated by amicroprocessor. Discrete com pone nts are used forthe voltage controlled oscillator (V.C.O.) nd bufferswhile integrated circuits (i.c.) are used elsewher e.The reference signal of 25 kH z is provide d by anoscillator divider i,c. which utilizes a 3.2 MHz crystalt o give the necessary stab ility . Only seven stages of afourteen-stage ripple-carry binary coun ter are used t o


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switching

T.R. diodeRx r

- bSidetone

Fig. 2.6 King K Y 196 simplified transmitter block diagram

J4I Y

Modulator-+

Fractional MHzdivider

switch

M H ~ 'o n t . M H ~ont.from pP from p P

.ic .input

4 V.C.O.hole MHz PrescalerPhase 4 4detector divider +40141

Fig. 2.7 King KY 19 6 simplified programmable dividerblock diagram

Limiter

J tBuffer

give the necessary division of 27= 128. This reference,together with the output of the programmabledivider, is fed to the phase detector wh ich is part ofan LC., he rest of which is unused. The pulsating d.c.on the outpu t of the phase detector has a d.c.component which after filtering is used t o con trol thefrequency of the v.c.0. by varactor tunin g. If there isa ~ n t h e s i i e rmalfunction, an out-of-loc k signal fromthe phase detector is used t o switch off the s.m.0.feed to the transmitter.The programmable divider consists basically ofthree sets of counters as shown in Fig. 2.7. Thebuffered v.c.0. ou tp ut is first divided by either 40 or41, the former being sowhen a discrete MHz selection

is made; i .e. zeros after the displayed decimal point.The prescaler which performs this division is a u.h.f.programmable divider (+ 1O/ 11) followed by adivide-by-four i.c. The whole MH z divider uses a74LS162 b.c.d. decade cou nter and a 74LS163 binarycounter which together can be programmed to divideby an integer between 118 and 145,hence th eprescaler and whole MHz divider give a total divisionof 4720 (40 X 118) to 5800 (40 X 145 ) in steps of40. Thus a required v.c.0. output of, say, 130.00 MHzwould be achieved with a division of 5200 (4 0 X 130)since 130MHz + 5200 = 25 kH z = referencefrequency.The 25 kHz steps are obtained by forcing the


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prescaler to divide by 4 1, the required n umb er oftimes in the cou nt sequence. Each time t he divisionratio is 41, one ex tra cycle of the v.c.0. frequency isneeded to achieve an ou tpu t of 25 kHz from theprogrammable divider. To see that this is so, considerthe previous examp le where we had a division ratio of5200 to give 130 .00 MHz, i.e. 520 0 cycles at130-00MH z occupies 4 0 ps = period of 25 kHz.Now a prescaler division ratio of 41 once d uring 40 psmeans 520 1 cycles of the v.c.0. outp ut occu py 40 psso the frequency is 5201/(40 X l om6) 130 .025 MHzas required. The prescaler ratio is controlled by thefractional MHz divider, again em ploying a 74L S 162and 74LS 163. The number of divide-by-41 events in40 ps is determined by th e kH z control code from themicroprocessor and can be anywhere from 0 to 39t imes. Therefore each whole megacycle can haveN X 25 kHz added w here N ranges from 0 t o 39. Thisproduces 25 kHz steps from 0 kHz to 975 kHz.

Microprocessor and Display The microprocessorused, an 804 8, contains sufficient mem ory for theprogram and data req uired in this application to bestored on the chip. In addition t o this memory and,of course, an eight-bit c.p.u., we have an eight-bittimerlcounter and a clock on board. Throughtwenty-seven I/O lines the 80 48 interfaces with theprogrammable divider, display drive circuits andnon-volatile m emory.

The 80 48 has been programmed to generate abinary code for the 'use' and 'standby' frequencies.The code, as well as being stored in the 8048, is alsostored in a 1400-bit electrically alterable read onlymemory (EAROM). This external memory iseffectively a non-volatile RAM, the data and addressbeing communicated in serial form via a one-pinbidirectional bus , the readlwritelerase mode beingcontrolled by a three-bit code. When power isapplied the microprocessor reads the last frequenciesstored in th e EAROM which are then utilized as theinitial 'use' and 'stand by' frequencies. In the even t offailure of the EAROM the microprocessor will display120-00 MHz as its initial frequenc ies. The EAROMwill store data for an indefinite period w ithou t pow erThe 'standby' frequency is changed by clockwiseor counterclockwise detent rotation of the frequencyselect knobs. 1 MH z, 50 kHz and 25 kH z changes canbe made with two knobs, one of which incorporates apush-pull switch for 50125 kHz step changes. Themicroprocessor is programmed to increment ordecrement the 'standby' frequency by the appropriatestep whenever it senses the o peration of one of th efrequency-select knobs.The code for the frequen cy in use is fed to th eprogrammable dividers from th e m icroprocessor.'Use' and 'standby' frequencies are exchanged onoperation of the momentary transfer switch. Whenthe transceiver is in the receive mode th emicroprocessor adds 11 -4 MHz to the 'use' frequencycode since the local oscillator signal fed to thereceiver mixer sho uld be this am oun t higher than thedesired received carrier in or der t o give a differenc efrequency equal to the i.f.Both 'usei an d 'standby' codes are fed to thedisplay drivers. The 'use' cod e represents thetransmit frequency and is not increased by 11.4MHzin the receive mo de. Each digit is fed in turn to th ecathode decoderldriver ,an i .c. conta ining a seven-segment decode r, decimal point and comma drivesand programmable c urren t sinks. The decimal pointJ and comma outp uts (i and h) are used to drive thesegments displaying ' l ' , '. ' and 'T' (see Fig. 2.10).The 'T ' is illuminated when in the transmit mode.The display is a gas discharge type with itsintensity controlled by a photocell located in the

i / 0 lines display window. As the light reaching the ph otocellevent counter decreases the curre nt being supplied to theprogramming pin of the cathode decoderldriver fromthe display dimm er circuit decreases, so dimming thedisplay.Fig. 2.8 8048 eight-bit microcomputer (courtesy King Time multiplexing of the display drives is achievedRadio Corp.) by a clock signal being fed from the microprocessor to


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14-bits Data register

Memory ReacWrite

I Decode I I1

MSB Units Address LSB

Fig. 2.9 Electr ically alterable read only me mo ry, e .a .r .0 .m.(courtesy King Radio C orp .)Display

Data 110110 buffer.--I I Data flowdecodelogic c3

Clock Clock

Sync. Clock

A1 A 2 A 3 A 4 A5 A 6 A 7 A 8

Anodedrive A1 A 2 A 3 A 4 A 5 A 6 A 7 A 8

(A1 to AS)

B.C.D.code

(a to i)

4 L aUse F -. Standby b4 b11110 sec.

Anodedriver

Fig. 2.10 King KY 19 6 simplified display drive blockdiagram

Cathodedecoderldriver~b A B C D

t h A A A

Multiplexer B.C.D. Dimmingcode current


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a 1 of 8 counter/rnultiplexer so tha t the ano de drives(A1 t o A8) are switche d sequentially. As th e anodedrives are switched the app ropriate b.c.d. info rmatio nfrom th e microprocessor is being decoded by thecathode decoderldriver, the result being that thenecessary segments of each digit are lighted on e d i g tat a t ime at approxim ately 11 0 times per second.A synch ronizatio n pulse is sent t o the m ultiplexerfrom the microprocessor every 8 cycles to maintaindisplay synchronization.

modulated off-resonant signal and an unmodula teddesired signal, the resultant aud io ou tp ut shall no texceed -10 dB with reference to th e output producedby a desired signal onl y when mo dulated 30 per cent(under specified signal levelloff resonance conditions)Undesired ResponsesAll spuriou s responses in band 108-13 5 MHz shall bedown at least 100 dB otherw ise, including image,at least 80 dB dow n.Audio OutputCharacteristics

The selected characteristics which follow are d rawnfrom ARINC Characteristic 566 covering airborn ev.h.f. com mu nications and S atcom Mark 1 . Detailsof Satcom and extended range a.m. are not included.System Units

1. V.h.f. transceiver;2. modula t ion adaptor /m odem - .m. provisionfor Satcom;3 . power amplifier - Satcom and ex tended range;4. pre-amplifier - Satcom an d extend ed range;5. control panel;6. remote frequency readout indicator - optional;7. antennas - eparate Satcom antenna .Note: 1 and 2 may be incorporated in one linereplaceable unit (1.r.u.).

Frequency Selection720 channels from 118 through 135.975 MHz,25 kHz spacing.Receiver muting a nd p .t.t. de-energization duringchannelling.215 channel selection.Channelling time: d 0 m s .ReceiverSensitivity3 pV, 30 per cent modulation at 1000 Hz to giveSt NJN 2 6 d B .SelectivityMinimum 6 dB points at f 15 kHz (* 8 kH z sharp).Maximum 6 0 dB points at + 3 1 5 kHz (+ 15 kHzsharp).Maximum 100 dB points at f 40 kHz ( 5 18.5 kHzsharp).Cross Modu lationWith simultaneous receiver input of 30 per cent

GainA 3 pV a .m. signal with 3 0 per cent modu lation a t1000 Hz will produce 100 mW n a 200-500 S2 load.Frequency ResponseAudio power ou tp ut level shall no t vary more than6 dB over frequen cy range 300-2 500 Hz.Frequencies 2 5750 Hz must be a t tenuated by a tleast 20 dB.Harmonic DistortionLess than 7- 5 per cent with 30 per cent modulation.Less than 2 0 per cent with 9 0 per cent modulation.AGCNo more than 3 dB variation with input signals from5 pV to 100 mV.TransmitterStabilityCarrier frequen cy within + 0.005 per cent underprescribed conditions.Power Output25-40 W in to a 52 !2 load at the end of a 5 fttransmission line.Side oneWith 9 0 per cent a.m. at 1000 Hz the s idetone outp utshall be at least 10 0 mW into either a 2 0 0 o r 500L?load.Mic. InputMic. audio inp ut circuit to have an impedance of150 i' 2 fo r use with a carbo n mic. or a transistor rnic.operating from the (approx.) 2 0 V d.c. carb on mic.supply.AntennaVertically polarized and omnidirectional.


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To match 52 S2 with VSWR < 1.5 : 1.Ramp TestingAfter checking for condition and assembly andmaking available the a ppro pria te power su pplies thefollowing (typical) checks should be ma de a t eachstation using eac h v.h.f.

1. Disable squelch , check backgroun d noise andoperation of volume control.2 . On an unused channel rotate squelch controluntil squelch just closes (n o noise). Press p.t.t.but ton, speak into mic. and check sidetone.3 . Establish two-way comm unication with aremote station using bo th sets of frequen cycontrol knobs, in conjunction with transferswitch, if appropriate. Check strength andqua lity of signal.

NB. Do not transmit on 121.5 MHz Emergency).Do no t transm it if refuelling in progress.Do not interrupt ATC-aircraft communications.

H.F. Communications4

Basic PrinciplesThe use of h.f. (2-30 MHz) carriers for com mun icationpurposes greatly extend s the range a t which aircrewcan establish conta ct w ith A eronautical M obileService stations. This being so, we find t ha t h.f.comm. systems are fitted to aircraft flying routeswhich are, for some part of the flight, out of range ofv.h.f. service. Su ch aircraft obviously inclu de pub lictransport aircraft flying intercontinen tal rou tes, bu tthere is also a m arke t fo r general aviation aircraft.The long range is achieved b y use of sk y waveswhich are refracted by the ionosphere to such anextent that they are ben t sufficiently to retu rn t oearth. The h.f. grou nd wave suffers qu ite rapidattenuation with distance from th e transm itter.Ionospheric att en uat ion also takes place, beinggreatest at the lo wer h.f. freq uencies. A significantfeature of long-range h.f. transmission is tha t it issubject to selective fading over narrow ban dw idths(tens of cycles).The type of m odulation used, and associateddetails such as channel spacing and freq uen cychannelling incr eme nts, have been the subject o fmany papers and o rders fro m users, bot h civil andmilitary, and regulating bodie s. ARINC CharacteristicNo. 559A makes interesting reading, in that it revealshow conflicting proposals fr om various auth orities(in both the legal and expert opinion sense) can existat the same time.

The c urrent a nd futur e norm is to use singlesideband (s.s.b.) mode of operation for h.f.comm unications, although sets in service m ay haveprovision for compatible or normal a.m., i.e. carrierand one or two sidebands being transmittedrespectively. This s.s.b . ransmission and receptionhas been described briefly in Chapter 1 an dextensively in many text boo ks. A feature of aircrafth.f. systems is that coverage of a wide band of r.f. anduse of a resonant antenna requires efficient anten natuning arrangements which m ust operateautomatically o n changing channel in order to reducethe VSWR to an acceptable level.InstallationA typical large aircraft h.f. installation consists oftwo systems, each of which comprises a transceiver,controller, ante nna tuning unit and antenna. Each ofthe transceivers are connected to the AIS for mic., tel.and p.t . t. provision. In addition outp uts to Selcal.decoders are provided. Suc h an installation is shownin Fig. 2.1 1.The transceivers contain the receiver, transmitter,power amplifier and power supply circuitry. They aremo unte d on th e radio rack and provided w ith a flowof cooling air, possibly augm ented by a fan . Atransceiver rated at 200 W p.e.p. needs t o dissipate3 00 W when operated on s.s.b. while on a.m. thisfigure rises to 500 W. Telephone and microphonejacks may be provided on the fron t panel, as might ameter an d associated sw itch which will provide ameans of mon itoring various voltages and c urrents.Coupling to the antenna is achieved via theantenna tuning unit (ATU). Some systems mayemplo y an antenna coupler and a separate antennacoupler control unit. The ATU provides,automatically, a match from the antenna to the 50 Cltransmission line. Closed-loop con trol of matchingelemen ts reduces the standing wave ratio t o 1.3 : 1or less (ARINC 559A).Since the ma tch must be achieved between line andantenna the ATU is invariably mounted adjacent tothe an tenna lead-in, in an unpressurized part of theairframe. For high-flying aircraft (m ost jets) the ATUis pressurized, possibly wit h nitrogen . Som e unitsmay contain a pressure switch which will be closedwhenever the pressurization within the tuner isadequate. The pressure switch may be used forohmmeter checks or, providing switch reliability isadequate, may be connected in series with the keyh e hus preventing transmiss ion in the event of aleak. Alternatively an atten uato r may be switched into reduce power.

Light aircraft h.f. systems in service are likely, for


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A.M.1 OF F0 .S.B.Mic. -+ No. 1Xmit No. 1 Selcal28V t.r.Tel.*

' AerialNo. 1 couplingp.t.t.

Controller

No. 2p.t.t.Aerialcoupling

+ ,Mic.dr +2

No. 2t.r.Tel.- No. 2 bXmit Selcal

Fig. 2.1 1 Typical dual h.f. installation

financial reasons, to have a fixed antenna coupler.Such a system operates on a restricted num ber ofchannels (say twenty ). As a particular channel isselected, appropriate switching takes place in thecoupler t o ensure the r.f. feed t o the antenn a is viapreviously adjusted, reactive components, whichmake the effective antenna length equal t o a quarterof a wavelength, thus presenting an impedance ofapproximately 50 52. The required final manualadjustment must be carried ou t by maintenancepersonnel on the aircraft.The anten na used varies greatly, depending on th etype of aircraft. Fo r low-speed aircraft a long wireantenna is popular although whip antenn as may befound on some light aircraft employing low-poweredh.f. systems. The aerodynam ic problems of wire

antennas on aircraft which fly faster than, say, 400knots, have led t o the use of notch and probeantennas w h c h effectively excite the airframe so thatit becomes a radiating e leme nt.Modern wire antennas are constructed ofcopper-clad steel or phosph or bronze , giving a reducedr.f. resistance compared with earlier stainless-steelwires. A covering of polythe ne reduces the effec ts ofprecipitation static. Positioning is norm ally a singlespan between forward fuselage and vertical stabilizer.Larger aircraft will have twin antennas while a singleinstallation, possibly in a 'V' configuration, is morecom mon fo r smaller aircraft. The r.f. feed is usuallyat the forward attachm ent via an antenn a mast. Therear tethering is by means of a tensioning unit.The an tenna mast is subject to p itting and erosion


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of the leading edge; a neop rene covering will providesome protection, nevertheless regular inspections arecalled for. Protection against condensation within themast may be provided by containers of silica gelwhich should be periodically inspected for a change incolour from blue t o pink, indicating satura tion.Hollow masts are usually provided with a water-drainpath which should be kept free from obstruction.The two most important features of the reartethering point are tha t the wire is kept u nder tensionand that a weak link is provided so as to en sure tha tany break occurs at the rear, so preventing the w irewrapping itself around the vertical stabilizer andrudder. On light aircraft a very simple arrangement ofa spring, or rubb er bungee, and ho ok may be used.The spring maintains the tension but if this becomesexcessive the h oo k will open and th e w ire will be fre eat the rear end. On larger aircraft a spring -tension ingunit will be used t o co pe with the more severeconditions enco untere d du e t o higher speeds andfuselage flexing. The unit lo ads the wire by means o fa metal spring, usually enclosed in a barrel housing.A serrated tail rod is attache d t o th e tethering po inton the aircraft and inserted int o the barrel where it issecured by a spring collet, the grip of which increaseswith tension. The wire is attached to a chuck unitwhich incorporates a coppe r p in serving as a weak linkdesigned to shear when the tension exceeds a bou t180 lbf. Som e units incorporate two-stage protectionagainst overload. Tw o pins of different s tren gth s areused; should th e first shear, a small extension (3116 in.)of overall length results, th us reducing tension andexposing a yellow warning band on the u nit.

Notch antenna s consist of a slot cut int o theaircraft structure , often at the base of the verticalstabilizer. The indu ctance of the no tch isseries-resonated by a high-voltage variable capacito rdriven by a phase-sensing servo. Signa l injec tion is viamatching circuitry d riven by a SWR ensing servo.Since the no tch is high 'Q' the inpu t is transformed t oa voltage across the no tch which is of the order ofthousands of volts. This large voltage provides th edriving force for current flow in the airframe whichserves as the rad iator .A probe anten na, which is aerodynamicallyacceptable, may be fitted at either of the wing-tips oron top of the vertical stabilizer. Again series tun ingprovides the necessary driving force for radiation.The probe an ten na, as well as the w ire anten na , isliable to suffer lightning strikes, so pro tect ion in th eform of a lightning arrester (spark gap) is fitted.Any voltage in excess of approximately 16 kV on theantenna will cau se an arc across the electrodes o f thehykoge-n-filled spark gap , thus p reven ting discharge&kyc. ,

through the h.f. equipm ent. Bujld-up of precipitationstatic on a ntenn as, particularly prob es, is dealt w ithby providing a high resistance static drain (abo ut6 M a ) path to earth connected between the antennafeed point and the ATU.It is imp ortant in dual installations that only oneh.f. system can transmit at any one tim e; this isachieved by m eans of an interlock circ uit. This basicrequirement is illustrated in Fig. 2.1 1 where i t can beseen tha t the N o. 1 p.t.t. line is routed via a contactof the No. 2 interlock relay, similarly with No. 2p.t.t. The interlock relays will be external to thet ransceivers o f t e ~itted in an h .f. accessory box.While one of the h.f. systems is transmitting the oth ersystem mu st be pro tected against induced voltagesfrom the keyed system. In addition, with someinstallations, we may have a probe used as atransmitting antenna for both systems and as areceiving an tenna for , say, No . 1 system. The No. 2receiving anten na might be a no tch . It follows that onkeying either system we will have a sequence ofevents which might proceed as follows.HF 1 keyed:1. HF 2 keyline broken by a contact of HF 1interlock relay;2. HF 2 antenna grounded;3 . HF 2AT U inp ut and o utp ut feeds grounded andfeed to receiver broken .HF 2 keyed:

1 . HF 1 keyline broken by a contact of HF 2interlock relay;2. H F I probe antenna transferred from HF 1 ;AT U to HF 2 ATU;3. HF 2 notch antenna feed grounded;4. HF 1 ATU input and ou tpu t feeds groundedand feed t o receiver brok en.

Controls and OperationSeparate controllers are employed in dual installations,each having 'in-use' frequency selection only. Oldersystems and some light aircraft systems have limitedchannel selection where dialling a particular channelnumber tunes the sy stem, including ATU , to apre-assigned frequency , a chan nellfreq uenc y chart isrequired in such cases. With modern sets, indicationof the frequency selected is given directly on thecontroller.The controls shown in Fig. 2.1 1 are those referredto in ARINC 559A ; variations are comm on an d willbe listed below.Mode Selector Switch. OFF-AM-SSB The ' turn o ff 'function may be a separate switch or indeed may n ot


13/25

be employed at all; switching on and off beingachieved with the master radio switch. The 'AM'position may be designated 'AME' (AM equivalent orcompatible) and is selected whenever transmissionand reception is required using a.m. o r s.s.b. plus fullcarrier (a.m e .). The 'SSB' position provides fortransmission and reception of upper sideband only.Although use of the upper sideband is the nor mfor aeronautical h.f. com munic ations some controllershave 'USB' and 'LSB' positions. In add ition 'DATA'and 'CW' mode s may be available. Th e forme r is forpossible futur e use of da ta links by h .f. using theupper sideband - he receiver is operated atmaxim um gain, The latter is for c .w . ransmission andreception, morse c ode , by 'key bashing', being theinformation-carrying m edium .Frequency Selectors Freque ncy selectors consist of,typically, four controls which allow selection offrequencies between 2.8 and 24 MHz in 1 kH z steps(ARINC 559A). Military requirements are for afrequency coverage of 2 t o 3 0 MHz in 0.1 kH z steps,consequently one will find systems offering 2 80 000'channels' meeting these requirements in full or28 000 channels meeting the extende d range b ut n otthe 0-1 kHz step requirement.When a new frequency is selected the A TU mu stadjust itself since the antenna characteristics willchange. Fo r this purpose th e transm itter is keyedmomentarily in order th at SWR and phase can bemeasured and used to drive the ATU servos.Squelch Control Normal contro l of squelchthreshold may be provided. As an alternative an r.f.sensitivity control may be used, but where Selcal isutilized it is imp ortan t tha t th e receiver o perates atfull sensitivity at all times with a sque lch circuit beingemployed only for aural monitoring and no t affect ingthe output to the Selcal decoder.Audio Volum e Control Provides for adjustment ofaudio level. Such a control m ay be located elsewhere,such as on an aud io selector panel, part of the AIS.Clarifier This control is to be found o n som e h.f .controllers. With s.s.b. signals while the phase of there-inserted carrier is of little con sequenc e it sfrequency should be accurate. Should the frequenc ybe incorrect by , say, in excess of + 20 Hzdeterioration of the quality o f speech will result.A clarifier allows for ma nual adjustm ent of t here-inserted carrier freq uen cy. Use of highly ac curateand stable frequency synthesizers make the provisionof such a control unnecessary.

indicator A meter mounted on the front panel of tcontroller ma y be provided in orde r to give anindication of radiated power.Block Diagram OperationDansceiver Figure 2.12 is a simplified bloc k diagraof an a.rn.1s.s.b. transceiver. Th e ope ration will bedescribed by function.Am plitud e Modulated Transmission The frequencyselected on the controller determines the output frothe frequ ency synthesizer to the r.f. translator whicshifts the frequency up and provides sufficient drivefor the power amplifier (p.a.). The mic. inpu t, afteamplification, feeds the mod ulator w hich produceshigh-level amplitude mo dula tion of the r.f. amplifieby the p.a. The re f, ignal is fed to the AT U via theantenna transfer relay contact.The PA o utp ut signal is sampled by the sidetonedetector which feeds sidetone au dio via the contac tof the deenerg ized sidetone relay and the sidetoneadjust potentiometer to the audio o utpu t amplifier.Single Sideband Transmission Low-level mod ulatiois necessary since there is no carrier to modulate atthe p.a. stage, hence the mic. in pu t, f,, is fed t o abalanced modu lator togethe r with a fixed carrierfrequency, fc, from the frequency synthesizer. Thbalanced mo dulator ou tpu t consists of bo th sidebanf, t f, and f c - f,, the carrier being suppressed.Th e required sideband is passed b y a filter t o the r.ftranslator a fter further amplification.If we consider an aud io response from 30 0 to3000 Hz we see that the separation betwe en thelowest u.s.b. freq uen cy and the highest 1.s.b.frequency is only 60 0 Hz. It follows that the filterused must have very stee p slurts and a flat bandpassA mechanical filter can be used in which a n inp uttransducer converts the electrical signal intomechanical vibrations, these are transmitted bymechanically resonant metal discs and coupling rodand finally converted back to an electrical signal byan outp ut t ransducer.Freque ncy translation is by a mixing processrathe r tha n a multiplicative process since if theu.s.b. f, + f;, were multiplied by N we wouldradiate a frequenc y of N ( c + f,) rather thanft t f, +f,. The am oun t by which the u.s.b. istranslated, ft , is determine d by the frequenc y selecton the c ontroller. Final amplification takes place inthe p.a. prior to feeding the r.f. t o the ATU.T o obtain sideton e from the p.a. stage a carrierwould need to be re-inserted. A simpler m eth od ,


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Audioto AIS

- 0/flATU

1 +b w

r- Squelch

Fig. 2.12 Typical h.f. a.m.1s.s.b. transceiver block diagram

A

which nevertheless confirms tha t a signal has reached signal, which is dealt w ith in th e same way as before.the p.a., is to use th e rectified r.f. to operate asidetone relay. When energized the contact of this Antenna Tuning Unit Figure 2.13 illustrates anrelay connects the amp lified mic. audio to th e ou tp ut autom atic ATU simplified block diagram. Onaudio am plifier . selecting a new frequency a retune signal is sent tothe ATU control circuits which th en:

gate

AmplitudeModulated Reception The received signalpasses from th e ATU via the de-energized antennatransfer relay contact to an re f. amplifier an d thenceto the r.f. translator. After the translator no rma l a.m.detection takes place, the audio so obtained being fedto the output stage. A variety of a.g.c. and squelchcircuits may be e mp loy ed.

Sidetonerelay

A Tuning (4elay P

1 . keys the transmitter;2 . inserts an atte nu ator in transceiver ou tp ut line(Fig. 2.12);3. switches on the tuning ton e signal generator(Fig. 2.12) and drives a tune warning lam p(optional) ;4. switches on reference phases fo r servo mo tors.Single Sideband Reception The circuit action o n The r.f. signal on the in pu t feed is mon itored by as.s.b, is similar to th at o n a.m. until after t he loading servo system and a phasing servo system. Iftranslator when t he translated r.f. is fed to the product the load impedance is high then the line current, I L ,detector along with th e re-inserted 'carrier' f, . The is low and the line voltage VL s high. This isoutput of the product detector is the required audio detected by th e loading servo discriminator which

quelchcont.A d

*T o r.f./i.f. stages.M .det.

,quelchsettingA.G.C.det.

Tonegen.

A

f- R xS.S.B.Filter R.F.4 lx ranslator ,

Af, + fm 4 L

- roduct + f~ Frequencydet. synthesizerfc + fm tc - f ControlwiresBalanced f~mod.

Mic.Modulator .

Tuningrelay


15/25

II AT U

Tune Tx 1Retune tone key ,

Fig. 2.13 Typical h . f . a.t.u, block diagram

11I,

Loading ' Control Icircuit -servo I

applies the appropriate amplitude and polarity d.c.signal to a chopper/ampIifier which in turn providesthe control phase for the loading servo m ot or . Theauto transformer tap is driven until the loadimpedance is 50 St.Should ILand VL not be in phase this is detectedby the phasing servo discriminator which applies theappropriate amplitude and polarity d.c. signal to achopperiamplifier which in tu rn provides the con trolphase for the phasing servo m ot or . The reactiveelements, inductance and capacitance, are adjusteduntil IL and VL are in p hase.As a result of the action of the tw o servo systems aresistive load of 50 S2 is presented to the co-axial feedfrom the transceiver. When bo th servos reach theirnull positions the control circuits remove the signalslisted previously,

II

Auto

CharacteristicsThe following brief list of cha racter istics are those ofa system which c onforms w ith ARINC 559A .Frequency SelectionAn r.f. range of 2.8-24 MHz covered in 1 kHzincrements.Method: reen trant frequency selection sys ern.Channelling time less than 1 s.

1 1 1 ;;:;;itd lPhasing I - -servo 1t

Mode of OperationSingle channel simple x, uppe r single side ba nd.

I vIIIIariablereactive

Sparkgap.

-

TransmitterPower output: 400 W p.e .p . (200W p.e.p.operational).Absolute maximum power outpu t: 650 W p.e.p.Mic. input circuit frequency response: not more than* 6 dB variation from 1000 Hz level through the range350 Hz to 2500 Hz.Spectrum control: com ponents at or belowf, - 00 Hz and at or above f , t2900 Hz should beattenuated by at least 30 dB.Frequency stability: k 20 Hz. Shop adjustment nomore often than yearly. Pilot con trol (e.g. clarifier)not acceptable.Interlock: on ly one transmitter in a dual systemshould operate at a time o n a 'firs tso me , first-served'basis, this includes transmitting for tuning purposes.ReceiverSensitivity: 4 pV max.; 30 per cent modulation a.m.( 1 pV s.s.b.) for 10 dB signal and noise t o noise ratio.A.g.c.: audio outpu t increase not more th an 6 dB forinput signal increase from 5 to 1000000 pV and nomore than an additional 2 dB up to 1 V input signallevel.Selectivity:s.s.b., 6 dB points at f, + 300 Hz and f, + 3 100 Hz,+ 35 dB points at f, nd f, + 3500 Hz.A.m.: toensure proper receiver operation (noadjacent channel interference) assuming operations on6 kHz spaced a .m. channels.


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Overall response: com pati ble with selec tivity but inaddition n o more tha n 3 dB variation between anytwo frequencies in the range 300-1500 Hz (forsatisfactory Selcal opera tion) .Audio out pu t: two-wire circuit isolated from ground,300 CL (or less) ou tpu t impedance supplying 10 0 mW(0.5 Selcal) into a 600 load.]Ramp Testing and MaintenanceWhilst regular inspection of all aircraft an tenn as iscalled for , it is particularly imp orta nt in the case ofh.f. antennas and associated componen ts. Anymaintenance schedule should require freque ntinspection of a ntenna tensioning units and te theringpoints in the case of wire antennas, w hl e for bothprobe and wire antennas th e spark gap should beinspected f or signs of lightning strikes (crackingand/or discolouring).A functional test is similar to th at for v.h.f. in thattwo-way com munic ation should be established with aremote stati on; all controls should be checked forsatisfactory operation and meter indications, if any,should be within lim its. Safe ty prec autio ns areparticularly im po rt ant since very high voltages arepresent on the antenna system with the resultingdanger of e lectric shock or arcing. No personnelshould be in the vicinity of the antenna whentransmitting, nor sho uld fuelling opera tions be inprogress. Rem embe r with many h.f. systems a changeof frequency could result in transmission to allowautomatic antenna tu ning.

SelcalThe selective calling (Selcal.) syste m allows a gro undstation t o call an aircraft or grou p of aircraft usingh.f. or v h . f . comm s withou t the flight crew havingcontinuously to monitor the station frequency.A coded signal is transmitted from the ground andreceived by the v.h.f. or h.f. receiver tu ned t o theappropriate frequency. The o utp ut code is fed to aSelcal decoder which activates aural and visual alertsif and only if the received code corresponds to thecode selected in the aircraft.Each transmitted code is made up of tw o r.f.bursts (pulses) each of 1 k 0.2 5 s separated by aperiod of 0.2 & 0.1 s. During each pulse thetransmitted carrier is 9 0 per cent mo dulated wit h tw otones, thus there are a total of four tones per call;the frequencies of the to nes determin e the code.

The tones available are given by t he form ulafN = antilog (0.054(N- 1)+ 2-O) ,

giving a total of sixteen tones between 312.6 and1479.1 Hz. The tones are designated by lett ersA t o Somitting I , N and O so a typical code might beAK-DM. The re are 29 70 codes available forassignment using the first twelve ton es, the a dditionof tones P, Q, R and S (1976) bring the total to10 92 0. Codes or blocks of codes are assigned onrequest to air carrier organizations who in turn assigncodes t o their aircraft either on a flight number oraircraft registration-related basis.Figure 2.14 illustrates a single Selcal system .Large passenger transport aircraft would normallycarry tw o identical systems. The decoder willrecognize a received combination of tones on any offive channels which corresponds to that combinationselected on the code select and annu nciator panel.When the correct code is recognized the chime switchand appropriate lamp switch is made . The lamp switchsupply is by w ay of an interrupter circuit so that thelamp will flash. A constant supply to the chimeswitch causes the chimes to sound once. Each lampholder, designated H F I , HF 1 1 etc. incorpora tes a resetswitch which when depressed will release the latchedlamp switch and chime switch . The tone filters in thedecoder will typically be mechanically resonantdevices.Variations in the arrangement shown an ddescribed are possible. Mechanically the c ontro l andannuncia tor panel may be separate units. Should theoperator require aircraft registration-related codesthere will be no need for code select switches, theappropriate code being selected by jumper leads onthe rear connector o f the decoder.Although five reset leads will be provided t he ymay be connected individually, all in parallel to asingle reset switch or t o the p.t.t. circuit of theassociated transmitter. In this latter case isolationdiodes (within the decoder) prevent 'sneak' circuits,i.e. keying on e transmitter causing one or moreothers to be keyed.The lam p and chime supplies shown can bechanged at the operator's op tio n. Possibilities are t oreverse the situation and have steady lights andmulti-stroke chimes, or have stea dy lights andsingle-stroke chime, in which case the interruptcircuit is not used.The Selcal systems which d o not comply withARINC 596 m ay n ot provide facilities for decoding offive channels simultaneo usly. A switch is provided onthe control panel with which th e single desiredchannel c an be selected; in this case only Selcal codesreceived on the corresponding receiver will be fed t o


17/25

Reset( 5 wires) Codeselect(4 x 4wires)

V.H.F.L l .H.F.L l .H.F.Self test I

Self testI drive:es

the decoder. Only one annunc iator lamp is required. tones A to S are numbered 1 t o 16 (0) the open wiresCode selection in an ARlNC 596 system is achieved will be as given by the corresponding binary numb er;by means of a 'b.c.d.' format. Each of the fou r tone e.g. tone M-12-1100, o with the wires designatedselectors has f ou r wires associated with i t ; for any 8,4 ,2 and 1 we see 8 and 4 will be ope n. Note this isparticular tone an appropriate combination of the not really b.c.d. but is nevertheless termed so.wires will be open c ircuit, the rest grou nde d. If the Testing of Selcal is quite straightforward. If

V.H.F. 1

V.H.F. 2

V.H.F. 3

H.F. 1

H.F. 2

circuit

Channelamps

SupplyFig. 2.14 Typical Selcal block diagram

4--+ Chime

I 4 switchTochimes


18/25

possible a test rig consisting of a tone gen erator inconjunction with a v.h.f. and h. f. transmitter shouldbe used, otherwise permission t o utilize aSelcal-equipped ground station should be sought.

Audio lntegrating Systems (A IS ) - ntercomIntroductionAll the systems in this bo ok e xhibit a variety ofcharacteristics but none more so than AIS. In a lightaircraft the f unc tion of the audio system is to providean interface betwe en the pilot's m ic. and tel. and theselected receiver and transmitter; such a 'system'might be little more than a locally ma nufacturedpanel-mounted junction bo x with a built-in audioamplifier and appropriate switching. In contrast alarge m ulti-crew passenger a ircraft has several

sub-systems making up the total audio system. Theremainder of this chapter will be concerned with theAIS on a Boeing 747.I t is unusual t o consider all the systems andsub-system s which follow as part of AIS, a termw h c h should perhaps be restricted to the systemw h c h provides for the selection of radio system audioou tpu ts and inputs and crew in tercomrnunications.However a brief description of all systems wh c hgenerate, p rocess or record aud io signals will be given.The following services comprise the complete audiosystem:

1 . flight interphone: allows flight deck crew tocomm unicate with each other or with groundstations;2. cabin inter pho ne: allows flight deck and cabincrew to communicate;Attendant's chimecall system

Audio --elcalsystem

Handsets

Headsets

Pass.addresssystem

' system II provisionsl 1

systemPTT-

4Audio+

Fig. 2.15 Boeing 747: typical communicat ions f i t(courtesy Boeing Commerc ia l Aeroplane C o . )

PA.L

Voice r-----..Voice r e c 1p ~ o v i s i o n s ,

.t

Cabin PTT-

crew callsystem

Pass-entertain.

interphoneAudio

i

Audio

(music)override

VOR/ILS NAV 'systemMarker beaconsystemLow rangeradio altimetersystemATC system - ServiceDM E system I I

Monitorsignals+


19/25

3. service interp hon e: allows ground staff tocommunicate with each other and also with theflight crew;4 . passenger aderess (PA): allows anno unc em entsto be made by the crew to the passengers;5. passenger entertainment system: allows theshowing of movies and t he piping of m usic;6. ground crew call system: allows flight andground crew to attrac t each other's a ttentio n;7. cockpit voice recorder: meets regulatoryrequirements fo r the recording of flight crewaudio for s ubsequen t accident investigation ifnecessary.

It should be noted that the above are not completelyseparate systems as illustrated in F ig. 2.15 anddescribed below. The d ividing lines be twee n

Table 2.1 Flight interp hon e facilitiesCAPT F/O FIE OBSI OBS2 M E .

ASP X X X X X XJack panel X X X X X +Int - R-Tp.t.t. X X X X X XHandheldrnic. X X Jack Jack Jack JackHea dset Jack Jack Jack Jack Jack JackBoom mic.headset X X X X X -

sub-systems of the to tal a udio system are somew hatarbitrary, and termino logy is varied; however the mask mic. Jack Jack Jack Jack Jack -facilities described are commonplace.lnterphoneFlight Interphone speaker x x - - -This is really the basic and most essential part of th eaudio syste m. All radio equipm ents having mic. A 'X ' indicates the particular unit or component is fittedinpu ts or tel. outp uts , as well as virtually all oth er at that station (colum n).audio systems, interface with the flight interph one 'Jack' indicates a jack plug is fitted to enable use o f thewhich may , in itself, be term ed the A IS. appropriate mic. and/or tel.A large number of units and com ponents m ake upthe total system as in Table 2.1 w ith abbreviated TabIe 2.2 Abbreviationsterms as listed in Table 2.2. Figure 2.16 shows theflight interphone block diagram, simplified t o th e CAPT - Captain a.s.p. - Audio Selector Panel

extent that only one audio selection panel (ASP), F/O - First Officer int. - nterphonejack panel et c. is show n. An ASP is shown in OBS - Observer r/t - RadiotelephoneFig. 2.17. m.e. - Main Equipment p.t.t. - Press to TransmitA crew member selects the tel. and rnic. signals Centrerequired by use of the appropriate co ntr ols /s~ itch es mic- A Microphone tel. - Telephoneon an ASP. The various aud io signals enterin g an ASPare selected by twelve combined push select and audio lines which, together with loading resistors involume controls. Each ASP has an audio bus feeding the inter pho ne accessory bo x, form an anti-cross talka built-in isolation amplifier. The v.h.f. and h.f. netw ork; if one crew member has, say, h.f.1 selectedcomm. AD F, interp hon e and marker aud io signals are on his AS P then the resistive netwo rk will greatlyfed to the bus via th e appro priate select but ton s and attenu ate say h.f.2 which would otherwise be audiblevolume controls. The v.h.f. nav. and DME audio is should ano ther crew mem ber have selected h.f.l and

fed to the bus when voice and range are selected with h.f.2.the Voice pu sh bu tton ; wi th voice only selected the Six mic. select bu tto ns are provided on an ASP;DME audio is disconnected while the v.h.f. nav. audio three v.h.f. comm., two h.f. comm. and PA. Additionis passed through a sharp 1020 Hz ban dsto p filter switches associated with mic. select and transmission(FL1) before feeding the bus. With the fail-normal are the boom-mask and r .t . i nt . p.t. t . on each ASP an dswitch in the fail only one audio channel can also p.t.t . button s on the han dh eld m icrophones,be selected (bypassing the amplifier) and t he PA jack panels and the captain's control wheelaudio is fed direct to th e audio-o ut lines. Radio (R/T-in t .).altimeter audio is fed direct t o th e au dio-o ut lines. To speak over interphone a crew member shouldThe above aud io switching arrangements are illustrated select interp hon e using the r.t.-int. s witch on thein Fig. 2.18. Note the series resistors in the inp ut a.s.p. which will conn ect mic. high (boom or mask)


20/25

II INPH

IGAIN I1 08 5 I I

TESTACK

I - I ACCESSORY CAROII i i L--L1--JACS PANEL

MUTING 1 I I1-~:ERDr--------I I i---- ---7 ---,-----!---:w I1 I1I VUF 1 VOR 1 ADk I HK R BC NVHf 2 HF I 4D F 3 A r cVHF 3 HF 1 OME 1 LR RAO ALTt L-J VOR I DME 2INTERPHONE SPEAKER 4U OIO SELECTOR PANEL

LOW RANGE RADI O ALTlMk TEN PUOlUPA AUDIO FROM P9 MICROPHONk SELfCTOR

F R O H BOOM MA SK SWITCHTO BOOM MASK SWITCH

U &2Fig. 2.18 Audio signal selection (courtesy BoeingCommercial Aeroplane Co.)Fig. 2.16 Boeing 747: flight interphone (courtesy BoeingCommercial Aeroplane Co.)

r " D C 7 I~:"I

NORM

Fig. 2.17 Audio selection panel (courtesy BoeingCommercial Aeroplane Co.)to the interphone mic. high ou tput feeding the flightinterphone amplifier in the interphone accessory bo x.Alternatively the captain can select interphone on hiscontrol wheel p .t .t . switch w hich will energize relayK2 thus making the mic. high connection as before.Note that the ASP r.t .-int. p.t .t. sw itch d oe s not relyon power reaching the ASP for relay opera tion (see

HEAOPHONEAUDIO

SPLAKER4UOIO

VUlCLt LURDE R*,,Li,O

CONTROL WHEELPTT IN 1

BOOM SPEAKER MUTEM1CROPHONE

MASKICROPHONEHIGH --p . - - ---LRPM I N TSELECTOR

CONTROLIMICROPMONLLDW

CONTROLTT - R I l

I ( H f2 CONTROL MICROPHONE LOW

I N T E R P ~ O ~ E~ c n o ? m o ~ CIGHTO FLIGHT INTERMC+4EAMPLIF~ER

HANO HELO MICROPHONC WlGH

Fig. 2.19 Microphone signal selection (courtesy BoeingCommercial Aeroplane Co.)


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Fig. 2.19). Interp hone mic. signals from all ASPs arefed to the flight interpho ne amplifier w hich combinesthem and feeds the amplified interphone au dio to allASPs for selection as required.Pressing a mic. select bu tto n on the ASP willconnect the corresponding system mic. input lines torelay K2 and to co ntacts on the ASP r.t .int. p.t .t .switch. Thus when a p.t.t. sw itch is pressed, the mic.lines will be made by either the contacts of K2 or bythe ASP p.t.t. sw itch in the r.t. position . In Fig. 2.19the h.f.2 select switch is shown as typical of all comm.select switches. When the PA select sw itch is pressedthe flight interpho ne mic. circuit is interru pted andPA audio is applied to the fail-normal switch; inaddition the rnic. lines to the PA system are m ade.Operation of any p.t.t . switch mutes both interphonespeakers to prevent ac oustic feedba ck.Cabin InterphoneThe cabin interphone is a miniature automaticteleph one ex change servicing several subscribers:the cabin attend ants and the captain. In additionthe system interfaces with th e PA to allowannouncements to be made.Numbers are dialled by pushbuttons on thetelephone type handsets or on the pilot's controlunit. Eleven two-figure numbers are allocated t o thesubscribers, plus additional num bers for PA invarious or all comp artm ents, an 'all-attendants' calland an 'all-call'. Tw o dialling code s cons ist of letters:P-P is used by an at tendant t o alert the pilot (calllight flashes on c ontro l unit and chime sou nds once)while PA-PA is used by the pilot to gain absolutepriority over all other users of the PA system. Thedirectory is listed o n the push -to-talk switchincorporated in each handset to minimize ambientnoise.All dialling code decoding and the necessary trunkswitching is carried o ut in th e c entral switching unit ,CSU (automatic exchange). The CSU also containsthree amplifiers, one of w h c h is perma nentlyallocated to the pilot on what is effectively a privatetrunk. Of the five othe r available trunks, tw o areallocated to the attendants, two to the PA system andone for dialling. (Note a trunk is simply a circuitwhich can connect two subscribers.)The cabin interphone and service interphonesystems may be combined into a com mon networkby appropriate selection on the flight engineer'sinterphone switch panel, captain's ASP and cabininterphone con trol unit. Any handset may then belifted and connected into the network (dial 'all-call').In a similar way the flight interpho ne c ircuits may beused to m ake specific calls over the cabin inte rphon esystem.

*ttenciantrs stations1 7typicaU hirnallight 'me/lightnsor n r nit6'Cd I light

control Control logic andunit switching circuits

Central switching unitllnMrphons1 V P A

Fig. 2.20 Boeing 747: cabin interphone (courtesy BoeingComm ercial Aeroplane Co .)

The system is more complex than has beensuggested above but a basic description ha s been givesupported by Fig. 2.20.Service InterphoneA total of twe nty-tw o handset jacks are located invarious parts of the airframe in order th at groundcrew can comm unicate w ith one anothe r using theservice interph one system . The system is rathersimpler than those considered above. Mic. audio fromall handsets, with 'press to talk' depressed, arecom bined in and amplified by the service interphon eamplifier in the interphone audio accessory box.The amplified signal is fed t o all hand set tels.Volume co ntrol ad justment is provided by a presetpotentiometer.With the flight engineer's inte rpho ne switchselected to ON the input summing networks for bothservice and flight interphone systems are combined.Al l mic. inputs from either system are amplified andfed to both systems.Passenger AddressThe system comprises three PA amplifiers, tape deckannunciator panel, attendant's panel, PA accessorybox, co ntrol assemblies, speaker switch panel andfifty-three loudspeak ers. The various PA messageshave an order of priority assigned to them: pilot'sannounc eme nts, at endant's ann ouncem ents,prerecorded announcements and finally boardingmusic. All PA audio is broadcast over the speakersystem and also, exc ept fo r boarding music, override


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I lN Tt RP ItOliE ACID10 ACCC_SSORY BOX- - - - I- - - - - - - - - - -

F L I GH T I N T E R P H ON EL U D l O I N P U l+ A U D I O

A U D I O **P

XIGEh SYS E M ~ R G E ~ C YRU A U D I O-0V DCPAA N N U N C I A T O R . S E L E C T T A P EP A N E L PA

C A N C E L AMPO 1C O N T R O L

M A I N C A B INSPEAKERSVIA SPEAKE RS C I T C H I N GP A N E L

C O N T R O L I

P A R A L L E L C O N T R O LI - ICABIN INT SV S A U D I O C *C O N T R O L I 1* IPAS E N S I ~ I V I T I AMP

IN O 1

C ON T R OL OU T P U TA S ' l A M P L I F I E UA U D I O7 M A I N CABINSPEAKERSPASSENGER V I A S P E A K E RS W I T C H I N G

S E N S I T I V I T V- MP :S"" P A N E L+L * M P O I NO 3&AMP NO 2. S W I T CH I N G A S Y -

Fig. 2.21 Boeing 747: service interpho ne (courtesy Boeing Fig. 2.22 Boeing 747: passenger address (courtesy BoeingCommercial Aeroplane Co.) Comm ercial Aeroplane Co.)

entertainment audio fed to the passenger stethoscopeheadsets. A prerecorded emergency announcementmay be initiated by the pilot or an atte nda nt, o rautomatically in the event of cabin decompression.A chime is generated when the pilot turns on 'fastenseat-belt' or ' no smokin g' signs.The passenger address amplifiers are fed via theflight or cabin interphone systems for pilot orattendant annou ncem ents respectively. Distribu tionof audio from the amplifiers t o th e speakers in variouszones depe nds o n t he class configu ration, since som eannouncements may be intende d for onl y a certainclass of passengers.The necessary distribution is achieved by means ofswitches on th e speaker switching panel. Au dio is alsofed to the flight inte rph one system fo r sidetonepurposes.Number 2 and number 3 amplifiers a re slaved t onumber 1 for all-class annou ncem ents. Shouldseparate class annou nce men ts be requ ired t he parallelcontrol relay is energized, so separating the n umb er 1audio from that o f number 2 an d 3 . The controlassemblies in t he PA accessory b ox con tainpotentiometers used to se t the gain of the PA

amplifiers. When the aircraft is on the ground w ithlanding gear locked dow n and g round pow er appliedthe level of speaker audio is reduced by 6 dB .The tape deck c ontains up t o five tape cartridgesapart from the necessary tape-drive mechanism,playba ck head and a pre-amp lifier. Boardin g music isselected at an attend ant 's panel while prerecordedannouncements are selected by means of twelvepushbuttons on the annunciator panel.Passenger Entertainment SystemThe passenger entertainm ent system of the Boeing747 and any other modern large airliner is perhapsthe most com plex of all airborne systems. It is alsothe system likely to cause most trouble and,fortunate ly, least l ikely to affect the safety of theaircraft unless bad servicing leads t o a fire orloose-article hazard. Even on the same ty pe ofaircraft a variety of services will be available sincedifferent opera tors will offer different entertainm entin a bid to cap ture more custom ers. In view of theabove commen ts, the following description isparticularly brief and does no t d o justice t o thecom plexity involved.


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~ u y , ~ ,P . A .override

Fig. 2.23 Boeing 747: simplified passenger entertainmentsystem

I) .... ...........v

Both movies and m usic are provided, the movieaudio being fed t o individual seats via the mus icportion of the system. Ten tape-deck channels, fourmovie audio channels and on e p.a. channel (totalfifteen) are provided using time multiplexing. A t imeinterval, ternled a frame, is divided into fifteenchannel times during which th e signal amplitude ofeach channel is sampled. The audio signal amplitudesare binary coded (twelve bits) and transmitted,together with channel identification, clock and sync.pulses, over a co-axial cable running throughout theaircraft.The music channels (five stereo, ten mona ural or a

'system', as can be seen from the sch ematic diagrain Fig. 2.24. The horn and flight-deck call butto nlocated in the nose wheel bay while the ground-crcall (wit h illumination) and au ral warning b ox areth e flight deck. Operation is self-explanatory fromthe diagram. Should horn o r chime sou nd, the grcrew, or flight crew respectively, will conta ct eachother using one of the interphone systems.

Zone Asub- Othermultiplexer submultiplexers

F L I G H T D E C K C A L L

-

MainmultiplexerTapedeck

mixture) are multiplexed in the main multiplexe r, theresulting digital signal being fed to six subm ultip lexer s 130 SEC TDI G R O C R E W C A L Lin series, the final one being terminated wi th a suitableload resistor. Movie and PA audio are multiplexed

-b

v v 7

with the music channels in the zone sub multiplexers ,each of which feeds thre e or four colum ns of seatdemultiplexers. Channel selection is ma de by thepassenger wh o hears the appro priate au dio over hisstethoscop e headset after digitil t o analogueconversion in the dem ultiplexer. Alternate zonesubmultiplexers are used as back-up in th e event o fprime su bm ultip lexer failure (class priorities exist iffailures mean som e passengers mus t have theentertainment service discontinued).The co ntrols necessary for activation of theentertainments system are located on a ttendants 'control panels.Ground Crew Call SystemGroun d crew call is hardly w orth y of t he title

Seats1 2 34 4 4 Channel select

Fig. 2.24 Boeing 747: ground crew call (courtesy BoeinCommercial Aeroplane Co.)

Seat

Cockpit Voice RecorderAn endless tape provides 30 min recording time foaudio signals inpu t on f our separate channels. Thchan nel in pu ts are captain's, first officer's and fligengineer's transmitted and received audio and cocarea conversation. Passenger address audio may bsubstituted for the flight engineer's audio in anaircraft certified to fly with tw o crew members.The m icrophone inpu ts should be from so-calle'hot mics' , i .e. microphones which are perman entlive regardless of the settin g of ASP or controlcolumn switches. The area microph one (which m

Other seat' ' emultiplexersther seat -columns

4 4 Audio1 2 3Seats

demultiplexer


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LandinggearrelayParkingbrakerelay

Fig. 2.25 Typical cockpit voice recorder block diagram

separate from th e co ntro l panel) is strategicallyso that it can pick up flight crew speech and

While the cont rol panel is situa ted in th e coc kpi t,recorder unit (CVR) is located a t the oth er end o faircraft where it is least likely t o suffe r d amage inevent of an accident. The CVR is constru cted soto withstand shock and fire damage, and additionallypainted in a fire-resistant orange paint to assist infrom a wreck .The recorded audio may be erased providing thegear and parking brake inte rloc k relayacts are closed. As a fu rt he r safeguard ag ainstntal erasure a delay is incorp orated in th e bulkcircuit which requires th e opera tor t o depress'erase' switch for tw o secon ds be fo re era sureTest facilities are provided for all fou r chan nels,

L J ~ W I 1 u a 1flt. inst.bus bar

separately or all togeth er. A playback head andmo nitor amplifier allows a satisfactory test to beobserved on meters or heard over a headset via jackplug sockets. Pressing the test bu tto n o n the controlpanel or the all-test but ton on the CVR causes thechannels to be m onito red sequentially.The power supp ly for the system should be from asource which provides ma ximu m reliability. Since thetape is subject to wear and thu s has a limited life, theCVR should be switched off when n ot in use. Asuitable meth od wou ld be to remove power t o theCVR whenever external ground power is connected.

Testing and Trouble Shooting the AudioSystemsVariou s self-test facilities ma y be provided'by which


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tones may be generated and heard over headsets.However, t o test properly all switches should beoperated and al l mic. and te l. jacks, as well asspeakers, should be checked for the required audio.This should be sufficiently lo ud, clear and noise-free.Amplifier gain presets in accessory boxes may need tobe adjusted, A full functio nal test is best don e b ytwo me n, although it is no t impossible for on e manwith tw o headsets and an exten sion lead to establishtwo-way contact between various stations.Faults can be quite difficult to find owing to thecomplicated switching arrangements. However thewide range of switching can be used to advantage inorder to isolate suspect units or interconnections,Disconnecting units provides a good m eth od o f

fmding short circuits or howls due to coffee-inducedte1.-mic. feedbac k (i.e. spilt liquid providing aconducting path between tel. and rnic. circuits).Where one has a number of units in series, e.g.demultiplexers in an entertainment system,disconnecting can be a particularly rapid me thod offault-find ing; it is usually best to split the run in halthen in half again, and so on until the faul ty unit orconnection is fou nd . Continuity checks on very loncables can be achieved by shorting to earth at one enand then measuring the resistance to earth at theother. The resistance t o ea rth should also bemeasured w ith the s ho rt removed in case a naturalshort exists.

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Aircraft Comm Systems