Unreliable Speed Indication

7/29/2019 Unreliable Speed Indication

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Unreliable speed indications

Operational Liaison Meeting


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2Unreliable speed indications

Background

Two recent fatal accidents on non-Airbusaircraft and several reports of incidents

attributed to erroneous airspeed and/or altitude

indications have prompted the need to improve

flight crew awareness to identify and tacklesuch a failure.


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Content

Architecture display architecture interface with flight controls

interface with flight guidance

conclusion about architecture Probes: what may happen?

Recommendations

Procedure

Conclusions


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Display architecture - A300/A300-600/A310

ADC 1 ADC 2

Normal display

Reconfigurations

STBY

probes

STBYPFD 2

(Airspeed

indicatoron A300)

PFD 1

(Airspeed

indicatoron A300)

CAPT

probes

F/O

probes


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Display architecture - Fly by wire A/CPFD 1 PFD 2

ADC 1 ADC 3 ADC 2

Normal display

Reconfigurations

STBY

CAPT

probes

STBY

probes

F/O

probes


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Effect on flight controlsA300 / A300-600 / A310:

Erroneous airspeed / altitude does not affect

the flight controls (conventional aircraft ).

Fly by wire aircraft: Depending of the failure, erroneous airspeed /

altitude may affect the flight control of fly by

wire aircraft. In the worst case, the aircraft

reverts to a conventional aircraft.


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Effect on flight controls fly by wire A/C)Each flight control computer (PRIM on A340/A330, ELAC

on A320) receives the speed from the three ADC.

ADC 1

ADC 3

ADC 2

speed PRIM ELAC

speed

speed

COMPARISON/

SELECTION



Effect on flight controls fly by wire A/C)

it is eliminated, without any caution.

Normal operations continue.

If one has deviated:

ADC 1

ADC 3

ADC 2

speed

COMPARISON/

SELECTION

speed

speed

PRIM ELAC



Effect on flight controls fly by wire A/C)

ADR DISAGREE caution is triggered.the flight controls revert to alternate law (without protections).

SPD LIM flag is shown on both PFD ( no Vls, no Vsw).

This situation is latched ( PRIM/ELAC reset has no effect).

If the two remaining values are no longer in agreement:

ADC 1

ADC 3

ADC 2

speed

speed

speed

.

DISAGREE

PRIM ELAC



Effect on flight guidance

AP / ATS are affected if ADC 1 or 2 are affected.

Possible AP /ATS disconnection depending on speed

inaccuracy.

A300/ A300-600 /A310

ADC 2 speed/altitude

ATS 2

AP 2

ADC 1speed/altitude ATS 1

AP 1



Effect on flight guidanceFly by wire aircraft:Each flight envelope computer (FE)on A340/A330 or flight

augmentation computer (FAC) on A320 receives the speed

and the altitude from the three ADC.

ADC 1

ADC 3

ADC 2

FE FAC

COMPARISON/

SELECTION

speed/altitude

speed/altitude

speed/altitude

On A320, same check is performed by the FMGC on the

speed and the altitude.




the related ADC is eliminated without any caution.

Normal operations continue (except that CAT 3 is no more

available).

If one speed deviates:

ADC 1

ADC 3

ADC 2

speed/altitude

speed/altitude

speed/altitude

COMPARISON/

SELECTION

FE FAC

Fly by wire aircraft:




autopilot and autothrust disengage.

If the two remaining ADC are no longer in agreement:

ADC 1

ADC 3

ADC 2

speed/altitude

speed/altitude

speed/altitude .DISAGREEFE FAC





When the disagree disappears:

the autopilot and the autothrust may be used again

on the A330 / A340.




Conclusion about architecture Single failure (one probe affected):

A300 / A300-600 / A310:

One display is wrong.

AP and ATS are affected with possible automatic

deactivation.


One display is wrong but,

no effect on flight control and guidance.


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Conclusion about architecture Dual failure - Two probes affected, giving

different wrong values:A300 / A300-600 / A310:

Autopilot and autothrust are affected with possible

automatic deactivation.


flight control normal law and protections, autopilot,

autothrust are lost; on A330/A340, FD/AP/ATHR are

again available if disagree disappears.

The crew is aware of the anomaly (ADR DISAGREE

ECAM caution).

The aircraft reverts to a conventional aircraft, in

manual flight.


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Conclusion about architecture Dual failure - Two probes affected, giving

identical wrong values:

This may not be detected by the systems

which may eliminate the correct value and usethe erroneous ones (only fly by wire aircraft).

The crews must be aware of thesymptoms in order to recognize such a

situation.


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Pitot probes

Total

pressure

ADM

Drain hole

Heatingresistor


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Pitot probes - heavy rain

speed indication is too

low or fluctuating

It returns to normalwhen water drains.

ADM

Total pressure

Measured pressure is

lower than total

pressure:


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Pitot probes - icingPitot icing may be due to:

severe icing condition (in this case the anomaly is

transient)

or, pitot heat failure.


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Pitot probes - icingADM

Total

pressure

with free

drain hole

speed indication

decreases.

If icing is transient, speed

fluctuations and erratic

A/THR are observed.

Measured pressure

gradually decreases :


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Total

pressure with

obstructed

drain hole

Measured pressure

remains constant

Pitot probes - icingADM


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Pitot probes-foreign objectaccumulation

ADM

Total

pressure

Measured pressure

remains constant:

Abnormal AP/FD behavior to

hold the speed:

pitch up in OPEN CL pitch down in OPEN DES( SPEED MODE A300/A310).

Speed is constant in level flight.

It increases in climb.

It decreases in descent.


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Pitot probes obstructedOther consequences:

The Mach number will vary in the same way as

IAS.

The altitude and V/S information remains

correct. The FPV remains correct.


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Static probes - total obstruction ongroundStatic pressure constant at airfield value.

Lift-off

speeddecreases

speedincreases

altitude remains constant

Normalindications


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Static probes - total obstruction onground Altitude is wrong

Speed and Mach are wrong

Flight Path Vector is unreliable


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RecommendationsPilots should suspect erroneous speed/altitude

information when they observe:

Fluctuations of indicated speed.

Abnormal correlation of the basic flight parameters

(speed, pitch attitude, thrust, climb rate).

Abnormal AP/FD/ATHR behavior.

Undue stall warning or overspeed warning.

Reduction of aerodynamic noise with increasing speed.

Increase of aerodynamic noise with decreasing speed.


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Crosscheck all three ADR outputs and

stand-by instruments.

Recommendations When erroneous speed/altitude information is

suspected:


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Crosscheck all three ADR outputs and

stand-by instruments.

Recommendations When erroneous speed/altitude information is

suspected:

Detecting an unreliable airspeed indication presents

some traps:

all indications may be consistent but equally

unreliable. indications may differ but attempting to assess the

correct indication may be hazardous.


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RecommendationsWhen erroneous speed/altitude information is

suspected:

Disconnect AP, FD and A/THR.


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RecommendationsWhen erroneous speed/altitude information is

suspected:

Pitch and thrust memory items have been

established for short term flight path control:

Intended to provide short term protectionagainstVls and Vmax until the QRH is accessed.

Fly target pitch attitude and thrust setting.


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Procedureto ensure safe flight before referring to QRH

AP/FD....................................................................OFFA/THR....................................................................OFF

FLAPS.......................MAINTAIN CURRENT CONFIG

SPEEDBRAKES.......................CHECK RETRACTED

L/G...........................................UP WHEN AIRBORNE


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Procedureto ensure safe flight before referring to QRHINITIAL PITCH ATTITUDE AND THRUST GUIDANCE:

A300 / A300-600 / A310:

To climb:

PITCH...with slats extended: 12.5

with slats retracted: below FL

100: 7.5

above FL 100: 5

THRUST................................................100% N1


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INITIAL PITCH ATTITUDE AND THRUST GUIDANCE:

A300 / A300-600 / A310:

to level off / maintain level flight (at safe altitude) :

PITCH ................................2.5

THRUST.......................... below FL 100: 72 % N1

FL 100 - FL 250: 82

% N1

above FL 250: 92

% N1

To descend:

PITCH ..............................- 2.5(nose down)

THRUST........................ IDLE



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INITIAL PITCH ATTITUDE AND THRUST GUIDANCE:

Fly by wire A/C:

With slats extended:

THRUST......................................................MCT

PITCH ATTITUDE...................................12.5

In clean configuration:

THRUST LEVER..............................CLB

PITCH ATTITUDE below FL 100...................10

PITCH ATTITUDE above FL 100.................5

aircraft climbs with the initial pitch and thrust (simpler than

A300/A310 to have common values for A320, A330 and A340).



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Procedure to continue the flight When the aircraft flight path has been

stabilized the crew should refer to the

unreliable speed indication procedure or , in

cruise, thrust setting for turbulence speed.

This permits to refine the initial pitch and thrustsettings

and prepare for the next phase of flight.


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Procedure to continue the flight Takeoff:

Pitch and thrust are provided for level flight at F and

S speeds for configuration change from Conf2 to 1

and from Conf1 to Clean.


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Procedure to continue the flight Climb , cruise and descent:

Pitch and thrust are provided for turbulence speeds

(speeds vary according to the FL)


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Procedure to continue the flight Cruise:

A detailed thrust table for the cruise is available inQRH 5.01 SEVERE TURBULENCE

Note that the crew may also use this table in case of

ADR disagreement to determine which ADR (s) is

wrong


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Procedure to continue the flight Approach:

decelerate from the descent speed to green dot speed:

increase pitch and thrust according to the table ( level flight

at green dot)

Green

Dot

Descentspeed

Clean conf


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Procedure to continue the flight Approach (contd):

Configuration changes are done in level flight

Green Dot S speed Vls conf2

+10

Vls conf 3

+10

5.5 7.5 7 6

Conf 1 Conf 2 Conf 3 (Conf full )Clean


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Deceleration to the speed for the next configuration

change is done in level flight by:

reducing the thrust towards idle, and

increasing the pitch to maintain the level flight


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how to decelerate to the speed for the next conf change ?

Actual

conf

Next

conf

Reduce towards idle

thrust

Adjust pitch to maintain

the level flight

When the pitch reaches

the pitch required to fly

the speed to select nextconf, adjust the N1 to fly

that speed


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Conf 1 Conf 2

Clean

5

59 %

Reducethrust,adjust pitchfor level off

When pitchreaches 7.5set 60% N1

pitch N1

7.5

60%


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Procedure to continue the flightFinal approach:

- Reach FAF in Conf FULL and Gear down

- At FAF, set the appropriate pitch and N1 to descend at -3

gradient at Vls + 10 kt.


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Conclusions A new procedure concept with memory items is

proposed based on extensive simulator testing

The FCOM has been revised to provide

expanded awareness information and guidance

material

The new procedure is incorporated in the QRH

for quick access.


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Documents

Unreliable Speed Indication