SPORTPLANE BUILDER Tony Bingelis

W

Spin-on oil filters must be safetied. Sometimes it is a challenge to find a way to doit, but do it you must.

When completed, your aircraft willbe subjected to many stresses in flightand on the ground; any of which canabnormally affect the integrity of themany assemblies making up your air-craft structure . .. and your safety.

These stresses are induced, for themost part, by engine vibrations, in-flight turbulence, in-flight man-euvers, and landing loads.

What keeps an aircraft from com-ing apart in spite of these unrelentingever present stresses in flight, and onthe ground, are the countless nuts,bolts, and other fasteners and devicesused to hold the airplane together.The unique thing about almost allthese a i rcraf t qual i ty fasteners istheir fail-safe nature.

Most builders know that using or-dinary hardware nuts and bolts inaircraft s t ructures is a dangerouspractice. This type of hardware ismade of mild steel and, for the mostpart, lacks the structural strength andcorrosion resistance of aircraft hard-ware. Furthermore, the hardwarestore varieties are generally recogniz-able by their coarse threads - and nutsthat lack the self-locking featurescommon to aircraft hardware. Al-though these hardware store nuts andbolts are plated and look nice andshiny, they are not very corrosion re-sistant, nor are they very strong. Inshort, don't use them in any aircraft

areas where structural integrity is animportant consideration.

It is embarrassing enough to havesomeone point out a missing screw onyour aircraft, but think how embar-rassing it would be to have somethingcome loose in flight and fall off.

Look most any store-bought air-plane over closely and you willprobably find at least one screw orsome kind of fastener missing from itsprop spinner, cowling, or landing gearfairings . . . makes you wonder what

else you would see under the cowling.This need not be typical of your

aircraft if you take the time to ensurethat every assembly you make, ini-tially, is a fail-safe installation.

And just what is a "fail-safe" in-stallation? Webster's Dictionary saysit is "incorporating some feature forautomatically counteracting the ef-fect of an anticipated possible sourceof failure."

To me this means that a fail-safeinstallation is one where each bolt,screw or pin is of aircraft quality, andis secured by a self-locking nut, acastellated nut properly locked with acotter pin, or is correctly safety wired,as the need may be.

However, all the fail-safe devicesin the world will do you no good un-less they are installed, and installedcorrectly, too!

I know of one homebuilder whomade a shocking discovery during hisfirst annual inspection, one year afterhis aircraft was duly certified, testflown and thereafter operated safelyfor more than 100 hours. He foundthe clevis pin connecting the controlcable to his right rudder pedal didnot have a washer or cotter pin to se-cure the installation. How lucky canyou get?

Another individual was equally aslucky. One day, coming in to land,he routinely reached down to apply

Gascolator design dictates how it must be safetied. Safety wiring the bowl of this pop-ular all-metal gascolator is often overlooked when installed by the amateur builder.

76 SEPTEMBER 1994

about 10 degrees of flaps for land-ing, but the instant he pulled up onthe flap handle the airplane vio-lently lurched into a left b a n k .Startled as he was he had the pres-ence of mind to quickly release theflap handle quickly, and the aircraftresumed level flight. Every time hetried to apply flaps the airplanewould bank v io l en t ly . It f i n a l l ydawned on him that his left flap hadsomehow become disconnected. Ano-flap landing was safely com-pleted. Inspection of the left flapassembly revealed that the bolt con-necting the flap actuator lever wasmissing. It was found in the bottomof the aircraft . . . there was no signof a nut.

Common fasteners can loosen andfall off, but even fail-safe hardwarewon't do you any good unless it is in-stalled - and installed correctly!

Consider this - every bolt, screw,rivet, or fastener of any type is im-portant to the structural integrity ofyour airplane or it wouldn ' t havebeen installed in the first place!

Here are a number of standard air-c ra f t qua l i t y hardware items ofproven reliability commonly used toproduce assemblies that are virtuallyfail-safe:• Aircraft bolts with a drilled head

(secure by safety wiring).• Aircraft bolts with a drilled shank

(install with castle nut andcotter pin).

• Self-locking nuts (many varieties -use only high temperature typelock nuts in the enginecompartment).

• Lock washers (split type or shakeproof varieties).

• ESNA Roll pins.• Safety Wire - corrosion resistant

(stainless) steel.• Cotter pins.• Double nut installations.

One FAA old timer once told methat whenever he saw a bolt or screwwith a drilled head he expected it tobe safety wired. This is, of course,understandable where a blind instal-lation is made, one where the bolt isthreaded into a housing and no nutis needed.

A typical example may be seen inthe two AN4H bolts securing theback plate to the brake cylinder as-sembly on some Cleveland brakes.However, what if a nut, a self-lockingnut, is used in an installation with adrilled bolt head? Must the bolt head,too, be safety wired, somehow?

My suggestion is why ask why.Simply use a bolt that is correct forthe application.

Shown here is a black anodized prop extension. Notice that the drilled bolt headsare safety-wired in pairs. It is most essential that the prop bolts be carefullytorqued with a torque wrench before safetying them.

Turnbuckle Installations

Most homebuilts will ut i l ize atleast two turnbuckles to connect therudder cables to the rudder pedals.

Some biplanes, of course, have manymore in other locations.

A turnbuckle is a nifty device as itallows you to easily adjust the lengthand tension of a cable assembly with-

CUT ENDS AND CLINCH AS SHOWN

CASTELLATED NUT ON DRILLED BOLT(PREFERRED METHOD)

COTTER PIN SECURED(ALTERNATE METHOD)

LONG THIN TYPE ISEASIER TO INSTALL WITHONE HAND IN HARDTO REACH LOCATIONS

^ 1/16" DIAMETER(LENGTHS 1/2" - 2 ")

1/8" DIAMETER(LENGTHS 1/2"-2 1/2")

1/32" DIAMETER;LENGTHS1/2"-1")

3/32" DIAMETER(LENGTHS 1/2" - 2")

CLEVIS PIN

PLAIN WASHER-

COTTER Pll>CORRECT CLEVIS PIN ASSEMBLY

ASSEMBLED(BOTTOM VIEW)

WASHERCLEVIS PIN

COTTER PINSECURED

FIGURE 1.TYPICAL COTTER PIN INSTALLATIONS

SPORT AVIATION 77

The crankcase oil sump drain plug must be carefully torqued and safetied asshown here. If the drain plug loosens or falls out, there goes the oil supply, andyour engine . . . and maybe you.

These two drilled bolt heads must be safety wired together as a fall-safe measureto be sure the brake cylinder housing does not come apart. It Is easy enough toforget to do this.

This throttle lever is Installed with a large washer to prevent the uni-ball rod endbearing from accidentally separating In service.

out disconnecting the entire installa-tion. However, this implies, too, thatit is extremely important to safetywire this type of device so it doesn'tloosen ... or come apart.

To ensure that a turnbuckle is ca-pable of developing its m a x i m u mstrength, no more than three threadsshould extend beyond either end ofthe barrel after it has been adjustedto the correct cable tension for thesystem. Then, and only then, will itbe ready for safety wiring.

Safety wiring a turnbuckle can be-come a study in complexity . . . if youpermit it. I've seen some unusualways to safety a turnbuckle, some ofwhich, in my estimations, are ridicu-lously complex.

For our part, we should, of course,abide by the FAA recommendationswhich apply to all civil aircraft. But,even the FAA offers a simple methodof safetying, so why not use it?

Actually, the different safetyingtechniques boil down to two basicmethods. The so called single wrapand the double wrap turnbuckle safe-tying methods.

Turnbuckles can loosen allowingthe control system to become unreli-able, or loosen to the extent that slackin the cable might lead to the develop-ment of control surface flutter.

The simple single wrap system ofsafetying a turnbuckle is consideredto be adequate for most homebuiltapplications and is illustrated inFigure 2.

Use of Safety Wire

First, here are a few pointers re-garding the use of safety wire. Safetywire is made of a variety of metalssuch as soft iron, brass, copper corro-sion-resistant steel, Monel metal andeven aluminum. For all practical pur-poses, I would suggest you use onlythe corrosion resistant (stainless)steel lock wire as it is fairly economi-cal and is readily available fromhomebuilt supply sources.

In general, safety wire comes in anannealed state, and the most fre-quently used size is .032" in diameter.However, 0.040" diameter wire is therecommended size wire to use forsafetying turnbuckles.

In some nonstructural applica-tions, like securing switches andcover plates, a smaller size wire maybe necessary where the hole for thesafety wire is too small to accept the.032" size.

Actually, in these applications theuse of a finer safety wire is O.K. andis more to serve as an indication that

78 SEPTEMBER 1994

The ball joints in exhaust systems arelightly clamped together with bolts.Even high temperature self-locking nutsshould not be used in such applicationsas they may back off. The correctchoice? Castle nuts and cotter pins.

the unit has been opened or, perhaps,otherwise tampered with than it maybe for structural security.

Safety wire can be twisted by hand,with a pair of pliers (tape the jaws toprotect the wire) or, preferably, witha safety-wiring tool.

Always install the safety wire sothat the tension on the wire will beincreased if the turnbuckle nut, boltor screw starts to loosen. Completetwisting the locking wire by forminga pigtail that will be about 4 to 6twists long after the wire ends havebeen cut off.

Do not over-twist the wires or theymay become over-stressed and break.

CORROSION RESISTANT SAFETY WIRE(040" DIAM) AFTER ADJUSTING

CABLE TENSION,NO MORE THAN 3

INITIAL BEND THREADS SHOULDSHOWCl

\\! ' — ill — 1 " w — l i '

INITIAL BEND

STEP 2

•PIN EYE(OR FORK END) K

1

4 TURNS OF WIRE

STEP 3

LOOP WIRE THRU FORKEND AND WRAP SHANK

FORK END

COMPLETE WITH,4 TURNS

COMPLETE ASSEMBLY

FIGURE 2TURNBUCKLE SAFETYING(SINGLE WRAP METHOD)

TON*

Bend the sharp cut ends of the pig-tail in, out of harm's way, so it willnot stick out and snag on something -something like the delicate flesh ofyour arms and hands as you attemptto work on something else.

Finally, don't attempt to reuse safetywire. The money thus saved won ' teven buy you a chunk of bubble gum.

Torque It First

The first step to obtaining a fail-safe installation is to be absolutelypositively sure that every bolt, nut,and screw you ins ta l l is properlytorqued before you safety wire it orotherwise secure it.

You can see where this is critical

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SPORT AVIATION 79

1DRILLED BOLT HEAD/ STAINLESS STEEL WIRE

(.032" DIAM)

LOOP A PIEC

OOF WIRE THRUDRILLED BOLTHEAD (1)

GRASP WIRES, AS SHOWN,SLIGHTLY BEYOND IN-TENDED ATTACH POINT (2)(TWISTING WILL SHORTESOMEWHA

NOTE-ALWAYS TORQUEBOLTS BEFORESAFETY WIRING

PULL ONE WIRE THRUBOLT HEAD (2) - GRASPWIRE END AND TWIST

—.TO FORM PIGTAIL>

NO MORE THAN10 TWISTSPER INCH AUTOMATI

RETURNNOB

PIGTAIL 4-6 TWISTS- BEND TO PREVENTSNAGGING

COMPLETE INSTALLATIONFIGURES

HOW TO USE WIRE-TWISTING PLIERS

The Lycoming engine dynafocal mount bolts are secured by castellated nuts andcotter pins. Inserting cotter pins into the lower attach bolts/nuts is very difficult,but it can be done.

when installing a propeller, but isn't itequally important when installing yourcrankcase oil drain plug and oil filter?

In each of these cases the torqueapplied must be within prescribedlimits - especially when installing

the propeller. Torque values aremeasured in either inch pounds,foot pounds.

For example, let's say you are us-ing an 8" wrench on a 3/8" boltinstallation. If you apply a 20 pound

If you wish to contact the authorof this column for additional infor-mation, please send a SASE to:

Tony Bingelis8509 Greenflint Ln.Austin, TX 78759

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force to turn the nut with thatwrench, you would be applying atorque value of 20 x 8", or 160 inchpounds. If you want to express thetorque value in foot pounds, dividethe 160 inch pounds figure by 12 andyou will get 13-1/3 foot pounds.

Obviously, to obtain accuratetorque values you should use a torquewrench rather than trust to your ownbuilt-in "armpit sensor." \\<

Final Reminder

It is not enough to use aircraft qualityfail-safe hardware and devices; theymust also be installed correctly to beeffective.

80 SEPTEMBER 1994