lock nut failure

[gotalltheanswers]

has anybody experienced a failure of a lock nut so that a throttle linkage has been disconnected in flight?

[stevef]

Aren't all engine (and flying control*) linkages secured by split pins in castellated nuts, not self-locking nuts?
I know of one case in which an HS748 throttle became disconnected because the pin hadn't been fitted. Fortunately it happened on the ground.

* On second thoughts, some light aircraft flying control rods are secured with stiffnuts. IIRC, a Cessna 206 accident some years ago was the result of a Kayloc in the elevator bellcrank connection coming undone, allowing the bolt to fall out.

[HAWK21M]

I've noticed most commercial aircraft having Castellated nuts with split/cotter pins for safety.
Tell us more on what transpired.
regds
MEL.

[Horwood]

I will preach the obvious and state that you must always use parts that the AMM and IPC specify. In the case of flight and engine control always use new stiff nuts every time that they are disturbed .THEY ARE ONE TIME USE ONLY.Another thing to consider is that of the source of the stiffnuts. In the case of engine and flight controls I would only feel happy using parts supplied by the OEM. There are many standard part suppliers out there and bogus parts are not unknown and difficult to spot even to the trained eye. A known history is everything.

[gotalltheanswers]

I'm looking into an accident where we observed the throttle linkage had become disconnected with no other signs of damage etc to explain it. The aircraft was a PA31 with a lycoming engine. The lock nuts are the nylon type

[draughtsman99]

Having been in engineering for 40 odd years I would echo the comments above re 'Nyloc' type nuts being 'once only' fastenings in any critical application.
There is an added problem if 'Nyloc' type nuts are used where they may be subjected to high temperatures - ie high enough to melt the nylon insert!
I may be a bit of a luddite, but wire or split pins on castelated nuts are still far safer than any insert type nut. They can be inspected visually and are there or not unlike inserts which can be degraded without showing any signs.

Tin hat on in case the manufacturers take offence!!

[stressmerchant]

I thought locknuts were not allowed on this type of linkage.

FAR 25.607(c) No self locking nut may be used on any bolt subject to rotation in operation unless a nonfriction locking device is used in addition to the self locking device

(FAR 23.607(c) identical)

[Von Klinkerhoffen]

Concur with Horwood and draughtsman99 . Self locking nuts , whether Nyloc or all metal can only be used once in primary control or critical bolted joint applications . They may be re-used elswhere provided that their locking ability is still intact . Indeed Nylocs should only be used in cool areas as the nylon does melt out releasing the nut ! Having said all that , as previously stated , ALWAYS follow what the aircraft manufacturer says in the AMM and IPC !

VK

[NutLoose]

There is a Lycoming SB of old that called for all self locking nuts to be changed to the good old mk1 splitpin.