The brief judder on the way up is the indication. In this case it happens only if the gear has been cycled several times immediately previously i.e. it will never be seen during normal flight operations. The aircraft did about 150hrs (airborne time) during the preceeding 12 months and was found to be doing this immediately during the Annual, not before.
The judder happens before all three legs are up against the rubber stops so cannot be a pressure switch issue, because the pressure reaches the limit (from memory, 1400psi) only when all three legs are up against the stops. It also cannot be a sticky actuator, because (a) again that would just cause another part of the gear to move instead and (b) the gear moves very freely when moved by hand (the hydraulics have to be used to move it just off the locks before it can be hand-moved).
The aircraft is running from external power, 28V 40A max. During normal operation, the gear pump draws 10-14A, but during the judder (1-2 seconds) it draws about 30A. This can be only due to
The particular gear pump used on the GT is made by Commercial Hydraulics and is quite cheap (Socata claim $1000, though they seem to have difficulty with working out the correct P/N) and has no serviceable parts.
Even the motor brushes are not sold as spares, which is obviously bogus, but at $1000 for a new one I don’t want to remove the pump just to take it apart to see if the brushes can be changed, especially as many cheap motors (e.g. ones used in King autopilot servos) wear the brushes at the same rate as the commutator so by the time the brushes are used up, the motor is scrap. It’s true that there is a procedure for rebuilding the commutator, and I have done that, but it is strictly for really difficult situations. It’s also “technically illegal” (according to one “Socata personality”, anyway) to service any subsystem for which there isn’t a published CMM (component maintenance manual). Obviously I think that’s nonsense and would change the brushes anytime, but at $1000 it’s not worth the effort given that the issue may be also elsewhere. And that way one ends up with a spare (repaired) pump on the shelf which is itself a very handy thing.
This video shows normal operation, with the judder only at the end, which was found to be typical. It looks like the motor needs to be warmed up to show the problem.
Not directly related, but I don’t think it’s a good idea to cycle the gear without at least a minute or two cool down between cycles.
Good point Michael
It seems to me something is wrong with the commutator. Have seen this on Cessna power packs a couple off times, where it even can cause failure of the motor to start, when the commutator is stopped on the wrong segments.
I would check the gear pump breaker as well – I once chased an intermittent / jiddery power-pack pump on a Cessna. After changing out the pump motor and no joy, I checked the breaker – sure enough, it was going bad. Sure wish I had started with it !
I´d check the harness around the landing gear, limit switches and so on. I don´t believe the pump is at fault as it is producing perfect power, nothing related to the commutator. You could run a direct wire to the motor for testing on the jacks. And tell us your findings.
VicI don’t think it’s a good idea to cycle the gear without at least a minute or two cool down between cycles.
If that is real, it would explain the issue 100%. That would however suggest that the motor is way too small for the job!
For a brief period, pre-GT, Socata were fitting a pump made by LHC, which is used in the TBM. This was apparently very good but cost more than 10k.
something is wrong with the commutator
I wondered about that too. However the issue seems fairly consistent with the number of cycles immediately preceeding the “not good” cycle. This suggests something is overheating, rather than the effect of the motor sometimes stopping on a duff spot of the commutator (leading possibly to an open-circuit or – more likely – a short-circuit winding).
I would check the gear pump breaker as well
That however should just cause
It could not account for the extra current drawn by the motor. There is no way to make something draw more current by putting anything in series with it.
You could run a direct wire to the motor for testing on the jacks
That is a really good point. I guess I was not in a great position to try that, due to
To me, the extra current draw, plus the ease of moving all the parts by hand, suggests it must be the pump (and probably the motor). The only other explanation is an esoteric pipe blockage somewhere.
I would check the gear pump breaker as well
That however should just cause insufficient power going to the pump, and massive heating inside the breaker
First off, let me say that electrics are not my forté.
With that out of the way: I believe that the “bad” breaker develops resistance from poor contact between the contact blades and results in the blades “chattering” but not heating up sufficient to open the breaker. This is exactly the situation I had and it drove me nuts untim I changed out the breaker !
The 30A “spike” correlated totally with the “hesitation” in the upward movement of the nose gear. It thus didn’t happen at the start of the cycle.
Also the issue was never seen if the pump was left unused for a few mins or longer.
