Peter wrote:
Nowadays you would use a 3 phase alternator, with an inverter driving it to form a starter motor
Considering we’ve just been discussing the very poor quality of software in electric aircraft, It’s re-assuring to see this kind of thing going on in ‘traditional’ aviation as well. How much would a grounding brush on the shaft cost? It’s a well known requirement for the very reason stated. It could even have a leakage warning built in.
Of course the damaging currents could be induced in the shaft itself, and ‘carbon dust’ seems a little convenient as an explanation, a bit like carb icing.
Given it’s a rare event, but Wow! Where did these people learn engineering? Not in consumer electronics, that’s for sure.
It is indeed super-rare – since book-keeping started in the mid-90es there have been about 40-odd instances of EDD (with damage ranging from minimal to worst-case bearing #1 failure). Over the whole installed PT-6 fleet it is a very small occurence rate, although sure enough the design could have been optimised.
Interestingly enough there is a helicopter accident investigation where it was determined that EDD occured when the engine was not running (but they could not determine what caused it).
Thanks T28, your analysis is spot on. I was just very very unlucky and the 40 or so instances of known EDD sounds about right. Negligible risk and strongly mitigated if the precautions described by Sebastian earlier in the thread are included in the maintenance programme.
Some programs you just can’t afford to stay off, like Williams. They force your hand. On program your TBO is 5000hr, off program it’s 4000hrs. Magically, the engine became more reliable by a 1000hrs as long as your check-writing ability wasn’t impaired…
I think PT6’s makes sense to be on, also some of the P&W jet engines for the Mustang and the mentioned Williams etc. But TPE’s do not make sense to be on.
Aveling wrote:
Considering we’ve just been discussing the very poor quality of software in electric aircraft, It’s re-assuring to see this kind of thing going on in ‘traditional’ aviation as well.
I for one do not think we are discussing the same thing:
The whole idea is not to have an unplanned flight termination which will be, in this context, the main safety-impacting event.
I don’t have the background on the PT-6, but definitely this particular incident is a problem detected during scheduled maintenance which is what scheduled maintenance is for. Inspections intervals have been planned precisely to avoid a safety impact. Perhaps there were a few in-flight shut-down events originally driving this required inspection?
Even if it would not stop an inflight gearbox damage it would surely show up in the chip detector driving further inspection in resulting on removal of the engine from service without a major safety event.
So by design logic (either inspections or chip detector functionality) , the problem is driven out of the safety realm , which is exactly what we want.
This was not the case in the A400M or Pipistrel Alpha-Electro.
What we are discussing here seems mainly a cost and reliability issue, not a major safety concern: whole different games.
This is not meant to say the design is good: only that its safety impact is minor. Then the push for a fix is more a commercial matter.
I wonder: in EASA land is it possible for private operators of the likes of TBM (with PT6-64 and a TBO of 3500 hrs) to operate “on condition” above TBO?
Since ELA2 is max MTOW 2000kg, the associated alleviations do not apply. In theory you could still go around the OEM recommendations, but in practice it is very difficult, especially for a one- aircraft operation. For a fleet operator, using historical reliability data you could in theory and progressively extend the interval, but I do not know any fleet operators of these types.
The “fleet” concept is important since it ensures a controlled environment. When you have multiple individual operators with wildly different, and often very low use, operational profiles, it is more difficult to have benevolent one-size-fits-all intervals.
We did that e.g. here and the view seemed to be that, N-reg, you could do it, but almost nobody does due to the effect on resale value.
Also, and this is the biggest issue with GA maintenance in Europe which drives most owners to using a company, you would need your own facility i.e. a hangar and a freelance engineer to do the work. None of the turboprop maint companies would touch this with a bargepole (I did ask one I knew well about it).
thank you all for the answers. I see that More company offers the following: “instructions for Continued Airworthiness” for pt6-64. Is not an STC but. according to them, it could potentially extend the TBO to. 8000 hrs. Anyone has any idea about that?
