DRIVESHAFT FAILURE

[[email protected]]

gear fail = the shaft between the input and output end of that gear breaks, however so.

so it is a shaft failure or the bolts that hold the gear to the shaft and not the gear breaking up - that makes more sense.

However, Nick's post indicates that the problem won't happen again since the shaft and gear were redesigned - therefore not much point in teaching it in the sim.

On the N1 governing by the DECU - with AC talking about matching N1 or Tq - I think that is confusing the ability to beep the engines up and down to match them in the way you prefer with what the DECU is designed to do (govern N1 and load share with the other engine)

[Ascend Charlie]

The B in the 76B stood for BALLS, which it had plenty of with the PT-6 and FADEC.

[gulliBell]

Originally Posted by [email protected]

1. so it is a shaft failure or the bolts that hold the gear to the shaft and not the gear breaking up - that makes more sense.

2. However, Nick's post indicates that the problem won't happen again since the shaft and gear were redesigned - therefore not much point in teaching it in the sim.

3. On the N1 governing by the DECU - with AC talking about matching N1 or Tq - I think that is confusing the ability to beep the engines up and down to match them in the way you prefer with what the DECU is designed to do (govern N1 and load share with the other engine)

1. I think that gear is 2 separate pieces that are bolted together. As NL pointed out the failure mode in the example cited was bolts coming loose. Gears in helicopter gear boxes can certainly fail and make metal for other reasons. I know, not long after when we got C model there was an AD about the possibility of loose bolts within the MGB. Sikorsky sent a maintenance team to open all the gear boxes in the new helicopters and check the torques on the bolts. They had some pretty impressive test gear with them. It is very rare event to see inside a S76 MGB.

2. As I mentioned earlier, we do not cover this malfunction in the simulator. It is discussed in class room training, more-so to get the trainees to think about and diagnose a set of circumstances which might not be covered in the RFM. Not all malfunctions are in the ECL.

3. There is no beep available to the pilot with DECU in automatic mode. The DECU gives you 107% N2 on both engines with N1 matched. That's it. The engine trim switch on the pilot collective - of which there is only one - only functions when the engine is in manual mode (i.e. using the MFCU controlling fuel flow, DECU controlled AFCU is disabled). It will beep up/down the engine/s in manual mode. Things get a bit tricky with the single eng trim switch and both engines in manual mode, particularly if there is a TQ split between the engines. This isn't usually practiced in the simulator, but it is easily manageable if you know what you're doing.

[212man]

On the N1 governing by the DECU - with AC talking about matching N1 or Tq - I think that is confusing the ability to beep the engines up and down to match them in the way you prefer with what the DECU is designed to do (govern N1 and load share with the other engine)

DECUs, and any governing system, only govern N1/Ng when the engine(s) are at idle. With the Nr in the normal operating range they govern N2/Nf. Matching N1 is something different.

[212man]

Originally Posted by Ascend Charlie

The B in the 76B stood for BALLS, which it had plenty of with the PT-6 and FADEC.

Pretty sure it doesn’t have FADEC.

[[email protected]]

212man - yes, my mistake, I wrote N1 when I should have put N2 - it comes from years of calling them Ng and Nf instead of N1 and N2 - doh!

Gulli - thanks, the Bell DECUs and Airbus DECUs clearly work in subtly different ways.

Since many aircraft have the facility to let you beep up Nr for Cat A, either automatically or via a switch/beep, I assumed the 76 was the same.

[gulliBell]

Originally Posted by 212man

DECUs, and any governing system, only govern N1/Ng when the engine(s) are at idle. With the Nr in the normal operating range they govern N2/Nf.

Not quite. After the engine is started, the DECU only controls N1 once the N2 exceeds 9%. Which is why, if you start the engine with the rotor brake on and then move the ECL from IDLE to FLY, the engine stays at idle.

And the other point. The DECU has no idea what the NR is...it doesn't even have an input for NR signal. The trigger for the DECU to govern N2 is when it senses the ECL in FLY position.

[212man]

Not quite. After the engine is started, the DECU only controls N1 once the N2 exceeds 9%. Which is why, if you start the engine with the rotor brake on and then move the ECL from IDLE to FLY, the engine stays at idle.

So you start with the rotor brake on and the engine sits there happily at idle N1 - what do you suggest is controlling that value?

And the other point. The DECU has no idea what the NR is...it doesn't even have an input for NR signal. The trigger for the DECU to govern N2 is when it senses the ECL in FLY position.

I didn't suggest it did sense Nr, but if you don't have Nr you won't have N2 and in normal conditions they are matched.

I was also talking in generalities about any governor, whether a FADEC, a DECU, an ECU or a hydro-pneumatic system using P3 air and bob weights. You start the engine and the N1 is governed then, as you increase the power the N2 increases and at a certain point it becomes the datum that is then governed.

The trigger for the DECU to govern N2 is when it senses the ECL in FLY position

Are you positive about that? In the S92 when you advance the throttles from IDLE to FLY the N2/Nr will rise and then reach the normal value of 105% at which point pushing the throttles forward further has no effect. This happens a couple of inches before reaching the FLY detent and clearly indicates that the FADEC has taken over N2 control. There also some flight checks that require the throttle to be retarded out of the FLY gate, but the N2 is still governed. It was the same in the 76A+ with its steam driven system.

(Note - I do know that in the S92 the Nf/N2 is referred to as Np, but was keeping it simple)

[Salusa]

Terminology

I think we should all think carefully about..

N1/Ng
N2/NP
N3/Nr?

Goes to show standardisation is required..

Hated Torque matching 212 - 3B engine with TCU. 412/212 with ITT trim better but can lead you away... PTGs obviously don't match perfectly and one OHC unit against a almost timex unit can be interesting. PWAC removed field cleaning from MM so orifice clean no longer an option

Back to original question.

Most twins have overspeed protection of some sort. Singles driveshaft failure will be interesting...

Revert to the pilots...

[[email protected]]

But Ng = gas generator rpm
Nf = free power turbine rpm
and Nr = rotor rpm

That is both logical and easily understood

[megan]

Nick's post indicates that the problem won't happen again since the shaft and gear were redesigned - therefore not much point in teaching it in the sim

Sometimes the dog of fate lifts its leg and urinates on the pillar of science. Never believe in the word never, as in, it can't happen. The only MGB problem in this regard I ever had was the rotor brake quill on a 212 snapping in half for some unknown to me reason. Bevel gear dropped and bounced around in the MGB and the brake disc left lying on the cabin roof. Any part can fail for any number of reasons, heat treating, tooling marks, corrosion, inclusions being a few. I'm sure some one once said a MGB can't fail and release the main rotor. We know better now, and still await the initiating cause.

[Ascend Charlie]

There was a case of a 412 in Oz, airborne for a check flight after service, and the drive shaft from the combining gearbox let go.

Two serviceable engines having a little overspeed, rotor having somewhat of an underspeed, but the cool captain pulled off a good auto and all was well.

[gulliBell]

Originally Posted by Ascend Charlie

..Two serviceable engines having a little overspeed, rotor having somewhat of an underspeed, but the cool captain pulled off a good auto and all was well.

The aircraft was damaged but they walked away...I think I read that the auto technique as described by the Captain was to develop the flare at the bottom until the tail stinger hit the ground, and then level off and land. And that's exactly what happened: the impact bent the tail boom but, as I said, they walked away.