(Auto)feathering the live engine?

[Clandestino]

I've recently heard of incident that happened to ATR. While investigation is still in progress, it seems that SCU1 failed immediately after take-off. TQ indication no1 went to zero but Np1, ITT1 and Nh1 remained at normal take-off values and eng no2 got uptrimmed. Fortunately, SCU recovered quickly so autofeathering did not occur. I wonder what would happen if ATPCS tried to feather engine that was still producing take-off power. My opinion is that it would load prop so severely that blades or hub failure would be inevitable but then perhaps I'm missing the part of the picture. Also I'd like to know if inadverent autofeathering ever happened before on any type of prop airplane and what were the consequences.

Many thanks for the replies.

[barit1]

I can say definitely that it happened at least twice on a flight test a/c I was familiar with, but it didn't have production-type systems.

We did standard overtorque checks and drove on.

[captainpaddy]

Happened before to ATR72. Exactly the same problem. The ATPCS in the 72 is triggered by the same source as the needle on the torque gauge. The digital readout comes from another source. (I can't remember now what units they come from) In this case the digital readout remained, although the needle dropped to zero, therefore a FDR reading of the torque was retained. The engine did fully auto-feather. I believe the torque eventually reached something like 170-180%, despite the fact that the crew realised it just as it happened and made an attempt to pull the power lever back. All props removed and scrapped, but stayed together. Surprisingly, the theory is that as the RPM reduces, the torque should peak and then drop off rapidly despite running at full power, so it is unlikely that the props would disintegrate. Torque is really the force resisting rotation, so both blade angle and RPM combine to produce it. I have been told that the RPM would decrease extremely rapidly and limit the torque achieved. The props are the most limiting factor and the engine should be fine unless the ITT limit was exceeded.

ATR maintenance manual lists the limits for overtorques and I believe about 145% torque or above requires immediate removal and replacement of all affected propeller blades. Below 145% (or so) various inspections are required to ascertain if any significant damage had been done. They are very strict inspections and it can be thought of as - below 145% the props must come off unless the inspection proves that they are still intact. Even at that I believe only a ferry flight may be allowed.

If you ever see a fluctuating needle on a 72 (particularly one that momentarily drops below 20%) think twice about going! I presume the 500's (42 or 72) are the same. Or, turn the ATPCS off and then work out the RTOW and realise you will have to leave all the PAX behind!

Very interesting to hear that it happened somewhere else.

Keep the eyes on that UPTRIM light!!!!

CP

[Capt Claret]

Had the prop on a DH8-200 with PWC123 engines fitted, autofeather just on lift off, whilst still producing take-off power years ago.

The highest Tq I recall seeing was 125%. Apart from Np, all other engine parameters were normal. The highest observed Tq exceedence didn't require a special inspection. The cause was determined to be a faulty signal from the EEC, in turn caused by a poorly fitting canon plug. The plug was repaired and we were on our way.

The hardest part from my perspective, was the mind numbing confusion caused by a feathered prop with Nh, TGT, FF all in the take-off range! :eek: It shouldn't have feathered, and it seemed to take for-ever to get past the contradictory indications. A legacy I guess from all prior multi training being in the aircraft (not sim) where the throttle would be retarded to simulate a failure and the confirmation would be from low TGT/Np/Nh.

[Plain Driver]

@ Claret:
The reason you never saw anything over 125 % TRQ is that the instrument simply does not indicate higher . You could have had much, much more. :uhoh:
The ECU stores the actual value which then can be retrieved by maintenance personnel. Also, the FDR should record the actual value.
The PW123 does have a system installed which will cancel Np underspeed governing from the ECU and uptrim the opposite engine. (Source of all info: AMM)
Supposedly this system should prevent damage to the engine/prop which had the unscheduled auto-feather.
Just my penny's worth.....
Regards, PD

[Stuck_in_an_ATR]

We had several similar occurences recently, on both the 45 and 72. All happened during climb, cruise, or descent and no engine was overtorqued (at least none had to be removed). All this happened with ATPCS disarmed :hmm: ATR is investigating, but still no answer...

[captainpaddy]

A torque display failure would be a non-event if the ATPCS is not armed. Auto-feathering does not occur and the actual torque is completely unaffected. In these cases, it is nothing other than an inconvenience. The only real critical phases are with ATPCS armed. i.e. From takeoff power selection until the PWR MGT is moved out of takeoff thereby disarming ATPCS. The only other likely phase is during a go-around for similar reasons.

[Stuck_in_an_ATR]

I'm not talking about torque indicator failure, but an actual uncommanded feathereing of the prop... In most cases it was transient, but in 2 (I think) it resulted in engine shutdown. Does anyone know, if other operators noted similar incidents?

[captainpaddy]

Sorry SiaA,

Didn't understand your original post. Do now though!;) I would be very worried if that is becoming in any way a regular occurence!!! Never heard of anything like that. We did however have many problems with the synchrophaser on the 42's particularly. It would occasionally rapidly coarsen the blade angle on the no 2 (slaved) prop and would give transient Tq and Np fluctuations. It normally was very obviously felt through the seat of the pants and could be extremely disconcerting but was always solved by selecting the synchrophaser off. I wonder whether you guys are experiencing something similar?

If I recall correctly, the synchrophaser has authority to adjust the Np by up 2.5%, but with the slight delay in the indication we were never sure if it was exceeding this value. 2.5% Np surprisingly causes a very noticable change in trim and Tq if it occurs rapidly. My own, admittedly completely unqualified, opinion would be that if was actually a feathering pump or something like that causing your problems, the Torque would almost certainly have been excessive. At climb power, particularly in a 72 at 102% or so, an Np reduction of just 8 or 9 % (assuming 77% Np) would cause a torque of 115% or more which would require maintenance action.

I'd be very interested to see what the investigation concludes.....

CP

[error_401]

We see Np fluctuations all once in a while on the 42-300's.

Mostly in climb be it turbulent or not. Remedy has been discussed before (Synchrophaser OFF - ON).

The A/C with the fluctuations also has a split of 0.75 % in the 100 % setting with PWR MGT in T/O position. Could the not properly registered Props be the reason?