View Full Version : Overtorque on PT-6 series engines

13th Oct 2008, 21:23
Hi all,

Anybody have experience of this event on a PT-6 powered fixed wing aircraft?

Its a 'practiced-for' event in the simulator of the type I fly (King Air)but neither the instructors nor the other pilots i've flown with can explain how or why it may occur.

I've looked for additional information online but can't seem to find anything.

Any information or steers would be appreciated.

Thanks in advance.

14th Oct 2008, 01:09
Exactly how do you practice it in the sim? And you're talking about overtorque not overspeed, correct?

I'm baffled, 3000 hours behind PT6s and I've never heard this discussed other than as a limitation. Overspeeds would be much more interesting.

14th Oct 2008, 01:49
Having operated PT-6 powered airplanes in the mid-sixties, some folks ignored the torque limitations.
It got quite expensive.
Greatly increased propellor reduction gear wear and, even worse, greatly increased power turbine wear...so much so, that on one engine I noticed, while it was in for overhaul, fully one fifth of the power turbine blades had been worn away...gone.
P&W Canada provided a circular power computer with each engine type, and its use was mandated by aircraft operators.

To ignore torque limitations with a turboshaft engine is foolhardy in the extreme.

14th Oct 2008, 01:51
May have seen it happen, The inverters on the aircraft were not switched on and therefore no torque indication, halfway down the runway levers against the wall the inverters were switched on and apparently the gaugues were not looking good, but it was hard to prove how much it had over torqued and for how long.

14th Oct 2008, 01:58
What would cause an overtorque barring pilot error? I'm guessing a loss of oil from the governer causing the prop to feather?

14th Oct 2008, 04:28
I'm guessing a loss of oil from the governer causing the prop to feather?

Not nesessarily so.
Take early models of the Cessna 425 Conquest One.
The torque gauges on these aircraft were supplied directly by oil lines directly to the torque gauges (similar to an engine oil pressure gauge, with added restrictions in the line to prevent gauge failure), and often they under-read by a significant degree.
Bleeding the gauge lines was a necessary service from time to time.
On later models, the engine torque gauges were AC powered, from a torque/oil pressure transmitter, located on the engine.
Whatever the failure mode, it is very necessary to observe limitations, otherwise, it can be an expensive proposition.

14th Oct 2008, 04:36
While overtorque is certainly undesireable, I still fail to see how this is a scenario for simulator training.

Waiting for enlightenment....

14th Oct 2008, 04:58
I think EPR is talking about a torque runaway.

Between the two main parts of the FCU is a shaft which if broken will lead to the FCU going to max flow. The only way to reduce power is to shut the engine down. There are three ways this can become a problem:

1. If this happens before rotation and the power levers are pulled back for the reject one engine will spool down and the other will not = tulips.

2. If this happens right after rotation and the engine is shutdown quickly the transition from rudder inputs into the runaway engine to rudder inputs into the good engine has been too much for some pilots. Wait for staging, get to a safe altitude and then shut down the offending engine.

3. Because rudder inputs will be required into the problem engine, a misdiagnosis of the problem is possible with an attendant shutdown of the good engine.

Practice in the sim is very simple. Overtorque before rotation = both condition levers to cutoff. After rotation = climb to safe altitude and follow engine shutdown drill.

Perhaps some more knowledgeable instructors may be in order. If the shaft in question breaks it will leak a blue dye out of the FCU. This can be caught on the walkaround and therefore should be taught.

ernie blackhander
14th Oct 2008, 10:44
If the shaft in question breaks it will leak a blue dye out of the FCU. This can be caught on the walkaround and therefore should be taught.

I thought that inspection was if the seal failed on the pump or fcu, fuel would wash the blue dye in the bearings out of the fuel pump/fcu interface drain. The shaft inspection was to remove the above mentioned drain and swab inside with cotton tip looking for rust.

I think that shaft breaking while running can also be refered to a a High Side failure

Empty Cruise
14th Oct 2008, 13:09
TQ runaway...

Experienced something similar on a B200 - I was unable to reduce TQ. It turned out to be caused by paint residue having entered the FCU actuating arm ball bearing...

Anyway, I also thought that shutting it down with the CL was the way forward, so to speak - and imagine my surprise when nothing happened upon closing the CL. Turned out the fuel pressure being generated by the EDFP was so high that the CL didn't have authority enough to close the valve.

Oh well, the good ol' fireswitch did the trick in the end, but still :confused: at the time.


14th Oct 2008, 16:56
Feather can be triggered by LOW PITCH STOP

14th Oct 2008, 20:27
Not sure about the point of this post...but if you overtorque or overtemp then it's time to see the engine guy...if you break the engine up there, well, shut her down and land...

ernie blackhander
15th Oct 2008, 12:10
Mootyman, was that a emb110 up topend

15th Oct 2008, 15:06
One of the boys had an FCU fail on one of our PT6A-42's. Whilst I didn't hear of the torque indications during the event, the ITT began to rise from the 750 that was set. He pulled the power back, got a temporary reduction before it began to approach redline again so he pulled the CL.

This all occurred at a reasonably high alt (FL280ish) which probably was the only reason the torque never featured in the account that I heard of the event.

From what I hear the episode took a grand total of about 5 seconds. In all, it sounds like it was well handled and saved a whole bunch $$'s

16th Oct 2008, 22:55
Thanks folks for the replies, I think my original post may have been confusing. When I mentioned 'overtorque' I did in fact mean runaway torque as opposed to the overenthusiastic handling of the throttles.

Simulator training involves runaway torque occuring on one engine with the outcome (abort/continue) depending upon the IAS. Below 80kts abort by closing both CL's. Above 80kts accelerate to V1/Vr and climb to safe altitude accepting the excessive force, at a safe altitude (usually MSA or 1000' AGL) commence shutdown.

Aharmin, wrt the blue dye that leaks from the FCU when the shaft is broken - would this only be visible with the cowling open or will it leak onto the ground beneath the aircraft? Also is it a feature of all PT-6's? Ours is -41 by the way.

17th Oct 2008, 21:55
As stated the dye may be visible on the bottom of the cowl but the most obvious way to catch it is by opening the cowl.

I would guess that all PT-6s have this but I am only familiar with -20, -21, -28, -42, and -60. All of these have the blue dye.

Keep in mind that this type of failure is extremely rare, far more so than an engine failure, rollback to idle, or stuck power lever cable.