Torque runaway question...

[ILLINI]

I was recently asked this questinon... what do you think?

You are flying in a King Air (PT-6 engines) and you notice a torque runaway in cruise flight. What do you do?

My initial answer was, assuming you notice the runaway the instant it happened, immediately shut down the engine as fast as possible... by pulling the condition lever to cut-off. Then just fly to the nearest suitable airport on the one remaining engine and land.

I was then told that while my answer was one of the options, it wasn't necessarily the best one. I was told the best thing to do would be to leave the engine alone and keep it running. The reasoning being that it's already ruined and will need to be taken apart anyway. Plus if you should happen to lose the "good" engine, the one with the torque runaway will still produce power to get you on the ground safely. Continue to the nearest suitable airport to land. On final shut down the affected engine, to avoid a large asymetrical thrust when you bring the power back. Basically, one engine will be producing alot more power than the other, and if you use reverse they will produce thrust in oposite directions.

My question is, 1) exactly how long does it take for an engine to destroy itself if it experiences a torque runaway? Is it possible to save the engine if you shut it down right away, or at least have a cheaper repair bill? Obviously the first concern is for the safety of yourself and everyone else in the plane, and nevermind how much it is going to cost to repair the plane. But I was just curious about this. 2) Assuming you have determined in advance that shutting down the engine would not affect the safety of the flight, would you shut it down immediately, or leave it running as was explained to me? Or do you have another answer?

I'm very interested to hear what others would do if faced with this situation. Thanks!

 

[Andy Neill]

I would pull the power back to normal operating range for torque. The reason you had high torque could have been for a variety of reasons from an inadvertant feather to a faulty fuel control. I'm aware of one situation where the torque exceed the limits by a significant margin for less than a minute, was left running at a reduced power setting to a safe landing. A post incident teardown of the engine revealed no damage.

Even if you pull all the power off, the engine is still a source of electrical power and bleed air.

 

[SDdriver]

I agree with the above poster. I fly PT6's regularly and if that happened to me I would probably just pull the power to idle on that engine, recheck the guages and if something was still out of limits I would shut it down. I have seen the result of a catasrophic (SP?) failure in a King Air 200 motor. The stacks on both sides had holes from where pieces were flying out of the motor. I wouldn't want one of those to come into the cockpit or damage something crucial to flight. Even though they are designed to contain a failure of that kind I wouldn't put all my faith in it. In other words

Flight Idle
Re-evaluate engine intruments
If still out of limits
Shut Down.
Land

 

[flint4xx]

Torque on the PT-6 is a measurement of oil pressure in the propellor housing. There is no connection to the engine as it is a "free turbine." If there was a runaway propellor, I would feather it and leave the engine running at idle for electrical power. If the engine ran away, then shut it down using the engine failure after V1 checklist. Your gauges will tell you which is happening.

 

[Phrogs4ever]

Just like any other aviation scenario question, you have to take in as much available information as possible and apply it to your current situation and make your best decision.

Initially, I would try to control the torque with the power lever. Then I would try to control the torque with the prop levers. Since we cruise at 1700rpm, and increasing prop rpms decreases torque, run the prop lever up to 2000rpms and see if it comes down any. If still unable to control the torque, I would consider how far out of limits we're talking. If we're 50ftlbs over red line that's not good, but if that sucker is whining and trying to twist itself into slinging metal then that's horrible. I would then shut the engine down.

Since I am a King Air instructor, and since I have had to shut an engine down on an IFR flight plan, I have practiced numerous engine out procedures, practiced flying and trim, approaches, and landings and have complete confidence in flying the plane single engine. But like Andy said, you have to know what you're giving up when you do that. If you give up a generator and bleed air source, you probably want to get and maintain vmc, and descend as required (of course with one engine you might not have to make the decision to descend, gross weight and conditions will dictate).

I haven't thought much about a runaway torque in flight, and our procedures don't address that condition. I basically applied a version of the prop overspeed procedure and general knowledge to try and control the problem. It was good headwork drill.