29th Apr 2012, 11:22
I currently fly Cessna Grand Caravans for an Indonesia air carrier. Some of the work we do requires us to fly over mountainous terrain, where it is not always possible to glide to a safe landing site in the event of an engine failure. A question that has been up for discussion recently is the best course of action in the event of an engine fire, whilst flying over this kind of terrain.
The discussion is regarding how long the PT6 engine should be kept running in the event of an engine fire. It is a well known fact that the firewall in the C208B can protect the cockpit and cabin for up to 20 minutes at very high temperatures, however there are of course other factors to worry about.
So here is the question:
Roughly how long can the engine mounts support the weight/thrust of the engine during an engine fire, before the heat weakens them to the extent that they can no longer support this weight/thrust?
If the engine were to dislodge from the aircraft, it is obvious that the centre of gravity will move so far aft that the aircraft will enter an irrecoverable stall. So it is important that the engine is shutdown before we reach this point, regardless of the terrain below. The question is, how long do we have before we reach this critical point? Can you provide any case studies/evidence to support your claims?
Thanks in advance.
Genghis the Engineer
29th Apr 2012, 12:05
I can only speak in general terms, I don't know the specific design. But, I've overseen a few engine mount designs.
The engine mounts are primarily steel, which is pretty tolerant to fire - but there are rubber or synthetic anti vibration mounts that take out vibration. As they burn through, their ability to vibration damp will steadily degrade and the cockpit environment will get steadily less pleasant until the vibration is horrendous. But, even when they've gone completely, I'd expect the surrounding steel structure to take the thrust loads.
So, my instinctive belief (and that's probably all you'll get since I don't believe this is a certification requirement), is that you should keep the engine running until landing, but expect steadily worsening vibration, which will probably render most of the structure at the front end unfit for future flying, but should keep intact until landing.
Big Pistons Forever
29th Apr 2012, 20:08
The good news is an engine fire is the least likely engine emergency for a PT6 turbine. The only fires I am aware of were a result of massive internal damage which resulted in the engine unable to produce any thrust and in every case went out when the shutdown procedure was carried out and the prop feathered.
One of the traps of discussions on how to deal with emergencies, is too much emphasis is often placed on extremely rare failure modes, at the expense of really getting proficient at dealing with the most likely failure modes. Since ice induced flame outs are probably not a problem in Indonesia, the most likely fail modes involve the engine failing to get enough/any fuel. Those are the ones you IMHO ought to be talking about with your CP and trainers and other Captains.
29th Apr 2012, 21:08
Agree with big pistons.
I have seen engines which a couple of nozzle lines have come off the burning cans and have been pissing fuel into the hot section. We only picked it up after shut down when fuel leaked from the bottom of the engine and vapour could be seen coming out the top.
30th Apr 2012, 01:08
I Also agree with Big Pistons.
I would expect that you would be on fire in a Caravan before enough fire had occurred to jeopardize the engine mount. The engine certainly would have stopped creating trust long before then. Though none of us likes to have to glide a powerplane, the Caravan actually glides fairly nicely with the prop featherd.
It is unlikely in the extreme that you would have a sustained fire after you had shut off the fuel. You'll be flying a whole plane to the ground (and feathered props really don't vibrate too much). As said, I would concentrate on all the other dangers which want to drag you back to earth prematurely...
30th Apr 2012, 01:51
Thanks for your input guys. I have emailed our training department with a summary of what you have said.
30th Apr 2012, 08:21
Your more likely to get a failure due to the operation of the seperator thing than an engine fire.
There doesn't seem to be a standard way of doing this amongst operators of PT6's. I havn't heard of a single in flight engine fire. But I have heard of numerous crashes due to SOP's or stepping outside the SOP's with the seperator. Long term ones where the engine gets shagged with FOD and also crashes after departure when its been changed and an engine failure has occured.