JMT, have you got any further info on that SW airplane that had to return to LAS the other evening? I haven't seen any more about it after SW said it didn't happen and the FTC guy said it did............as well as some?/one? tower person and several civilian witnesses that said they saw the flames. Inquiring minds and all that........................???

Sure, I was at work in the hot seat when it happened on Thursday afternoon and Saturday when we finished working on it, and Sunday morning when it first flew again. This kind of thing doesn't happen a lot. The media is pretty hot on airlines so they focused on it. And there is an interesting situation that feeds the local media as Southwest officially denies the fire.

Lesson One. A jet engine is a self contained blow torch. It generates a lot of fire, creates a lot of heat, and the efficiency of the engine is determined by how much heat energy can be converted into kinetic energy to turn the turbines, compressor, and fan blades that result in thrust. (that's really important, so read it again) Newer fan jet engines have these really big fan blades on the front. The actual jet part is much smaller in diameter than previous generation engines. This design is the result of engineering to use all the energy they can to turn that big fan up front. It is a very efficient engine as there isn't a lot of heat left by the time the exhaust gases come out the back of the engine. It isn't smart to walk behind a jet engine, but if you did, the wind would blow you down long before the heat burned you. And that wind would be unbreathable air that would smell like burning kerosene.

Lesson Two. A jet engine is an ugly contraption. It is a beautiful piece of engineering but not much to look at. There's not much inside it except a bunch of blades. There's a lot of gizmos called accessories on the outside of the engine that sense temperatures, sense pressure, meter fuel, control air flow, tap off air for deice, tap off air for air conditioning, convert spinning energy into electricity, and other fun stuff. These little add ons make it an even bigger and uglier contraption and a more marvelous piece of engineering. Once it is mounted to the airplane, the airplane has a nacelle or cowling that wraps around it that makes it look smooth and aerodynamic. Think of it like the hood and front fenders of a car. Airplanes need that cowling around the engine for streamlining which converts to efficiency. Airlines often paint it to make blend in with the plane and become a non focal point.

Lesson Three. In the cockpit, the pilots have control to allow fuel to the engine, and they have a control to select a power setting. They have gauges to indicate the speed the engine is spinning, a temperature indication at the turbine, an indicator to show how much fuel is flowing to the engine, and a vibration indicator. Of course they have oil temp, oil pressure, and other basics.

Because the engine is a blow torch, very hot air is expected to be blown back towards the tail pipe. Because the engine is efficient, much of that heat is dissipated as energy to turn the turbines. There is little concern for fire out of the tail pipe because there isn't much behind the engine to worry about being burned and if fire makes it out of the tailpipe the engine isn't performing well and will be indicated by too much fuel flow, too high temperature, and/or too slow engine speed. The engines do have fire detectors. They have several of them. They are all located inbetween the cowl and the engine. If there is a leak in the jet engine that is blowing into the cowling or if there is a fuel leak in the cowl, it could start a fire and burn up the wing, and that would be bad. So inside the cowling is where all the fire detection equipment is mounted.

There are a number of reasons for engine failure. It could become unbalanced by a bent blade, a loose blade, a broken blade, ingestion of a foreign object, a bearing failure, or an engine that got too hot and started to melt a blade. As the engine spins about 35,000 rpm, it doesn't take much of an out of balance condition to fail.

So what happened to the SWA engine? We don't know. It will go to a shop where they will disassemble it. All we know is it had a failure and we replaced it with a different engine. As the engine failed on takeoff, the heat was not being converted to energy to power the plane, so the heat came out of the back of the tailpipe. It more than likely came out as fire for several seconds as the engine died. The tower saw fire, several witnesses on the plane and the ground saw fire. Since it was all blowing out of the tailpipe there was no fire indication in the cockpit. I don't know exactly what the pilots saw, but the temperature went way up and the speed slowed down rapidly. The engine shut itself off in a matter of seconds as the temperature, speed, and pressure sensors couldn't determine a fuel flow to match the power setting input from the pilots. The pilots obviously felt the loss of power, the felt a slight yaw, they heard the engine winding down, and I suspect by the time they looked down at the engine instruments, it was halfway to shutdown, and they just cut the fuel off to the engine. Because the temperature was way hot, they knew not to try to a restart.

So the media was correct, fire was seen. The airline is correct because the pilots and the black boxes show no evidence of fire.

This is a fairly rare event. I see about 1 per year. We usually catch failures early as the engines are running hotter than normal or the vibration increases or the engine is slow to react to pilot inputs. We change out the engine with a fresh one and send the old one in for overhaul. What was interesting in this event was that we took the plane down the by cargo area where the county makes us change engines and there was a Northwest 757 getting an unscheduled engine change. Our nearest spare engine was in Texas so it took a couple of days to get it out here on a flat bed 18 wheeler. By the time we finished our engine change, Allegiant was doing an engine change down there too. So it was a bad week for engines in Las Vegas. Southwest got caught because it was prime time and the media is alert to airline mishaps.

There are very very few scenarios where fire would constantly blow out of the tailpipe to what the public would call an engine fire. Nothing in the engine is flamable, so there's nothing to burn. It would need constant fuel flowing and the heat not being used to spin the turbines and compressor fast enough and the temperature would be way up. The computers would cut the fuel off as all of the input sensors would be out of range for computing. Even if the fuel didn't shut off, the temperatures would melt the turbines, they wouldn't spin the compressor, and there wouldn't be enough airflow to sustain a fire. The turbine blades would melt down quickly. There were reports of fire, but all those witnesses saw was the process of a shutdown.

OTOH, what a pilot calls an engine fire is a big deal. If there is a leak in the jet engine and it acts like a blow torch that could be ugly. If it burned a hole in the wing, that's where fuel is stored and that's not good. If there was a fuel leak of some kind in the cowling and it ignited, it could heat up the fuel in the wing to unhappy results. If a pilot sees an engine fire warning, he immediately pulls the fire handle for that engine that shuts off electricity, hydraulics, air, and fuel to that engine. There are fire extinguishers for the engines that spray halon into the cowling area to put the fire out. If there were an engine fire, chances are the public would never see it.

Any questions?

Thanks, Joe, that was really an interesting answer. It explained a lot of things we wondered about as casual (me) and business (Toby) travelers. I enjoyed the lesson. It was really relevant with all of the other aircraft problems we have been reading about lately. Thanks, again.

Fire after all, is such a negative word!
I prefer the term, uncontrolled combustion!
I say this sort of tongue in cheek, but it's true!

BTW, Thanks for the in-depth explanation Joe, That was quite a lot to type out for us, but you explained it well! Thank You

The other place where you could see a big flame is if there is a fuel leak into the by-pass air stream. This would then ignite when the hot turbine section exhaust mixed back with the bypass. While dramatic, I don't think this would be immediately extremely damaging.

This would not be dissimilar to the afterburner flame on military jet engines that mix fuel in with the hot section exhaust to get extra thrust through the rocket effect.

I'll defer to your expertise, but I don't see how that could happen on the planes I work on. The pylon that connects the engine to the wing does not have access to the fan duct, and the wing above the fan is a dry bay with no fuel. If there were a fuel leak in the pylon, the pylon drain is right above the exhaust.

Also, with no scientific proof, I would think fuel in the fan case would be too lean of a mixture.

Here's something that is really special. See the video link.

One really kewl way to see a jet engine on fire is the very first start after an engine change. A fresh engine out of overhaul is prepared to be stored until needed, so all of the fuel lines are filled with a preservative, and the engine itself is sprayed with an anti corrosion solution. On the very first start, those fluids are burned off. It ALWAYS creates some nice flames from the tailpipe.

The fire you will see on the video is not uncommon. Sometimes it is more fire. Usually, it's a little less. This fire does not damage the engine at all. Remember that the engine is a blow torch. At the stage of this fire, it's barely a fire and nowhere near the intensity needed to constitute a blow torch. Along those same lines, it doesn't hurt the airplane. It's just a few seconds of fire, not a torch. The fire actually gets blown out as the blow torch effects take place.

http://www.youtube.com/watch?v=EDc8o_h-dn0


The video doesn't do justice to the smoke that comes out at first. Sometimes you can't even see the airplane as it gets covered in the smoke.

One night working in the control tower at NAS Alameda circa 1970, we had a couple of Navy Jets take off with full afterburner and it really had the look of flames coming out the engines but they are controlled flames like on a gas oven.

Anyways the phones started to light up from people who saw a flaming streak in the night. After fielding about 50 of these calls and explaining what was happening I finally told one of the callers 'Yes miss your correct. We've have had some missles attack us and you should take cover".


IN those days they could not trace calls etc, so I always had deniability if I needed it.

I remember at another air base telling a drunk who called in one time."Yes sir your correct. Those strange lights are a UFO that just landed at our air base and were going out to see just what these aliens want".

He never called back.