After seeing this video of a 757's engine catching fire because of a bird, I wonder:
- What happens to the engine?
- Why does it periodically exhaust fire?
- Is this some safety feature?
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7$\begingroup$ I guess that is a compressor surge cycle initiated by the ingestion of the bird and associated damage. The fire is presumably from an excess of unburned fuel due to the pilots/FADEC not cutting engine power at take-off. It looks more like a mode of natural oscillation rather than a safety feature! $\endgroup$– RedGrittyBrickJul 21, 2014 at 12:43
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5$\begingroup$ Note that the engine is not really on fire. The flame is only in the normal flow path and will stop once the fuel is cut off. $\endgroup$– Jan HudecJul 21, 2014 at 19:00
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1$\begingroup$ Just as a matter of note, you can also get a compressor surge if you use excessive reverse thrust for the forward speed that you have, and especially as the engine nears time for overhaul. $\endgroup$– TerryJul 22, 2014 at 4:53
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5$\begingroup$ Because birds are flammable? ;) $\endgroup$– egidJul 22, 2014 at 6:52
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2 Answers
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As RedGrittyBrick has commented, what happened in that video is a compressor surge. This is not really an engine fire, but a temporary backfire. To understand what is happening, look at this diagram of a turbine engine:
The air is compressed in the front of the engine, fuel is injected and ignited, and then everything flows out the turbine section and the exhaust nozzle. The only thing supporting the compression at the front of the engine is the rotation of the compressor blades. There are many different things that could cause the compression to fail. In the event of a bird strike, these blades can be damaged.
When that happens, there is highly compressed air in the combustion section, and there is no longer enough compression to prevent it from exiting the front of the engine. This causes the air to exit both the front and rear of the engine. The disruption in air flow happens too quickly for the engine control systems to react. This means the engine is still pumping enough fuel to run the engine at normal thrust, but there is no longer enough air flow to burn all that fuel in the combustion section. The result is that the fuel burns through the turbine and exhaust section of the engine, and behind the engine.
Once this compressed air has exited the engine, it will generally recover. The engine is not on fire, so once the excess fuel burns away, the flames are no longer visible. However, if the problem is still present (and damage from a bird strike would not go away), then the engine surges over and over until it is shut down or set to a lower, more stable power setting.
So this is not so much a safety feature as physics taking over, and there is no real fire of immediate danger to the airplane. The engine system will automatically shut down the engine if it senses there is anything potentially very dangerous or that completely prevents the engine from running. While compressor surges can eventually damage the engine, it's not really a catastrophic event, and it is up to the pilots to assess the situation and shut the engine down if necessary. Reducing power to idle may sometimes be sufficient to stop the surges.
During a compressor surge, the flames are confined within the engine. There should be nothing to burn here besides fuel, so cutting off fuel will stop the fire. Actual engine fires are generally caused by flammable fluids leaking into hot areas around the engine, or the hot engine gases escaping from the air flow sections into surrounding systems. There are detection systems to alert the pilots of this situation, and the engine area is designed to contain a fire until the pilots can use the fire extinguishing systems to put such a fire out or at least get the plane back on the ground.
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8$\begingroup$ I would stress that the engines is not really on fire during compressor surge/stall. When fuel is cut off, the engine just flames off and stops and no extinguishing is needed. $\endgroup$ Jul 21, 2014 at 18:56
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1$\begingroup$ Also, the engine in the video did show an oscillating surge, alternately compressing air almost like in normal operation, and then much less. Since the turbine is sort of blocking the flow path out back, it needs a minimum compression performance to work, and with the damage this performance was no longer possible. Once the compressor pumped air back, this air would not exit quickly enough through the turbine, but would partially flow back through the compressor. Now the power to keep the compressor turning decreased, the flow went back to normal, and the cycle repeated. $\endgroup$ Jul 21, 2014 at 20:29
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$\begingroup$ So how would a pilot not looking at the engine to see the fire detect surge from his instruments in order to take corrective action e.g. reducing power to idle etc. Is there a surge warning alarm or something like it in the avionics? $\endgroup$ Feb 2, 2019 at 8:00
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$\begingroup$ @curious_cat That sounds like a good separate question. $\endgroup$– userFeb 2, 2019 at 21:50
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When turbine engines ingest birds or other objects the result is Foreign Object Damage (FOD). The metal parts of these engines are operating under extreme stress very close to the absolute limits of what metallurgical technology can produce. The flames you see are bits and pieces of the object coming out of the engine along with bits and pieces of the engine. Think Capt. Sullenberger and miracle on the Hudson or Capt. Al Haynes and crew with UAL 232. Turbines are designed to have only two things pass through them air (of various temperatures) and fuel. Anything else causes problems up to and including total failure.
Hope this helps!
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$\begingroup$ UAL 232 was a fan disk disintegration due to metal fatigue, resulting in loss of hydraulics and subsequent loss of controllability. There are very few birds, and not really a whole lot else either, at FL370. $\endgroup$– userFeb 2, 2019 at 21:56
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$\begingroup$ My point is that anything (other than air or fuel) passing through a turbine will mostly likely cause damage and flaming debris to exit the engine. Also, it may cause the engine to fail in a complete self destructive manner. Birds have been encountered at some incredible altitudes (not FL370) but ash and pumice from volcanoes have damaged windows and engines at similar levels. Jon $\endgroup$ Feb 8, 2019 at 5:54
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$\begingroup$ @JonCaples: Actually, birds have been encountered at FL370. They were found by - guess what - getting sucked into a jet engine. $\endgroup$– VikkiMar 23, 2019 at 21:49