In an airliner, such as an A380, what indications do the pilots have that an engine has thrown a fan blade?
Will that always result in destruction (of the engine)?
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asked Apr 30, 2015 at 15:57
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$\begingroup$ Incidentally, the most prominent incident with the A380 actually wasn't a thrown fan blade, but actually a thrown turbine disc. The engine housings are designed to contain a thrown blade, but they won't (and didn't) stop pieces of a turbine disc. The root problem actually wasn't immediately apparent to them due to the flood of warnings they received as a result of many hydraulic and electronic control lines being severed by pieces of the disc that punctured the wing in several places. $\endgroup$– reirabCommented May 1, 2015 at 2:12
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4 Answers
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engineer and pilot here - formerly of JPL. I've seen blade loss tests and any modern high performance engine will violently destroy itself if a blade is lost. You can see tests of this on youtube for example at that link or by searching on "jet engine blade-out test" (without the quotes). (User fooot gave a longer example video as well.) In these tests they typically use a (very small, prebalanced) explosive charge to separate one blade from the spindle and then the engine vibrates itself to pieces.
Modern engines are constructed near the very limits of the materials used. In fact there is a special joint venture company in the US that makes a lot of the fan blades and which exists just for this purpose. They use special tools to X-ray every blade and measure the center of gravity as the slightest bubble in composites or weight anomaly would be enough to destroy an engine. When a blade is lost, the off center mass causes an engine to vibrate itself violently to death.
In person you can't really hear anything but the vibration and destruction at these tests - I couldn't even hear the explosive charge when it went off (although it seems audible in some of those videos). As fooot noted above, the pilots are informed through the usual engine-out detection methods such as loss of telemetry (and it may start a fire which would be noted and dealt with by fire suppression systems on a larger jet).
Many companies including, I know, Rolls-Royce will invite the public to fan blade out tests to show how their engine bodies control the resulting carnage. It might be a fun afternoon if you're the type to visit aviation stackexchange :) They post videos online for the same reason and you can see a lot of very nice slow-motion videos on the subject.
answered Apr 30, 2015 at 19:09
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$\begingroup$ I think it is very improbable that any engine manufacturer would let Joe Public anywhere near a fan blade off test. Even company employees won't be there unless they have direct involvement in carrying out the test. In any case, waiting around for several hours (or even days) of test preparation for an event that makes a rather loud bang and a few seconds of pretty flames, but with nothing else to see until the high-speed video is available, isn't really "fun." I've been there in an official capacity. $\endgroup$ Commented Apr 30, 2015 at 22:12
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1$\begingroup$ Hi alephzero. I've been to one too, and there were some members of the public (a city councilman and the local news channel among others). I agree it was over fast and we watched from over 100 yards away. I admit that I can't seem to find any mention of public invitations with a quick google search, and I went in the early nineties, is that really over 20 years ago now? Wow. When I went I had only the loosest excuse to be there (JPL engineer, working together with somebody working on nozzles at the jet company parent corp) $\endgroup$ Commented May 1, 2015 at 1:29
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$\begingroup$ Attitudes to public relations have changed over time. These days, the last thing you need is for a no-win-no-fee lawyer to use an eyewitness account from a clueless member of the general public as evidence in court that your products are "unsafe". Of course companies sometimes put official videos of successful tests on Youtube, but you won't get to see the failures! $\endgroup$ Commented May 1, 2015 at 2:28
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Turbofan engines are designed and tested to contain a fan blade failure. However, this is a violent event and will cause serious damage to the engine. The engine and surrounding components are subjected to large forces and vibrations, and the engine can ingest debris from the damage to the fan, causing further damage. There will likely be an investigation, and the engine will probably be scrapped due to the widespread damage to different components.
To the pilots, it will seem like a bird strike, but probably worse. There will be a loud noise as the fan blade separates and collides with the fan case and following blades. The fan will then be out of balance, so heavy vibrations will follow as the shaft loses its rotational energy. The pilots will be able to tell which engine has failed by the loss of thrust, as well as the indications of vibration and failure on the instruments for that engine.
If the fan does not get stuck, it will continue to windmill, which will result in ongoing vibration from that engine.
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If you lose a "large" blade like a fan blade, or a "heavy" blade like most turbine blades, the engine vibration monitors will instantly go off the scale and the engine control system should shut the engine down automatically. To give some idea of the forces involved, losing a fan blade from a large turbofan will create an force of about 100 tons on the engine, oscillating at a few thousand RPM until the rotor slows down. In a worst case scenario, the aftermath is enough to shake the entire aircraft to the level where walking about in the passenger cabin is difficult. But aircraft have survived that and landed safely, and for long distance flights over water the plane is certified to survive the consequences for up to 3 hours flight time before reaching the nearest land.
On the other hand, if the tip of 10mm long high pressure compressor blade cracks off in the engine core, you might not even know about it until the next time the engine is stripped down for a full inspection.
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$\begingroup$ nice to have force numbers, upvoted! Also, I agree about difference between turbing blades and fan blades. $\endgroup$ Commented May 1, 2015 at 1:30
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$\begingroup$ The time that an aircraft is certified to fly with an engine out over water varies greatly by aircraft. For some aircraft, it's only 90-120 minute, while for the 777 it's 5.5 hours. I don't think quad-engine aircraft have a limit at all. $\endgroup$– reirabCommented May 1, 2015 at 1:44
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1$\begingroup$ Sure, it doesn't make much sense for a short-haul aircraft to be certified for long-haul operations. But there are two separate issues here. Shutting down one engine out of four is less of a problem than one out of two, but the issue of vibration from a failed engine that is windmilling hazarding the aircraft is the same for both. AFAIK the incident which prompted a change to the airworthiness regulations to cover that scenario was a 747 that broke a fan blade in flight. The plane survived, but the crew seriously considered the option of ditching into the Pacific ocean. $\endgroup$ Commented May 1, 2015 at 2:18
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1$\begingroup$ If I told you everything I know about that incident (and not just from reading the official reports), I would have to shoot you afterwards ;) $\endgroup$ Commented May 1, 2015 at 2:31
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1$\begingroup$ @alephzero Are you implying that information related to the investigation of an engine failure on a commercial airliner is classified? That seems extremely unusual (and I used to work in a jet engine testing facility that dealt with all manner of classified engines.) $\endgroup$– reirabCommented May 2, 2015 at 22:23
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Provided the engine survived the event, we are looking at all the engine parameters from the cockpit displays. Those indications are different depending on engine manufacturer. N1, N2, EGT, Oil pressure, ITT, engine vibrations indicator (VIB) and source, EPR (...). Just by looking at the numbers you cannot tell for sure if a blade separated. And it all depends also from the compressor stage the blade separated (first, last...) or if it was a fan blade.
