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I have a question related to this question and this video. The question says that sometimes a fan blade failure can occur.

  • My question now is, if a fan blade separates or a malfunction occurs, is the engine still repairable?
  • I think the damage to the engine's structure would be quite high?
  • According to the video a fan blade malfunction occurred on start-up, how could a malfunction appear on start-up?
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This depends on the failure: A fan blade can be damaged by birds, debris, or aircraft parts (pitot probe for example). In this case the blades are dented, in which case it is possible to replace the damaged parts, inspect the engine and re-use it.

For this kind of damage it has to be disassembled and inspected by the engine manufacturer.

Inspection of blades is done with a microscope: For example, I have seen an HPT blade declared unserviceable because of a small scratch.

Obviously if the repair costs more than a new engine you don't repair the old one.

Many more things can happen in startup:

  • The engine is run on full power
  • There is a lot of possible interactions (like birds for example) that you don't have in a steady cruise at 40,000 feet.

Aside from debris, you can also have a compressor stall like this.

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  • $\begingroup$ remember reading about an early F/A-18 test flight where they had a bird strike, animal damaged the front fan, several blades went right through the engine (together with deep fried bird parts) and out the other end. Apart from the initial bang and the engine running a bit hot and losing partial power the pilot noticed little. $\endgroup$ – jwenting Oct 31 '14 at 10:36
  • $\begingroup$ I think it depend of the engine itself, military engine are very strong engines. See what can be happen in commercial engine : youtube.com/watch?v=ji4nZ5k82uk $\endgroup$ – Kromen Oct 31 '14 at 11:27
  • $\begingroup$ of course it does. And on what hits it, what breaks, etc. etc. And a pretty solid dose of luck... $\endgroup$ – jwenting Oct 31 '14 at 11:53
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    $\begingroup$ Your link to "a compressor stall like this." broke $\endgroup$ – Ferrybig Nov 16 '18 at 16:09
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Jet engines are fairly expensive, accounting for a large part of the cost of an airliner. So there will be some incentive to repair the engine if possible.

Whether an engine is repairable is dependent on the failure. If it happens at startup and mostly causes a bunch of smoke, like in the linked video, then the damage may be fairly limited and will probably be repaired. If damage is suspected, a borescope inspection is generally done to check the inner parts of the engine for damage as well. If a failure happens at full thrust, the engine is probably finished. Aircraft and engines are designed for this failure, but a full blade failure will cause extensive damage. Parts that get loose and enter the engine core will cause further damage through the engine.

Also, serious failures are generally investigated, especially if they are considered uncontained. This will mean at least extensive downtime for the engine, and may or may not end up being repaired.

There are many malfunctions that can occur at startup. The engine has been shut down for some period of time, during which:

  • Parts cool down and contract
  • Debris can enter the engine
  • Things can leak or get damaged

Whatever has changed will be discovered when the engine is started. Also, the problem in the video seemed to only appear after taxiing for some distance, so the engine could have ingested FOD at some point. It's not clear whether the 10 minute hold is due to ATC or the pilots noticing abnormal engine parameters.

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Depending on the jet engine type and on the cause and amount of damage, an engine repair or module replacement could be carried out on the wing. If damage or further investigation is needed, or if the aircraft is needed quickly, the engine could be removed to the airline's engine shop and parts/module/modules replaced on site.

Further damage may dictate the engine goes back to the manufacturer's overhaul shop or an approved overhaul facility: The engine will be stripped and each part inspected for serviceability. The cost of repair will be calculated. If the repair cost is higher than a new engine cost, the insurance company may deem it "beyond economical repair" and the engine is scrapped.

If the repair cost is acceptable then the engine may be repaired or fully overhauled depending on the operating hours/cycles already accrued in service: A low-hour damaged engine may just be repaired, whilst it would be more economical to fully overhaul a high-hour damaged engine to bring it back to "nil hours since last overhaul status".

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