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I have read that jet engines, at least the under-wing ones, are attached to the airplane by shear nuts and bolts so that in case of unstable forces in or on the engine, the engine would fall off to prevent further damage to the airframe. Have there been any incidents and or accidents where the engine has fallen off the plane in accordance to this design feature?

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American Airlines flight 191 , and Investigation video had that happen. There are quite a few examples of engine separation, both fatal and nonfatal if you search on Google. – Ron Beyer Feb 5 at 3:15
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When I was flying there was a magazine with a feature about close calls - I can't remember if it was Flying or AOPA Pilot...but there was a story about a crew in Alaska flying at night that had their engine catastrophically fail and, as the pilot told ATC (who thought this was an engine failure), "the engine departed the airframe." It may have been a prop driven aircraft, but the article was fascinating nonetheless... – Tim Feb 5 at 12:27
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Not only in reality but even in movies: see Donnie Darko. – DarioP Feb 5 at 13:33
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@Gusdor Its not a requirement in that its mandated by a regulatory body, its a design feature. – Ron Beyer Feb 5 at 14:17
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@jean Hmm... The pilot not noticing an engine completely missing seems rather hard to believe. It seems like the engine instruments all reporting zero and the need to apply large rudder inputs in order to fly straight would be pretty noticeable. – reirab Feb 5 at 19:35

Engine shearoff, though rare, has happened in a number of cases.

  • Boeing 747F experienced a number of engine falloffs:

    • On Dec 1991, a China Airlines' flight 358, a 747-200F lost an engine near Taiwan and crashed.
    • On October 1992, a El Al 747-200F crashed after takeoff due to engine seperation, at Amsterdam, The Netherlands.
    • Another 747F from Evergreen Airlines lost an engine over Anchorage, Alaska soon after.
    • On October 2004, a Boeing 747-132SF of Kalitta Air lost an engine while climbing and landed without further incident.
  • Engine seperation has been reported on 737s too:

    • In November 2007, Flight CE723, a Nationwide 737-200 lost an engine during takeoff; the aircraft was landed without further incident.

    • On December 1987, USAIR FLT 224, B737, lost an engine during climb; the aircraft was landed succesfully.

There are cases where engine seperation has occurred in B707, B727, and DC-10. aviation-safety.net has a list of engine seperation incidents.

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CE723: “I heard this huge bang, and he said ‘that’s our engine that’s just fallen off’" Thats why you train for an engine failure at V1 :) – Jonathan Walters Feb 5 at 13:03
    
The El Al flight tried to return to Schiphol and didn't make it because control structures failed... and it ended up inside an apartment building. Being a cargo plane with a crew of just 4, most of the fatalities (39 of 43) were on the ground. – Floris Feb 5 at 23:01

ELAL flight 1862 crashed in Amsterdam on the 4th of October 1992 as a result of 2 engines shearing off. The number 3 engine sheared off shortly after take-off due to metal fatigue related failure of the fuse pins. After separation, the number 3 engine hit the number 4 engine, shearing it off as well. Combined with damage to the leading edge of the right wing the aircraft became difficult to control at low speeds and subsequently crashed on the attempt to return to Schiphol Airport.

Official accident investigation report (PDF)

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The Boeing 727 acquired a reputation for shedding engines. The process was ice buildup on the right side due to leaking lavatory plumbing, the ice falling off and into the No. 3 engine where it caused damage to the fan blades. The resulting imbalance lead to vibrations, and the engine was designed to shear off in that case.

This New York Times article describes one case where a 727 lost its engine over Florida. The article continues:

Cases of engines falling from airplanes are rare. In 1974 a National Airlines 727 lost an engine near Sierra Blanca, Tex. In 1985 an American Airlines 727 flying from Dallas to San Diego lost an engine near Deming, N.M.

I would expect this happened also to some Russian airplanes, but here the reporting is less up-front, so I know of none.

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In addition to the cases where engines fell off in flight, there have also been cases where the engines sheared off due to impact forces during a crash, which is the primary reason for attaching to the wing with shear pins. Some examples:

July 2013, an Asiana 777-200ER crashed short of the runway. Both engines separated.

April 2013, a Lion Air 737-800 crashed short of the runway. The right hand engine sheared off.

February 2009, a Turkish Airlines 737-800 crashed, both engines separated.

December 2008, a Continental 737-500 departed the runway on takeoff. One engine separated.

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And as a negative example, in en.wikipedia.org/wiki/Air_Canada_Flight_621, failure of the engine to separate cleanly led to a fire in the wing and a fatal crash... – DJohnM Feb 5 at 21:02
    
I don't get it. You're saying that it's a design goal that engines can shear off during a crash? I don't see why that is even desirable. – Nate Eldredge Feb 6 at 5:56
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@NateEldredge possibly removing as much mass (therefore energy) as possible helps? And the fewer massive, possibly still-spinning turbines near passengers the better. – Robert Grant Feb 6 at 13:08
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@NateEldredge -- having the engine mount fail in a controlled way protects important wing structures from being damaged by the engine tearing off in a crash. – UnrecognizedFallingObject Feb 6 at 16:28
    
@NateEldredge There are fuel tanks in the wings. If the engine strikes something during the crash, or is dragging along the ground, it might break the wing and spill fuel. Designing the engine to shear off avoids this problem. – David Richerby Mar 27 at 15:59

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