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?
Engine shearoff, though rare, has happened in a number of cases.
Boeing 747F experienced a number of engine falloffs:
- On Dec 1991, 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 separation, 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 separation has been reported on 737s too:
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.
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.
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. This helps to prevent excessive loads from breaking the wing structure, which would cause a fuel leak from the tanks in the wings.
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.