El Al Flight 1862 - Causes

Causes

In the event of excessive loads on the Boeing 747 engines or engine pylons, the fuse pins holding the engine nacelle to the wing are designed to fracture cleanly, allowing the engine to fall away from the aircraft without damaging the wing or wing fuel tank. Airliners are generally designed to remain airworthy in the event of an engine failure, so that the plane can be landed safely. Damage to a wing or wing fuel tank can have disastrous consequences. The Netherlands Aviation Safety Board found, however, that the fuse pins had not failed properly, but instead had suffered metal fatigue prior to overload failure. The Safety Board pieced together a probable sequence of events for the loss of engine 3:

1. Gradual failure by fatigue and then overload failure of the inboard mid-spar fuse pin at the inboard thin-walled location.
2. Overload failure of the outer lug of the inboard mid-spar pylon fitting.
3. Overload failure of the outboard mid-spar fuse pin at the outboard thin-walled and fatigue-cracked location.
4. Overload failure of the outboard mid-spar fuse pin at the inboard thin-walled location.

This sequence of step-by-step failures caused the engine and pylon to break free, then shot forward and by sheer chance, Engine #3 flew back and knocked outboard engine 4 and its pylon off the wing as well and inflicting serious damage on the leading edge of the right wing, including the control surfaces (flaps) that Captain Fuchs later tried to extend in flight.

Research indicated that the plane had only managed to maintain level flight at first due to its high air speed (280 knots). The damage to the right wing, resulting in reduced lift, had made it much more difficult to keep the plane level. At 280 knots (520 km/h), there was nevertheless sufficient lift on the right wing to keep the plane aloft. Once the plane had to reduce speed for landing, however, it was doomed; there was too little lift on the right wing to enable stable flight, and the plane banked sharply to the right without any chance of recovery.

The official probable causes were determined to be:

The design and certification of the B-747 pylon was found to be inadequate to provide the required level of safety. Furthermore the system to ensure structural integrity by inspection failed. This ultimately caused – probably initiated by fatigue in the inboard midspar fuse-pin – the no. 3 pylon and engine to separate from the wing in such a way that the no. 4 pylon and engine were torn off, part of the leading edge of the wing was damaged and the use of several systems was lost or limited. This subsequently left the flight crew with very limited control of the airplane. Because of the marginal controllability a safe landing became highly improbable, if not virtually impossible.