Investigation Summary
The investigation revealed that the separation was initiated by a fatigue crack in the inboard forward midspar fitting of the No. 4 engine pylon. This crack had likely been present for some time but was not detected during maintenance. The Boeing 707's pylon design was intended to be fail-safe, but the investigation found that the design did not sufficiently account for the effects of corrosion and fatigue on multiple load paths simultaneously. When the No. 4 engine broke away, its trajectory caused it to collide with the No. 3 engine, tearing it from the wing as well. This left the aircraft with no power on the right side and a breached fuel tank, leading to a massive fire. The captain's ability to maintain control of a heavily asymmetric, burning aircraft and land it on a runway is considered a remarkable feat of airmanship.
Final Conclusions
The accident was caused by the separation of the No. 4 engine pylon due to a fatigue crack in the forward midspar fitting that went undetected during inspections. The subsequent loss of the No. 3 engine was a direct result of being struck by the departing No. 4 engine. Contributing factors included the inadequacy of the pylon's fail-safe design under certain fatigue conditions and the difficulty of detecting such cracks with existing maintenance procedures.
Video Analysis
Trans-Air Service Flight 671: Double Engine Separation and Emergency Landing — Analysis of the 1992 Boeing 707 incident where fatigue failure caused both right-wing engines to detach in flight. The crew successfully landed the burning aircraft at Istres-Le Tubé Air Base with no fatalities.
Watch on YouTube ↗If playback is blocked, the owner has disabled embedding — use the link above.
Systems & Failure Modes
Engine Pylon Fittings
The structural components connecting the engine to the wing; the forward midspar fitting failed due to fatigue.
Fire Warning System
The system correctly identified fires in the engine nacelle areas, though the engines themselves were no longer attached.
Fuel Dumping System
The crew attempted to dump fuel to lighten the aircraft for the emergency landing.
Interesting Facts
- 01The aircraft was a 28-year-old Boeing 707 that had previously been operated by several airlines including Pan Am.
- 02The separation occurred during a period of severe turbulence which likely provided the final stress to the fatigued pylon fitting.
- 03The No. 4 engine struck the No. 3 engine as it fell, causing both to be lost within seconds.
- 04The landing was performed with the right wing actively on fire and the landing gear extended.
- 05All 5 people on board (3 crew, 2 passengers) survived without injury.
Safety Actions & Advisories
Pylon Inspection Revision
Source ↗The BEA recommended more stringent and frequent non-destructive testing (NDT) of Boeing 707 pylon midspar fittings to detect fatigue cracks before they reach critical length.
Fail-Safe Design Review
The investigation prompted a review of the 'fail-safe' certification of older aircraft pylons, ensuring that multiple load path failures are considered in maintenance intervals.