What happened
On March 31, 1992, a Boeing 707-321CH, registration 5N-MAS, operated by Trans-Air Limited Company, was performing a cargo flight from Luxembourg to Kano, Nigeria. While climbing through flight level 320 over the Drôme region in France, the aircraft encountered severe turbulence. During this period, the crew heard a loud double bang, and the aircraft experienced a sudden right roll.
Immediately following the impact, the two right-hand engines separated from the wing. The crew experienced fire warnings for both engine number 3 and engine number 4, and noted that the engine throttles had moved forward on their own. The aircraft suffered a cabin depressurization, and the cockpit was filled with significant noise from the fire warnings and the warning horn.
The crew declared a Mayday and initiated an emergency descent. To manage the aircraft's weight, the flight engineer and a maintenance man performed fuel dumping. The crew identified the Istres military airbase as a suitable diversion field. During the approach, the captain struggled to maintain control of the aircraft due to the asymmetric thrust and aerodynamic changes.
The aircraft touched down on the Istres runway at 190 knots. Due to the loss of hydraulic pressure, the crew had to use the emergency braking system. The landing resulted in the bursting of the left main gear tires, and the aircraft eventually veered off the left side of the runway. A fire on the right wing was extinguished by the Istres fire brigade. All 5 crew members and passengers evacuated safely, and the cargo remained intact, though the aircraft was destroyed by the fire.
The investigation
The investigation focused on the structural failure of the engine pylons and the subsequent sequence of events. Investigators examined the wreckage and the flight recorders to determine why the engines had detached. The inquiry also reviewed the effectiveness of the air traffic control communications and the crew's management of the emergency.
Findings
- The primary cause of the accident was the fracture of the right inboard engine pylon fitting.
- This fracture was caused by an undetected fatigue crack in the inboard midspar fitting.
- The failure of the engine number 3 pylon caused the engine to be propelled outboard, where it struck engine number 4, causing the second engine to tear away from the wing.
- Existing Airworthiness Directives (AD) requiring periodic visual inspections of the midspar fittings were insufficient to detect the hidden fatigue crack.
- While the crew's management of the distress situation showed some defects in handling the extreme stress, these factors did not contribute to the accident itself.
Safety action
- The BEA recommended that inspections of the current midspar pylon fittings be modified to allow for the detection of hidden cracks, or that the fittings be systematically replaced with reinforced versions.
- The BEA also recommended that air traffic controllers receive specific periodic training to better manage high-stress emergency and distress situations.