Twenty years after the most notorious accident in the Boeing 767’s history, US investigators are faced with explaining another fatal dive involving a twinjet which has maintained an impressive safety record over its near-40-year service life.

The Atlas Air 767-300F had been inbound to Houston on 23 February when, during descent to its cleared height of 3,000ft, contact with the jet was suddenly lost at about 6,000ft while it was travelling at 240kt.

US National Transportation Safety Board investigators, based on preliminary video evidence, have indicated that the aircraft failed to recover from a “steep nose-down attitude”, without elaborating on the possible reasons for the excessive pitch.

Investigators will be tasked with establishing whether the cause was operational or mechanical, and determining whether the presence of a band of poor weather – around which the aircraft was being vectored – contributed to the upset.

“It’s a mystery,” says NTSB chairman Robert Sumwalt. “And the NTSB has 52 years of solving mysteries such as this.”

Just one of the six previous total losses involving 767s in flight, since the type’s entry into service in 1982, has been attributed to mechanical failure, when a thrust reverser on a Lauda Air jet deployed during cruise in May 1991.

Three aircraft were destroyed as a direct result of air piracy, including the two 767s which were flown deliberately into the World Trade Centre towers during the events of 11 September 2001.

Controlled flight into terrain was responsible for the 2002 loss of an Air China 767 during approach to land at Busan.

But the most controversial accident was that involving an EgyptAir 767-300ER which, having departed New York JFK, crashed into the Atlantic Ocean in October 1999, after entering a steep dive from its cruising altitude of 33,000ft.

Egyptian authorities had fiercely maintained that a mechanical problem had contributed to the accident and the investigation resulted in extensive NTSB analysis of the 767’s elevator systems, including ground tests on an instrumented aircraft, to determine possible failure scenarios.

While multiple scenarios were evaluated – including four which warranted particular attention – the inquiry found that the scrutiny “did not reveal any evidence” of a failure condition in the elevator system which could have caused the initial pitch down or prevented the crew’s recovery from the subsequent rapid descent.

US investigators, supported by evidence from the flight recorders, believed one of the pilots had deliberately put the aircraft into its fatal dive – triggering a clash with Egyptian counterparts, who continued to claim that failure in the elevator mechanism was responsible.

During the inquiry the US FAA ordered inspections on particular 767 elevator components, shear rivets designed to break in the event of a jam, but insisted that the order was unrelated to the EgyptAir probe.

The situation was exacerbated when, before the completion of the EgyptAir investigation, the NTSB probed a March 2001 pitch control incident which occurred to an American Airlines 767-300 as it descended through 6,000ft on approach to Paris.

Its crew found the jet did not respond as expected to control column input, and the pilots resorted to using horizontal stabiliser trim to control pitch.

US investigators could not find any discrepancies in the stabiliser components – including bellcranks, power control units and shear rivets – and tests suggested the jet might have been affected by water entering the empennage and freezing on the elevator control system. A similar incident had occurred on another 767 during approach to Zurich a month after the American event.

Cirium’s Fleets Analyzer lists four 767 hull losses on the ground as a result of fire, including one in which an American Airlines aircraft had suffered uncontained engine failure during its take-off run from Chicago in 2016.

Two other 767s were destroyed on the ground in 1990 during the conflict surrounding the Iraqi invasion of Kuwait.

Source: Cirium Dashboard