B734 / MD81, en-route, Romford UK, 1996

B734 / MD81, en-route, Romford UK, 1996

Summary

On 12 November 1996, a B737-400 descended below its assigned level in one of the holding patterns at London Heathrow in day IMC to within 100 feet vertically and between 680 and 820 metres horizontally of a MD-81 at its correct level, 1000 feet below. STCA prompted ATC to intervene and the 737 climbed back to its cleared level. Neither aircraft was fitted with TCAS 2 or saw the other visually.

Description

On 12 November 1996 (the same day as the fatal mid air over New Delhi), a B737-400 descended below its assigned level in one of the holding patterns at London Heathrow, in Instrument Meteorological Conditions (IMC), to within 100 feet vertically and between 680 and 820 metres horizontally of a MCDONNELL DOUGLAS MD-81 at its correct level. Neither aircraft was fitted with Airborne Collision Avoidance System (ACAS).

Synopsis

The following is an extract from the UK AAIB report:

"…MD81 was requested to report reaching FL140, but replied that it was already at that level. Accordingly the controller then cleared the B737 to descend to FL150. At 1641:55 hrs, this instruction was correctly acknowledged by the B737 crew, including the correct cleared flight level 150."

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