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).
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."
"At1642:07 hrs, the B737 had about 5 nm to run to the LAM VOR and the MD81 was turning back westward inbound to LAM and was 0.6nm north of the B737. At this time, the B737 was some 1,800 feet above the MD81. From this point, the LATCC radar display datablocks associated with each aircraft (i.e. flight number,destination and altitude data) became overlapped and could not be deciphered by the controller. This is not an unusual occurrence when aircraft are adjacent in holding stacks and did not cause concern to the controller at that stage. At 1643:18, the B737 reported that it was taking up the hold at LAM. At this time,it was directly overhead the MD81, 700 feet above it. The aircraft then turned left together in the holding pattern. At this stage, the LATCC Short Term Conflict Alert (STCA) system operated indicating that the B737 and the MD81 had lost the required separation. The STCA system indicated the callsigns of the conflicting aircraft to the controller, but not the respective flight levels. The controller therefore requested each aircraft in turn to confirm its Flight Level. The MD81 was questioned first and responded level at FL140. Immediately following this at 1643:30 hrs, the B737 was questioned and replied that it was 'out of FL143'. The aircraft was then informed that its previously cleared level was FL150 and was instructed to climb immediately back to that level as there was traffic immediately below it and to expedite the climb. Both aircraft were in cloud and neither crew saw the other aircraft. At 1644:30 hrs, the B737 reported level at FL150. The minimum permitted vertical separation in the holding stacks is 1,000 feet."
"…The seriousness of the incident was highlighted when the LATCC Separation Monitoring Function(SMF) data became available a few minutes after the event…The closest distance between the aircraft, derived from recordings of ground based radar facilities, was 100 feet vertically and between 680 and 820 metres horizontally, at 1643:54 to 1643:59 hrs. The B737 had executed a slightly smaller radius turn than the MD81, which resulted in the B737 being slightly ahead and to the left of the MD81 at the time of closest proximity, with both aircraft having left bank applied for the turn, about 25° in the case of the B737."
For further information, see the full AAIB Report