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| Date: | Tuesday 12 November 1996 |
| Time: | c. 16:44 LT |
| Type: |  Boeing 737-406 |
| Owner/operator: | KLM Royal Dutch Airlines |
| Registration: | PH-BTG |
| MSN: | 27233/2601 |
| Year of manufacture: | 1994 |
| Engine model: | CFMI CFM56-3 |
| Fatalities: | Fatalities: 0 / Occupants: 77 |
| Other fatalities: | 0 |
| Aircraft damage: | None |
| Category: | Serious incident |
| Location: | near Lambourne VOR -
 United Kingdom
|
| Phase: | Initial climb |
| Nature: | Passenger - Scheduled |
| Departure airport: | Amsterdam-Schiphol Airport (AMS/EHAM) |
| London-Heathrow Airport (LHR/EGLL) | |
| Investigating agency: | AAIB |
| Confidence Rating: |  Accident investigation report completed and information captured |
The Boeing 737 (B737) and DC-9-81 (MD81) were inbound to London Heathrow from Amsterdam
(Netherlands) and Aarhus (Denmark) respectively. The incident occurred while both aircraft were
under the control of the Lambourne (LAM) Sector Control (frequency 121.225 MHz) at the London
Air Traffic Control Centre (LATCC), West Drayton. The controller at the time was a trainee on the
sector, being supervised by a qualified mentor controller.
The LAM holding facility is based upon the LAM VOR, ground track 267°M inbound with left turns
and outbound leg timing of 1 minute up to and including FL140 (maximum speed 220 kt), or 1.5
minutes at FL150 and above (maximum speed 240 kt).
Due to single runway operations, there was extensive holding in progress for Heathrow arrivals and
the MD81 had already entered the LAM holding pattern at FL170 when it was transferred to the
control of this sector at 1638 hrs. It was instructed to maintain FL170 and was passed an
Expected Approach Time (EAT) of 1649 hrs. Two minutes later, at 1640 hrs, the B737 came onto
the frequency, routing inbound to LAM descending to FL170. It was instructed to enter the hold at
LAM. Meanwhile, the MD81 had been stepped down progressively until, at 1641 hrs, it was
instructed to descend to FL140 and to expedite descent. At 1641:45 hrs the 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.
At 1642:07 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 0.6 nm north of the B737. At this time, the B737 was some
1,800 feet above the MD81. From this point, the LATCC radar display data blocks associated with
each aircraft (ie 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.
Accident investigation:
 |
|
Sources:
https://assets.publishing.service.gov.uk/media/5422eec2ed915d1374000235/dft_avsafety_pdf_501723.pdf
https://www.jetphotos.com/photo/261478 (Photo)
History of this aircraft
Other occurrences involving this aircraft
| 23 July 2023 | C-FFNE | Canadian North | 0 | near Ottawa, ON |  |
min |
| Engine failure | ||||||
Location
Revision history:
| Date/time | Contributor | Updates |
|---|---|---|
| 11-Aug-2025 17:19 | Justanormalperson | Added |
| 11-Aug-2025 17:19 | Justanormalperson | Updated [Accident report, ] |
| 11-Aug-2025 17:28 | Justanormalperson | Updated [Departure airport, Destination airport, ] |
Corrections or additions? ... Edit this accident description
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