Northwood mid-air collision
Encyclopedia
The Northwood mid-air collision happened on 4 July 1948 when a SAS DC-6, registration
SE-BDA and a RAF Avro York, serial number
MW248 collided over Northwood, London close to RAF Northolt
. Thirty-nine passengers and crew of both aircraft died in Britain’s worst mid-air collision.
transport operated by 99 Squadron
of the Royal Air Force
was on a flight from RAF Luqa
Malta
to RAF Northolt
North West of London with six crew and the High Commissioner for the Federation of Malaya Sir Edward Gent
who was returning to London.
A Scandinavian Airlines System
Douglas DC-6 was flying from Stockholm also to RAF Northolt with a crew of seven and 25 passengers. Then weather in the area was bad and at 14.45 Northolt tower controller gave the Swedish aircraft permission to descend to 2,500 ft. At 14:52 the DC-6 reported Just passed 2,500 ft; going down the controller reminded him that he was only cleared the 2,500 ft and not to descend. At 14:59 the DC-6 had decided to divert to Amsterdam and informed the tower, the DC-6 was cleared to leave the area at 2,500 ft at 15:03 although this was not acknowledged by the DC-6.
At 14:12 the York was given permission to enter the Metropolitan Zone at 5,000 ft over Woodley (which is near Reading). At 14:38 the York was told to circle Northolt at 5,000 ft and at 14:50 was cleared to come down to 4,000 ft. This was within a minute of the DC-6 reporting itself at 2,500 ft. At 14:54 (three minutes after the DC-6 report at 2,500 ft) the York was cleared down to 3,000 ft. Nothing was heard from the York after 14:45 and it did not acknowledge further clearance down to 1,500 ft at 15:05.
The permission for the York to descend was given at least a minute or two after the DC-6 had been cleared from the area but neither aircraft acknowledged the last messages. At 15:03 the two aircraft had collided about four miles to the North of Northolt aerodrome. An investigation officer from the Ministry of Civil Aviation later reported that the York was above the DC-6, and the DC-6 had been climbing. The starboard wing of the DC-6 had penetrated the York on the starboard side behind the freight door and detached the York's tail unit.
Both aircraft crashed into some woods, bursting into flames on impact. After fire and rescue crews put out the fires the Avro York
was completely destroyed by the crash and the only part of the DC-6 that was still intact was the rudder and tailplane, with the rest of the DC-6 being destroyed by the fire. All 7 passengers and crew of the Avro York died and all 32 passengers and crew of the DC-6 also died, bringing the total number of deaths to 39, the worst mid-air collision
in British aviation history.
It was announced a week after the accident that a public inquiry would be held into the accident, only the third such inquiry held in the United Kingdom for an air accident. The inquiry was chaired by Sir William McNair and opened on 20 September 1948.
The inquiry report was published on 21 January 1949 and in one conclusion found that the height separation in force in the Northolt area of 500 ft provided an inadequate margin of safety and recommends that it be increased to 1,000 ft for the Metropolitan Control zone. The report also discusses the standard setting for altimeters (known as the Regional QFF) that had been introduced in May 1948 for aircraft over 1,500 ft within control zones and that any error in setting the barometric pressure of one milibar gave an error of 28 ft. While the inquiry was satisfied that the air traffic control system was satisfactory it raised three operational errors of concern which may have contributed to the disaster.
The court found no evidence of error by the Swedish crew although it noted that the erroneous QFF may have caused the altimeter to be wrong by one millibar. Although there was evidence of a failure to adhere to proper radio communications procedure it probably was not a factor in the accident. The report said there was reason to believe that the York's altimeters were a lot higher than the Regional QFF, this may have been caused by using the wrong QFF sent early by the controller or the altimeters were still set to the standard mean sea level barometric pressure.
None of the evidence established the cause of the collision but in the opinion of the court of inquiry the cause would probably be found in one of the factors mentioned. It also noted that although the air traffic system was satisfactory not all of the procedures involved appeared to have been equally promulgated.
The court recommended:
Aircraft registration
An aircraft registration is a unique alphanumeric string that identifies a civil aircraft, in similar fashion to a licence plate on an automobile...
SE-BDA and a RAF Avro York, serial number
United Kingdom military aircraft serials
In the United Kingdom to identify individual aircraft, all military aircraft are allocated and display a unique serial number. A unified serial number system, maintained by the Air Ministry , and its successor the Ministry of Defence , is used for aircraft operated by the Royal Air Force , Fleet...
MW248 collided over Northwood, London close to RAF Northolt
RAF Northolt
RAF Northolt is a Royal Air Force station situated in South Ruislip, east by northeast of Uxbridge in the London Borough of Hillingdon, West London. Approximately north of London Heathrow Airport, the station also handles a large number of private civil flights...
. Thirty-nine passengers and crew of both aircraft died in Britain’s worst mid-air collision.
The collision
On 4 July 1948 an Avro York C.1Avro York
The Avro York was a British transport aircraft that was derived from the Second World War Lancaster heavy bomber, and used in both military and airliner roles between 1943 and 1964.-Design and development:...
transport operated by 99 Squadron
No. 99 Squadron RAF
No. 99 Squadron of the Royal Air Force was a bomber squadron in both first and second world war. At present it operates the Boeing C-17 Globemaster III from RAF Brize Norton, the RAF's air transport hub. The squadron was the first RAF unit to receive the Avro Aldershot, Handley Page Hyderabad,...
of the Royal Air Force
Royal Air Force
The Royal Air Force is the aerial warfare service branch of the British Armed Forces. Formed on 1 April 1918, it is the oldest independent air force in the world...
was on a flight from RAF Luqa
RAF Luqa
Royal Air Force Luqa was a flying station and location of RAF Mediterranean Command headquarters of the Royal Air Force on the island of Malta during World War II...
Malta
Malta
Malta , officially known as the Republic of Malta , is a Southern European country consisting of an archipelago situated in the centre of the Mediterranean, south of Sicily, east of Tunisia and north of Libya, with Gibraltar to the west and Alexandria to the east.Malta covers just over in...
to RAF Northolt
RAF Northolt
RAF Northolt is a Royal Air Force station situated in South Ruislip, east by northeast of Uxbridge in the London Borough of Hillingdon, West London. Approximately north of London Heathrow Airport, the station also handles a large number of private civil flights...
North West of London with six crew and the High Commissioner for the Federation of Malaya Sir Edward Gent
Edward Gent
Sir Gerard Edward James Gent, KCMG, DSO, OBE, MC, was the first appointed Governor of the Malayan Union in 1946.-Life:Sir Edward was born in 1895 was the son of John Gent and Harriet Frankland Randall. His real name is Gerard Edward James Gent, but he changed it to Edward James Gent, the reasons...
who was returning to London.
A Scandinavian Airlines System
Scandinavian Airlines System
Scandinavian Airlines or SAS, previously Scandinavian Airlines System, is the flag carrier of Denmark, Norway and Sweden, and the largest airline in Scandinavia....
Douglas DC-6 was flying from Stockholm also to RAF Northolt with a crew of seven and 25 passengers. Then weather in the area was bad and at 14.45 Northolt tower controller gave the Swedish aircraft permission to descend to 2,500 ft. At 14:52 the DC-6 reported Just passed 2,500 ft; going down the controller reminded him that he was only cleared the 2,500 ft and not to descend. At 14:59 the DC-6 had decided to divert to Amsterdam and informed the tower, the DC-6 was cleared to leave the area at 2,500 ft at 15:03 although this was not acknowledged by the DC-6.
At 14:12 the York was given permission to enter the Metropolitan Zone at 5,000 ft over Woodley (which is near Reading). At 14:38 the York was told to circle Northolt at 5,000 ft and at 14:50 was cleared to come down to 4,000 ft. This was within a minute of the DC-6 reporting itself at 2,500 ft. At 14:54 (three minutes after the DC-6 report at 2,500 ft) the York was cleared down to 3,000 ft. Nothing was heard from the York after 14:45 and it did not acknowledge further clearance down to 1,500 ft at 15:05.
The permission for the York to descend was given at least a minute or two after the DC-6 had been cleared from the area but neither aircraft acknowledged the last messages. At 15:03 the two aircraft had collided about four miles to the North of Northolt aerodrome. An investigation officer from the Ministry of Civil Aviation later reported that the York was above the DC-6, and the DC-6 had been climbing. The starboard wing of the DC-6 had penetrated the York on the starboard side behind the freight door and detached the York's tail unit.
Both aircraft crashed into some woods, bursting into flames on impact. After fire and rescue crews put out the fires the Avro York
Avro York
The Avro York was a British transport aircraft that was derived from the Second World War Lancaster heavy bomber, and used in both military and airliner roles between 1943 and 1964.-Design and development:...
was completely destroyed by the crash and the only part of the DC-6 that was still intact was the rudder and tailplane, with the rest of the DC-6 being destroyed by the fire. All 7 passengers and crew of the Avro York died and all 32 passengers and crew of the DC-6 also died, bringing the total number of deaths to 39, the worst mid-air collision
Mid-air collision
A mid-air collision is an aviation accident in which two or more aircraft come into contact during flight. Owing to the relatively high velocities involved and any subsequent impact on the ground or sea, very severe damage or the total destruction of at least one of the aircraft involved usually...
in British aviation history.
Investigation
The inquiry report was published on 21 January 1949 and in one conclusion found that the height separation in force in the Northolt area of 500 ft provided an inadequate margin of safety and recommends that it be increased to 1,000 ft for the Metropolitan Control zone. The report also discusses the standard setting for altimeters (known as the Regional QFF) that had been introduced in May 1948 for aircraft over 1,500 ft within control zones and that any error in setting the barometric pressure of one milibar gave an error of 28 ft. While the inquiry was satisfied that the air traffic control system was satisfactory it raised three operational errors of concern which may have contributed to the disaster.
The court found no evidence of error by the Swedish crew although it noted that the erroneous QFF may have caused the altimeter to be wrong by one millibar. Although there was evidence of a failure to adhere to proper radio communications procedure it probably was not a factor in the accident. The report said there was reason to believe that the York's altimeters were a lot higher than the Regional QFF, this may have been caused by using the wrong QFF sent early by the controller or the altimeters were still set to the standard mean sea level barometric pressure.
None of the evidence established the cause of the collision but in the opinion of the court of inquiry the cause would probably be found in one of the factors mentioned. It also noted that although the air traffic system was satisfactory not all of the procedures involved appeared to have been equally promulgated.
The court recommended:
- The broadcast of the Regional QFF should be done on time and as a priority.
- All clearances into a control zone should include the Regional QFF and any local reading should not be given.
- Altimeter setting messages should be sent on their own and not included in other messages to avoid confusion.
- Air traffic procedures should be uniformly applicable to all users.
- Air traffic officers should be examined periodically.
- Ensure that there is no possibility of controllers confusing future Regional QFF with the current QFF.
- RAF crews should be given more information on procedures in the Metropolitan control zone.