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|Date||4 July 1948|
|Site||Northwood, London, England, United Kingdom|
|Operator||Scandinavian Airlines System|
|Flight origin||Schiphol International Airport, Netherlands|
|Destination||RAF Northolt, London|
|Type||Avro York C.1|
|Operator||99 Squadron, Royal Air Force|
|Flight origin||RAF Luqa, Malta|
|Destination||RAF Northolt, United Kingdom|
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.
The collision 
On 4 July 1948 an Avro York C.1 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 to RAF Northolt with a crew of seven and 25 passengers. The 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 to 2,500 ft and not to descend. At 14:59 the DC-6 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 was cleared from the area but neither aircraft acknowledged the last messages. At 15:03 the two aircraft collided about 4 miles (6.4 km) 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 was climbing. The starboard wing of the DC-6 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 recommended 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 above 1,500 ft within control zones and that any error in setting the barometric pressure of one millibar 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 failure of the RAF area control to broadcast the Regional QFF at the published times; the inclusion of the landing forecast sent to the York of a local QFF capable of being understood as the Regional QFF at a time when the landing forecast was not immediately required; and the transmission by the Northolt approach controller to the Swedish aircraft of an erroneous QFF
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.
In November 1948 after the inquiry had closed the Ministry of Civil Aviation increased the vertical separation distance between aircraft in control zones from 500 ft to 1000 ft.
- "39 Feared Dead In Air Collision – York and Skymaster crash near Northolt." (News). The Times (London). Monday, 5 July 1948. Issue 51115, col A, p. 4.
- "Air Collision Inquiry – Last Instruction to Pilots" (News). The Times (London). Tuesday, 21 September 1948. Issue 51182, col A, p. 6.
- "Air Crash Inquiry – Ministry Official's Theory" (News). The Times (London). Tuesday, 28 September 1948. Issue 51188, col A, p. 2.
- Fisher, Barbara (7 July 2008). "Families return to air disaster now almost forgotten". Uxbridge Gazette. Retrieved 11 April 2011.
- "Northolt Air Disaster – A Public Inquiry." (News in Brief). The Times (London). Tuesday, 13 July 1948. Issue 51122, col C, p. 3.
- Flight 3 February 1949, pp. 129–130.
- "The Northolt Air Crash – Three mistakes as possible cause" (News). The Times (London). Saturday, 22 January 1949. Issue 51286, col G, p. 4.
- "Airport Control – Wider separation of aircraft" (News). The Times (London). Thursday, 11 November 1948. Issue 51226, col B, p. 2.
- "Civil Aviation News: York and DC-6 Accident Report". Flight, pp. 129–130.
- "The Northolt Disaster". Flight, 8 July 1948, p. 30.
See also