The 1948 Northwood mid-air collision took place on 4 July at 15:03 when a Douglas DC-6 of Scandinavian Airlines System (SAS) and an Avro York C.1 of the Royal Air Force (RAF) collided in mid-air over Northwood in London, UK (then in Middlesex).

The DC-6, registration SE-BDA and named Agnar Viking, was on an international scheduled flight from Stockholm via Amsterdam to London's RAF Northolt.

At the time of the collision the DC-6 was ascending, as the pilots had minutes before having to decide to leave the stacking and divert to Amsterdam.

[1] The aircraft, with Swedish registration SE-BDA and named Agnar Viking, was brand new and had first flown earlier that year.

On board were six crew members and the High Commissioner for the Federation of Malaya Edward Gent, who was returning to London.

Air traffic control issued atmospheric pressure measurements (QFE), allowing the aircraft to synchronise their altimeters.

[1] The DC-6 decided to divert to Amsterdam at 14:59 and informed the tower; it was cleared to leave the area at 2,500 feet at 15:03 although this was not acknowledged by the DC-6.

[11] 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.

[12] 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.

[12] The report stated there was reason to believe that the York's altimeters were set a lot higher than the Regional QFF.

This may have been caused by using the wrong QFF sent earlier by the controller or that the altimeters were still set to the standard mean sea level barometric pressure.

[12] It also noted that although the air traffic system was satisfactory, not all of the procedures involved appeared to have been equally promulgated.

This had largely focused on the problems with icing, but the collision at Northwood brought the attention of the risks of too little vertical distance between aircraft in the stack.