Investigation Summary
The accident occurred within the Metropolitan Control Zone, where aircraft were stacked at 500-foot intervals. At the time of the collision, there were four aircraft in total in the Northolt stack, including the two involved in the accident. The SAS DC-6, 'Agnar Viking', had been cleared to 2,500 feet but reported passing that altitude and descending further at 14:52, prompting a correction from the controller. Meanwhile, the RAF York was cleared to descend from 4,000 feet to 3,000 feet at 14:54. At 14:59, the SAS crew decided to abandon their approach to Northolt and return to Amsterdam. They were cleared to leave the area at 2,500 feet at 15:03. However, the aircraft collided at that exact time approximately 4 miles north of the aerodrome. Investigation revealed the York was above the DC-6, and the DC-6 was in a climb. The DC-6's starboard wing struck the York's starboard side, severing its tail. The inquiry highlighted significant issues with altimeter settings (QFF), noting that the SAS crew had been broadcast a faulty pressure reading and the RAF crew may have been using an outdated or incorrect setting, leading to vertical overlap despite the assigned separation.
Final Conclusions
The Court of Inquiry, chaired by William McNair and published on January 21, 1949, concluded that the cause was likely 'human fallibility' involving both ground control and flight crews. The inquiry found that the 500-foot vertical separation was inadequate for safety. It also identified three operational errors: the issuance of a landing forecast to the RAF crew that could be confused with a regional pressure setting, the failure to transmit the regional QFF on schedule, and the broadcasting of an incorrect QFF to the SAS crew. While no specific pilot error was definitively proven, the report suggested the York's altimeters were likely set significantly higher than the actual Regional QFF.
Video Analysis
1948 Northwood Mid-Air Collision News Report — A retrospective on the 1948 collision between a Scandinavian Airlines DC-6 and an RAF Avro York over London, which remains the UK's deadliest mid-air collision.
If playback is blocked, the owner has disabled embedding — use the link above.
Airframe & Maintenance
Scandinavian Airlines System SAS Agnar Viking — Douglas DC-6 · SE-BDA
Systems & Failure Modes
Altimeter (QFF/QFE)
The inquiry found that errors in barometric pressure settings (QFF) likely caused the aircraft to be at incorrect altitudes; a 1 millibar error results in a 28-foot altitude discrepancy.
Air Traffic Stacking
A procedure where aircraft are held at specific altitudes in a vertical 'stack' while waiting for landing clearance, regulated by the Metropolitan Zone controllers.
Interesting Facts
- 01This was the first fatal accident in the history of Scandinavian Airlines System (SAS).
- 02It remains the deadliest mid-air collision to occur in the United Kingdom.
- 03The collision occurred in poor weather conditions with low visibility.
- 04The vertical separation between aircraft in the Northolt stack was only 500 feet at the time.
Safety Actions & Advisories
Increased Vertical Separation
In November 1948, the Ministry of Civil Aviation increased the minimum vertical separation distance between aircraft in control zones from 500 feet to 1,000 feet.
Standardization of Altimeter Procedures
The inquiry recommended that altimeter setting messages (QFF) be sent as priority, standalone transmissions to avoid confusion with other air traffic instructions.