What happened
The incident involved a four-engine aircraft named Agnar Viking that was on approach to RAF Northolt. The crew faced reduced visibility due to adverse weather conditions. After receiving clearance from air traffic control to descend to 2,500 feet, the flight crew could not visually acquire the airport and elected to return to Amsterdam-Schiphol Airport.
While climbing to an altitude of 3,000 feet shortly after this decision, the aircraft collided with a Royal Air Force Avro 685 York C.1 registered MW248. The second aircraft was completing a flight from Malta-Luqa and carried three crew members and four passengers. Following the impact, both airframes lost control, dove into the ground, and crashed in a wooded area in Northwood, approximately four miles north of RAF Northolt. Both aircraft were destroyed, and 39 people were killed.
Findings
Investigators concluded that the loss was most likely attributable to human fallibility regarding the control of aircraft from the ground or in flight. A key finding was that the 500-foot vertical separation standard in force at Northolt provided an inadequate safety margin, leading to a recommendation to increase this to 1,000 feet for the Metropolitan Control Zone. The inquiry also examined altimeter settings (QFF) introduced in May 1948 for aircraft above 1,500 feet within control zones, noting that a barometric pressure error of one millibar resulted in an altitude error of 28 feet.