Britain's worst mid-air collision happened over London. Thirty-nine people lost their lives when an RAF transport struck a commercial flight over Northwood, then part of Middlesex but now in the London Borough of Hillingdon.
On 4 July 1948, an RAF Avro York C1 was flying in from Malta, while a Scandinavian Airlines System DC-6 was on route from Stockholm. Both were attempting to land at RAF Northolt, but were ultimately diverted to other airfields because of low cloud.
Just after 3pm, the two aircraft collided. The starboard wing of the passenger plane hit the transport's tail, sending both crashing into the woods below. All seven passengers and crew on the RAF aircraft, and all 32 onboard the Scandinavian plane were killed.
The impact destroyed both planes, except for a tail section of the DC-6. The wreckage was strewn over a large area, with significant debris near Mount Vernon Hospital in Rickmansworth Road, Northwood, and in woods beside Battlers Farm in nearby Harefield. Remarkably, a charred envelope was recovered from the site and forwarded on by the post office. As recently as 2008, blackened tree stumps could be seen at one of the impact sites.
The subsequent investigation found no definitive cause for the accident, but poor visibility, shortcoming in air traffic control methods, and badly calibrated altimeters probably contributed.
The tragedy remains the worst mid-air collision in the UK, and is also the worst aviation accident of any kind in what is now the Greater London area. Yet, like so many horrendous accidents in the capital's history, it has slipped from collective memory into near-obscurity.