V9977 was an Handley Page Halifax II that had been sent to the Telecommunications Research Establishment at RAF Defford to be used as a flying testbed for the H2S radar.
On the afternoon of 7 June 1942, one of its Rolls-Royce Merlin engines caught fire and led to the aircraft crashing near the England-Wales border, killing all eleven crew-members. Among the dead was Alan Blumlein of EMI, who was well known as the inventor of stereophonic sound recording and the 405-line television system used in the UK until 1985.
Investigators determined that improper engine maintenance/assembly procedures caused the accident. It remains the deadliest crash in the history of military test flight in the UK.
The crash
On the weekend of 6 and 7 June, Lovell and the team met with Alan Blumlein and two of his associates from EMI to examine the system with an eye to beginning production. After the EMI team left to return to their hotel, Lovell flew in V9977 and received strong returns from Gloucester, Cheltenham and several other towns at previously invisible ranges.
The EMI team decided they should see this for themselves, and took off in V9977 at about 2:50 pm on the 7th heading for the Bristol Channel. At 4:20 pm the aircraft was seen over the Forest of Dean with its outboard starboard engine on fire. Shortly after, the left wing broke off and the aircraft rolled over and crashed in a field on the Courtfield estate in Lydbrook near Welsh Bicknor on the north side of the River Wye. All aboard were killed.
News of the crash did not reach Defford until 7:35. At 9 pm, Lovell and O'Kane were driven to the site to retrieve the top secret cavity magnetron from the wreckage.
Source: Wikipedia