140 Wing of the Royal Air Force’s Second Tactical Air Force was selected to carry out the raid using Mosquito Fighter Bomber Mk VIs. The Wing was based at RAF Station Hunsdon in Hertfordshire and consisted of 18 Mosquitos from No. 464 Squadron Royal Australian Air Force, No. 487 Squadron Royal New Zealand Air Force, and No. 21 Squadron Royal Air Force. The Wing Leader was Group Captain Percy Charles Pickard (winner of the Distinguished Service Order with two bars and the Distinguished Flying Cross). Though Pickard was an experienced pilot and leader, he was inexperienced in low-level attacks and had only 10 hours ‘ worth of specialized low-level conversion training at Hatfield.
The Mosquitos of 487 Squadron were to target the prison guards’ mess hall and to breach the prison’s outer wall in two places. 464 Squadron was tasked with breaching the main walls if no prisoners were seen escaping. As requested by those prisoners already aware of the proposed mission, 21 Squadron was assigned to bomb the prison and all in it. Prepared and ready to execute Operation Jericho beginning on 10 February, the raid was to be led by Air Vice-Marshal Basil Embry. Escort for the Mosquitos was to be provided by 14 Hawker Typhoons from 198 Squadron and 174 Squadron Royal Air Force.
Because Air Vice-Marshal Embry was involved in the planning for the upcoming invasion at Normandy, he could not be risked during the attack. Group Captain Pickard took his place. The mission was delayed by very poor weather, which worsened after 10 February with low scud clouds and snow across northern France. But by 18 February, it was not possible to wait any longer, and the 18 Mosquito fighter bombers, along with a single PR (photo-reconnaissance) Mosquito, were prepared for the mission.
The crews were briefed at 0800 local time under high security. This was the first time any of them had been made aware of the target. Pickard was to bring up the rear of the second wave of aircraft, to assess the damage and to call in 21 Squadron if necessary. In the event of anything happening to Pickard’s aircraft, the crew of the PR Mosquito would broadcast the signal instead.
The final decision to carry out the attack was made two hours before the deadline for striking the target and the Mosquitos took off from RAF Hunsdon, into weather worse than many of the crews had previously experienced. Four of the Mosquitos lost contact with the remainder of the formation and were forced to return to base. When one Mosquito turned back toward RAF Hunsdon with engine trouble, only nine Mosquitos were left to carry out the main attack.
