On one of these missions to attack Comines Power Station on 24 March 1942, flying Supermarine Spitfire Mark Vb (serial number "AB281") from RAF Hornchurch, Ash was one of three of the squadron's pilots shot down by Jagdgeschwader 26 he crash-landed at Vieille-Église, about 15 miles from Calais, and was smuggled by the French Resistance to Lille and onward to Paris. He was arrested in Paris at the end of May 1942 and imprisoned at Oflag XXI-B, Szubin. In September 1942, he exchanged identities with an army private and joined a fatigue party, from which he escaped, only to be recaptured the same night. In the spring of 1943, Flight Lieutenant Ash and 32 others escaped from Oflag XXI-B through the latrine tunnel with Harry Day and Peter Stevens. With a companion, he tried to reach Warsaw, but was recaptured four days later. Shortly afterwards, he was transferred to Stalag Luft III, Sagan, where he was an active member of the escape committee. For the next 21 months, when other ranks were being transferred from Sagan to Stalag Luft VI, Heydekrug, Ash changed his identity and accompanied them. Under his direction a tunnel was later made for a mass escape, but the tunnel was discovered when 10 prisoners had got away. Ash continued the attempt and eventually gained his freedom. He boarded a goods train for Kovno, but was discovered by station guards and returned to Sagan.