The PQ convoys used CAM-ships (Catapult-Aided Merchantmen)
As I recall, they simply blasted a Hurricane off a steel trestle on a rocket-powered dolly and it flew off and intercepted whatever was coming, usually a Fw200. This needed to be timed right because if the Fw200 saw the Hurricane launch, it would run away and the Hurricane wasn't fast enough to catch it.
Assuming a successful interception, the Hurricane would then stooge around the convoy until his fuel ran out, at which point he baled or ditched, as there was no way to land the Hurricane back on the launch ship; what fun that must have been in arctic waters...
They were known as 'Hurricats'; I can probably dig up more accurate info once I get home.