No. 1661 Heavy Conversion Unit RAF

Conversion units and operational conversion units (OCU) were training units of the Royal Air Force.

With the introduction of new heavy bombers, the four-engined Short Stirling, Avro Lancaster and Handley Page Halifax, the Royal Air Force introduced heavy conversion units (HCU). The heavy conversion units began forming in late 1941, to qualify crews trained on medium bombers to operate the heavy bombers before final posting to the operational squadrons. Some of the heavy conversion units were involved in bombing operations over Germany.

After the end of the Second World War, the role of the HCUs was taken over by the operational conversion units. Although the units had nominal bases, it was usual for different flights and individual aircraft to be detached nearer the operational bases.

Royal Air Force Operational Conversion Units (OCU) are training units that prepare aircrew for operations on a particular type or types of aircraft or roles. Some OCUs have a shadow, or reserve, squadron designation which is used if the unit has a war role.

Typhoon - RAF Coningsby

Tornado - RAF Lossiemouth

Hawk - RAF Valley

Support Helicopter (Puma & Chinook) - RAF Benson

Hercules - RAF Brize Norton

ISTAR - RAF Waddington

Tutor - RAF Wittering

Some aircraft types which are operated by a single squadron, which includes most transport aircraft, and most ISTAR aircraft, are not big enough to need a dedicated OCU Squadron for their training requirements - they may only have a couple of students at any time. Therefore, smaller squadrons also incorporate training facilities to allow them to process aircrew onto their aircraft type. Some roles, such as ISTAR, require more rear crew for the aircraft, like weapon systems officer (WSO)s and weapon systems operator (WSOp) - their training can be more generalised, and this is carried out by 54(R) Squadron at RAF Waddington. This reduces the training requirement on the individual ISTAR squadrons, by providing aircrew who require only aircraft conversion training when they reach their squadron. Pilots are trained directly onto their aircraft type by the frontline squadrons using training 'Flights'. An example of this is the OCU flight of 24 Squadron, which trains new Hercules aircrew onto the aircraft.

...
Wikipedia