Just to confirm/agree with NLH. It was definitely the 11F9 course for the Nimrod MR2 but may have been 11F4 for the Nimrod MR1, although there was no change to the hardware in between the aircraft upgrade. Can’t remember if it was a one or two day course but it was programmed in as part of the OCU with the specialist instructor travelling to RAF St Mawgan to provide the lessons. The capability was removed early in 1992, shortly followed by the Nimrod OCU moving to RAF Kinloss. Both bases and the NDB ‘fish boiler’ now confined to history (as well as the Nimrod MR1, MR2, MRA4 and AEW3)!