The BOI report describes Nimrod systems and their operation in some detail and the Inquest was held in open court. Therefore it is churlish to deny DV this basic info on the basis that it is either sensitive or a risk to national security. To some degree it is in the MOD's interest to let him and other interested parties know what we are doing. By doing so, we would obviate an unnecessary response to a FOI request, at best, or be accused of hiding information, at worst.
DV, here is your answer:
The Release to Service prohibits anyone from opening the crossfeed valves in flight. It also prohibits the opening of the crossfeed valves on the ground when any engine is running above 80%. This limit does not change the well established (and safe) engine start procedures in use since all Mks of Nimrod entered service. The limitation only affects some start fault diagnosis techniques. We initially start the APU which then supplies the air, through the crossfeed duct to No 3 engine. Provided that the area behind No 3 engine is clear for it to be accelerated, the APU will then be shut down and the No 3 engine will be set to 75%, normally. If air conditioning is required in the cabin due to high temperatures, the No 3 will be set to 80% and cabin air will be switched on. The No 3 engine then supplies the air to start the other 3 engines in sequence: 4, 2 then 1. Once they are all started, the crossfeed valves are closed and the No 3 engine is set to idle RPM (52%). Simple and safe.
Regards
Ed Sett