Sorry Brian. I suspect that I was misreading the tone of some of your earlier posts.
I do not recall the exact details, but essentially it was as follows.
One engine (let’s say Engine 1) would be connected to the main gearbox by a normal freewheel unit. This would permit the engine to drive the gearbox, but would prevent the gearbox from driving the engine.
The other engine (let’s say Engine 2) would be connected through a selectable clutch and freewheel unit. This was essentially two concentric drive shafts. The one through the centre was permanently connected through a freewheel unit to the accessory gear train in the main gearbox. The outer shaft was connected through the selectable clutch and freewheel unit to the main drive train in the main gearbox.
With “Acc Drive” (Accessory Drive) selected Engine 2 could be started to drive the main gearbox accessories such as oil pump and hydraulic pump, without driving the main rotor. In this way it was possible to ensure that the gearbox was lubricated and the flying control hydraulic systems were charged before rotor engagement. In this conditions Engine 2’s gas generator and free power turbine would both be running.
Engine 1 would then be started with the rotor brake still on. In this condition its gas generator would be running but its free power turbine would not.
After post-start checks, the rotor brake would be released and Engine 1 would start to turn the rotors.
In order to engage engine 2 to the rotors the pilot must first ensure that the number 2 freewheel was in a freewheeling condition. To do this he would advance the speed select lever on Engine 1 and retard it on Engine 2. He would then select “Main Drive”. This would cause the selectable clutch to engage with the main drive train. The pilot would then advance the Engine 2speed select lever and match the two torques so that both engines were driving the rotors.
The great risk with this system was that if the pilot selected “Main Drive” with the speed select levers in the wrong positions, there would be a very loud “BANG” when the selectable clutch engaged and immediately took up the load of driving the rotor.
I can recall one occasion when a pilot was asked to do a single engine run. He started Engine 2 in ACC Drive and the maintainers carried out a few checks. The maintainers then asked him to engage the rotors. He selected Main Drive without starting the other engine. The rest as they say, is history.
It is quite possible (indeed highly probable) that I have got the engine numbers the wrong way around, but the process was pretty much as I have described it.
Last edited by Keith.Williams.; 19th March 2008 at 09:11.