ODEN, normally we cruise in 3cue/4 axis as the AFCS pegs the collective on MCP-2%. I suppose there must be very slight hunting in turbulence but its not detectable (probably just the series actuator moving slightly, the pilot's collective lever doesn't move).
What I was referring to was when you want to cruise significantly below MCP. If you do that in 3cue/4 axis the AFCS is trying to maintain a particular IAS, so unless its totally smooth there will inevitably be changes in collective to maintain the target speed. This is not a big deal and certainly totally acceptable for instrument approaches, night visual approaches etc, however its probably not best practice for extended cruising. For extended cruising significantly below MCP, ie with IAS datum set to below max cruise speed (eg to save fuel, meet an EAT or in strong turbulence) IMO its best to fly 2 cue / 3 axis, though there is nothing to stop you flying 3cue/4axis if you wish.
We also sometimes use 2cue/3axis when slowing down eg for a rig landing. With the aircraft doing up to 165 on descent it takes a fair bit of slowing to get to 10kts committal speed! If you use the IAS datum to slow down its a fairly vigorous deceleration which is perhaps not optimal for passenger comfort, especially if you stop the deceleration at various points and/or fiddle with the IAS datum. The alternative is to fly 2cue eg with ALT or VS on the cyclic, and slowly lower the collective from cruise power to a low value.
On some helicopters this is a slightly dangerous as you can fall off the back of the drag curve and descend into the sea with the nose up in the air, but 225 has a nice safety feature in that if you are flying 2cue, as the speed drops below about 60 kts the AFCS automatically moves the ALT or VS to the collective and engages IAS on cyclic at the current speed, pulling in some power to stop the speed loss and stabilise the flight path.
HC
Last edited by HeliComparator; 19th Oct 2009 at 22:43.
Reason: typo