Originally Posted by
DOUBLE BOGEY
HC - I do not want to disagree with you but in my view, pressing the Trim release in IMC, especially during a dynamic manoeuvre like a night rig take off, is surely a poor alternative to using at least the ATT mode of the AFCS.
It shouldn't be a matter of WANTING to disagree or not, one has to say what one thinks!
For a night rig takeoff (in an EC225, which I think is what we are both talking about?), pressing GA around rotation time and letting go of the cyclic is, I suspect we both agree, the best way of flying it.
But presuming we want to fly it manually for some reason:
Case 1: no pressing of Trim release. Cyclic is moved forwards to rotate. Hover follow-up trim auto-trims the cyclic so as to maintain in-trim. If you eventually press GA and let go, everything is hunky dory. BUT that presumes not much wind.
If there is 40kts wind (not that uncommon!) no more follow up trim. Now you move the cyclic forward to rotate, airspeed increases so flap back requires the cyclic to be moved further forward. At say 80kts you decide to press GA and let go. The cyclic jumps back to its hover position, the aircraft rears up starts to lose speed. You then start to fight it and a Dancopter EC155 - like scenario ensues. If you operate the beep trim, you beep the IAS datum not the stick position or attitude datum. This is a dreadful way to operate.
Case 2: you rotate whilst pressing trim release. The aircraft is more stable in this mode*, there is no non-linearity of feel associated with the cyclic breakout forces. You can press GA at any time, under any wind conditions, and let go. The AFCS will smoothly take over and carry out its GA function. This technique works every time.
You mention not wishing to lose ATT mode, but the purpose of ATT mode is to hold a previously selected attitude. How can this be a useful feature during a dynamic manoeuvre such as a rig takeoff?
*It's important to bear in mind that the behaviour of an EC225 vs an AS332L when flown against the spring trim, is completely different. With an AS332L operated against the spring trim, there are 2 components acting - a SAS type function, and also a function trying to return the aircraft to the datum attitude. Therefore the deviation in pitch or roll is roughly proportional to the stick deflection away from its trimmed position.
It's quite different on the 225 and hence I suspect on the 155. As soon as override detection occurs any attempt to maintain the datum attitude is disabled. The axis reverts to SAS and a fairly wobbly SAS at that. It is therefore quite difficult to maintain say 30 deg bank flying against the trim in an EC225 - it is wobbly and you typically find the stick back within a few mm of its trimmed position in that non-linear breakout force area.
By contrast if you bank to 30 deg with cyclic trim release depressed, it is a doddle to hold it steady. Plus you can let go to scratch your nose at any time, and nothing happens.
The idea that pressing trim release in IMC is a heinous crime is a hang-up from the 332L and is therefore not relevant. But old habits are so hard to kill!