Well, here is the basis of my argument for the use of IAS hold -
Using Alt Acquire:
Good:
1. Autopilot will raise nose to try to level off at MDA or other selected altitude.
2. Autopilot will try to maintain a constant ROD during the approach.
Bad:
1. Pilot has to use secondary effects of controls to maintain IAS. e.g. in the descent, autopilot is trying to hold 600fpm ROD by changing pitch attitude. Aircraft hits a downdraft; ROD increases so autopilot commands a pitch up to reduce ROD, nose up attitude reduces airspeed. Pilot increases collective to increase speed just as downdraft decreases, ROD reduces to less than 600fpm due to loss of downdraft, nose-up attitude and increased collective. The autopilot now reacts by pitching nose down, airspeed increases again etc. etc. etc. Possible solution is to allow the airspeed to vary either side of datum (“a slightly messy airspeed”) but the approach will now be less stable.
2. On level off, if handling pilot is late bringing in the power then the autopilot will increase pitch attitude to hold MDA. If the approach is being carried out at 80kts then this pitch up will bring the speed back towards 65kts but if the power is still too low, at 65kts the autopilot will gently flash at you whilst allowing you to descend towards the ground. The aircraft is now descending, nose high with a possibly reducing airspeed – just about every parameter is wrong and the autopilot can’t help you until you get the speed back above 65kts!
3. At the MAP, if a go around is required, the handling pilot has to use the GA button to commence the go-around and at the same time, raise the collective for the climb (not a problem for a typically well co-ordinated helicopter pilot ;-). Again the nose comes up to initiate the rate of climb and if the collective increase is not co-ordinated well, then the airspeed can go either way.
Using IAS Hold:
Good:
1. Autopilot will hold a steady airspeed and near constant attitude.
2. The autopilot will always be operating within its effective airspeed range.
3. The handling pilot can use the primary effect of the collective to control altitude/ROD.
4. Since collective is used to level at MDA etc. the power will automatically be set for the desired airspeed. Alt Hold can be selected at this point and the airspeed will remain stable.
5. In the event of a missed approach, subject to a suitable airspeed being selected (e.g. 80-100kts in our case) the go-around can be initiated just by increasing collective and the airspeed will remain stable or can be adjusted to the required value.
6. Descent, once visual, is carried out by gently lowering the lever.
7. Pilot workload is massively reduced (in my experience/opinion).
Bad:
1. The autopilot will make no attempt to prevent you flying into the ground/water until the radio altimeter gets scared and in our case goes to Alt Hold at approx. 65ft.
From what I have seen, we are drawn to utilising the Altitude Acquire mode because we feel it will prevent us from flying into the ground. In so doing, we have to fly the helicopter in a non-intuitive fashion that is completely different to how we would fly it manually (i.e. secondary effects of the collective to control speed). This in turn increases the workload, produces a less stable approach and potentially takes the autopilot out of its operating regime.
Standard operating procedures and AVAD bug settings mitigate the risk of descending below MDA whether the approach is hand flown, flown on Alt. Acquire or IAS hold.
Cheers
Tees