Following the recent events in Shetland and the ensuing special bulletin I thought it might be worth raising this question again.

Cheers

TeeS

Why not use ALTA and IAS and NAV or at least HDG?

Hi Albatross - you can't have both ALTA and IAS engaged at the same time on a three axis machine.

Cheers TeeS

Excuse me - My bad,! Remedial reading course scheduled.

4-Axis A109
 

The A109LUH has a 4-axis AFCS. We use ALTA and IAS for a non-precision approach (along with either HDG or LNAV). The ALTA mode uses VS, which can be trimmed to a desired value, but still provides altitude protection. In the LUH, ALT is maintained on the pitch channel above 60KIAS unless IAS is also engaged, in which case it automatically changes to the collective channel. If the IAS drops below 60KIAS with only ALT engaged, ALT will again chop to the collective channel.

Instrument flying - 4-axis is the way to go.

I missed this thread 1st time around, but clearly it is better and more intuitive to use IAS. Collective then controls height, which is intuitive, missed approach just requires the collective to be raised. However I suggest it also depends on the desired speed. If it is anywhere near Vy then definitely IAS. However if it is at the high end of the speed range, then VS /ALT Acquire. This correlates with the need to manually control height with collective at low to medium speed, (medium-high if you like), but the absolute necessity to control height with cyclic at or near Vh.

The machine we fly (a 4 axis AW139), ALT will work all the way down to the hover if you wish...just like the RHT (or RADALT hold). As an example, if both radalts failure during an Automatic Hover, ALT will automatically engage after a very brief period....does a reasonable job although just not as "crisp" as RHT hold.

Cheers

U.S. Copter GPS SIAP's have tight design constraints as they are generally to hospitals or other special use facilities in urban areas where numerous obstacles are present that impact the approach design.

These constraints include:
--the TAA "T" leg length is generally only 3 NM;
--no segment of the approach may be flown in excess of 90 KIAS;
--final and missed approach segments max airspeed is 70 KIAS.

The latter is a fairly demanding criteria since three-axis autopilot systems are generally limited to a minimum speed of operation not much below 70 KIAS.

Another factor is that these procedures are routinely performed Single Pilot.

The technique I have found works best for accurate flying of the approach procedure and smooth ride for the passengers is to select IAS and establish 70 KIAS prior to arrival at the IAF, controlling altitude with collective. Subsequent altitude changes/rates of descent, up to and including descent to the MDA, are controlled with collective while closely monitoring altitude as the primary parameter.

Smooth but positive collective reduction for descents and leading the altitudes appropriately to level off at step-down altitudes and MDA will establish the helicopter on assigned altitudes while minimizing the cyclic pitch changes necessary to maintain the selected IAS.

If a missed approach is required, simply increasing collective will establish the helicopter in the climb at the prescribed 70 KIAS for the missed approach segment.

In summary, for routine instrument approaches to airports, i.e., ILS or non-precision approaches where other traffic may be a factor and approach design criteria is more expansive, altitude preselect modes work best. However for Copter GPS-constrained approaches, IAS seems to work best in my experience.

"how does the ec135 handle alt hold below 60 knots?"
It doesn't - at 60 knots and reducing, the annunciator will flash 5 times to tell you it's about to drop out - if you do nothing with the collective it will do just that.

"When you push the ALT button, the AP holds the current altitude, doesn't it?"
Yes it does.

"How exactly does the ALT PRE (aka ALT.A) logic work? In other words: Which buttons/knobs do you push in which order, if you use the ALT.A mode?"

Rotate the ALT.A knob to the desired setting and then press it.

So I assume the following:
...after pressing the ALT.A knob the ALT mode gets armed (is there any sign that shows that?) Yes, a little light next to the ALT.A Knob - the small circle between NAV & ALT.A in your photo until you are 300ft within selected alt. Then ALT mode engages and the ALT button illuminates "ON". Is that correct so far? Yes

I furthermore assume that you can control the v/s in ALT.A mode with the hat switch on the collective No, the hat switch on the cyclic - it's a 3 axis autopilot so everything is done via pitch & Roll - the Hat Switch on the collective steers the landing light.
and that ALT.A mode gets disengaged and current altitude will be hold if you press the ALT button BEFORE the selected altitude is reached?
Yes, but not very tidy if you have a high rate of climb on as it is done with a pitch change not collective.
Just in case these assumptions should be correct, does the V/S button illuminate "ON" while ALT.A mode is active? No.