Altitude Acquire or IAS Hold?

[TeeS]

In the hope of starting a discussion:

Flying a modern 3-axis autopilot equipped helicopter, and carrying out a non-precision approach, is it safer/better to utilise Alt. Acquire or IAS Hold to carry out the descent (and the go-around for that matter)?

Let's assume:

1. The autopilot won't work for you much below 65kts airspeed except in IAS Hold.

2. You may wish to reduce airspeed to around 80kts during the approach.

Cheers

TeeS

[OvertHawk]

i'd be tempted to use the ALT (acquire then hold) function for the approach and set the speed with the lever.

For the go around, i'd use the IAS to hold Vtoss or VY then pull the available power with the lever to get the best available climb.

Interesting thing to think about for someone who is fortunate enough to have 4-axis.

OH

[TiPwEiGhT]

Same as above for me. I use the ALT pre-select/ALT hold modes and control speed with the power. At MDH I will rebug the ALT pre-select for the first missed approach altitude. Rarely use IAS hold.

TiP

[212man]

Better to hit MDA at a slightly messy IAS than to hit the ground at a very precise IAS.....

[Joebootx]

I would also agree as above. Get on down to MDA. Once ALT has captured, slow down a bit. Set the missed approach altitude in the window. Hope you see something :E

[Jack Carson]

In the EC-135 you have to be careful when operating in any altitude mode at airspeeds near Vmin. At low power as the aircraft approaches bucket airspeed the cyclic will unsuccessfully attempt to hold altitude. Near or below the bucket airspeed the aircraft may continue to descend. There is no aural warning of this condition as the autopilot is functioning properly. The Alt enunciator on the PFD will flash. This condition may have contributed to a collision with the surface in Florida a few years back. If you want to demonstrate this just engage Alt hold and slow the aircraft with collective. You will see that the aircraft will very slowly and insidiously begin a descent while still above Vmin.

[Aerobot]

I'd use Alt rather that IAS, but I wouldn't do the classic dive-and-drive unless I were really sure of the terrain. The EC135 I'm flying these days has a default 500 fpm descent to target altitude, but I'd pickle the speed back so that I got to MDA only a little before time.

[TeeS]

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

[Shawn Coyle]

How do you guys do it?? I mean, that's exactly the subject of my next article in Vertical - you must all be psychic or something.
I have to say I'm impressed that there are helicopters out there with Altitude Acquire - most don't have this feature that's in all FW...

[TeeS]

Hi Shawn

I look forward to reading it. Please excuse my ignorance but is the magazine readily available in the UK/Ireland? If not, would there be any chance of a copy of the article once it is published?

It's become one of my pet battles after watching far too many pilots fly a very nice hand flown approach and then struggle like mad when asked to do one using the autopilot! The autopilot is supposed to reduce workload not increase it! - Whoops, back off soapbox now :)

Cheers

TeeS

[malabo]

Partly sim training, where there is an engine failure on every approach and every overshoot. Biggest problem becomes understanding the autopilot system and keeping the speed above Vy -tall order for most pilots. Engaging IAS becomes the solution of choice for instructors trying to get their students to pass a checkride in a couple of hours of training.

In the real world nobody flies IFR at the speeds that I see pilots or SOP call for in training. Normal ILS is flown at 140/150, with the occasional slowing down to 90 inside the FAF if the weather is crap.

Pilots in sims always want to fly it a Vy - guaranteed banishment to Siberia by a controller if you try that in real life. Slow flying means not much margin for error before you get on the backside of the power curve, or below IFR coupled airspeed, stupendous wind drift correction, etc. Again by pilots that never fly to minimums except in a sim on a checkride.

I like ALT PRE engaged to a VS or vertical profile a-la-139 style, and ALT to hold an altitude (or Rad Alt offshore). Throw in the IAS if four axis but generally who cares as long as you are fast enough to stay ahead of the Airbus a mile behind you that seems to never have a problem getting in at 150 knots with an eye reference point 50' above yours.

I've actually seen the argument in this thread carried to the point of some pilots not coupling to a vertical mode at all in cruise, and maintaining altitude with a fixed collective and hat trim! On a four axis helicopter.

[Gomer Pylot]

I prefer IAS hold. I've seen too many pilots crash in the sim because they had ALT engaged and forgot to add enough power to stay ahead of the power curve, resulting in going below Vmin, and losing control. There should be two pilots watching for MDA, so that should never be a problem. Letting the FD control the airspeed makes it a little easier to control everything else. At MDA, you just increase collective to maintain altitude, and the airspeed stays the same, minimizing necessary control inputs. I only use ALT hold in cruise, and not always then. In heavy turbulence, it will put the aircraft in unusual pitch attitudes, and makes the ride very uncomfortable. If there is turbulence, I disengage the altitude hold and let the aircraft float up and down a hundred feet or so.

[ShyTorque]

Depends which type you fly. The A109 is three axis and it doesn't have an altitude preselect facility, only IAS and VS.

You have to be a little careful how VS is used on the approach at "top of drop" because the aircraft can exceed VLE.

[eurobigchopper]

Definitly for a Eurocopter ALTA equipped helicopter, we should use ALTA for a non precision approach (the VS mode is not anymore used in normal flight ).
That will prevent the aircraft to overshoot a MDA and help the pilot in the (especially Single pilot )heavy duty job of instrument approach . The use of Collective pitch (3 axes ) or IAS (4 axes)should be determined by SOP in each company according to aircraft ....... and Captain decision .........

[TeeS]

Hi EBC

I am not sure what you mean by the VS mode is not anymore used in normal flight but on our eurocopter, when you press ALTA it engages V/S until you are within 300ft of target. Saying that, I agree V/S is not a mode I like to use, for the reasons I stated earlier.

The point I am trying to make is that with a 3-axis autopilot neither Alt. A or IAS will stop the aircraft hitting the ground if the pilot doesn't pull the lever up enough. However, Alt. A will increase the pilot's workload making it more likely that he won't pull the it up!

A four axis machine is a completely different matter and I totally agree Alt. A is the way to go if that is what you are flying.

Cheers

TeeS

[Jack Carson]

I was a fan of using AltA in the EC-135 until I began training GPS Copter approaches. Most pilots were unable to precisely control the airspeed at 70 KIAS during altitude changes and in large turns. Using A/S hold kept the airspeed at exactly 70KIAS making the required power adjustments for altitude control very predictable. From that point forward we trained to A/S hold for all non precision approaches for procedure standardization.

[ShyTorque]

A109 is 2 axis pitch and roll. Yaw has single SAS but not coupled to FD


AW109 Power
Avionics

AC system (2 inverters)
ADF
AFCS 3-axis duplex
DME
Gyrocompass
ICS pilot / co-pilot
Pilot / co-pilot LCD EADI / EHSI NAV instruments
Transponder (mode S)
Vertical gyro #1 & #2
CVR/FDR (Cockpit Voice & Flight Data Recorder)
HF/SSB radio
Moving map
NVG compatibility (internal & external lights)
IRIDIUM satellite telephone
Stormscope
TAS
ELT (3 frequencies)
Flight director & autotrim
FLIR
GPS intefaced with EHSI
Marker beacon
Radio altimeter
Stand by ADI
VHF-COM/NAV #1 & #2
VHF/FM radio
VHF/UHF - AM/FM radio
Weather radar displayed on the LCDs



The 109 Grand is the same. The GrandNew is 4 axis.

[gwelo shamwari]

Gomer
S76A - What's an autopilot?

S76C+/++
Approach - 4q : NAV, IAS, Alt Pre then Alt Selected
Go Around - Go Around button then when established NAV/Bug, IAS, Alt Pre/Alt Selected
Cruise - NAV, Alt Selected

In bumpy conditions in cruise AS, being 4q just causes the AP to hunt, so 3q (trim, NAV, Alt) works best and then just set airspeed with the collective.

Most if not all the guys I have flown with do it like this.

[ShyTorque]

This discussion is centered around FD commands do not confuse the two functions. Try flying A109 with feet off pedals during power changes.

Yes, I'll try this next time. thanks for the advice. ;)

[Jack Carson]

I don’t know exactly what dash number autopilot or 109Es had but there was a note in the autopilot supplement that limited the GA mode to indicated airspeeds greater than 115 knots. Anything slower provided a good entry into an unusual attitude situation.

[TeeS]

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

Cheers

TeeS

[albatross]

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

[TeeS]

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

Cheers TeeS

[albatross]

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

[Thax]

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.

[HeliComparator]

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.

[Turkeyslapper]

Is there a minimum speed for the ALT mode to operate or will it hold the
altitude even in a hover


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

[arismount]

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.

[Fortyodd2]

"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.

[Fortyodd2]

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.