dnj, in the absence of yaw damper/rudder boost, I think may be the reason, the KAP140 has the limitation that you can’t use it when asymmetric. Also blue line is outside the KAP140 operating speeds for the DA42.
RobertL18C wrote:
A typical scenario is to engage VS with too high a RoC at too low a speed, placing the aircraft on the back side of the drag curve, then using HDG change to turn. As a rate based autopilot cannot adjust bank angle for speed, an advantage of attitude based autopilots, the aircraft may over bank and stall. This is one of the reasons why rate based autopilots have a specific IAS speed envelope for operation.
VS is a mode which requires attention, particularly in climb. Not any of the conventional AP´s I know have any speed input whatsoever, so they WILL overspeed or stall the airplane when misused. The Avidyne AP and the Garmin 700 AP are the only GA AP´s I know which have IAS input at all, which would be necessary to generate protections or to have IAS vertical modes (FL Change).
All a VS mode does is maintain a Vertical Speed by pitching up and down vs a particular power setting. Therefore, naturally, if you let VS active you need to monitor your airspeed. The AP won´t do this for you. The 55X will ring an alarm if it is not capable of keeping the selected VS, however it knows nothing of the reasons why.
Consequently, if you leave your AP in VS mode, after the initial selected VS is reached and held, with a given power setting IAS will progressively decrease in climb or increase in descent. Uncorrected, it will fly the airplane into a stall eventually in climb and into overspeed on descent. Therefore, VS is not really a mode which is very unproblematic and actually much more difficult to use as a IAS based pitch mode.
In order to have either protections and/or pitch modes which take into account Airspeed, the AP must take IAS into account. None of the legacy GA AP´s do that. AFAIK only the Avidyne DFC90 as well as the Garmin 700 upwards can do that.
On the airliner AP´s or on the latest digital AP´s for GA, climbs and descents are often done very differently than VS. Those AP´s use a mode where pitch is steered by airspeed. The AP will rise or lower pitch in order to maintain a pre-selected IAS or, in jets, Mach Number at high altitude.
In order to fly an optimum climb profile, the flight path is a consequence of power setting vs IAS. IAS get´s set to the appropriate Vy, power at Climb Power. The VS you see is the result of the two and will slowly decrease with altitude. At the top end, if unattended and no Alt Capture is done befoe reaching the altitude where power and speed will balance out, VS will reduce towards zero but the airplane will not stall unless Vs (increasing with Altitude) catches up with Vy. Vy also changes with altitude, s certain tolerance should be kept.
In a descent in this mode, power is reduced to descent power (usually the minimum to avoid shock cooling in our planes) and IAS set towards a pre-defined descent speed, usually at the high end of the green arc or, in calm weather, somewhere below Vne. With the reduction of power, the AP will lower the nose in order to maintain the selected airspeed. If the airplane gets too fast, it will pitch up, if it´s too slow it will pitch down.
This FL/CHG or IAS mode depending on the make of the AP is quite a lot nicer and safer than flying VS. Until recently however, it was in the domain of airliners.
What a protection in pitch mode will do is to monitor either IAS or AOA or both and take corrective action if the airplane will either go too fast or too slow. Protections do not take into account the pre-existing mode but will kick in at any time values reach pre-determined limits. Protections are no home free solutions, all they do is monitor values and counteract them if you go too close to them. usually that is quite useful but it can go wrong, particularly if for some reason IAS gets lost or is erratic. One consequence of that was seen in AF447.
they may have to remove the health and safety restriction on the AP while asymmetric
Can you elaborate?
dnj good one, they may have to remove the health and safety restriction on the AP while asymmetric first
And, in fact, not even then. If you hold a rudder input or have the rudder out of trim it will happily fly in a (non-turning) sideslip
Indeed. This behaviour would be useful however if one engine inop in a twin, as it allows zero-sideslip flight (bank into good engine), on autopilot. Whereas an attitude based autopilot, will revert to zero degrees bank if roll mode is engaged at less than X degrees, so no way to command the autopilot to fly with the usual 3 degrees zero-sideslip bank, except perhaps by using heading mode.
I’m surprised that the GFC7000 on the DA42NG doesn’t have a facility to fly zero-sideslip OEI. This would provide useful extra performance as it’s hard to fly zero-sideslip perfectly when flying manually.
Another quirk of the S-TEC 60-2: in altitude hold mode it will initially lose 50’ of altitude in a turn. Why? Because it doesn’t command the required back-pressure on the elevator until it senses the altitude loss. It’s really a very straightforward PID feedback loop (maybe even PD), with lots of damping to avoid over controlling.
RobertL18C wrote:
The slower you are the less bank you need to achieve a given rate of turn, therefore the rate based system whacks in a one size fits all bank which gives rate one at a given speed, but gives more than rate one at slow speed. Is this correct?
No. It’s the other way around. Since the rate base A/P uses the rate of turn to sense roll attitude, it will give exactly the bank required for a rate one turn at the speed the aircraft happens to have. It’s the attitude based A/P that has to use a one size fits all bank — unless it has additional sensors to measure turn rate and/or airspeed.
The only way a rate based A/P could overbank would be if rolls so aggressively that it goes past the correct bank angle before the aircraft has started to turn. But that would be unlikely to happen — both as it would be bad design and as the turn coordinator will sense roll rate as well as turn rate. If anything, the rate based A/P I’m used to (KAP140) is sluggish rolling into a turn.
This sluggishness also means that it doesn’t do a great job in turbulence. Adequate, certainly.
However I think they adjust bank to achieve a rate one turn consistent with speed from the air data computer. Not sure this occurs with a rate based system, although if the S-TEC 60 can drive a FD it must achieve a similar outcome. On the S-TEC 60 are we sure it is not using the attitude indicator output to integrate the single cue (eye brows) with NAV and baro data, ie the rate based AP servos are out of this loop?
Yes, sure. There’s no input to the S-TEC 60 from the AI.
What it cannot do is determine the absolute bank angle – except for the wings level, zero rate-of-turn condition.
And, in fact, not even then. If you hold a rudder input or have the rudder out of trim it will happily fly in a (non-turning) sideslip, with the TC correctly indicating no turn and wings banked away from the rudder deflection.
Rwy20 the rate based AP do a better job with GPSS steering, but you are right, the turn co ordinator/accelerometer sends conflicting messages, especially in turbulence, and they tend to hunt and over bank. May be the way they are set up. Hence my query whether the KAP140 DA42 combo is really certified to CAT 1 for commercial ops.
dnj wrote:
No – it provides whatever control input is required to maintain a rate one indication on the TC, regardless of speed.
Having flown mostly behind the GFC 700 and DFC90 variants of autopilots, I was surprised to see in the latest plane I rented with an STEC 55X how bad this AP performs in different situations. We finished flying all approaches by hand, because it was just too unnerving watching the autopilot try to hold a localizer. It also definitely wasn’t good at doing rate 1 turns. Frequently overshot the localizer on interception due to wind, and at one time in cruise it started banking to the left without apparent reason until I disconnected it.
