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Rate based autopilot quirks

I thought having read Peter’s original Know your autopilot article that a thread on the innards of rate based autopilots might be useful, not least to correct any misconceptions that I have.

At the heart of the rate based autopilot is a wing leveller with a pick up from the turn co-ordinator. As the turn co-ordinator does not detect attitude changes but only turn rate, accelerometers are used to detect changes in bank angle, basically telling the autopilot whether one wing is low or not. Pretty basic and the old Mooneys had a wing leveller on all the time, you used a switch on the yoke to allow you to roll manually. From this basic system which was not exposed to the failure path of the vacuum gyro systems, the rate based autopilots with added features (S-Tec, KAP140, etc) tended to replace the old attitude based systems (Piper/Century Altimatics, Peter’s Bendix King 225, etc).

Now the GFC 700 has introduced attitude based digital technology that is not exposed to the vacuum gyros technology, but this system is only available to higher end GA or relatively new aircraft.

Before discussing the quirks it may be worth mentioning the strengths: simplicity, the turn coordinator does not tumble and is very reliable, the wing leveller mode is available even when your gyros or glass have failed. The turn coordinator for the G1000/KAP140 is hidden behind the MFD.

In terms of quirks: the rate based autopilot is less smooth in turbulence, the two axis variety use baro and accelerometers to control pitch and ALT hold, they are not typically linked to a yaw damper, and they can’t be paired with a flight director. In addition there is no SPD or FLC mode, which results in potential for loss of control events if the pilot does not have the aircraft correctly trimmed, configured and with the appropriate power setting before engaging the autopilot. In fact KAP140 recommends use of CWS (control wheel steering) before engaging VS mode.

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.

The KAP140 with the DA42 is certified to CAT1 minima on a coupled ILS, whether this means DA42 are allowed to go below single pilot 800m approach minima on an AOC is worth knowing. Technically it would be allowed as there is a suitable autopilot fitted. However the lack of smoothness in turbulence of rate based autopilots makes me wonder whether this is wise. This lack of smoothness also results in the recommendation to use CWS and ROL mode in turbulence.

Oxford (EGTK), United Kingdom

Interesting, but some of the generalisations here are incorrect. The S-TEC 60 is rate based and can drive a Flight Director. A yaw-damper is available also. It doesn’t have a wing-leveller mode, and in the event of a vacuum failure it must be disabled because the HI or HSI, which is the only way (absent GPSS steering module) to command the roll mode, cannot be used. (Ask me how I know).

RobertL18C wrote:

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.

Don’t you mean the other way around? A rate based A/P will inherently adjust bank angle for speed as it is controlled by the turn coordinator which indicates rate of turn, while an attitude based A/P has no way of adjusting bank angle for speed unless it also has an airspeed sensor.

ESKC (Uppsala/Sundbro), Sweden

I suspect he meant to say “a rate based autopilot cannot adjust rate-of-turn for speed”. The conclusion is correct though.

Now the GFC 700 has introduced attitude based digital technology that is not exposed to the vacuum gyros technology, but this system is only available to higher end GA or relatively new aircraft.

That isn’t quite the whole story because the only vacuum gyro in common use which is absolutely required by an autopilot is the KI256 used with King units such as the KFC225. And that can be replaced too, with the EFD1000+EA100, G500+GAD43, G600, or the new KI300. So it is possible to have a certified solution without any vacuum. There is also a non-STCd “solution” in the form of an electric version of the KI256, made by Castleberry Instruments.

More e.g. here

Administrator
Shoreham EGKA, United Kingdom

Jarvis wrote:

I suspect he meant to say “a rate based autopilot cannot adjust rate-of-turn for speed”. The conclusion is correct though.

I still don’t see why an attitude based A/P would be less likely to overbank and stall.

ESKC (Uppsala/Sundbro), Sweden

Jarvis thanks for unmangling my anglais.

A_A you can have a stall/LoC event with an attitude AP if you are in VS mode and insufficient thrust. Also above FL250 modern attitude based systems switch to half bank mode. 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? Or put another way the FD component is attitude based?

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?

Oxford (EGTK), United Kingdom

I still don’t see why an attitude based A/P would be less likely to overbank and stall.

Both will command whatever bank angle is necessary for a rate one turn? However an attitude based AP will have a maximum bank limit, and a corresponding speed above which a rate one turn will not be achieved. A rate based autopilot cannot do this as it does not know the bank angle, unless it has airspeed data.

the turn co-ordinator does not detect attitude changes but only turn rate

This is not really true, as a turn coordinator is sensitive to roll rate as well as rate-of-turn and therefore can detect changes in bank attitude. What it cannot do is determine the absolute bank angle – except for the wings level, zero rate-of-turn condition.

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 provides whatever control input is required to maintain a rate one indication on the TC, regardless of speed.

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.

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