KAP 140 autopilot and electric trim

[Nick - A2A]

Hello all,

I recently read this report into the fatal 2001 crash of a C182S in the UK which, I believe, led to the KAP 140 autopilot software being redesigned. (For software version 03/01 and later, the ‘AP’ button must be pressed and held for 0.25 seconds to engage the autopilot.)

The report does highlight that the manual electric trim in the C182 (and presumably the C172) should only be operable when the avionics bus is powered. With the current A2A set-up, the electric trim will work even with the battery master ‘off’. In the real aircraft, I understand one could isolate the electric trim servo either by switching the avionics off, or preferably by isolating just the autopilot by pulling its breaker (which we can’t do in the A2A Cessnas).

Similarly the ‘PT’ annunciation on the autopilot screen should flash to indicate trim direction when the electric trim servo is in motion if I understand correctly. (Presumably trim motion via either the yoke switches or the autopilot is annunciated in this way.) On the A2A KAP 140, it’s present but not modelled...



Very minor points I know, except for training specific autopilot procedures, but maybe something which could be addressed in an update?

Either way, the accident report in question makes for though-provoking reading. I'm not sure if it also prompted introduction of the KAP 140 voice message “TRIM IN MOTION, TRIM IN MOTION..” which according to the manual is also a feature of the real thing after >5 seconds of pitch trim operation.

Thanks,
Nick

[Piper_EEWL]

That's a very interesting point there Nick. Is that also the reason why the avionics switch is switch to the "ON" position in the cockpit inspection part of the walk around? I have been wondering why this needs to be done since this is not something that I know of in the planes I fly in RL. Maybe the autopilot is running a self test including checking the electric trim servo when switched on?!

Thanks

[Nick - A2A]

Yeah, I believe so Sebastian. The accident report includes the following paragraph (MET = manual electric trim):

Deliberate autopilot engagement on the ground is included within the pre-flight procedures recommended in the Cessna 182 Pilot's Operating Handbook at Section 9 Supplement 11. The test procedures are required before every flight and commence after the autopilot has completed its automated pre-flight tests. The pilot is instructed to test the MET in both directions and to check that pressing the 'A/P DISC/TRIM INT' button adjacent to the MET switches interrupts the MET nose-up trim function but does not isolate it. The next test is to engage the autopilot with the AP button before moving the control wheel in pitch and roll to confirm that the autopilot can be overpowered [...] The autopilot should then be disconnected with the 'A/P DISC/TRIM INT' button on the control wheel before the elevator trim is set to the take-off position manually.

Not sure how many pilots of KAP 140 equipped aircraft actually carry out these tests though...

Cheers,
Nick

[Piper_EEWL]

Very interesting. Thanks for extracting that piece from the report Nick (I was again to lazy to read the whole thing ).

It sounds like an awfully complicated procedure though!
I'm not convinced that a lot of pilots do this as part of the preflight checks. I'm not even sure a lot of pilots with KAP 140 equipped planes know about that. That's one of the reasons I don't like to much automation in GA aircraft especially when people do not fly them all the time or fly many different types. Because there is no way you know about all the quirks of the plane if you don't fly it a lot of fly to many different ones. There is a reason why airline pilots can't be current on many different types at once!

Thanks for sharing!

[Oracle427]

It takes about 10 seconds and I do this on my run up checklist on every flight. I don't want to end up a statistic.

Also on all aircraft I fly there is and avionics fan that you need to check audibly. Turning on the avionics makes this fan turn on.

[Piper_EEWL]

Oh ok. It sounded like it takes more time or is more complicated. Thanks for that input Oracle!

And thanks for the explanation with the avionics fan. Always good to talk to people who have experience in real planes compared to the ones simulated!

[Oracle427]

The procedure from our checklist with a 172S using a GFC700 autopilot is as follows. It is very similar to the procedure with the KAP140, but the steps involving the MET are not in this checklist. I used to fly a 172S with a KAP140, but I don't have a copy of that checklist anymore.

This is done as part of the before takeoff checks where the run-up is performed. The below steps take a few seconds to perform even with the additional steps to try actuating the MET using the only left, then only the right and then both switches together.

AP - Engage
Flight Controls - Check (Verify AP can be overpowered in both pitch and roll)
AP Trim Disc Button - Press and verify AP disengages and aural alert is head)
Flight Director - Off
Elevator Trim Control - Set for takeoff

[Piper_EEWL]

I see. I wonder why the A2A Cessnas don't show any of that stuff in the documentation?!

I have to admit that if never flown a plane in RL as PIC that had an autopilot. So I've never heard of that procedure.

[Oracle427]

You can't feel the AP resist you so I would say it is a moot point for simulators.

I don't know of any sim flight controls with a split trim switch, so again it would not make sense to test that.

It's been 2 years since I've used a KAP140, so I forgot if it makes a sound when it is disconnected. That might be the only relevant portion of the test.

[AKar]

In FSX/P3D it would need quite some custom work on the trim system to make these details meaningful. In current simulation, there is just a trim. In reality, these airplanes (when so equipped) have a mechanical trim that can be operated by hand, and also an electrical trim switch that drives the same system by a servo. So, the trim is indeed operative without any electrical power whatsoever if one just moves it by hand.

-Esa

[Piper_EEWL]

Very informative thread.

Thanks guys

[AKar]

Either way, the accident report in question makes for though-provoking reading. I'm not sure if it also prompted introduction of the KAP 140 voice message “TRIM IN MOTION, TRIM IN MOTION..” which according to the manual is also a feature of the real thing after >5 seconds of pitch trim operation.

Oh this is a neat feature really, the good old McDonnell Douglas used similar system that gives a warning horn and an aural "stabilizer motion" warning in some conditions, triggers of which I don't recall. It is especially important in airliners: in GA planes the trim in practice just alters the control forces while in airliners, where the whole tailplane is repositioned, it can easily overpower any elevator input there is available.

-Esa

[Piper_EEWL]

Oh this is a neat feature really, the good old McDonnell Douglas used similar system that gives a warning horn and an aural "stabilizer motion" warning in some conditions, triggers of which I don't recall. It is especially important in airliners: in GA planes the trim in practice just alters the control forces while in airliners, where the whole tailplane is repositioned, it can easily overpower any elevator input there is available.

-Esa

Yes I remember that from the PMDG MD-11 . I think that you got that warning when you trimmed the aircraft to late into your before takeoff preparations. There were probably other triggers that I don't remember. To long since I've flown it.

[AKar]

I had to check from MD-11 manuals. Apparently the warning will come on if the stabilizer moves continuously 1° on the ground or 2° in the air, and will stay on until the stabilizer stops.

Anyways, interesting that there is a similar system in KAP 140. I wonder if there would be a way to simulate electric trim separately from the manual one, likely not too many practical ways.

-Esa

[Nick - A2A]

Some interesting comments chaps. In the sim I admit I don't bother with any of the autopilot pre-flight checks because, as Oracle says, they don't serve much purpose. This in one area where reality and FSX diverge somewhat: in the sim once the autopilot is engaged we can't overpower it at all and the only way to regain control of the aircraft is to disconnect it. Seeing as how switch failures aren't yet implemented in Accu-Sim to the best of my knowledge, presumably we'd have no problem doing this. (On second thoughts, switch failures do seem to be craftily implemented in my aging joystick, so maybe I should be doing these checks after all! )

As Esa points out, for the most part there's probably not any useful benefit in distinguishing between manual and electric trim in the sim. However, the autopilot should show us when the electric trim is in motion, and strictly speaking those yoke trim switches shouldn't work when the aircraft is unpowered.

Cheers,
Nick

P.S. As an aside, the risk of taking-off with the autopilot actually engaged is probably the best reason for checking the AP disconnect button on the yoke as part of the pre-flight (especially if one's been 'fiddling' with it). It can make for an interesting take-off due to the inability to overpower those virtual servos!