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Avidyne DFC90 and Garmin GFC700 autopilots, and behaviour with a frozen pitot tube

This thread picks up from here

I have changed my procedure to turning it on for every flight turning onto the runway and turning it off after landing. In the beginning I left it off on sunny or cloudless days, but then I indeed found myself in clouds and noticing all of a sudden that the pitot heat was off, so I would turn it on all the time.

I once had a pitot heat failure in a Cirrus with Avidyne and the DFC90 autopilot. I was flying on the autopilot in below zero degrees clouds when all of a sudden the AP pitched the aircraft down (envelope protection feature). Seemingly the pitot tube had frosted up and the system did not get any airspeed data in anymore from there and thus the indicated airspeed was gone and the DFC90 pitched all the way down to protect me. What the DFC90 did not know is that I was cruising along at cruising speed and that it took me almost to Vne in seconds this way.

Two things I took away from this all: (1) leave the pitot heat on. You really need that airspeed data, even in a glass cockpit with all the bells and wistles and (2) even if you fly on the AP, be aware of what can still go wrong in the cruise or … don’t trust the DFC90 AP :-)

EHRD, Netherlands

I once had a pitot heat failure in a Cirrus with Avidyne and the DFC90 autopilot. I was flying on the autopilot in below zero degrees clouds when all of a sudden the AP pitched the aircraft down (envelope protection feature). Seemingly the pitot tube had frosted up and the system did not get any airspeed data in anymore from there and thus the indicated airspeed was gone and the DFC90 pitched all the way down to protect me. What the DFC90 did not know is that I was cruising along at cruising speed and that it took me almost to Vne in seconds this way.

That is an outrageously brain-dead systems design. It means that the SR22 will probably disintegrate due to aerodynamic forces if the pitot tube ices up or otherwise fails. Why didn’t they take into account aircraft pitch? There is no way you can be stalled if the pitch is the normal cruise pitch i.e. 2.5-3 degrees. Admittedly somebody might somehow manage to do an AF447, so you would bring in the static data too and if that is OK then why pitch down? The SR22 is not a helicopter… The system has access to all this info.

Administrator
Shoreham EGKA, United Kingdom

I first had a light go on to warn me of issues with the pitot heat, so I was aware something was wrong there. However, I was in IMC and could not instantly get out. I just kept my hand nearby and flew on. Then the pitot tube iced up and the AP all of a sudden from one second to the next was missing airspeed and pushed the nose way down. Since I was alert, I instantly disengaged and managed. The AP has envelope protection and that is why it was pushing the nose down. It cannot handle the throttle, so you all of a sudden are diving towards mother earth as it thinks you are about to stall.

EHRD, Netherlands

I have to get back after i have investigated the DFC90. But i (think) i know that it uses both attitude and speed, that’s also why it will fail if the PFD fails.

In any case: pitot tube failure and warning must result in hand flying by pitch and power.

Handflying is known of course. That is why I was alert and with my hands about at the stick. But it does appear not good that the AP pitched the aircraft down all of a sudden due to lack of airspeed data.

Peter: I would not call it a near disaster :-). I just immediately disengaged and managed fine. Was just pointing out that the behavior of the DFC90 AP. I doubt the same would have happened with the Cirrus Perspective Autopilot Controller GFC 700.

Last Edited by AeroPlus at 27 Nov 09:28
EHRD, Netherlands

It wasn’t a disaster because you were sharp, and expecting it

I did wonder if the GFC700 does the same thing. As I said, there is no need for that behaviour when the system has attitude and altitude inputs already.

Administrator
Shoreham EGKA, United Kingdom

@ AeroPlus
First: I do know that you are a more experienced IFR pilot than i am, and I am far from instructing you.

But, AFAIK the A/P reacted absolutely within the programmed parameters. The DFC90 manual explictely states that the envelope protection will not work correctly in icing conditions (and also says the DFC90 is not approved for icing conditions because EP will not work) because of the “lack of AOA data”, so I guess (while most of us do it…) it’s probably a mistake to have it on. (I use it in light icing conditions always, but like you, my hand is never far from the yoke)

What I do NOT understand is: While the A/P itself is attitude based the EP seems to use IAS only. But if you press the “Straight & Level” button the attitude information is used to turn the airplane around. But why isn’t the attitude used for the envelope protection? More investigation necessary ..

(We had a discussion about S & L in another thread, and in the meantime i have found out that the programming behind the S&L button is more complex than pressing “HDG” and “ALT”, it does more than that).

Of course, the AP should probably not be on in icing conditions, but in the Cirrus with Garmin Perspective I have it on anyways, but am more alert. However, I would not say that the indicent with the DFC90 was in “heavy” IMC conditions. It just was enough to obstruct the pitot tube. I would indeed expect the design to be this way that it would also take the attitude into consideration. It pointed the aircraft basically down towards mother earth. I can imagine that in slow flight in the circuit and getting too low an airspeed, it might want to point the nose down as the AP cannot control the throttle. When flying at altitude and at a cruising speed, I would think that the AP should behave different. I suspect it to be the difference between an AP designed/marketed by Avidyne as compared to Garmin …

EHRD, Netherlands

I bet the GFC700 will do exactly the same, because it also doesn’t have an AOA input. What’s interestting is that if you had pressed the S&L button it would have recovered the attitudem because that feature takes the attitude, the g-load, speed atc into account. WHY the EP doesn’t is still a mystery (to me that is!)

I know a couple of pilots who had both the DFC90 AND the Perspective/GFC700 – and they all liked the preciseness, FD and some other stuff of the DFC90 better. Is your experience different? I can’t say anything because I have 10 minutes experience with the GFC700. My best bet would be that the technology is very similar.

We have to keep in mind that all these systems are a compromise.

AoA is not the same as pitch (attitude). All decent autopilots know the current pitch. There is no point in pitching down to maintain a (defectively determined) airspeed if the pitch is obviously pointing the nose down, at say minus 10 degrees, especially if the altitude (also very much known to the autopilot) is also going down.

It seems to be a stupid design.

AoA wouldn’t be any good anyway because an AoA probe can freeze up just as easily as the pitot tube, if the heater is not working. But pitch and roll are easy to determine, from gyros.

In both cases 5L de-ice fluied was fine and 30-120 minutes to get the Ice off

What fluid did you use? I might need some for winter flying away from base… and the “garden centre” pump-up dispenser

Administrator
Shoreham EGKA, United Kingdom
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