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

AoA is not the same as pitch (attitude).

Thank you ;-) (Did i say it’s the same?)

It seems to be a stupid design.

That’s not my opinion. The Manual explicitely states that you have to handfly in icing conditions )in a big box actually). And for good reason, because despite of the envelope protection it might kill you if the autotrim system trims the airplane without you realizing that it will be completely out of trim when you disconnect.

Bothe the GFC700 and the DFC90/100 are both the best GA autopilots

What’s important: You can use the DFC90 for hand flying in FD mode, because the envelope protection is not active then.

I have flown with both autopilot systems and of course the DFC90 is a lot better than the STEC55, but I prefer the GFC700. Don’t ask me why :-)

EDLE, Netherlands

@ aeroplus

That would be the interesting part, but it’s pretty difficult to find facts, although I tried. Of course all the Avidyne pilots say the DFC90 is better … vs. all the Garmin guys (yawn! ;-)

By technology the two autopilots seem to be very similar, and I think that the EP features are based on the same inputs and sensors. I studied both manuals but could not find “the” difference. One thing that’s DEFINITELY better with the Garmin system is that the GFC700 will not quit in case of a PFD failure (AHRS) because there’s two AHRS systems, while there’s only one with the Entegra system (R9 has two aswell)

From what I know now (and believe me i let it on too) it would be good to fly with the FD in case of icing. You will feel any abnormal trim condition and can react and both systems have very capable flight directors.

I have flown extensively with the DFC90 and GFC700. I prefer the Garmin due to the vertical features. I also think it is a bit more intuitive to use. But both are very good.

EGTK Oxford

Yes, you are right – the Avidyne doesn’t have the VNAV feature, so I have to dial in the one or two approach altitudes for an LPV … but that’s complaining on a high level when you come from an old Warrior ;-)

Flyer59 wrote:

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.

Are you sure? I wouldn’t expect it to take a plane outside of an envelope (well, what the autopilot thinks is the envelope).

Peter wrote:

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.

Yes, it can freeze as well but the system isn’t designed to protect you from icing. At best it’ll be able to recognize something’s wrong and leave it to you (turning off autopilot and envelope protection). The logic is obviously simple and there might be a reason for it (something to do with certification would be my first guess).

There is a big difference between the two autopilots:

If Garmin are to be believed, their servos are brushless (i.e. stepper motors, or 3 phase brushless motors) which means they should last “for ever” whereas the DFC90 uses the old STEC servos which use old DC brush motors.

STEC have tried to block Avidyne’s usage of their servos, with a poison letter Opinions vary if this is legally valid, but for sure STEC can refuse to service a servo which they claim has not been installed IAW their IM, etc if they find out i.e. if your avionics shop tells them, and the shop has to fill in a form for STEC on which the AP model is stated. I have some stuff here, under “a different autopilot?”.

Shoreham EGKA, United Kingdom

The other big difference in the event of invalid FD data, a stall warning, invalid sensor data, is that the GFC700 will just disengage the autopilot with the associated visual and aural alerts which don’t cancel until acknowledged. It won’t nose over the aircraft. This can be seen as either a feature of a failing in that the autopilot does not have the same sort of envelope protection. But it avoids the AeroPlus problem of aiming at the ground when the pitot freezes.

Last Edited by JasonC at 28 Nov 08:35
EGTK Oxford

The algorythms inthe AHRS Systems, especially the ones in the Callman filter, do not only require inputs from the accererometers and solid state gyros, rate based or not, but they require a speed to support the equations. Airspeed does it best. If that fails, GPS speed could take over, but AFAIK this is not certifiable. So keep that pitot heat on. Most planes with two AHRS systems have two pitot tubes for that reason. If inly one is installed, the whole attitude system incl AP is depending on a littke f….g heatpad in the pitit tube when flying in ice.

EDTQ / Sarentino

What’s true is that Garmin installations use Garmin servos, while Avidyne uses the existing servos, three in the SR22: roll, pitch and pitch trim.

The type of servos has nothing to do with the envelope protection.

EDIT: I just found out that the Perspective equipped SR22s use Garmin GA80/81 brushless servis. I was not aware of this since the parts look pretty much the same. I am sure those servos last longer. My roll servo (S-TEC) was worn out after about 1000 hours.

Actually those servos last much longer if used with the S-TEC 55X autopilot which doesn’t care about the start-up voltage. But the DFC90/100 monitors the servos and will start showing failures once the start-up voltage goes above 2.2 Volts.

Last Edited by Flyer59 at 28 Nov 10:17
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