Hi!
I wonder if anybody else has experienced the same behavior with the GFC 700 or whether I am doing something wrong. Noticed it on two recent flights.
When climbing manually to a target altitude also preset using the ALT knob and then engaging the AP I will be in ROL and PIT mode, so far so good. I am already on or close to my target altitude. Now pressing the GFC 700 ALT button will arm the ALTS mode but not activate it, and the plane will remain in PIT mode and not go on to capture the selected altitude. Only option being to press the ALT button again to put the FD into ALT hold mode, which will most often end up being not the target altitude but close. So that isn’t a big issue but it somehow annoys me to have the wrong setting in the FD window and the danger of ending up in PIT mode instead of ALT, which should of course not happen when paying close attention, but there is no point in introducing unnecessary error sources either.
Am I missing something or is this the expected behavior?
Reading up on the GFC 700 manual again I saw that it would be possible to correct the altitude using CWS after ALT hold mode is engaged. Still seems a lot of effort for a simple problem.
All the best
Fabian
I don’t understand. How close are you when you hit ALT? If you are already through then it cannot of course capture it. And by the way I would never be in ROL and PITCH mode on the GFC700. Choose a better mode eg HDG and VS or FLC.
For example yesterday I had 2000ft set and was at 1980ft when I hit ALT, I would have expected it to go into capture mode when the actual altitude is within 50ft of target altitude, based on the GFC700 guide. Maybe a misunderstanding on my part?
Works perfectly fine when having the AP climb in e.g. VS mode to a selected altitude which is different from the present altitude.
I think you are hitting it too close to the target altitude. It is too late to capture. 20ft? Try hitting it 500ft before and see if it works. I am sure it will.
I’d guess that it has to be armed before reaching 50 feet away from target where it switches to capture mode. You can experiment with it if you want. Why do it so late?
Pushing ALT when in climb/descent is never a good solution in my experience. It will tell the AP to use the barometric pressure at the time you push as the one to stablize on. When you’re in a climb at that time, it means it will immediately overshoot it and then have to correct by putting the nose down.
Better to program a target altitude or reduce VS when approaching.
I probably should elaborate the use case/situation in more detail: I usually hand fly the plane for a quite a while after departure (I don’t have that many hours on the C172, so hand flying practice is great), but at some point I want the autopilot engaged (usually on a longer flight), so at this point I am already established on my target altitude, plus/minus however precisely I am holding that altitude. Now I can go into ALT hold mode by pushing ALT twice but this will lock in the offset I had when activating the mode, usually between 20 to 40ft.
Pushing ALT means selecting the current altitude. What you want is altitude preselect.
If I understand you currectly: pressing ALT (preceeded by say HDG or whatever, for the desired lateral mode) during cruise will lock in, as Achim says, the current baro altitude when the ALT button was pressed.
It won’t lock in the preset altitude.
This may be counter-intuitive but that is how “classical” autopilots have worked for decades. I believe airliner ones work the same way.
There are perhaps 2 ways to move the plane to the exact altitude you want:
I do this all the time when I get fed up with hand flying. Just press AP HDG ALT when at the target altitude and with the heading bug correctly set up.
achimha wrote:
Better to program a target altitude or reduce VS when approaching.
+ 1
