Yes; that’s true.
I tend to use PIT mode to reach high altitudes. 5 degrees works well and it won’t stall, and will eventually level off. 5 degrees is something like FL200, at best power.
What’s wrong with a simple pitch mode?
huv wrote:
I would think not, although I have never climbed that high in a non-turbo’ed acft using IAS/FLC mode.
This is correct. Think about it, to chase a V/S that is too high, the plane raises the nose and speed bleeds off. If a climb at a selected indicated airspeed is not possible, the nose lowers to try to gain the speed. This means that you may not make the altitude but assuming your speed is not too low, you won’t stall.
Timothy wrote:
What’s wrong with a simple pitch mode?
Not much I guess, if the pitch mode is there. Rate based A/P’s do not have this mode, and you still have to adjust pitch down as you climb, or you will/may eventually get on the back side of the curve and stall. But if available, pitch mode is less critical than VS mode, so that is a good point.
What I miss on many autopilots is the ability to select a horizontal mode without engaging a vertical mode. Several times, in at least two different aircraft, I have had vertical mode failure (servo failure, friction in elevator trim due to cold temperature) and so could not use the autopilot at all. I see no reason why not just revert to one-axis operation and enjoy HDG and NAV A/P capabilities while taking care of the altitude yourself.
What’s wrong with a simple pitch mode?
Yes – what I said above; the PIT mode. It’s good.
on many autopilots is the ability to select a horizontal mode without engaging a vertical mode
KFC225, for sure. If the pitch servo burns out, the whole unit is useless. You can still use the FD (Flight Director) mode and hand-fly it, but I think only if the servo burnt out in just the right way which is not detected in the power-up tests 
Actually I don’t know which of the KAP/KFC units you can have just the lateral axis, with the pitch axis duff and having failed the power-up test (if there is one). Does anyone know about the DFC90/GFC700?
Peter wrote:
Yes – what I said above; the PIT mode. It’s good.
Where we go for our recurrent training they call it “PIT = pilot in trouble”-mode. It has basically the same shortcoming as vertical speed in that it will try to hold that pitch attitude until the aircraft enters a stall. Not as bad as vertical speed though which will increase pitch with decreasing speed.
what_next wrote:
it will try to hold that pitch attitude until the aircraft enters a stall
Surely this can only happen if quite an aggressive initial pitch is selected (high enough that critical AOA is reached in level flight)? Otherwise all that would happen is the aircraft would slow and stop climbing.
This raises a different question – what is a common climb pitch angle in your aircraft? The highest I’ve seen in a light single in sustained climb is around 25º in a very overpowered Vans RV (guess why I’m building one…). Some jets seem to go off the ground at incredible rates.
Stupid autopilot logic
Why is it possible, on any autopilot I have ever seen, so be at 5000ft, set 7000ft on the preselect, and set -500fpm on the VS, and it just goes and flies you into the ground?
Apparently it is also the same on at least some airliner autopilots.
It is also quite easy to end up with the ARM mode deselected so the preselect altitude is never captured.
Is there some certification requirement for “standard logic”?
Because it is better than having forgotten about the 7000 ft on the preselect, trying to enter a descent and wondering why it bloody refuses to descend? Because the preselect cannot be turned off?
From the way I know it, most AP’s do not have a direct connection between the altitude preselect and the vertical speed mode. That connection only becomes active once the altitude set is armed. In many installations, the preselector is an option, not the standard. So VS operates independently and has no direct crosscheck with the selected altitude.
There are also procedures which are used which demand a VS in opposition to the set altitude: In Non Precision Approaches a lot of operators set the missed approach altitude into the preselector once they left the initial approach altitude and then descend on VS to MDA. If they have to go around, they then arm the selected altitude so the airplane will level off at the missed approach altitude.
Clearly your concern is very valid that under such conditions, a plane can fly into the ground in VS mode. This however can also happen if you don’t arm the altitude set in the preselector (and has several times).
