Peter wrote:
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?
Would be nice if AP stops VS at min sector altitudes (if GPS is available and away from airports and you are not conducting a cloud-break and going under icing and going under clouds…)
Mooney_Driver wrote:
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.
I would say that’s the recommended way of configuring the autopilot during any instrument approach.
(Setting the MAP altitude in the preselect – not using VS mode if a glidepath is available, obviously)
Airborne_Again wrote:
I would say that’s the recommended way of configuring the autopilot during an any instrument approach.
Right, makes sense. And therefore the question Peter is asking is pretty much answered.
I don’t think so; that is just one particular way to fly the missed, I don’t fly it that way, and I very much doubt that is why APs work that way.
IMHO they work that way due to long tradition of not containing a microprocessor (a.k.a. “software”) and without “software” it is really complicated to detect the case in my #1 post. You would need a PCB full of 74xx logic.
A lot of GA APs have no software at all. The later ones (say the KAP150 – e.g. around here) have a micro for the front panel functions but the control loop is analogue. The later ones, from the late 1990s like the KFC225, were what later became fashionably called “digital autopilots” and had the whole thing done in software (badly, in the case of the KFC225, which has a nasty bug which burns out the servos by thrashing them, helped along by the servos having a current limit set high enough to melt the armature winding in some tens of seconds).
And the only way the proposed logic could be implemented (nicely) is if the altitude preselect is integrated all in one box, and there is a clear VS mode, which again the early ones didn’t have.
It is also quite easy to end up with the ARM mode deselected so the preselect altitude is never captured.
No separate „ALT SEL/ARM“ on modern garmin autopilots. It’s part of the ALT preselector.
Why VS is uncoupled from ALT SEL is an interesting question. Non precision approaches and historical logic maybe?
Even with integrated all in one boxes it’s not that simple. Some descent modes (VNAV) will respect the altitude preselect restriction, some others (VGP) not.
T28 wrote:
Even with integrated all in one boxes it’s not that simple. Some descent modes (VNAV) will respect the altitude preselect restriction, some others (VGP) not.
Which makes sense to me. VGP is like ILS. VNAV is needs ALT SEL as „backup“ verification.
All the autopilots I have used outside the GA arena have had the logic that the VNAV could not override the selected altitude ( until APP was active ) and VS was armed when the selected altitude was moved 300 ft away from the current altitude when in altitude hold.
I would not suggest anything should override the selected altitude.
What I would do if designing an autopilot (I’ve designed hundreds of electronic products; some pretty complex) would be to somehow warn that the current preselect will never be achieved.
Also it is too easy to turn off the ARM accidentally so you will bust the preselect altitude unless watching it, or fly into the ground if descending. In fact a negative VS without ARM is just dumb AFAICS.
This stuff was designed decades ago, before decent user interfaces were easy to do (they need software to do well) and in the days when pilots were real men, carried Fate is the Hunter in their pocket, and had to be sorted from the sheep 
Also it is too easy to turn off the ARM accidentally so you will bust the preselect altitude unless watching it,
How? There’s no more ALT SELECT button, the ALT selector arms it automatically.
