Airborne_Again wrote:
Airborne_Again
02-Jan-16 12:50
#16
Peter wrote:
Are there any autopilots which have ALT mode and which don’t do that?
I wasn’t aware that the KAP140 did that. The documentation doesn’t mention it. I’ll have to try in the air sometime
It doesn’t. But you can use the trick Bosco mentions. Push the Up or Down button. For each push, altitude will change by 20 ft while staying in ALT hold mode. It works on many autopilots.
huv wrote:
It doesn’t.
I just reread the Bendix/King KAP140 manual and it actually states that the ALT mode maintains “pressure altitude”. This suggests that changing the BARO setting will not cause the aircraft to climb or descend to maintain indicated altitude. But I’ll check next time I fly.
But you can use the trick Bosco mentions. Push the Up or Down button. For each push, altitude will change by 20 ft while staying in ALT hold mode. It works on many autopilots.
That I knew – because it is mentioned in both the POH and autopilot manuals.
I just reread the Bendix/King KAP140 manual and it actually states that the ALT mode maintains “pressure altitude”
All the autopilots which work off a pressure encoder (a standalone encoder, or an encoding altimeter) will fly pressure altitude – because that is what it is getting from the encoder.
However some will also be reading the altimeter subscale. The King ones I have seen (KFC225 included) get an analog voltage from a multiturn potentiometer which is behind the subscale knob (as I wrote earlier). The autopilot then adjusts the encoder value for this difference away from 1013 and maintains that.
One would need to look at the wiring diagram for the actual aircraft. It is also possible, especially on a retrofit job, that the altimeter pot connection has not been wired up… That sort of thing happens a lot in retrofits. For example I know a guy who installed ~ 100 GNS430 boxes without ever connecting the HSI’s course pointer back to the GPS. He got away with it because none of the people were using much of the system’s capabilities.
Peter wrote:
For example I know a guy who installed ~ 100 GNS430 boxes without ever connecting the HSI’s course pointer back to the GPS. He got away with it because none of the people were using much of the system’s capabilities.
Wouldn’t that even break the VOR function of the GNS430? I mean, you set the VOR radial using the course pointer.
Yes, but… what is a VOR radial? 
Peter wrote:
Yes, but… what is a VOR radial?
Surely some of the 100 aircraft owners would have had an IR?
