Peter wrote:
It might or it might not do that.You can demonstrate this with a spare KI256 and connect a vac pump to it. Or just look at your own one after engine shutdown. Sometimes it just stays more or less level. I have often seen mine end up within 5 degrees of level.
Well actually it may of may not do many things depending on the failure.
Here’s a good example :
Question: I have the Bendix King KFC150 Autopilot with Flight Director, for which there are two principle units available for the AI: The KI256 Flight Command Director (air driven, which I have installed) and the KI 254 Electric Flight Command Director (electrically driven, would be availalbe as a used part).
Both have the same functions… I have an alternate instrument air pump installed in the Beech. So if I read this thread correctly, I should keep the KI256 because it has a standby vac pump, alas backup, while changing to a KI 254 would not. We had that discussion a while ago…
I don’t think you are allowed to trade a KI256 for a KI 254.maybe I am wrong (need to check the installation manuals and A/P approvals)….in any case you degrade your redundancy to single point of failure (electric), the alternate air pump is not a full independent redundancy system neither. An AI (electric) with a complete independent power source is if you have a separate vacuum system and indicator.
The only electric replacement for a KI256 is a unit from Castleberry which is TSOd but has no STC. I know – I have one in the RHS. You would need an STC for the particular autopilot system which is using the KI256, or do it via a field approval (N-reg) which is likely to be damn complicated. The KI300 (previous threads) doesn’t exist yet and may never exist.
when an vacuum AI fails it just “keels over” to the side
It might or it might not do that.
Indeed I had a mooney before with KFC150 KI256 which was acting up. The AP was slowly descending (pitch down) and the KI256 flight director was pointing the reverse amplifying the error when e.g. in ALT HOLD which is a pressure input. This is part of the Autopilot Loop feedback inner an outer loop, which is then messed up. No gyros don’t tumble they can degrade slowly after bearing wear……..
Michael wrote:
the cloud tops were 800 ft so the ceiling must have been below 400 ft. …
Assuming all of the above is true, I would classify that as reckless and irresponsible.
For the record: the ceiling was at 800 ft as in “OVC008”, during departure. Slightly higher during approach, as I recall.
Here you go. KI256 1 week after shutdown
similar to mine. It doesn’t tilt a bit.
And here is my KI256 after shutdown, yesterday. Never twice the same
Never happens with glass instruments. Just saying.
