Strange effect when doing AP preflight check

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Jarek
Senior Airman
Posts: 180
Joined: 31 Aug 2018, 14:59

Strange effect when doing AP preflight check

Post by Jarek »

While doing preflight check for AP on the ground, noticed some strange effect. With engine running it looks fine (by the book, so moving hdg bug will move control surfaces). Gyro is up to speed in this case.
However without vacuum (engine not running), if the same procedure is repeated, control surfaces on v-tail fall into strange flutter-like effect. So this raises a question - how KFC 200 should will react if there is loss of vaccum pressure?

alan CXA651
Senior Master Sergeant
Posts: 2438
Joined: 15 Mar 2016, 08:23

Re: Strange effect when doing AP preflight check

Post by alan CXA651 »

Hi.
The horz gauge is air driven , so loose air you loose horz , then autopilot will fail as it has no reference for control of aircraft , also if engine fails you would be handflying or should i say glideing , and looking for a safe place to put the aircraft down.
regards alan. 8)
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Jarek
Senior Airman
Posts: 180
Joined: 31 Aug 2018, 14:59

Re: Strange effect when doing AP preflight check

Post by Jarek »

Hi, it does not fail. When you loose vacuum, AP will start to command high frequency up/down signals to the tail servos. Please shut down your engine with AP on and wait until vacuum drops to zero. AP will stay on (P3Dv4 here).
However, I could not find any reference that it should fail - only that HDG warning flag on KI525 should pop out due to electrical failure or inadequate gyro speed.This flag is an advisory to manually disconnect AP, as guide says:
It is desirable to disconnect the autopilot under the following conditions:
1 HDG flag comes into view
2 System is in Fast Slave
3 During manual slaving
The system has the capability to supply the autopilot with an automatic disconnect signal under these conditions.

My understanding of it is that auto-disconnect capability seems to be there, but it depends on the implementation in particular aircraft. In current implementation, HDG flag hides as soon as avionics master is on even with no vaccum. So there are 2 problems in fact.

alan CXA651
Senior Master Sergeant
Posts: 2438
Joined: 15 Mar 2016, 08:23

Re: Strange effect when doing AP preflight check

Post by alan CXA651 »

Hi.
Horizon gauge is air driven , the auto pilot will fail as it has no horizon to hold or FD so controls ocilate but you have to manually disconnect the autopilot , or the aircraft would become unstable , look on page 100 of the manual.
regards alan. 8)
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Jarek
Senior Airman
Posts: 180
Joined: 31 Aug 2018, 14:59

Re: Strange effect when doing AP preflight check

Post by Jarek »

Sorry, I don't believe that real AP with so obvious failure will try to break off my tail ;-) This may happen to you mid air if you loose engine or vacuum pump. I see two possible options:
1. loss of vacuum -> trigger red HDG flag on KI -> trigger AP disconnect. (just like it does when you disable FD)
2. With no vacuum, flag will pop out and gyro will tilt to one side. AP will command roll (max deflection) to counteract (like it does now if you don't start engine), and aileron/tail will stay in this position - so either you have to overpower or disconnect. If you don't, AP will stay on - so no change to current behavior in this case.

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some1 - A2A
A2A Lieutenant Colonel
Posts: 1828
Joined: 03 Aug 2008, 11:56

Re: Strange effect when doing AP preflight check

Post by some1 - A2A »

We'll look into it :)
Michael Krawczyk

Jarek
Senior Airman
Posts: 180
Joined: 31 Aug 2018, 14:59

Re: Strange effect when doing AP preflight check

Post by Jarek »

Nice. Thank you :)

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