KI-256 Low Vacuum Causing KFC200 A/P Issues

[MooneyMike280]

All,

An FYI I wanted to pass along related to the KFC200 A/P in my M20J.  After having the engine overhauled (and new vacuum pump installed), I have been experiencing intermittent issues with the pitch control in altitude hold mode. At times, a bank left or right would cause pitch deviations (some quite severe) - and even in level flight, these oscillations could start.

The vacuum annunciator lights were never on and the AI showed up and erect quickly with the proper attitude displayed. Decided to check the vacuum last week during a Tachometer installation and had a vacuum gauge hooked up to the system. The vacuum was at the very bottom of the gauge (like ~ 4.5"). This was was regulated to a little above 5.5" or so, hoping this would reflect a change in A/P performance.

After flying for approximately 3 hours, this does reflect a difference in the A/P operation. The altitude has not wandered or pitched at all. I sure hope this continues until a good option is available for the KI-256 replacement!!!

So, for others having this problem, this may be something to check (even with normal indications on the AI).

Mike

 

[carusoam]

Do you have a panel mount vac gauge?

My O didn't get one. The PO had it installed...

Best regards,

-a-

[MooneyMike280]

No panel mounted gauge, just the annunciator lights (HIGH / LOW VAC). Would be a nice additional though...

Mike

[CaptRJM]

I have 4.5" of vacuum during normal operations. My KFC-200 and KI-256 operate perfectly at this setting.  

Did you perform a Vacuum Leak Down Test to see if your system is holding vacuum and not slowly leaking?  By raising the vacuum you may be masking the real problem.

I'm not an AP but seeing if you have a hidden problem may be warranted.

[kortopates]

Without looking it up, I recall 4.5" as the proper setting. There should be label on the King HSI as to its vacuum requirements and I remember it stating 4.25-4.75". If correct, 5.5" is too high and will shorten the instruments bearing life. But if it helped, it likely means its borrowing a little more time if the real problem is the bearings have started to go which manifest itself as a lazy HSI. Especially if it takes a few minutes to erect after startup.

CaptRJM is spot on in that the first step is always to perform a vacuum leak down test before adjusting the vacuum up. But I really doubt you have a leak if it was originally at 4.5"

[carqwik]

Wouldn't the problem be in the KI-256?  The vacuum system has nothing to do with the autopilot itself...(or am I mistaken?)

[kortopates]

12 minutes ago, carqwik said:

Wouldn't the problem be in the KI-256?  The vacuum system has nothing to do with the autopilot itself...(or am I mistaken?)

Absolutely, I referred to the HSI incorrectly merely because I have yet to see a Air driven KI-256 in a Mooney, every installation has been an electric KI-256. But vacuum KI-256's exist and same would apply. But yes the vacuum system has nothing to do with the AP which makes it real hard to associate with an AP issue unless the gyro is being lazy as I suggested as a possibility.

[jlunseth]

The HSI is the 525A which is electric, it is slaved to the flux gate.  The KI 256 is the air driven (vacuum) Attitude Indicator.  The KFC uses the signal from the 256 (its an attitude-based AP).  I haven't had the "persistent low" problem the OP describes, but I have had the vacuum fail (VMC) and its exactly as the OP describes, pitch excursions because of the unstable gyro.  

I might still have the gyro checked though.  On my gauge, 4.5 is at the bottom of the green range or just slightly outside it.  Maybe not an issue, but could be some friction in the gyro.

[CaptRJM]

The way it was simply explained to me when I was first introduced to the KFC-200 is the HSI is the brains but the Flight Director is the brawn.  The autopilot is only interpreting the guidance from the KI-256.

I do contend that if the system was not operating properly at 4.5" but is now functioning well at 5.5" there probably is a slight leak somewhere in the vacuum system.  The vacuum switch that operates the annunciator is a notorious source for leaks.  It is also possible that the gauge that was used to adjust the vacuum was not accurate.  Running excessive vacuum will accelerate wear on vacuum powered instruments.  

[kortopates]

44 minutes ago, jlunseth said:

The HSI is the 525A which is electric, it is slaved to the flux gate.  The KI 256 is the air driven (vacuum) Attitude Indicator.  The KFC uses the signal from the 256 (its an attitude-based AP).  I haven't had the "persistent low" problem the OP describes, but I have had the vacuum fail (VMC) and its exactly as the OP describes, pitch excursions because of the unstable gyro.  

I might still have the gyro checked though.  On my gauge, 4.5 is at the bottom of the green range or just slightly outside it.  Maybe not an issue, but could be some friction in the gyro.

Thank you John, I had it totally backwards between which is air and electric.

The little sigmatek vacuum gauges range is universal and does not match the spec's of King KI-256; plus they are commonly off by an inch. But they are very good for seeing a change in vacuum.

[jlunseth]

12 minutes ago, kortopates said:

Thank you John, I had it totally backwards between which is air and electric.

The little sigmatek vacuum gauges range is universal and does not match the spec's of King KI-256; plus they are commonly off by an inch. But they are very good for seeing a change in vacuum.

Yes, the gauge is not much.  I read it more like a binary switch, is the vacuum on or not.  

[MooneyMike280]

The vacuum was checked off of the back of the KI256. I agree increasing the vacuum is probably masking an issue in the KI256 - I have had the plane for almost 22 years and have never worked on the KI256. It is old and probably needs an overhaul. I hope to be able to hold out until an option is available to replace it - like a Garmin G5 driving the KFC200 system - hope that happens...

Mike