Vacuum System Question

[Earl]

My mechanic is losing his patience with me and I need some sage advice.  My vacuum pressure was running around 4.0 to 4.3 and the annunciator light came off and my AI was slow to come erect.  So I had him turn it up to around 4.8 to 5.0 and my AI came erect right away but my annunciator light was still coming on and mostly staying on. So I had him turn it down and the light is flickering again and my AI is slow to come up.  I can't believe the sensor has such a small range of operation without coming on.  At the end of the day if I had to choose the lesser of two evils I will take the higher pressure and the proper AI operation, especially in the morning when I need to get in the air.  I have an 86 Mooney 252.  Any suggestions?

[carusoam]

POH specs for the M20R (other vacuum systems should have near identical requirements???)....

Section VII - Systems.... Anunciator Panel

9.0 HI/LO VAC 

"Designated vacuum range"

Low = 4.25 +/- .25  in Hg

High = 5.5 +.2 / - 0.0 in Hg

below 4.25 - blinking red light

above 5.5 - steady red light

"in either case, gyros should not be considered relieable during this warning time"

There is a reference to "airborne Service Letter no 31"  Unfortunately I do not have the reference 

 reference does not help much: http://www.parker.com/literature/Fluid%20Systems%20Division/Original%20Document%20Storage/Service%20Letters/SL-31.pdf

[MooneyMitch]

I might suggest having the system checked for leaks.  There was a very good article by Wayne Fischer in a recent MAPA log and also on Mooney.com regarding this very subject.   A leak is what he found.  Here is the path to Wayne's article on www.mooney.com  Once on the site, open "NEWS", scroll down to "NEWSLETTER ARCHIVES",  then pull up Decemober 2009 for his article.

 

[Earl]

The POH for the 252 is not nearly as specific but for sure the light goes on and stays on when vacuum falls below around 4.3" or above around 5.0".  More importantly I am bothered by the slow erection of the AI when vacuum is closer to 4".  I get no blinking light when it is low and only a steady light when it is low or high.  Either way the light comes on when it is within the proper range of operation which really bugs me.  Sounds like the system didn't need adjustment but the annunciator panel does.

[Kwixdraw]

Just an opinion but I think it would be a good time to let an instrument repair shop check it over and see if you might be needing some work on the AI. I sure wouldn't want to be depending on one that is acting quirky.

[donshapansky]

I had the same thing happen and as time passed I noticed when the OAT dropped into the 40's on start-up, the time for the AI to erect took longer and usually would not right itself until I was on the takeoff roll.

I kept flying it for several more flights until one day coming home at altitude the autopilot would not track on the 'GPS' roll steering function, it just slowly started to drift right of course.

I then realized the AI was listing to the left, the next day I took it into the avionics shop and got a yellow tagged KI 256 from Mid Continent for $3,750.00, ouch! The shop had tried several rebuilds from other sources with poor reliability results. That was 250 hrs ago and the replacement is much quicker to erect, much more steady in operation.

 A side note was, I blamed the autopilot trim servo for over correction on climb or descent commands from the K297 rate controller and over correction in cruise, especially in turbulence, well that all disappeared with the AI being changed out! 

I hope that helps you find the problem and be careful on the rebuild, I'ved talked with pilots since that have had poor results with low cost rebuilds, one guy was on his 3rd and it was in trouble.

Regards: Don

[donshapansky]

I forgot to mention my indicated vacuum never is much above the green minimum, 4.4 maybe. However the shop calibrated test device showed the real number to be 4.75 so don't assume the aircraft gauge is necessarily on the money.

 

Don

[Jeff_S]

Man, doesn't anybody in this forum have a sense of humor? Am I the only one who sees that a faulty erection is certainly an Indicator of a bad Attitude?

;-)

[Jeff_S]

And let's not even START talking about vacuum pumps!

[Earl]

You know Jeff, several of my posts had to be edited before submitting for fear of the obvious double entendre.  You found it anyway.  :-)