VG-21 Squadron

Maintenance Items

MAINTENANCE ITEM #26   - NEWSLETTER #53 


Vacuum System Problems


It could be the gyro horizon refuses to erect. Or it could also be that the gyro compass just keeps moving. Considering the age of the instruments and the unusual attitude the plane ha been subjected to and the fact that the vacuum gauge reads in the middle of the green band, it must be a bad instrument.


In my case, it was the GH. So, I obtained a factory overhauled GH from a local instrument overhaul shop and I do the exchange. But the replacement GH acts strangely as it was slow to erect. I requested another GH from the vendor and they gave me another unit but suggested I check out my vacuum system while I did the exchange. I did per their request and changed all of the hoses and filters, finding nothing wrong. Lo and behold, the second GH reacted the same way.


Now Iím out of ideas so I call in the FBO on our airport that has an avionics and instrument technician. He came over to our hangar, hooks up a vacuum source at my planeís firewall to test the system. What he finds is, powering one of my 2 gyros at a time, all is well, but when both are connected, the Gil is no good. After two hours of trying different things, he gives up and requests I bring the plane over to his shop.


At this point, I decide to remove the instrument lines, regulator, gauge, hoses and test them on the work bench. When this was done, I found some nearly invisible cracks in the back plate of the gyro compass. Could this be the problem?.


To find out, I cleaned the plate with MEK and applied 5 minute epoxy as a temporary fix. Sure enough, that was the problem.


I guess we are never too old to learn. All of this cost about $700. I got the following info from the gyro repair shop: Those little one inch vacuum gauges, as a norm, read low and if you have a gyro pump failure, you stand a 50/50 chance of a gyro failure in the near future.