HSI Question

[Kyle]

I have a Century NSD360A HSI in the 170. It runs off the left side Venturi, has a regulator set to 5"/hg and is fed NAV info from the Garmin SL30. Over the last few weeks it's been behaving strangely in 2 ways:

1) Flying along straight and lever, pick a heading, say 180. The heading card reads 180 and the course needle is also set to 180. You execute a 180 degree turn and the heading card and the windshield strap mounted compass both match up at 360. The course needle will be anywhere from 10 - 20 degrees off. It won't stay at whatever heading you initially set it to. This happens consistently.

2) Flying along straight and level and the next thing you know is the heading card start’s spinning at an alarming rate. Vacuum gage reads about 6 to 6 1/2"hg so I know it’s getting the 5 of the regulator setting. It stops spinning in two ways - a) on its own or b) you push the heading selector knob in and re-set the card to the desired course. This doesn’t occur under identical conditions - sometimes it's during ground taxi and sometimes it's cruise flight and is also very intermittent.

Anyone ever experience this before?
Thanks,

Kyle T.

[HA]

time for overhaul, or your filter is really dirty. but the cheap easy thing isn't usually what fixes a problem.

[FredMa]

Yeah, the cheap and easy fixes are only found to be the problem after the expensive parts have been changed with no success.

[blueldr]

Sounds to me like you're losing gyromatic stability from low rpm. How long since the last overhaul?

[FredMa]

I believe what blurldr means is that that gyros don't work unless they are turnng at the proper speed. I used to see this all the time when towing or moving aircraft with gyros shortly after shut down. If you turn power on and move it with gyros not up to speed damage to the instruments can result. We would have to pull the gyro circuit breakers (electric gyros) in order to move the aircraft or wait about 20 minutes for them to spool down. Since yours are vacuum operated I would check to see that the proper vacuum is actually reaching the instrument. Even with the proper vacuum indicated there could be a restriction or kink in the line going to the instrument or as previously mentioned a dirty filter. If it isn't something simple like that you are probably looking at an instrument repair.

[Kyle]

It's only been 3 years since O/H, and a new filter last year. I'll check the plumbing, regulator and filter first but you guys may be right that it's time for an overhaul. It is happening both in cruise and ground operations which makes it harder to pinpoint. Not that electronic stuff dosent go south too, but I wish somebody made a stand alone electronic HSI that was as small as the top half of the Aspen Avionics set up... The Century HSI is deep, heavy and supported by those small panel shock mounts. It puts a capable instrument in the panel without altering the look to much. Maybe they would use my plane as a test ship

N8087A Panel

[GAHorn]

Small, instrument panel mounted electric gyros are notorious for short life spans.
Vacuum gyros are much more durable, until you get into high dollar remote mounted
electric gyros.