DI doing strange things...

[Ringway Flyer]

We took advantage of the fine wx today to fly from Barton to Pembrey, however when we started the engine, the DI started to rotate clockwise. It increased in speed as the engine speed was increased and slowed down as the engine speed was reduced. After about 2 minutes it eventually stopped and behaved normally, so we decided to continue with our trip. The fault hasn't recurred and it worked fine all the time we were out.

The aircraft is a PA28 Archer III. Can someone (a) offer an explanation, and (b) reassure us that it isn't an indication of imminent failure. The AI showed no signs of anything untoward.

[muduckace]

I am sitting here in a big jet avionics shop talking with my co workers talking about your problem. Assuming your aircraft has a DG for the DI (HSI), the most likely culprit would be the DG. Possibly vibration induced or recessing (only a coincidence), once the DG gets up to speed to supply a heading platform the forces it is exerting may be actually stabilizing it. One Idea was a combination of the DG spinning up combined with voltage increase as the engine was brought up to speed.

Just keep that GPS backup with fresh battery's

More help if we had details on your avionics package and the age of the aircraft.

[Ringway Flyer]

Thanks for your comments! The aircraft was new in 2001 and has two Garmin 430s, which were off at the time. This has never happened previously and was, perhaps, as you imply, just a series of coincidences?
RF

[stevef]

I could be wrong but I thought the Archer III DI was driven by a vacuum pump. If so, was/is the suction gauge reading normally? There might be a regulator fault, going by the rpm-related symptoms.

[Ringway Flyer]

Yes, it is vacuum driven and yes, suction was normal.... The autopilot worked correctly.
RF

[NutLoose]

As you say vac was ok and the Art Horizon was ok, it will be a bit of corrosion formed on the bearings so they would initially have some stiction in them not allowing to fully run up the gyro hence the spinning display, we get it every now and then on our PA 28's etc and is not normally a problem until it becomes common then it in to get it sorted, I bet yours has some inactivity between flights etc

[Ringway Flyer]

Well there is some inactivity - it gets out about once every 2 weeks on average - but recently we have flown on the 2nd, 4th and 11th of this month. But what NL says does sound plausible. Guess we'll just have to keep an eye on it.... Thanks for all your comments.
RF.

[theavionicsbloke]

A high rate of presession in a vacuum driven DI is due to either.
a) Lack of vaccum pressure at the DI. (Check Vaccum Pressure Indicator. Needat least 5". Low pressu suggests a blocked filter, a leak or pump problem. Be aware! some DI'd may have a seperate filter.
Note: The DI is the most sensitive instrument here. Other equipment is more tolerant.

b) Bearing wear (ie USGyro).

I would recommend that you:
a) confirm pressure is good on the indicator (as above).
b) check that the pipe to the DI is not blocked or kinked.
c) If all of the above is ok then remove the DI and send for bench test and repair.

Dave Goodison at 'Aircraft Instruments Ltd' should be able to check this out on the bench for you.

Bear in mind that precession varying with engine rpm change is also indicative of a vacuum regulator fault.

[Ringway Flyer]

Avionicsbloke - all noted, thanks. We flew on Monday and everything worked fine. The suction is just over 5, where it usually is, (about 5.25). The only other thing, which may or may not have had anything to do with it, was that P1 adjusted the DI before starting the engine. Could this have 'confused' it, I wonder?

Anyway, all the comments are much appreciated, and we will keep a close eye on all parameters and act as and when anything changes.

RF.