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what makes the heading indicator drift?

Posted: 16 May 2018, 08:13
by cessna lover
I have it set so the gyros and stuff drift which means they lose heading after a while. but what causes it to do that? and why when the engine is not running there is a thing in the attitude indicator that says gyro what is that and what make it move? I have an unquenchable thirst for aviation knowledge and hope to get more.

Re: what makes the heading indicator drift?

Posted: 16 May 2018, 08:33
by Oracle427
The gyro flag is an indicator of a loss of vacuum to the instrument. It is very important as a loss of vacuum will not be readily apparent.

The AI is vacuum driven. A vacuum pump pulls air out of the instrument case and fresh filtered air is drawn into the case past the gyro. This will cause the gyro to spin up to operating RPM in the tens of thousands.

Gyro drift or precession is caused by friction against the gyro gimbal bearings. In the case of the AI, the gyro has components built in to make it self correct for precession.

Even without precession the gyro compass would still need to be adjusted to remain aligned with magnetic north. The magnetic fields lines are not perfectly straight and the rate of drift will be larger at higher latitudes as one gets closer to the pole.

Re: what makes the heading indicator drift?

Posted: 16 May 2018, 08:39
by cessna lover
oh ok so the flag in the AI is a visual aid for when you lose the vacuum system that little flag will show you along with the VAC gauge?

Re: what makes the heading indicator drift?

Posted: 16 May 2018, 09:24
by Oracle427
Correct. If that flag is down it means a major loss of vacuum. It is possible to lose a lot of vacuum yet not have that flag drop. I've seen AIs get a little "laggy" sure to a weak vaccuum, though the flag was still up.

The real danger is when the vacuum is cut off and the instrument gradually spools down and gradually drifts off to one side without the pilot ever catching on. This is why it is so important to keep scanning the entire instrument panel and cross reference what
all the instruments are telling you to make sure the story makes sense.

They still can get mechanical issues that can cause similar issues without the flag dropping down. I find that happens more frequently.

Re: what makes the heading indicator drift?

Posted: 16 May 2018, 09:27
by cessna lover
right. I have in FSX failed the AI once to see if I would catch it fail. holy mosses is it hard to fly with out that instrument.

Re: what makes the heading indicator drift?

Posted: 16 May 2018, 10:09
by Oracle427
Are you flying in IMC conditions? I never bother to look at the AI in VMC conditions except to make sure it is working correctly. At night in VMC I will certainly use it.

You have a much better AI outside that can never fail as long as you are in VMC.

It isn't too hard to fly without the AI, but it takes practice and the knowledge of how to use the various instruments combine with a proper scan. Fixate on one instrument at the expense of a loss of control. No one instrument is capable of telling you everything you need to know.

If you lose the AI, you can use the turn coordinator to keep the wings level in combination with the compass to maintain a course. Be careful about using the gyro compass if the vacuum system caused the failure of the AI.

The ASI in combination with the Altimeter will tell you if you are managing your pitch properly. The VSI together with the ASI will help you maintain a steady rate of climb or descent. This isn't something you can learn by trail and error, best to read up on instrument flying procedures as well as partial panel procedures. This is something that flight simulator combined with A2A is EXCELLENT for practicing. I use it to complement my RL flying and my CFI notices it.

Re: what makes the heading indicator drift?

Posted: 16 May 2018, 10:18
by cessna lover
no I wasn't in IMC but I was just playing around with the failures in FSX.

Re: what makes the heading indicator drift?

Posted: 16 May 2018, 10:54
by Oracle427
As noted, the AI is of little to no value in VMC. The real AI outside is all you need. Your eyes should be outside the cockpit. With quick glimpses issue to check the altitude, heading, etc.

Try an AI failure in actual IMC for a challenge! :)

Re: what makes the heading indicator drift?

Posted: 16 May 2018, 11:02
by cessna lover
well VMC is no problem at all. but when I tried the instrument training on FSX OMG I could not get any thing right. lol