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4 Answers

What is the maximum allowable precision error DG for IFR?

Asked by: 2054 views Aircraft Systems, FAA Regulations, Instrument Rating

Is Three (3) degrees every fifteen (15) minutes correct?  Can't find a reference.

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4 Answers

  1. Skyfox on Feb 21, 2016

    Yes, you’re correct. According to “The Pilot’s Manual 3: Instrument Flying” by Trevor Thom:

    “The heading indicator should be checked at least every 15 minutes, and reset to the correct magnetic heading by reference to the magnetic compass (and compass correction card) during steady straight-and-level flight. For the heading indicator to be acceptable in normal operations, this correction should not exceed 3° in 15 minutes.”

    However, I can’t find any source directly from the FAA that says the same thing.

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  2. Mark Kolber on Feb 21, 2016

    I don’t know if it is an airworthiness rule.

    An FAA reference is the Pilot’s Handbook of Aeronautical Knowledge:

    Because of precession caused by friction, the heading indicator creeps or drifts from a heading to which it is set. Among other factors, the amount of drift depends largely upon the condition of the instrument. If the bearings are worn, dirty, or improperly lubricated, the drift may be excessive. Another error in the heading indicator is caused by the fact that the gyro is oriented in space, and the Earth rotates in space at a rate of 15° in 1 hour. Thus, discounting precession caused by friction, the heading indicator may indicate as much as 15° error per every hour of operation.

    That tells me an unslaved DG or HSI operating in perfect condition can be expected to precess up to 15° in an hour. More than that, it is telling me I should start being concerned about the condition of the gyro.

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  3. Frank Diaz on Feb 21, 2016

    Thanks. I rent a C172 and ten minutes after take-off the DG was already 20 degrees off and it’s really annoying, especially trying to do practice instrument approaches. Every 10 – 15 minutes the DG gets was off.
    I was looking for a reference so if I wrote it up as a Squawk they can try to fix it.

    Thanks again.

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  4. Skyfox on Feb 22, 2016

    I was once up in actual IFR shooting instrument approaches in a rental C172 when the same thing happened to its heading indicator. Every few minutes it would be 10 degrees or more off. I wasn’t comfortable with that so I notified tower that my next approach would be my last one, and just did timed turns with the mag compass. Everything else proceeded exactly as normal.

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