Welcome Guest. Sign in or Signup

4 Answers

What is the maximum allowable precision error DG for IFR?

Asked by: 1914 views Aircraft Systems, FAA Regulations, Instrument Rating

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

Ace Any FAA Written Test!
Actual FAA Questions / Free Lifetime Updates
The best explanations in the business
Fast, efficient study.
Pass Your Checkride With Confidence!
FAA Practical Test prep that reflects actual checkrides.
Any checkride: Airplane, Helicopter, Glider, etc.
Written and maintained by actual pilot examiners and master CFIs.
The World's Most Trusted eLogbook
Be Organized, Current, Professional, and Safe.
Highly customizable - for student pilots through pros.
Free Transition Service for users of other eLogs.
Our sincere thanks to pilots such as yourself who support AskACFI while helping themselves by using the awesome PC, Mac, iPhone/iPad, and Android aviation apps of our sponsors.

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.

    +1 Votes Thumb up 1 Votes Thumb down 0 Votes



  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.

    +2 Votes Thumb up 2 Votes Thumb down 0 Votes



  3. Frank Diaz on Feb 21, 2016

    Mark/Skyfox,
    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.

    0 Votes Thumb up 0 Votes Thumb down 0 Votes



  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.

    0 Votes Thumb up 0 Votes Thumb down 0 Votes


The following terms have been auto-detected the question above and any answers or discussion provided. Click on a term to see its definition from the Dauntless Aviation JargonBuster Glossary.

Answer Question

Our sincere thanks to all who contribute constructively to this forum in answering flight training questions. If you are a flight instructor or represent a flight school / FBO offering flight instruction, you are welcome to include links to your site and related contact information as it pertains to offering local flight instruction in a specific geographic area. Additionally, direct links to FAA and related official government sources of information are welcome. However we thank you for your understanding that links to other sites or text that may be construed as explicit or implicit advertising of other business, sites, or goods/services are not permitted even if such links nominally are relevant to the question asked.