When I was learning to fly (on PA38s) I was taught to check the Direction Indicator against the magnetic compass every 15 minutes or so, because the DI can drift. I may not have done it that frequently after I got my license, but I still figured it's a good idea.

Do crew of airliners and other complex aircraft still do this? Or is there something else that fixes the problem?


1 Answer 1


The main difference is that airliners and other more complex aircraft will have much more sophisticated navigation equipment. Instead of a "simple" vacuum-powered gyroscope, they have things like ring laser gyros that are constantly measuring acceleration and orientation of the aircraft.

There is still an amount of drift, but it's much less. Honeywell claims a drift of less than 0.0035 degrees per hour, so after a full day that's still less than 0.1 degrees. The system is initialized before each flight and on modern aircraft it's constantly adjusted based on input from GPS and DME.

Airliners are also usually flying between waypoints under ATC supervision rather than following headings on their own. If there were a heading alignment issue, ATC would notice. But it would probably only be on a single unit and cause enough position error that the system would detect the error and alert the crew.

  • $\begingroup$ The gyros are also synched to a magnetic source. $\endgroup$ Commented Apr 30 at 3:13
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    $\begingroup$ @MichaelHall Not true for larger airliners with a full ADIRS/ADIRU (e.g. Airbus, Boeing), which derive magnetic heading from IRS true heading and a global magnetic declination model. Smaller airliners with a simpler AHRS (e.g. Dash-8 Q400) can slave their heading to the magnetic measurements from the flux valve. $\endgroup$
    – Bianfable
    Commented Apr 30 at 6:09
  • $\begingroup$ @Bianfable, consider my comment revised to "may" be synched... Anyway, this additional detail would be good to capture in an answer. $\endgroup$ Commented Apr 30 at 14:49

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