ATA 34-21, Inertial Reference System

Some confusion between the flight crew and maintenance that caused a delayed flight.

The issue stemmed from the HDG (heading) display on the IRMP (IRS Mode Control Panel) in comparison to the heading indication on the navigation displays.





The mode control always shows "true" heading. Any differences noted are due to the IRS's accounting for the magnetic deviation for the current location.

Some aircraft are equipped with a Mag/True switch.



When placed to True, will have the control panel and navigation heading displays matching.



The "M" or "T" next to the digits display will indicate the current mode.