IRS & AHRS / mag heading
 

1) how does the IRS get the magnetic heading?

2) in the MD80, does the AHRS give the mag hdg or it is necesary to have a flux valve?

IRS has a mag deviation database that it adds to true to give mag heading.

I cannot speak of the MD family, but in the 737 'Classic' the database appears to be well out of date, giving a variation error in mainland Europe of around +4 degrees.:eek: Makes "What is your heading?" a challenging question.:)

In a NG (presumably with a later database) is a far easier question.

On the MD-80 there are 2 compass systems driven by flux valves. These provide outputs to the RMIs, HSIs, VORs and DFGCs.

The AHRS also gets a heading reference from the Compass 1 or 2 system - then the AHRS becomes a highly accurate heading and attitude reference using 2 gyros and 3 accelerometers. There are 2 AHRS units in each aircraft.

Newer IRS units generally get their references internally by measuring the earth's movement over a period of time during initial start up and computing the location by calculating the minute lateral accelerations, which would vary predictably according to your lat/lon. Heading inputs assist in speeding the initial calculations, but are not required for NAV use with an IRS. These modern IRSs take a few minutes to initialize (longer at higher latitudes) - while the MD-80 AHRS system is up and ready in under 1 minute.