Had an a/c today (747) just before push f/o had HDG annunciated with a 6 deg error on the HSI, switching compass cured it as did switching between INS and RADIO but it was a no go, cycled cb's it momentarily cleared but came back again.
Then somebody piped up, yeah had that before, it'll go on pushback, once on the taxi way. Lo and behold after pushing 6 deg error disappered and HDG warning went.
Apparently it has happened with the same a/c on the same stand before. There are metal blast barriers to the front with some metal buildings approx 30M beyond that, also a 777 to the right and another 747 to the left.
Any ideas as to what can cause this or have I already answered myself?
