6 deg compass error Phenomena

[spannersatcx]

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? :O

[QAVION]

"Any ideas as to what can cause this or have I already answered myself? "

I think you've already answered yourself :D

747's with their antiquated fluxgate systems regularly suffer from this. On a related theme... In our engineering office, we have a set of tables listing the expected/acceptible compass deviations for particular gates at our airport. Perhaps you could get your Maintenance Watch guys to organise one for yours? ;)

Cheers.
Q.

[Plastic Penguin]

I agree, think you have answered yourself. I used to get it when flying the classic but only certain airframes on certain stands..... maybe my outfit sold you one of ours!

Still doesnt seem right to despatch with a no go item, knowing it will clear :D

[spannersatcx]

Still doesnt seem right to despatch with a no go item, knowing it will clear

Capts decision, and they would come straight back if it didn't go away. Sometimes you have to be flexible!

[mono]

Still doesnt seem right to despatch with a no go item, knowing it will clear

Assuming the engineer wouldn't dispatch it, he would have got in the flight deck for a check swing and as soon as the a/c had gotten back far enough from the stand for the defect to clear would have realised it was a stand problem and released the a/c.

It would be a good idea, spanners, if you write to the airport authority and to your technical/quality department to advise that this stand/aircraft combination causes compass errors. Could save several hours delay one day if this info is circulated.