Compass Deviation Cards 744
 

Thought this gentleman's question would get a better answer here...

Compass Deviation Table

I have noticed differences, with/without power, but then we have cards with both values.

Rgds.
NSEU

The original question is on a forum entitled "744 Forum" and so appears to relate to the B747-400. ;)

That being the case, starting with the digital Boeings, in common with all the digital Boeings from B737 upwards, there is only one compass installed in the aircraft - the standby compass. With twin FMCS's supplied with heading information from triple IRUs, backed up with Rho-Rho updating from DMEs and/or GPS, magnetic compasses have joined the Dodo as far as commercial airliners are concerned.

The AMM accuracy requirement for standby compasses is +/-5° and a correction card is issued when a new compass is installed. After that a compass swing is a bit superfluous as any pilot who does his scan properly will immediately notice any error and report it. My last three companies have deleted the routine compass swing from the schedule with the regulator's blessing: the AMM Compass swing procedure is only used when installing a replacement compass and the "annual" swing is a simple check airswing, conducted during normal operational flight.


Having said that, other less well equipped or older obsolescent types still use magnetic compass systems and in that case they require annual check swings, compass adjustment and the issue of a correction card. Usually, the check swing procedure requires the aircraft to be in a normal flight condition - for example: engines running, busses powered and services such as windscreen heating, navigation lights, landing lights or other equipment that can affect the compass flux detectors must be switched on. These requirements differ between aircraft types so an exhaustive list isn't possible. Thus, it may be necessary to produce either more than one card, or else a composite card to account for different configurations (e.g. night and day). High power transmissions on HF are intermittent so, although they may affect the compass reading, this is temporary and isn't usually included in correction tables.

When the Deck Goes Dark
 

What does your all-digital, all-glass airplane have for navigation and directional guidance when on just battery power, as has happened?

I've known airliners to be grounded by the authorities for the deviation card not having a date of check.

GB

Standby Instruments powered from the standby bus, powered by the emergency sources - in order of preference - the APU, The HMG which is itself powered by the RAT that provides centre system hydraulics, (without which the flight controls would be useless and you're all gonna die) and finally, all else failing (i.e. the APU won't start and the HMG fails even though there is hydraulic pressure) the battery powered static inverter. The standby magnetic compass, with its pitiful accuracy of +/-5° is enough for an emergency diversion.

As to regulators grounding an aircraft for not having a valid deviation card, too right; but we do have them. As I said we perform a swing and issue a card upon fitting a new standby magnetic compass (its the same as the ones you can buy for a car, and only a little better for accuracy) and it gets an annual check air-swing. The crews fill in the data on the air swing record sheet and we enter the data on a swing sheet, do the arithmetic to extract the A, B and C figures and re-validate the correction card.