Friday, October 29, 2010

True, Magnetic and Whatever

I'm up north, and I have a whack of GPS equipment on board that tells me which way is which, so the lowly whiskey compass at the top of the windshield doesn't get much regard. Not that I disregard it. A compass is a very cool device, incredibly low tech, but working all over the planet without power. I can imagine an advanced technological society that had lost the knowledge of compass navigation. They could have built so many structures, electronic devices and power transmission lines that compasses didn't work most places, and have so much signage and technological assistance that no one used them anyway.

The Earth's magnetic field lines, with which a compass aligns, encircle the planet emerging from and disappearing into the poles. My compass is a hemispheric shell suspended by a point in liquid (the "whiskey") and free to rotate and tilt to align with the local magnetic field. I can't use it to roll out on a heading because the turning errors are enormous at this latitude, and even in level unaccelerated flight the tilt of the bowl is such that it's not easy to read.

The HSI is a kind of stabilized compass, as it gets its information from a magnetic compass, but right now mine isn't working. It hasn't been removed from the aircraft, because we're planning to go south to Edmonton soon. There's an avionics shop with an excellent reputation there, and the AME wants them to examine the HSI in situ. That would allow them to spot if it's something simple and removes the risk and cost associated with removing and shipping it, especially as we're still orbiting the black hole of postal and shipping services. In the absence of an HSI, I'm setting the copilot side directional gyro; habit and original training makes me look at the compass to do this, even though I have far more information available from the GPS and INS.

I'm tracking 270 degrees true, and the compass reads 260 degrees, which is obviously a magnetic reading. The chart says the local magnetic variation is 23 degrees east of true. The compass correction card says for 270 degrees steer 272, and is reasonably consistent from south through west, and it's dated last month. The computer says I'm crabbing four degrees to the north. How do I put that together to make any sense?

270 true minus 23E suggests we're tracking 247 degrees magnetic, but we're crabbing four degrees north, which makes a heading of 251 magnetic, for which I should steer 253 magnetic. So there are seven degrees missing. I'm willing to believe it's the result of all the extra kit that wasn't running when the compass was swung, because we stow some of the valuable non-aviation equipment before giving the airplane over for extended maintenance. Either that or the universe is wrong.

Eastbound on the exact reciprocal track of 090 true, the compass reads about 035, but I didn't write all the associated numbers down, so I can't play with them.


Frank Van Haste said...

Dear Trix:

When N631S came to me six years ago the one thing that had to be replaced ASAP was the awful vertical card compass. (It seems to me that a compass that relies on itty-bitty gears is a bad idea.) It stuck and it slipped and it jittered and generally did everything that a well-behaved whiskey compass ought never do.

There has to be one thing in the panel that will NEVER lie to you - and that one thing, offering "ground truth", is the magnetic compass. If you can't trust the compass, you can't trust anything.

I replaced the VCC with the wholly admirable SIRS Navigator compass, which has since provided yeoman service.



Jeremy said...

Hi Frank,

Well as alluded to above, the compass can easily lie to you if the deviation is different from that calculated when the compass card was filled out - for example if on-board equipment changes or even if the passengers have large metal objects in their luggage!

I actually vote for "the universe is wrong" as the cause for Aviatrix's compass calculation gaps. I plan to use that answer to explain to my boss why the report is late, too.

Sarah said...

I always notice even in airliner cockpit photos, the compass is top dead center. Perhaps so when everything else goes dark & wrong they'll have that. One perhaps "tall" tale I've heard: if you've got nothing else and find yourself descending into a cloud deck - turn South to maximize the sensitivity to turning, hold heading & trim speed.

I've noticed huge compass errors in a certain club a/c, the speculation being that it was induced from the landing light current.

Frank thanks for the compass endorsement, I've been considering them. The vertical card compass in my glider is worthless- sticky and it lies. It's not that it's of any use, but it's right in my field of view and broken.

Unknown said...

I once ferried a little biplane from West Texas up to the Toronto area. It had no electrics at all except for a starter. GPS was new then but I had one that I'd borrowed. It took about ten batteries, which, being a poor pilot, I cheaped out on. The GPS was useless but the compass worked. Well if you flicked it... It was only half full. Good memories :-)

Unknown said...

Did you remember to factor in the "rate of change?" A lot of people don't.

Some charts will indicate the date for when the Magnetic Variation was calculated and it's rate of change. I've see the date as old as from 1996 on some charts.

So, if the rate of change is 1.2 degrees east per year and it has been 10 years since the last calculation, that's over 12 degrees of difference!

I believe the VNCs don't include this information, however.