Wednesday, May 29, 2013

Traffic Twelve o' Clock

I've come to pick up an airplane from routine maintenance, but the compass swing is due in a few days, and who knows how many days I'll be on the road. I'm asked to help maintenance perform the check before I depart. The compass swing is a test and calibration to ensure that the magnetic compass is accurate to a few degrees on all headings, and to provide offset information on what compass heading to steer to be exactly on each heading. It's a bit of an anachronism now, but the compass is a completely independent navigation system, not needing electrical, pitot-static, hydraulic, vacuum or anything else in the airplane to do its job. If it's to be my last best hope when everything else fails, let's spend a few minutes a year making sure it can do its job.

The typical aircraft compass, indeed every one I can ever remember using, is a dome that is free to pivot and swing about its apex, inside a container of fluid, to damp its oscillations. It used to be called a "whiskey compass" but I think the fluid is now something thicker than whiskey. I had one leak once and I seem to recall the fluid was more oily than volatile. The points of the compass are displayed around the edge of the dome, and the whole thing is mounted in the middle of the windshield, usually near the top, and less frequently down by the dashboard. It's in front of the pilots, so that the pilot is looking at the back edge of the dome. That means that if the airplane is pointed north, the edge of the dome with the S on it pojnts north, facing the windshield and the edge with the N on it faces the pilot.

In the olden days, the pilot drew a line on a map from A (where she was) to B (where she wanted to be), used a protractor to determine the bearing of the line, used a circular slide rule to determine the bearing required to correct for wind drift, converted that bearing from true (aviation map grid lines are meridians of longitude, which run towards true north) to magnetic, and then consulted the compass card to determine the direction to steer in order to fly that heading. The compass card is a piece of paper, often a sticker now, but some compasses have a little bracket to retain a piece of card. It looks like this:

So if the pilot calculated she needed to steer 135 degrees magnetic to compensate for the wind, based on that compass correction card she should steer 136, splitting the difference between the correction for 120 and 150. If I were making a trip over water or featureless terrain with only compass bearings to guide me, I would have to remember to do this. I can hardly imagine depending on a mere compass to that extent. First I have to read the compass indication, as the dome bobs and sways in turbulence. Then I have to transfer that reading to the gyroscopic heading indicator. Then I have consult the correction card to see what to steer, and then I have to steer accurately by the heading indicator. If the error introduced by the other three steps doesn't exceed the required correction, I'm doing very well. I don't remember the last time I looked at a compass card for a purpose other than to see if the compass swing was due. As this one is.

My role consists of taxiing the plane. I, an AME, and an apprentice pile into the airplane and taxi off towards the compass rose like we were in a pick-up truck. I get a clearance, first, as I have to go through a controlled apron to get there, but the compass rose itself is down a taxiway that leads to a runway not in use, so there's no one else taxiing on it. I'm trundling along, looking for the correct turn off to the compass rose, when suddenly a non-aircraft vehicle is barrelling towards me on the taxiway. I can't suddenly veer off to the side when you're taxiing an airplane. The steering radius isn't that great, and I'm so wide it wouldn't make much difference. I can't honk. I have to admit being momentarily stunned, so before I could gather words to key the mike to say "yo don't crash into the airplane!" or "Ground, there's a vehicle at my twelve o' clock." (What would YOU say>?) the vehicle driver has looked up from whatever task took his attention, and done a hard right off the taxiway into the grass beside it.

Well, um, that works. I probably should have advised the ground controller of the near miss at that point, or made a phone call later, but I didn't. If you're the one who almost crashed an airport vehicle into a taxiing aircraft, you probably owe me one. Buy one for your nearest pilot just to keep your karma in line. We just went and did the compass swing.

At the compass rose, one of the maintenance guys gets out and the other one stays in. The one on the ground directs me to line me up exactly with the painted lines on the pavement while the other one adjusts the compass with a little screwdriver. With all the avionics on, I turn to north. He adjusts the compass so it reads exactly north. I turn to west. He makes it read exactly west. I turn to south. He takes out half the error. I turn to east. He takes out half of that error. And finally I turn to each heading in turn so the values for the compass correction card can be recorded. We return to the hangar without encountering any more crazy drivers, and after a bit more paperwork, the airplane is good to go.

Here's the cheater's way to do it.


World'sMostAnxiousPerson said...

Whaddya mean "in the olden days"? :) Some of us are still struggling to figure out Nav and are made to do it the hard way ;)

Sarah said...

"Traffic at 12 O'clock"? I should be OK. It's only 9:45 now. hyuk hyuk hyuk...

So a compass swing is a periodic check in Canada? As far as I know, in the US it need be done only when something material has changed, or it becomes obviously incorrect. And in practice, it's sometimes a low priority item even then. It's a little strange - I'd think the mag compass pretty hard to use in the north anyway.