Galen and I went out to Belen today, to both check out some work we did on the compass and so that I could shoot some approaches. I hadn’t been flying much, and I’m much more rusty than I’d really like. I thought I was getting back in the swing of things, as I made three reasonable landings at Belen, but then had an awful landing at ABQ on runway 3. I came in a little fast, so flared a bit to slow down, over flared and was slow and high, with the nose high. Pushed the nose over and started pushing in power to go around. Air speed came back up, I had 8000′ of runway left, and was in a reasonable attitude, so I pushed the nose while pulling the power, flared again, and the back wheels squeaked on. Nose came down, and we rolled out. In the end, bad part was that I missed the F-1 exit and had to go to F-3, so I had a long high-speed taxi. Contributing factors to the bad landing were:

• Frustration from problems flying the vectors given from approach due to compass problems.
• EFIS going insane when I loaded a synthetic approach to 3 into the EFIS and then Galen loaded the ILS approach to 3 in the GNS430 without progressing past the IAF (lots of warnings, but should not have been an annoyance).
• Approach to a 150′ wide runway after flying approaches to the 60′ wide runway at E80.

I’m not sure why, but I haven’t been comfortable in 813T since we brought it back from Colorado. I’m not sure if it’s having someone in the right seat (the CO time was all solo time while test flying). It’s getting better and I’m now starting to fly ahead of the plane instead of just trying to keep it doing the right thing. But all the little things built up this afternoon to me being a bit behind the plane while landing on 3. At least things are going in the right direction. On the good side, with one exception, I kept the nosewheel off the ground for as long as able. The one exception was the second approach at Belen. Galen had his foot resting on the right rudder pedal. We started drifting to the right after touchdown and I was feeling stiff resistance on the left rudder pedal when I tried to correct. I let the nose wheel down in case I had to brake the left wheel (which is touchy at those speeds). So at least that part seems to be getting better. And takeoffs were very smooth, although I still come out of ground effect faster than Galen likes. I prefer to establish a Vy climb with a shallow climb, while Galen prefers to stay in ground effect until Vy then establish the climb. I’m shallow enough on the climb that Galen appears satisfied on the point, so I’ll take that.

We’ve had a problem with the plane the last week with the magnetic heading reported by the EFIS. On both Monday and Tuesday this week, we were vectored into ABQ by approach. Both times, the EFIS reported a heading +/- 5 degrees of the assigned heading but GPS reported a very different ground track. The ground track matched what the controller was seeing, but the heading matched what the controller expected to see (so we weren’t going the direction the controller expected, which is bad). At the time, the only compass we had in the plane was the EFIS, since it’s a magnetic compass (gyro stabilizd) and that’s all the FARs require. We decided to order a traditional wet compass to help debug the problem and to make sure we had a backup in case of power failure. At this point, it’s probably worth a note on the directions used in planes:

Magnetic heading

The direction the plane is pointed, relative to magnetic north. Generally determined with a magnetic compass and/or a directional gyro calibrated to a magnetic compass. Headings assigned by approach / center controllers are in magnetic heading

True heading

The direction the plane is pointed, relative to true north. Generally determined in flight by finding the magnetic heading and adjusting with the local variance. Aviation charts are all relative to true north, leaving the pilot to convert between true and magnetic north.

Ground track

The direction the plane is actually going, almost always given relative to magnetic north when determined by an instrument (like a GPS), but in true north when doing preflight ground track planning based on charts.

The traditional method for determining magnetic heading is a wet compass, a card with a small magnet sitting in alcohol so that it spins freely and points towards north. Magnetic compasses are hard to use because they move an awful lot just due to vibration in the airplane. They also spin ‘backward’. When you’re pointed north, the compass reads north. To the left on the card is the reading 30, but to turn to 30, you make a right turn. This requires more thought than one really wants in a high stress environment and if the plane is in turbulent air, getting a reading off the compass can be near impossible. So most planes have what is called a directional gyro (DG) installed. The DG doesn’t have any concept of which way is north. But it uses gyroscopes to determine if you’ve turned left or right and how much. So if you set the DG to have a known heading (like the compass reading before takeoff), it will smoothly display the current heading. And it’s mechanically set to have the proper right/left turning orientation. There are some problems, like slow drift over time, but it basically works as long as you calibrate it against the compass every now and then.

N813T doesn’t have a traditional DG, instead it has gyros coupled with a magnetometer and a computer uses both inputs to develop a stabilized heading indicator on the EFIS. If either the gyros or the magnetometer are wonky, then the heading is going to be wrong. The EFIS control software is supposed to detect this and return an error to the pilot. It had been doing this (we the pilots just didn’t know where to look for that error), which caused us to spend most of yesterday working on the problem. We got that problem fixed and so we test flew it today. On departure, we noticed that the EFIS again didn’t match the ground track or mag compass. Odd, but ok. I knew there wasn’t much wind, so I just flew the ground track indicated on the GNS430 GPS (and about 3 other places), and it was fine. On the way home, however, the EFIS heading, compass, and GNS430 all had different headings. Figuring out which to fly was problematic and the source of much confusion for me the pilot. Thankfully, the vectors were pretty simple, so I was able to guestimate with landmarks and get the plane lined up for runway 3 without getting close to other traffic.

On the rollout after the crappy landing (and since I missed the early exit, it was a long rollout), I noticed the wet compass was definitely off and started thinking about something my instructor in B’ton said — that if the compass ever seemed wrong, double check by first turning off high current electrical items. I shut off the landing light and the compass swung right back to the proper heading. Now, I always turn on the landing light when in the vicinity of the airport or when in a practice area to make it easier for other traffic to see me. Not a big deal in N813T, since it has strobes, but is a problem on the C-150 I used to fly. So the entire flight, the landing light was turned on. In testing after shutdown, there is about a 20-30 degree swing in the wet compass caused by the landing light. And the EFIS computer, attempting to compensate for the heading reported by the magnetometer, will eventually report the new (wrong) heading, similar to the wet compass.

Ok, so now we know there was something wrong with the magnetometer wiring which we fixed. And we know the problem persisted, but in a different way. We know the landing light causes that problem. So we now at least understand the problems we’re seeing. I noticed that the ground wires for the landing, taxi, and nav lights were wired to the wing tip rather than back to the central grounding block (close to where the +12V line is routed). This set off alarm bells in my head, so I started a discussion with Perk and Chris about the problem. Sure enough, Perk breaks out some math that seems to indicate that the proximity of the +12VDC line for the landing and taxi lights and the lack of a matching ground line back could cause a magnetic field in the plane that would cause a swing of a compass in the right ballpark. So it looks like we should ground back to the cockpit instead of the wing tip and this might help quite down the magnetic compass interference. Way too much time working on the plane, but it was kind of cool to see math being used to figure out an electrical problem…

In other news, I now have 60.1 hours of pilot in command time and 55.9 hours of dual received time. Which means that I have more time being in charge of the plane than I have being taught how to fly the plane. Note that these times are not mutually exclusive. I have about 9 hours of time that is both pilot in command and dual received, for things like checkouts in new planes. But it’s still fun to think about…