So it turns out that the SD card that was in the data logger wasn't the one that I usually use, and is one that, even though the data logger seems to be able to write to it, nothing else I own is able to read it. Two card readers, three digital cameras with a mass storage USB mode, and I tried the two card readers in another computer, but I can't get it to acknowledge that there's a card there. However, if I put it back into the data logger, the LEDs flash exactly like they do with the SD card that I can read. So maybe the data is on the card, but I can't read it.

Anyway, I have two guesses about why the plane flew into the trees:

1. The GPS fell off. Since the compass is in the GPS, it wouldn't have known which direction it was going. However, I didn't notice the GPS had fallen off until it was in the tree, so I'm not sure that it didn't just fall off because of the crash.

2. I remember at one point disconnecting the battery and reconnecting it. I'm not sure which flight this occurred before, but if it was the one with the crash, then the GPS may have simply not been online yet. However, I believe that in that case it would have thought it was at latitude 0 longitude 0, and thus it would have flown in a completely different direction. However, another possibility is that it had a few satellites but not a good position fix yet and so it thought that the start of its landing path was in the trees.

Anyway, I thought of a few things to change before the next flight:

1. Make the plane refuse to start flying until the GPS is ready. Presently I have to realize it might not be ready and watch the LCD on the RC controller.

2. The gyroscope already gives a good indication of how the plane turns, and the compass just corrects for drift. So I can make it sound an alarm if the gyroscope and compass ever suddenly find themselves out of sync. Thus, if the compass did fall off at the start of the flight, there would be an alarm to tell me and I could react sooner.

3. Record the data sent over the radio that is received on the laptop. It's exactly the same data that's written to the SD card, so currently I don't record it because the radio loses data and so the SD card is a better source. However, not only did the SD card fail in this case, but if I'd never gotten the plane out of the tree, the data received over the radio would be better than nothing.

I wish I could read that SD card though because I'd really like to know for sure what went wrong.