I lost my Mentor to a crash tonight, which is disappointing because it's my first crash. I wouldn't be so disappointed, but I don't know what even caused it. I'm hoping someone can shed some light.
Here's my set-up.
Futaba 6EX 2.4gHz radio set.
APM2 running 2.50
3dr 433 Telemetry
External MediaTek GPS
Lipo 4000mAh 3S
Had been flying for quite a while, and was on the last flight of the last battery. (plenty of charge left though, 11.6 volts right before I took off. Anyway, rest of the flights had gone great. I had finally gotten the PIDs dialed in so FBW-A worked flawlessly. Even ran a couple rectangular patterns in Auto. Navigation needed some tuning, but that is a different subject.
So I take off in stabilize switch to FBW and one tight climbing right turn and then a gradual climbing right turn and all of a sudden I get an uncontrollable pitch down, and since I was still climbing at a high power it shot toward the ground quite quick. Tried to switch back to stabilize mode, but I think I was too late.
Data log exists up until just before impact. Funny thing is that it shows pitch up the entire decent, when I know for a fact that it was nose down during the whole incident. Hit the ground at 28 m/s. Destroy the fuselage of the plane, but all electronics survived. Servos and APM seem to function normally after hooked the back up on the bench. APM shows correct pitch and roll.
Did my APM lock up or will I never know what happened?
The tight turn probably caused a centrifugal force that caused the pitch to read wrong. The APM uses only basic corrections for centripetal forces in the AHRS.
You probably shouldn't do tight turns until there's an advanced AHRS system in place.
I've done many tight turns, spins, rolls and loops with v2.50 and never had a problem. Tom, the only thing I can think is perhaps an elevator linkage came loose as you say the servo's seem fine? Were the wings level on the descent?
When I say tight turn, I meant it relative to the second part of the turn. It wasn't super tight, and I highly doubt that was the problem, I had control well into the gradual turn. Wings were fairly level on descent, it was pretty far away and happened fast. The log shows they were close, but I'm not trusting the log since it also showed about 15 degrees pitch up. Elevator linkage was still intact and connected. The APM must have thought it was pitched up and thats why it ran itself into the ground.
I found one other post in the 2.50 release discussion of someone having a similar problem. I wonder if its something that got changed in 2.50?
Further investigation make me believe that since I was in FBW-A, the APM somehow got confused and thought it was holding max pitch up, while in reality, it was pitched down. But since it thought it was at the max pitch, it couldn't do anything to arrest the descent. Elevator input and output confirms that I have full up elevator on the controls, but the APM was commanding neutral. I know I flipped the switch to go back to stabilize, but I'm not confident that I did it in time. Telemetry showed FBW through the crash, so I'm not sure if it didn't respond, or I just flipped it as it hit the ground.
Anyone have any idea what would cause it to sudden lose track of it's pitch? Works fine now on the bench.
Sad to hear about your crash, but it happened to me many times also.
Did you try to use a geo-fence to avoid the crash by imposing a minim flight altitude?
I did see on another post you want to find a way to enable geo-fence on 6 channels TX.
I am in the same situation, 6 channels TX, ArduPlane, not spare channel and I want to have an option to use geo-fence, see below my post, not yet any answer..
Do you have any solution so far for you to enable/disable geo-fance?
Check your logs again and see if you lost GPS lock before the dive. There is a bug in 2.50 where AHRS gets screwed up if GPS is lost.
One other thing and you are not the only one who has done this, why don't you switch to manual and pull out of the dive. If the AHRS is messed up, it's messed up in all modes that use it. At this point only manual control will save you.
It never lost GPS lock, at least it shows 3d Fix the whole time. I wanted to switch to manual, but the way I had the mode switches set up I had to flip two switches (not ideal at all), so I was unable to get it to manual quick enough. Probably a lack of experience caused that.
Criro, like Greg said, the only thing that could have saved me was manual mode. I thought for a while that Geofence would have, but once you think about it, even if it went below the minimum altitude the autopilot wouldn't have been able to increase pitch to pull out of the dive because it thought it was already at max pitch up.
Did you get an answer to this one ? It sounds like a problem I have with a Copter but I havent found an answetr!!