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.
Multiplex Mentor
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?
Replies
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.
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.
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.