Quad fails to hold altitude

Quad fails to hold altitude.

After two normal flights, on the third flight my quad started to lose altitude shortly into an Auto mission. It descended slowly until it collided with the long grass. Raising the throttle had no effect. Throttle IN went up to 100% while Throttle OUT remained at about 40%. Fortunately it was undamaged. The battery was still almost full. I was using an APM 2.6 running ArduCopter V 3.2.1.

The data showed that both the Barometric Altimeter and the GPS Relative Altitude were in agreement. They were both following the commanded altitude, which was 16 metres. Yet the quad was coming down. I do not see how these quite independent measurements could both be wrong. A barometer error did come up, but only after the quad tipped over on contact with the ground.

Probably a pressure effect, it’s usefull to share a log to others can help. Its your apm covered?

Yes, there is foam inside the APM cover and the barometer normally works correctly. What doesn’t make sense to me is that the barometer AND the GPS both continued to claim it was at 16 metres throughout the descent to the ground. I did notice that the CMD data stopped being recorded at about the time (I think) when it started to descend. It should not have descended because the next way point was at 16 metres and the battery had plenty of capacity left, but even if it decided to descend for some reason surely the Barometer and the GPS would show that it was coming down.

I should have attached a file but I’m embarrassed to admit that I can’t see how to do that ! Perhaps you could advise me.