Kamikaze initiated by KAP140
This happened to me on a flight with a KAP140-equipped DA40. As a flight instructor, I had a student converting to the DA40, and he was new to the KAP140. When trying to select lower altitude from FL90 on the KAP140, he must have hit the BARO and turned the QNH setting way off. He then managed to set correct target altitude, toggled the VS key, which made the A/P enter VS mode, and selected 500 fpm down. The A/P obeyed, IAS was around 125 K and all was well.
Then my student checked the BARO setting. It was around 1070, and QNH was around 995, a huge difference. He turned the knob (a lot), setting 995 as the proper BARO setting for the descent. Then I noticed the trim wheel moving steadily forward, the nose lowered and in very few seconds the airspeed approached Vne. I pressed the copilot's A/P disconnect botton, steadied the aeroplane and retrimmed, then re-engaged the autopilot, which now worked fine again. We were IMC, and perhaps due to c/s propeller, light turbulence and a generally low noise level the student had not noticed the dive in time to react.
On the next flight with the same aeroplane I managed to re-create the upsetting KAP140 behaviour. During descent on A/P in VS mode, I changed the BARO setting. Moderate (realistic) changes produced no reaction, but after turning to an absurdly high setting like 1070 and then turning down to the actual setting of around 1000, the same thing happened - a dramatic dive towards redline.
Apparently there is no warning in the manual against changing BARO setting during descent - which would indeed be a nuisance, especially IFR, since descend clearences to a low flight level are often extended, while descending, to an altitude below transition altitude, requiring the pilot to set QNH.
The autopilot works flawlessly in every other respect as far as I can tell. I would like to hear from someone with similar experience or perhaps an explanation.
Last edited by huv; 25th October 2012 at 09:35.