Flight crew reported an uncommanded abrupt pitch up during descent. The flight crew disconnected the auto pilot and continued to land at destination airport.
Synopsis
Flight crew reported an uncommanded abrupt pitch up during descent. The flight crew disconnected the auto pilot and continued to land at destination airport.
Narrative
We were beginning our descent from FL220 to honor a crossing restriction to cross the ZZZZZ way point at 6;000 feet. After dialing in 6;000 feet in the altitude alert and selecting ALT SEL on the autopilot panel; we noticed the ALT light was still illuminated on the Flight Mode Annunciator. The PF tried to initiate the descent by moving the pitch rocker on the autopilot panel nose down to descend. Nothing happened. The PF then thought he should de-select the ALT button. The aircraft suddenly and very dramatically pitched down resulting in one passenger injuring her hand as it flew up and hit the ceiling. It appeared that the autopilot ALT HOLD function was still engaged as the airplane was trimming nose down even though the ALT SEL button was engaged. The PF noticed that the TRIM annunciator light in the FMA momentarily illuminated after moving the pitch rocker to nose down. We both thought that when ALT SEL was selected with a lower altitude in the alert; the airplane would come out of ALT HOLD and allow the pitch wheel to change the aircraft's pitch attitude in order to manage a descent. In past flights in this aircraft I don't recall seeing; both; ALT Preselect and ALT Hold illuminated on the FMA simultaneously. We immediately regained control of the aircraft and returned to a normal pitch attitude and rate of descent. We chose to hand fly the aircraft for the remainder of the descent and approach.Autopilot pitch mode malfunction.Unless we were completely mistaken and misunderstood the autopilot pitch operation; nothing I can think of other than maintaining the equipment in good working order. Over many previous routine flights operating this aircraft; this is the first time I experienced a radical autopilot pitch over.
Second reporter narrative
Autopilot Pitch Hardover.Second Trip after qualification. Leg 2 of 2 for the day. I was CA in left seat. IP in right.Aircraft level at FL 220 approaching top of descent; AP engaged in ALT HLD (annunciated). Also ALT PRE-SELECT was annunciated; even though we had been at altitude for some time; and had climbed to FL 220 and AP had captured the altitude on its own. That same dual annunciation had been displayed on leg 1 after we reached cruise altitude. NAME (IP) said he thought that was a little odd. I had used the vertical descent wheel/rocker to initiate climb/descents on the AP on this AC the same as with other I had flown at ZZZ with no problem.ATC cleared us to descent to 6;000 feet. NAME dialed 6;000 feet into the altitude monitor window. Since ALT PRE-SELECT was already annunciated; I believed the AP was armed to leave altitude. I made a few nose down pulses to the vertical descent wheel/rocker. The AP annunciations did not change and the AC did not start down. I was a bit confused; but since NAME was talking on the radio at the time; I couldn't ask him. I made another nose down pulse to the vertical descent wheel and the ELEV TRIM annunciation momentarily illuminated; but we stayed at altitude with both ALT HLD and ALT PRE-SELECT annunciated. Remembering my days from flying some old B-727s with old autopilots that would not leave altitude if the ALT HOLD switch was on; I then selected the ALT HLD push button on the pedestal control panel thinking that my nose down inputs had not been registered. (On the older B727s I occasionally flew; any altitude input made w/ ALT HOLD selected would be ignored.). The AC immediately snapped into a negative G nose hard over. I was pushed against my shoulder harnesses and I could hear the passengers in the back exclaiming. I grabbed the control yoke and immediately disconnected the autopilot. The nose had pitched over somewhere in the 30 degree nose down range (a lot of brown on the ADI); and airspeed was increasingly close to red line. I pulled the power; ensured the wings were level; and made a moderate pull to put the nose back on the horizon. ( I flew aircraft in the military and was cognizant that we were VMC with no terrain as a factor; we had ATC clearance to descend; and with people in the back I didn't want to add a 'high G' pullout to their negative G experience.)The passengers had been snacking on food brought onboard; and as the result of the negative g excursion; food and beverages were thrown all around the cabin. I hand-flew the AC continuing the descent and talking to ATC while NAME checked on the passengers. One passenger complained that she had hit her hand on the ceiling and it was swelling and painful. Not trusting the autopilot and electric trim; I continued to hand fly using manual trim only for the duration of the flight. We flew an uneventful actual IMC ILS arrival and approach landing normally. I am very baffled by this event. Granted I don't have much time in the King Air; but with every other AP climb and descent I have made on the King Air; once PRE-SELECT ALT is illuminated; a simple activation of the Vert Spd wheel/rocker causes the AC to climb or descend. INCLUDING on this aircraft. Another pilot informed me that on this particular AC; he believed ALT HOLD must be manually deselected prior to starting an altitude change. I didn't do that earlier in the day flying this ship. Why would this occasion be different? Plus; why would the PRE-SELECT ALT annunciation remain illuminated after an altitude was attained? After landing; we ran through some dry run tests with Maintenance personnel; and every time we pressed the vertical speed wheel/rocker; ALT HOLD disconnected. One further observation: The autopilot will disconnect whenever the electrical vertical trim is activated. Is this system really designed so that a vertical input to the elevator control can be input with the autopilot engaged; yet be maskeduntil the autopilot/ALT HOLD is disengaged? We had flown through some heavy rain earlier; and perhaps accumulated moisture caused issues w/ this older circuitry. Maintenance personnel must ensure that this autopilot is functioning as designed. If this design requires ALT HOLD to be specifically and manually deselected prior to selection of any altitude change; it should be stressed in training and be placarded to warn crews.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.