Narrative:

I was left seat on empty leg after passenger drop off at las. I am usually right seat and PNF; but this was my sic recurrency flight with check airman; who is also a senior captain. I hand flew the airplane beautifully from takeoff at las to FL220. At some point while cruising at FL220; check airman asked me to engage autoplt. I believe that it was he who actually hit the engage button. He told me to select heading mode; which I did. This is old style instrumentation ('steam gauges') -- the only indication of mode selected is on autoplt panel on glare shield. Sometimes in bright light; the mode button lights are hard to see. Immediately after selecting heading mode; the airplane pitched down significantly. I attempted manually to raise the nose; but no response. We were losing altitude. I called twice for check airman to disengage autoplt. He did not. I then disengaged using the yoke mounted disconnect. During this sequence; we lost about 400-500 ft of altitude. I then climbed (hand-flew) back up to FL220 in a matter of seconds. I did not see any other mode light illuminated -- only engage and heading; during this episode. We later re-engaged the autoplt and it worked adequately; although it did seem to re-trim more than normal to hold altitude; sometimes deviating more than 150 ft; while it was engaged. There was no traffic conflict. In fact; as soon as we re-arrived at FL220; we were instructed to descend to 16000 ft. (Mins before engaging the autoplt; we did get a master warning which we re-set and which the captain/check airman said was mach trim related. After reset; we did not get the warning again.)

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Original NASA ASRS Text

Title: LEAR 55 PITCHED DOWN AFTER AUTOPILOT WAS ENGAGED AT CRUISE ALT; RESULTING IN AN ALT DEVIATION.

Narrative: I WAS L SEAT ON EMPTY LEG AFTER PAX DROP OFF AT LAS. I AM USUALLY R SEAT AND PNF; BUT THIS WAS MY SIC RECURRENCY FLT WITH CHK AIRMAN; WHO IS ALSO A SENIOR CAPT. I HAND FLEW THE AIRPLANE BEAUTIFULLY FROM TKOF AT LAS TO FL220. AT SOME POINT WHILE CRUISING AT FL220; CHK AIRMAN ASKED ME TO ENGAGE AUTOPLT. I BELIEVE THAT IT WAS HE WHO ACTUALLY HIT THE ENGAGE BUTTON. HE TOLD ME TO SELECT HDG MODE; WHICH I DID. THIS IS OLD STYLE INSTRUMENTATION ('STEAM GAUGES') -- THE ONLY INDICATION OF MODE SELECTED IS ON AUTOPLT PANEL ON GLARE SHIELD. SOMETIMES IN BRIGHT LIGHT; THE MODE BUTTON LIGHTS ARE HARD TO SEE. IMMEDIATELY AFTER SELECTING HDG MODE; THE AIRPLANE PITCHED DOWN SIGNIFICANTLY. I ATTEMPTED MANUALLY TO RAISE THE NOSE; BUT NO RESPONSE. WE WERE LOSING ALT. I CALLED TWICE FOR CHK AIRMAN TO DISENGAGE AUTOPLT. HE DID NOT. I THEN DISENGAGED USING THE YOKE MOUNTED DISCONNECT. DURING THIS SEQUENCE; WE LOST ABOUT 400-500 FT OF ALT. I THEN CLBED (HAND-FLEW) BACK UP TO FL220 IN A MATTER OF SECONDS. I DID NOT SEE ANY OTHER MODE LIGHT ILLUMINATED -- ONLY ENGAGE AND HDG; DURING THIS EPISODE. WE LATER RE-ENGAGED THE AUTOPLT AND IT WORKED ADEQUATELY; ALTHOUGH IT DID SEEM TO RE-TRIM MORE THAN NORMAL TO HOLD ALT; SOMETIMES DEVIATING MORE THAN 150 FT; WHILE IT WAS ENGAGED. THERE WAS NO TFC CONFLICT. IN FACT; AS SOON AS WE RE-ARRIVED AT FL220; WE WERE INSTRUCTED TO DSND TO 16000 FT. (MINS BEFORE ENGAGING THE AUTOPLT; WE DID GET A MASTER WARNING WHICH WE RE-SET AND WHICH THE CAPT/CHK AIRMAN SAID WAS MACH TRIM RELATED. AFTER RESET; WE DID NOT GET THE WARNING AGAIN.)

Data retrieved from NASA's ASRS site as of May 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.