CE680 flight crew experiences failure of the autopilot to track the localizer after capturing. The autopilot is disconnected and a hand flown approach to landing ensues. The aircraft had a history of pitching down while coupled on ILS approaches.
Synopsis
CE680 flight crew experiences failure of the autopilot to track the localizer after capturing. The autopilot is disconnected and a hand flown approach to landing ensues. The aircraft had a history of pitching down while coupled on ILS approaches.
Narrative
Flying in heavy rain and light turbulence; we were at 4;000 FT being vectored on a wide right downwind for ILS 33L. Just inside of JANNS (IF) we were given a heading of 300 and descent to 2;000 FT; cleared for ILS 33L. The autopilot was engaged in a heading mode and a vertical descent was initiated. The 'approach' was armed. At approximately 3;000 FT; still descending to 2;000 FT the localizer came alive on both sides and then captured. The landing gear was lowered. However; the turn back to 330 to center the CDI was very slow and we flew through the course. The aircraft continued to turn back to a heading of about 360 and again flew through the localizer. However; this time it was not turning back to track inbound. I notified the pilot flying to turn back to the left to recapture the inbound course. At the same time; I turned the heading bug back to a heading of 300 but the aircraft was not turning and the flight director was apparently not following the heading bug. The pilot flying then disconnected the autopilot and began an immediate left turn. As we were passing a heading of about 330 in our turn back to 300; ATC notified us to make a turn back to 300 and intercept the approach. While hand flying; the PIC was able to level off at 2;000 FT and then capture the ILS. He hand flew the aircraft and we broke out of the clouds at 800 FT and landed. Prior to this event; pilots flying this aircraft have reported; while captured on an ILS with the autopilot engaged; a sudden nose down pitch and have had to disconnect the autopilot to maintain control of the aircraft. A human factor might be that once we observed the localizer to be 'captured' and intercepting the inbound course we did not expect it to become un-captured and fly right back through the course without remaining captured.
Second reporter narrative
I left the autopilot on and turned the heading bug to 300 and armed the approach; we intercepted the localizer and glidescope between JANNS and GRAFE; I called out 'capturing the localizer and glideslope' the First Officer called out the same for his side. I verified this on the PFD. I remember at one point the heading and course was the same (after maybe 1 or 2 left and right turns); I felt the approach was stable. ATC informed us to slow to minimum approach speed; so I started to reduce the throttle and work on getting the gear and flaps down and requested flaps 15. Shortly afterward the First Officer said ' hey are you going to turn back?'; referring to the heading that was now close to 360 and continuing to turn to the right . I was shocked; I had no idea why the airplane was in a right turn; at first I thought the controls might have been jammed (my hands have been on the throttle and yoke for quite sometime).I looked at the autopilot display panel and there we no lights illuminated (no heading; no LNAV; no alt lights); I am pretty sure I did not even see a yaw damp light on - no lights were illuminated. I thought; how could the airplane turn with no input (there as no aural tones from the autopilot or FMS). At this point I did not know if the autopilot was on; off or the controls jammed. I pressed the red disconnect button and disregarded what the command bars said and turned the airplane to the left to rejoin the ILS; as I was turning back to the left ATC called to say 'we must have gotten a little lost' and gave us a vector of 290 to rejoin the ILS. I continued to hand fly the approach and the rest of the approach/landing was uneventful.I have since called Cessna who has called Honeywell. They are going to download the MDAU and run some checks. This airplane has had hard overs on several other ILSs (pitch down up to 20 degrees); and is apparently a fleet wide problem and Honeywell is working a fix. I am very concerned; in my 25 years of flying I have NEVER had an autopilot make an uncommanded turn and not let the pilot know or just turn off.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.