A BE-400's aileron trim malfunctioned and caused an uncommanded roll while descending through 18;000 FT on approach with the A autopilot engaged. An emergency was declared; while the crew overrode the malfunction with the normal trim switch.
Synopsis
A BE-400's aileron trim malfunctioned and caused an uncommanded roll while descending through 18;000 FT on approach with the A autopilot engaged. An emergency was declared; while the crew overrode the malfunction with the normal trim switch.
Narrative
We were descending out of 18000 FT on our approach when my First Officer noticed that we had a yellow box on the PFD (it was an A) as I was looking up the squawk in the checklist. I noticed that we were off course by 45 degrees and the plane kept turning to the right. I told the First Officer to disengage the autopilot and hand fly back on course. We then noticed that the right roll trim had runaway and my First Officer hit the trim interrupt button and kept it depressed. I went to the checklist and read the roll trim emergency checklist. I selected the right trim motor and tried to retrim with success. I declared an emergency with ATC and told them our problem along with how many souls and fuel we had on board. They vectored us around and we landed with no further incident. The fire crash crews met our plane as we rolled to a stop. We then taxied back to the FBO. This airplane had a similar incident 30 days prior to this event. I was told by maintenance on the ground that a golf cart had damaged the right roll trim section of the plane and was released by maintenance. It looks like this plane needs a whole new right roll trim replaced.
Second reporter narrative
On descent; I glanced at the PFD and noticed a little yellow box with an 'A' in it. I brought this to the Captain's attention and he pulled out the checklist for corrective action. Before he had time to locate the correct page he commented that I was turning off course. Selecting HDG mode on the autopilot failed to turn us back on course; so I manually deselected the autopilot to hand-fly us on course. When I deselected the autopilot; the plane rolled quickly and violently to the right. I pressed the Trim Interrupt button on the yoke and called for the runaway trim checklist. I quickly pulled the thrust to idle to reduce some of the airload on the control surfaces because it was very difficult to hold opposing force on the yoke. We declared an emergency; requested CFR; and completed the checklist. Luckily we were able to use the L/R Trim Select switch which allowed my yoke switch to move only the right trim tab. We moved the right tab into a neutral position which allowed for normal control force. The Captain informed the passengers of the issue; and we landed without incident. We were met by CFR; but did not need assistance. We found out after the flight that this same situation happened on the same plane a few weeks prior. Maybe more flight tests should have been performed before returning it to service; but I am sure they wouldn't have sent an airplane back into service if they thought it could be a recurring problem.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.