BAe 125 Series 800
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance; Flight Crew Overcame Equipment Problem
Environment - Non Weather Related; Aircraft
At FL400; two hours after takeoff; the First Officer's control stick began several forward and aft movements as it attempted to trim to maintain altitude. Suddenly; without the autopilot disconnecting; the nose pitched up and [the aircraft] began an immediate 1500 foot per minute climb. The First Officer disconnected the autopilot but the aircraft reached between FL404 and FL405 before the climb was stopped. We immediately notified ARTCC of the altitude diversion and requested a lower altitude to FL280 (out of the RVSM airspace). The autopilot was not reconnected.We noticed the pitch trim to be extremely difficult to operate with 'catches' in the control. Extreme stiffness and then a 'catch' then stiff movement again. The aileron trim was not moveable; frozen in position. On landing we noticed the aileron trim seemed free; and the pitch trim was easily operated.Speculation may lead to an iced control system. Weather in departure airport was light rain; overcast at 2100 feet; winds 180/15 with 10 miles visibility and 30.17 inches of mercury. We did encounter rain on the climbout; but had been clear at FL400 for over an hour. The temperature at FL400 was -40C.Complete a maintenance inspection and determine the cause of the flight control/trim malfunction.
BAE125-850XP First Officer reported possible flight control icing; which led to an altitude excursion at FL400.