Date: 201712
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: BAe 125 Series 800
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Autopilot
Component problem: Design
Reporter organization: Corporate
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew FLC Overrode Automation; Flight Crew Returned To Clearance
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: We were level at our cruise altitude of FL280. ATC issued a descent to FL240. The PNF selected the new altitude in the altitude preselect and we each confirmed the new altitude as per company SOPs. I (PF) reduced thrust slightly; selected VS mode; and rolled in 300 fpm down. My technique is to initiate a shallow descent for the first 200-300 feet; then increase the rate of descent to a normal target of 2;000 fpm down once the aircraft has started down. I find that this creates a comfortable transition for the passengers and allows me time to smoothly set a target power setting for descent. After selecting VS and 300 fpm down; the aircraft began a slow climb; which I thought would soon reverse itself. Going through 28;200 ft I selected ALT to capture the altitude and stop the climb; but the aircraft kept climbing. Passing through 28;300 ft I disconnected the autopilot and pushed the nose over. In order to keep from plastering the passengers on the ceiling I smoothly transitioned from a climb to a descent; which resulted in us reaching a maximum altitude of 28;500. There was no traffic in our vicinity so I wasn't concerned about an immediate conflict. I quickly returned through FL280 and continued the descent to FL240. I reselected the autopilot on and the rest of the flight was uneventful. I have been flying this airplane for 16 years and it has a tendency to abruptly pitch up during descent if it senses the airspeed is too close to redline. In this case I was cruising at .76 with the redline at .80; which is usually enough of a buffer for a normal descent. I suspect the autopilot pitched up to reduce airspeed and to avoid a potential overspeed condition; but I'm not sure why selecting ALT didn't capture our current altitude. This is about the worst case of altitude control I have seen with this airplane; but it is something that all of our pilots are aware is a potential issue. The autopilot has been checked numerous times by different maintenance shops over the years and no faults are ever found. I think it is a built-in protection that just doesn't work very well; poor design.


Second narrative: This aircraft has a known issue where it occasionally pitches up during descent as if the autopilot is trying to protect itself from an overspeed. We typically stay 10-15 knots from the barber pole at all times to avoid this situation. Neither pilot has ever seen this particular issue when first initiating a descent. Collins equipped Hawkers never have these problems.


Synopsis: Hawker 800 flight crew reported the autopilot started to climb after the crew initiated a descent. The problem was caused by a built-in speed protection feature of the autopilot.