History of Flight
The pilot, who also owned the aircraft, had delivered N666JH to Wycombe Air Park on 22 July 2003 for its annual service. On the day of the accident, he arrived at the airpark with the intention of flying the aircraft back to his home base at Rochester. The fuel tanks were just over half full and were not refueled. The pilot booked out for a flight under Visual Flight Rules (VFR) and did not file a flight plan. While taxiing towards Runway 24, another aircraft was landing, and the local weather was discussed over the radio. The landing pilot described the cloud base as between two and three hundred feet. The pilot of N666JH acknowledged and stated that the weather at his destination was much better. He departed from Runway 24, climbed to about 200 feet above ground level, then commenced a right-hand turn and disappeared into cloud. The aircraft was heard flying overhead and departing to the east, consistent with the planned route to Rochester. Two minutes after takeoff, the pilot transmitted, "GOING OVER TO THAMES NOW, TWENTY TWO HUNDRED FEET". No further transmissions were received. The aircraft was last seen in a descending, right-hand turn with the engine running and moving extremely fast, about 800 metres from its point of impact. An ear witness heard the sound of an aircraft followed by a loud thump and then silence.
Radar Data
A radar head at Heathrow Airport recorded most of the flight. It showed the aircraft departing Wycombe Air Park and flying a climbing right-hand turn through 250° back to the airfield overhead. It then tracked approximately 100° for 75 seconds, climbing to and maintaining 2,500 feet amsl. During this period, the aircraft speed steadily increased to 140 kt groundspeed. The straight and level flight ended with a tightening right-hand turn and a rapidly increasing rate of descent. At the last radar position, approximately 100 metres from the point of impact, the aircraft had a rate of descent of over 16,000 ft/min and had turned through 180° since departing straight and level flight.
Search and Rescue
No one in the vicinity of the airfield or crash site appreciated that the aircraft was in difficulty, so there were no reports to emergency services around the time of the accident. Later that evening, a relative reported the aircraft overdue. The Distress and Diversion cell at the London Terminal Control Centre (LTCC) decided to await airfield opening hours the following morning before initiating tracing action. A radar replay from the Heathrow radar head was studied, revealing the aircraft's flightpath. A Search and Rescue helicopter was deployed to search the area where the aircraft disappeared below radar coverage. The wreckage was located close to that point at 1015 hrs the morning after the accident.
Pathology
It was not physically possible to determine the presence of any pre-existing disease, but there was no evidence of toxicological factors that may have contributed to the cause. The aviation pathologist's opinion was that the pilot's age and complex medical history suggested that incapacitation may have been a causal factor.
Meteorology
An aftercast from the Meteorological Office at Bracknell showed a light, moist, westerly flow with a weak cold front moving slowly across the area. Cloud was overcast with the base lowering to 800 feet amsl in drizzle. The accident site was approximately 400 feet amsl. The cloud base element concurred with the pilot report of a cloudbase between 200 and 300 feet over Wycombe Air Park (500 feet amsl). Surface wind was from 200° at 8 kt, and wind at 2,000 feet amsl was from 230° at 12 kt. At Rochester, ahead of the cold front, weather reports described good visibility and little cloud below 5,000 feet.
Accident Site and Wreckage
The aircraft crashed into woodland about half a mile from Hard to Find Farm near Marlow, Buckinghamshire. Damage to treetops showed that the right wing had cut a swath through trees, indicating a tight right turn consistent with a spiral dive. Further tree damage showed the aircraft at an attitude of 65° of right bank with 30° nose-down pitch. The right wing continued to cut through trees before the aircraft struck the ground, tearing the right wing from the fuselage. The forward section left an impact crater approximately 2.6 metres in diameter and 0.5 metres deep, consistent with a very high rate of descent at an estimated speed of about 200 kt. The aircraft pitched over on its nose and to the left before rolling over, suffering severe fragmentation. Several items detached from the airframe, the furthest approximately 30 metres away. The bulk of the aircraft came to rest around a large oak tree, about 10 metres from the initial impact crater.
Within the wreckage trail were two of the three propeller blades, both with extensive damage to their leading edges. The relatively undamaged third blade, thrown during hub breakup, landed about 18 metres to the right. Inspection of the fuel injection servo revealed a witness mark from the butterfly valve on the body of the unit, indicating it was at the full throttle position at impact. Despite extensive fragmentation, all main structural elements and flying control surfaces were located. Failures were consistent with high impact forces and did not show signs of in-flight failure or detachment.
Autopilot Modification
The autopilot (AP) fitted to N666JH was a two-axis Honeywell KAP-140. Prior to a modification (KAP140-M1 per Cessna Service Bulletin 02-22-01), pressing the AP, HDG, or ALT buttons would immediately engage the AP. After modification, the AP button must be pressed and held for 0.25 seconds to engage, intended to reduce inadvertent engagement. The pilot/owner was aware of this modification and had requested it many times. The modification was carried out during the annual maintenance. The AP computer was removed, shipped to an avionics facility at Kidlington for software reprogramming, then re-fitted and tested on the ground. Thorough bench and ground testing showed satisfactory operation. After the accident, other aircraft with the same modified software were flight tested with no reported problems.