Occurrence
On a scheduled flight from Saskatoon, Saskatchewan, to Prince Albert, Saskatchewan, a Beech 99A aircraft, registration C-FDYF, was at approximately 4,000 feet above sea level when the crew selected flaps for approach. A bang was heard from the rear fuselage, followed by an uncommanded pitch-up to a near-vertical attitude. The aircraft stalled, nosed over, and entered a left spin. The crew countered the spin, but the aircraft descended in a near-vertical dive. Through full-up elevator and power adjustments, the pilots regained a near-horizontal attitude.
The crew extended the landing gear and issued a Mayday call, stating they were conducting a forced landing. The aircraft struck a knoll, tearing away the belly cargo pod and landing gear. It bounced, traveled approximately 180 metres, contacted a barbed-wire fence, and slid to a stop about 600 metres from the initial impact point. The crew and passengers sustained serious but non-life-threatening injuries. All occupants exited through the rear main cabin door. The accident occurred during daylight at 1802 central standard time.
Pilot and Weather Information
The pilots were certified and qualified per regulations. All required flight control checks were completed, and the aircraft operated with normal aerodynamic flight loads. The reported weather at Prince Albert at 1800 CST was: wind from 120° at 14 gusting to 21 knots, visibility 15 statute miles, few clouds at 6,000 and 9,000 feet, temperature 21°C.
Investigation Findings
The aircraft was equipped with a Fairchild A100 cockpit voice recorder, which was examined by the TSB Engineering Branch. The crew had activated the Narco ELT-10 before impact, and the emergency signal transmitted throughout the occurrence.
Inspection of the horizontal stabilizer trim control system revealed that the trim actuator had fallen from the upper airframe mounting structure, allowing the stabilizer to move freely in flight. The mounting bolts were installed through the airframe brackets but not through the mounting lugs of the trim actuator. Contact marks on the mounting lug assemblies, channel and plate assemblies, and worn rivets indicated the upper lugs had been positioned ahead of the normal location. The spherical bearings of the lugs were positioned above and in contact with plate-to-channel attachment rivets. Marks on the rear of each mounting lug confirmed the bolts were installed behind the lugs, trapping the assemblies between rivets and bolt shanks, approximately ¾ inch ahead of the correct location.
Maintenance History
The occurrence flight was the 12th flight after heavy-maintenance inspections involving disassembly, inspection, and reassembly of the wings and tail. During this process, the horizontal stabilizer trim actuator was replaced. The unit was installed and tested for functionality and adjusted through multiple trimming ranges.
The mounting structure consists of four channel-and-plate assemblies aligned fore-and-aft between a former at station 418.900 and a bulkhead at station 428.335. The installer, kneeling and holding the 10-pound actuator above and ahead, cannot see the mounting lugs. Bolts are installed by feel. The actuator can be positioned with lugs forward enough for bolts to be inserted behind them. The lugs can jam between rivets and bolts, leading to a false belief of correct installation.
Inspection Challenges
Dual inspections of the flight controls were completed after installation. The inspectors confirmed bolts were through the airframe channels with appropriate washers and nuts, but the confined space and component shape made it difficult to verify the bolts passed through the lugs. The incorrect installation went undetected.
Functional testing on the ground did not identify the problem because the actuator is under compression while on the ground. In flight, the actuator experienced compressive and tensile loads. The rivets holding the actuator gradually wore away, leading to detachment and loss of pitch control.
Findings as to Causes and Contributing Factors
During flight, the horizontal stabilizer trim actuator worked free of the mounting structure, and as a result, the flight crew lost pitch control of the aircraft.
During replacement of the horizontal stabilizer trim actuator, the upper attachment bolts were inserted through the airframe structure but did not pass through the upper mounting lugs of the trim actuator.
The improperly installed bolts trapped the actuator mounting lug assemblies, suspending the weight of the actuator and giving the false impression that the bolts had been correctly installed.
Dual inspections, ground testing, and flight testing did not reveal the faulty attachment.
Findings as to Risk
The nature of the installation presents a risk that qualified persons may inadvertently install Beech 99 and Beech 100 horizontal stabilizer trim actuators incorrectly. There are no published warnings to advise installers that there is a potential to install the actuator incorrectly.
Safety Action
On 02 May 2003, the TSB issued an occurrence bulletin (A03C0094) detailing factual information. On 20 June 2003, a Safety Advisory was forwarded to Transport Canada. Transport Canada issued a Service Difficulty Alert (AL-2003-07, 2003-07-17) based on the bulletin. Transport Canada contacted the FAA, requesting assistance and suggesting a service letter and maintenance manual warnings. Raytheon Aircraft issued King Air Communiqué No. 2003-03 to alert operators and maintenance personnel.
This report concludes the TSB investigation. The Board authorized release on 20 January 2004.