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23rd of April 2003, Blog #596

A Beechcraft 99A (Build in 1969) was operating a domestic scheduled flight from Saskatoon Airport to Prince Albert Airport (both in the province of Saskatchewan Canada). With a crew of two and 4 passengers, the aircraft descended towards its destination after a short flight.

(The distance between the airports is approximately 84 miles ( 134 km),

The aircraft after salvage (Source; http://www.iasa.com.au/ © Unknown)


The reported weather for Prince Albert Airport was recorded by the crew as;

  • Wind - 120º at 14 knots, gusting at 21 knots

  • Visibility - 15 miles

  • Clouds - Few at 6000 and 9000 feet

  • Temperature - 21ºC

While on the approach, at an altitude of approximately 4000 feet, the flaps were set to the approach setting, at that time a loud noise (described as a "bang") was heard from the rear of the aircraft. The aircraft started an uncommanded pitch-up to a nearly 90º nose-up attitude, causing the wings to stall. This made the aircraft pitch over and started a spin to the left. The crew managed to recover the aircraft from the spin, however, the aircraft was now in a near vertical dive. The crew applied full nose-up elevator input and with the aid of changing power settings were able to recover the aircraft from the dive to a nearly horizontal flight attitude.

The aircraft in better days (Source baaa.acro.com © Unknown) The landing gear was lowered and a Mayday call was made to ATC, stating that they would be making a forced landing. The initial impact was on a small round hill, this ripped the belly cargo pod and the landing gear from the aircraft, for a short while the aircraft became airborne again, touching down approximately 180 metres from the initial impact, crashing to a barbed wire fence and sliding on its belly for another ~400 metres, coming to a stop about 600 metres from the initial impact point.

All occupants sustained serious (Non-life-threatening) injuries but evacuated the aircraft through the main cabin door.

The Transportation Safety Board of Canada (TSB) investigated this accident and during the examination of the wreckage it was found that the stabiliser trim actuator was not connected to its mounting structure, causing the stabiliser to move freely in flight. Further investigation revealed that the mounting bolts were in place, during the installation of the trim actuator the mounting bolts had not passed through the mounting lugs of the trim actuator. Arrows indicating installed bolts and This caused the mounting lugs to be actuator missing trim jammed between structural rivets and (Source; http://www.iasa.com.au/

the shanks of the mounting bolts © Unknown) The mounting lugs can not be seen during the installation of the trim actuator, the bolts are installed by feel. The accident flight was the 12th flight after a heavy maintenance inspection of the aircraft, during which the trim actuator was replaced.


On the 2oth of January 2004, the TSB published their report, in this report (which served as the source for this blog) they identified the following findings as to causes and contributing factors;

  1. .During the 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.

  2. During the 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.

  3. The improperly installed bolts trapped the actuator mounting lug assemblies, suspending the weight of the actuator and giving a false impression that the bolts had been correctly installed.

  4. Dual inspections, ground testing, and flight testing did not reveal the faulty attachment.

Drawing from TSB report depicting the correct and incorrect installation of the stabiliser trim actuator (Source & © TSB)


More details on this accident, including more details on the technical details and safety actions taken, can be found in the TSB report which can be accessed by clicking on the .PDF file below;

B99A 23-Apr-2003 Flt Ctrl Failure
.pdf
Download PDF • 252KB






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