Aviation Safety Letter 1/2005
Jammed Throttle Levers or Pilot Technique?
A Safety Information Letter from the Transportation Safety Board of Canada (TSB)
On June 3, 2004, during training exercises, the captain of a Beech 99 Airliner could not advance the power levers during a balked approach procedure. During the overshoot, the captain had called for power, gear and flaps up, but found that the power levers were stuck when an attempt was made to move them. The gear and flaps had been raised by the training pilot in the right seat, and with reduced power, the aircraft began to settle to the ground. The training pilot reached over to assist the captain, and pushed the power levers forward. The engines eventually spooled up and the overshoot was completed. Unbeknownst to the crew, the aircraft's belly pod had touched the runway prior to the overshoot. The belly pod damage was later discovered during a maintenance inspection.
During the debriefing and examination of the aircraft after the incident, it was discovered that if the power levers are lifted or raised slightly during movement, they hit a detent and become jammed. The occurrence captain had been sitting low in his seat and was using the palms of his hands to push up and forward on the power levers. Because of his upward movement, the levers hit the detent and jammed. When the training captain reached over and pushed the throttle levers forward, it was in a sliding motion that bypassed the detent.
The operator checked its remaining fleet of Beech 99 and King Air A100 aircraft and found this condition can occur on all of them if the power levers are sufficiently raised. The operator has since included this awareness as part of its ground school training syllabus. It is not known what models of the Beech 99 and King Air aircraft that this anomaly extends to; however, jamming of the power levers in any model could result in serious consequences if immediate power is required and not available. The foregoing is provided for whatever follow-up action is deemed appropriate.
Transport Canada (TC) reviewed this incident and is of the opinion that the throttles were operating normally, there was no "jam" and there was no aircraft defect or design problem. The design of the idle stop is such that the throttles may only be retarded into reverse after they are lifted and, conversely, that they will only move forward into positive thrust when they are in the lower position. In this case, when the pilot inadvertently pushed the throttles up, they did not move forward because they were blocked by the forward idle gate. This behaviour of the throttle "system" is similar to that in all the King Air family of airplanes. The throttles were "blocked" by the gate but were not "jammed." In this case, the throttles operated as they were designed.
It can be accepted that a pilot might apply enough upward pressure on the throttle levers to encounter the face of the "gate" and have forward travel blocked, thus preventing application of more than idle power. However, in this case, the crux of the problem was the upward force coupled with the procedure whereby the wheels (and flaps) are raised prior to full or maximum continuous power being applied. The procedure, as described in the letter above, is contrary to the recommended procedure in the flight manual, and is not good airmanship. The flight manual implies that not only is the power to be applied first, but a subsequent rate of climb is to be achieved before raising the gear. Leaving the gear down until after the power is set ensures for the proviso that the aircraft will touch on the wheels if the sink rate is not arrested in time. TC agrees that "jamming" of power levers would need to be investigated and corrected, but believes that in this case "jamming" is not an appropriate categorization of the event. The more relevant lessons to be drawn from this incident are shortcomings in pilot technique, safe training practices, crew resource management, standard operating procedures and adherence to aircraft flight manual procedures. Raising the flaps and gear before establishing climb power and speed is clearly poor airmanship and contrary to the aircraft flight manual. These issues have been addressed with the operator involved. - Ed.
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