Sequence of events
On 21 February 2003, at 1615 Western Standard Time, shortly after commencing the take-off run on a charter flight from Lake Johnston to Perth, Western Australia (WA), the crew of the Cessna Aircraft Company 441 Conquest, registered VH-LBZ observed the right engine exhaust gas temperature and power increasing without pilot input. There were no other abnormal indications or illuminated warning lights accompanying the unsolicited power increase from that engine.
The aircraft was crewed by a pilot in command (PIC), as the pilot flying, and a supervising pilot. The PIC rejected the takeoff by attempting to reduce power from both engines with the power levers. While the left engine reduced to idle power, the right engine remained at maximum power. The aircraft veered to the left, and both pilots applied the aircraft brakes and attempted to maintain directional control. The PIC acted to move the fuel control from AUTO to MANUAL mode by selecting the fuel computers OFF, and selected both engine stop buttons to initiate engine shut down. As the PIC was selecting the condition levers to EMER SHUT-OFF, the aircraft entered the runway strip at an estimated speed of about 65 kts.
Due to vibration of the aircraft over the rough terrain, the crew could not confirm whether the right engine stop button had been fully depressed, or whether the condition levers had been fully selected into the SHUT-OFF detents prior to the aircraft impacting a sand mound. The PIC secured the aircraft and the pilots and passengers evacuated from the aircraft uninjured.
The wind was reported to be a `light' north-north-easterly, with a crosswind component estimated at 3 kts. The crew of the aircraft reported no wind gusts immediately prior to, or during the occurrence.
The aircraft came to rest nose-down in soft sand approximately 45 m to the left of the runway strip, and 750 m from the brakes release point. Inspection of the damaged aircraft by a salvage team revealed that the nose landing gear had collapsed, the main landing gears had sunk into the soft sand, and that both propellers struck the ground. All engine control runs were confirmed to be intact, and correctly rigged. On-site examination of the right engine control and indicating systems was unable to explain the uncontrolled increase in power from that engine during the occurrence. The investigation was unable to confirm the pre-occurrence continuity of the electrical harness that activated the fuel control unit (FCU) manual mode solenoid valve or the position of the computer mode switches (either AUTO or MANUAL) prior to the pilot securing the aircraft.
The PIC had flown about 161 hours on the Conquest, all of which were flown in the last 90 days. There was no indication that the PIC had flown another type during that time. The supervising pilot had a total of about 1,000 hours on type at the time of the occurrence.
Temporary revision 10 to the aircraft Pilot's Operating Handbook (POH) included the actions for an Uncommanded Power Increase In-flight (takeoff, climb, cruise, approach or landing) and On [the] Ground During Takeoff Roll or Landing Rollout. Actions in response to an in-flight power increase included reducing the appropriate condition lever and selecting the fuel computer switch OFF. The memory actions in response to an uncommanded power increase on the ground included:
Directional Control - MAINTAIN WITH DIFFERENTIAL BRAKING AND RUDDEREngine Stop Button (Engine with High Torque) - Push
Power Levers - FLIGHT IDLE
The right engine FCU and a number of other engine components were examined at the engine manufacturer's facilities in the United States under the supervision of the National Transportation Safety Board. It identified fine red dust deposits in, and located downstream of the FCU in-line P-3 air filter, and a blocked flow restrictor (Viscojet) within the FCU. Advice from the engine manufacturer was that:
- When in the MANUAL mode, a blocked Viscojet under rare conditions could act like a check valve, trapping P-3 air within the FCU control bellows chamber;
- When operating in the AUTO mode, and with uninterrupted electrical power to the FCU manual mode solenoid valve, the engine should have been controllable, and decelerated when the PIC retarded the condition levers to idle; and
- When in the AUTO mode, an interruption to the electrical power supply to the manual mode solenoid valve could lead to P-3 air being trapped in the control bellows.
The effect of P-3 air being trapped in the control bellows chamber was that pilot control of the fuel supplied to the affected right engine was not possible.
The operator reported that the occurrence aircraft history included extensive operations in the hot, dusty areas of northern and inland WA. At the time of the occurrence, the operator was cleaning the P-3 air filter element every 100 hours of engine operation. That action halved the aircraft maintenance manual requirement to clean the element every 200 hours.
It appeared that, over time, operation of the aircraft in the northern WA environment had resulted in the inability of the P-3 air filtration system, and element cleaning regime to prevent fine red dust from infiltrating the internal, precision parts of the FCU.
Two possible explanations exist for the loss of control of the engine:
The trapped P-3 air in the control bellows meant that there would have been no control in MANUAL mode and the PIC's reported attempt to select MANUAL mode would, if successfully carried out, not have returned fuel control authority to the pilot.
The PIC's stated action to lower the fuel computer guards would have placed the computers in AUTO mode. For an uncontrolled fuel supply to the right engine in the AUTO mode to be a possibility, it necessitated an interruption to the electric power supply to the FCU manual solenoid valve. The investigation could not confirm whether such an interruption in power supply had occurred in this case, nor could it be ruled out.
The PIC's initial reaction to reduce engine power and select engine management to MANUAL mode, although understandable given the circumstances, was not in accordance with the memory items promulgated in the temporary revision to the POH. The partially successful PIC's action to reduce engine power from both engines resulted in an increased power difference between the left and right engines, and contributed to the difficulty experienced by the crew to maintain aircraft directional control. While the reasons for the difference between the actions promulgated in the POH in response to this emergency, and those taken by the PIC could not be explained, it appeared that the PIC might have initially carried out the actions for an uncommanded power increase when in-flight.
It was unlikely that any action taken by the PIC, other than an immediate and successful shut down of the affected right engine, would have allowed him to maintain directional control, and prevent the aircraft from departing the runway during the take-off roll. Although not in accordance with the temporary revision to the POH, ultimately, the PIC activated the engine STOP button and selected the condition levers into the SHUT-OFF detents in order for that to occur.
On 17 December 2003, after consultation with, and agreement from, the airframe manufacturer, the engine manufacturer promulgated Service Bulletins TPE331-A73-0266 and 0267 recommending maintenance action affecting the Viscojet, and a functional test able to detect early blockage of the Viscojet.
The operator has addressed the risk of Viscojet blockage resulting from its operations in the northern WA environment by increasing the frequency at which it cleans the P-3 air filter elements to every 50-flight hours.
In addition, the operator has reviewed its engine shut down procedures in the case of an Uncommanded Power Increase during Takeoff in Conquest aircraft. While the airframe manufacturer indicated reluctance to amend the published procedure to include movement of the condition levers into the SHUT-OFF detent, indication was given that, if time and circumstances permitted, emergency action could be used as a back-up to the existing priority of activating the STOP button.
After consideration, the operator has amended its emergency procedures to include movement of the condition levers into the SHUT-OFF detent. The Civil Aviation Safety Authority has accepted that amendment.
PIC reported that the takeoff was commenced with the fuel computer
guards raised, in accordance with published temporary revisions to
the Pilot's Operating Handbook. He also reported that in an attempt
to de-select the fuel computers he lowered the fuel computer
2 The runway strip is defined as that portion of ground between the runway and the fly-over area which is in a condition that ensures minimal damage to an aeroplane which may run off a runway during a take-off or landing.
3 Engine manufacturer advice was that the action to activate the stop button prior to retarding the power levers was because an action to initially select the emergency shutdown position would result in a momentary power surge from the affected engine. There was the potential for that power surge to add to any control difficulty experienced by the pilot.
|Date:||21 February 2003||Investigation status:||Completed|
|Time:||1645 hours CST|
|Release date:||04 November 2004||Occurrence category:||Accident|
|Report status:||Final||Highest injury level:||Minor|
|Aircraft manufacturer||Cessna Aircraft Company|
|Type of operation||Charter|
|Damage to aircraft||Substantial|
|Departure point||Lake Johnston, WA|
|Departure time||1615 hours CST|
|Role||Class of licence||Hours on type||Hours total|
|Pilot-in-Command (AICUS)||Senior Commercial||160.6||1764|