On 4 June 2004, the pilot of a Cessna Aircraft Company U206A planned to conduct a private flight from Lakeside Airpark to Proserpine aerodrome and return, a total distance of 40 km. The pilot was the only occupant on the flight to Proserpine which he described as uneventful. Three passengers boarded at Proserpine for the return flight via Laguna Whitsunday Resort, a diversion for sightseeing that added a few minutes flight time.
The pilot described the pre-flight engine run-ups and takeoff at Proserpine as normal and said that he was operating on the right fuel tank as he climbed the aircraft to 1,500 ft above mean sea level. About 4 minutes after departure, the aircraft flew parallel to the resort's airstrip construction site, and then passed the golf course and marina. The pilot reported that, for short periods of less than a minute he had banked the right wing 30 degrees down to optimise the view for his passengers, but he could not recall the extent to which opposite rudder1 was applied during those times. Shortly after passing the marina, when the aircraft was flying straight and level and was over water at about 1,200 ft, the engine failed.
The pilot selected the right half (coloured yellow) of the auxiliary fuel pump switch to LO and changed the fuel selector position from the right to left tank. There was no response from the engine so he changed the position of the fuel selector a number of times and selected the left half (coloured red) of the fuel pump switch to HI for short periods.
Figure 1: VH-OWA auxiliary fuel pump switch (centre).
The pilot reported that he didn't refer to the fuel flow gauge, but was convinced that fuel wasn't being supplied to the engine. By this stage the altimeter was indicating 750 ft and he began searching for a specific landing area. The pilot glided the aircraft in a north-westerly direction towards a flat area between the marina and the resort golf course. The wind was from the south-east at about 10 kts. He continued with his attempts to restart the engine and managed to transmit a Mayday late in the approach. Flap was not extended beyond 5 degrees and the pilot reported that he was too busy during the final stage of the approach to hold the spring-loaded flap control switch down.
The stall warning activated just before the aircraft landed heavily on a flat area about 20 m before a 1.4 m high embankment. The propeller dug into the bank and the aircraft overturned, resulting in substantial damage. Witnesses and resort staff attended the scene and helped the pilot and passengers out of the aircraft. Emergency services attended from Proserpine and treated the four occupants, who were seriously injured.
The pilot reported that he was not manipulating any engine controls or switches immediately prior to the engine failure and that there were no prior indications of the failure. An extensive examination of the aircraft including the fuel system, ignition system and engine did not reveal any contaminants or defects that would have contributed to the engine failure. The aircraft's fuel tanks contained approximately 50L of fuel per side, which was about 1/3 of each tank's capacity. Laboratory testing of samples from both tanks identified the fuel as aviation gasoline (AVGAS) and did not identify any characteristics that would have contributed to the engine failure.
The Owner's Manual stated that when selected to LO, the auxiliary fuel pump would only operate when the starter was engaged. However, the pilot advised that when priming the engine prior to starter engagement, the auxiliary fuel pump selected to LO consistently produced at least a 12 gal/hr fuel flow. While this indicated that the aircraft's auxiliary fuel pump wiring did not conform to the manufacturer's specifications, damage to the aircraft prevented an assessment of the pump output and exactly how the fuel pump switch was wired.
A Civil Aviation Safety Authority airworthiness directive (AD), issued by the then Australian Civil Aviation Authority in 1979, mandated action in accordance with a service information letter issued by the aircraft manufacturer. The service letter specified provision of a fuel flow stabilisation placard and associated procedure card in that aircraft model. Although the aircraft logbooks indicated compliance in 1985 with the requirements of the AD, neither the placard, nor the procedure card that included in-flight engine restarting procedures, was in the aircraft. The restart procedures specified that the auxiliary fuel pump be selected to ON or HI until the indicated fuel flow was in the green arc, then it should be selected off. The pilot reported that he was not aware of the requirement for the placard and procedure card, nor was he aware of the type-specific in-flight engine restarting procedures. The copy of the owner's manual that was in the aircraft did not include any emergency procedures.
The pilot related that about 8 months prior to the accident the aircraft had sustained a significant power loss while cruising straight and level at 6,500 ft in calm conditions. He had applied the same engine restart procedure that he used during the accident sequence and after a series of engine power fluctuations eventually accomplished a sustained restart at about 3,000 ft. The pilot believed that there had been a vacuum or blockage and the only way he got the engine to run satisfactorily was to rotate the fuel selector between the left and right tanks using a small amount of boost. There was no reason identified for that power loss and the aircraft operated normally for a further 70 hrs. During that time, a periodic inspection was carried out and no aircraft defects that could have contributed to the power loss were identified.
The owner's manual included the following caution in the description of the fuel system.
... with 1/4 tanks or less, prolonged uncoordinated flight such as slips or skids can uncover the fuel tank outlets, causing fuel starvation and engine stoppage. Therefore, with low fuel reserves, do not allow the airplane to remain in uncoordinated flight for periods in excess of 1 minute.
The ATSB recently completed an investigation into an engine failure resulting from fuel starvation that involved a similar aircraft type (Cessna 207, ATSB report 200403210). That investigation found that the in-flight engine restart procedures published by the manufacturer were not followed, but the engine restarted after a significant height loss of about 700 ft.
1 Application of rudder in the opposite direction to a lowered wing inhibits the development of a turn, resulting in a slip that can allow flight on a straight track. An aircraft that is slipped is considered to be in uncoordinated flight.
It was likely that, in lowering the right wing while passing the airstrip and golf course, the pilot 'slipped' the aircraft in order to counter the aircraft tendency to turn. However, the period of any uncoordinated flight could not be determined. Although the quantity of fuel in the selected tank exceeded the '1/4 tanks or less' quoted in the Owner's Manual caution about uncoordinated flight, there was still a risk of fuel starvation and engine stoppage with 1/3 tank capacity. Given the absence of any evidence of pre-accident aircraft defects or engine mis-handling, it is possible that the engine failure was due to uncovering of the right fuel tank outlets and introduction of air into the fuel system.
While the previous power loss event was of interest to the investigation the lack of specific information about that event meant that a link with the engine failure could not be established.
The pilot's response to the engine failure was based on a generic procedure that was inconsistent with the in-flight engine restarting procedures produced by the aircraft manufacturer. Although the investigation could not determine the status of the auxiliary fuel pump wiring, the information available indicated that the LO function of the fuel pump was capable of producing significant fuel flow. It was likely that sustained use of the auxiliary fuel pump, instead of the momentary use specified by the engine manufacturer, provided fuel flow that exceeded the engine's requirements and prevented a restart. Had the pilot referred to the fuel flow gauge, he could have ascertained the amount of fuel being supplied to the engine, and responded accordingly. The pilot's lack of awareness of the manufacturer's procedures could be attributed in part to the absence of the applicable placard and procedure card in the aircraft and the absence of training in type-specific emergency procedures.
The pilot delayed the search for a specific landing site until the aircraft had descended to about 750 ft, because he was initially focussing on restarting the engine. This was primarily due to the pilot's perception that the engine failure was similar to the previous engine power loss event and his expectation that the engine would eventually restart. Although the pilot managed to reach a clear area, an earlier diversion to a specific landing area after the engine failure would have reduced the risk of a forced landing in a less favourable location.
The tailwind during the glide approach, and the limited amount of flap extension had a positive effect on the aircraft's glide range. However, those two factors contributed to a relatively higher touchdown speed, which increased the risk of aircraft damage and occupant injury. Due to the prevailing wind conditions and the track to the landing area, a tailwind during landing was unavoidable. The spring-loaded flap switch combined with high pilot workload made it difficult for the pilot to further extend flap in the late stage of the approach.
The Civil Aviation Safety Authority advised that an article highlighting the importance of placards and emergency procedures had been submitted to the editor of Flight Safety Australia and will be published in a forthcoming edition.
|Date:||04 June 2004||Investigation status:||Completed|
|Time:||1311 hours EST|
|Location:||83 km NW Mackay, (VOR)|
|State:||Queensland||Occurrence type:||Engine failure or malfunction|
|Release date:||23 December 2004||Occurrence class:||Technical|
|Report status:||Final||Occurrence category:||Accident|
|Highest injury level:||Serious|
|Aircraft manufacturer||Cessna Aircraft Company|
|Type of operation||Private|
|Damage to aircraft||Destroyed|
|Departure point||Proserpine, QLD|
|Destination||Lakeside Airpark, QLD|
|Role||Class of licence||Hours on type||Hours total|