Forced/precautionary landing

The Skyfox Gazelle had undergone maintenance to rectify a flooding carburettor. The left side carburettor bowl had been removed and one of the floats found to be low. The carburettor was then removed and both carburettor floats replaced. The carburettor was re-installed, but when the fuel pump was turned on to prime the fuel system, the carburettor again flooded.

The maintenance organisation reported that the needle and seat were then adjusted until the flooding ceased. No evidence of flooding was found when the engine was ground run to full power, without the electric pump operating. Before commencing a subsequent test flight the pilot conducted another satisfactory full power check. However, shortly after take-off at about 200 ft, the engine lost power. The pilot commenced forced landing procedures, but the engine regained full power at about 100 ft. The pilot re-commenced climbing and returned to the airfield. But once again the engine lost power so the pilot force landed the aircraft on the airfield.

The resulting heavy landing bent the landing gear and damaged a wing tip. During a subsequent ground run the engine was able to attain full power. The left side carburettor was again removed and sent to the engine manufacturer's agent for disassembly and inspection. The inspection revealed that although the carburettor float level was slightly incorrect, it was not considered sufficient to cause the engine to lose power. However, the carburettor air duct was found to be holed and both the needle and float mechanisms were excessively worn. Those components were replaced, and the carburettor re-installed. The fuel system was then pressurised using the electric fuel pump.

The left side carburettor was found to be operating correctly, but the right-side carburettor now flooded. The right side carburettor air duct was subsequently found to be holed, and the needle and seat were excessively worn. It is considered that the combination of holed air ducts and worn carburettor needles and seats probably resulted in an excessively rich fuel mixture, with an associated loss of engine power. The maintenance organisation has supplied a report to CASA.

The helicopter had just finished being used to survey cattle and was returning for a landing. The pilot reported that the engine governor was turned on and that during a steep turn downwind, he allowed the helicopter's airspeed to decrease to approximately 20 kts. As he lowered the helicopter's nose to accelerate downwind, the Low Rotor RPM warning horn sounded indicating that the main rotor RPM had decreased below 96%. The pilot reported that he immediately lowered the collective, fully opened the throttle and further lowered the helicopter's nose to recover airspeed. As the helicopter approached the ground, he selected a decelerative attitude, a manoeuvre that would normally recover some main rotor RPM. However, in this case, the main rotor RPM did not recover. When the helicopter had descended to approximately 3 ft AGL with an airspeed of approximately 55 kts, the pilot placed it in a sideways quick-stop manoeuvre. This was done to decrease the helicopter's air and ground speed before it contacted the ground. The pilot reported that he then levelled the helicopter before running it onto a sand ridge. Despite these actions, the Low Rotor RPM warning horn continued to sound throughout the descent and landing.

After sliding along the ground for approximately 20 ft, the helicopter somersaulted. After the dust had settled following the crash, the pilot reported that he checked the welfare of his passenger and then checked the engine. The pilot reported that the engine exhaust was cool enough to hold and that the rocker cover was only warm to touch.

The pilot reported that, on a ferry flight back to Hyden from Northam, following a .37 periodic maintenance inspection, he encountered fog ahead along track. As a precautionary measure, the pilot decided to land in a paddock to wait for the fog to clear. He made several passes over the paddock to ensure it was suitable before attempting to land. As the aircraft touched down, the right main wheel struck a rock, hidden in grass, causing the right oleo to collapse from overload.

The pilot reported that the helicopter landed heavily in a carpark, after the engine lost power during takeoff. On lift-off, engine indications were normal with 28-29 inches boost while in the hover. As the helicopter accelerated, the pilot felt a deterioration in performance and observed that the boost had fallen to 15-18 inches. During the subsequent landing, the helicopter's main rotor blades struck a tree and both landing skids were bent. The helicopter remained upright. Post flight examination found that the turbo charger had failed.

The pilot reported that he was conducting a practice forced landing when the engine stopped. At 3,000 ft he had applied full carburettor heat, closed the throttle and commenced a glide descent. During the descent he applied power on four occasions to re-warm the engine. At about 600 ft AGL the engine stopped along with the geared wooden propeller. The pilot carried out trouble checks and declared a Mayday. He twice engaged the starter in an attempt to restart the engine but was unsuccessful. He subsequently reported that the starter seemed to be sluggish.

The aircraft touched down in a paddock on a downhill slope. It rolled about 30 metres, crashed through a farm fence, passed under a powerline, crossed a road, encountered a ditch and overturned.

At the time of the accident weather conditions were: CAVOK, outside temperature about 15 degrees Celsius, wind calm, visibility 10 km plus, nil precipitation.

This was the second accident to VH-IDB in the same area due to the engine stopping during practice forced landings. The previous accident occurred on 14 May 1997. Verbal reports of other instances have been received of Skyfox Gazelles experiencing engine stoppages after the throttle was closed and engine RPM reduced to idle.

After the first accident, the engine was inspected by a licensed aircraft maintenance engineer (LAME). Then, after the second accident with the same engine installed, it was inspected first by the LAME and then by the engine manufacturer's agent. No fault was found with the engine after each accident. Post accident tests on the starter, the battery and the engine have failed to detect a reason for the sluggish starter anomaly reported by the pilot.

Safety Action

As a result of complaints about the stalling of engines at idle, Airworthiness officers from the Civil Aviation Safety Authority (CASA), in conjunction with the aircraft manufacturer, conducted an investigation which included test flights in the Gazelle.

It was found that the throttle mechanism had enough free play (also known as backlash) to reduce RPM below idle and stop the engine. This could occur if extra force was applied when pulling the throttle back to idle speed. To address this problem, the aircraft manufacturer issued Service Bulletin (SB) No. 20 on 28 October 1997. This SB provides the following information: "Idle speed may inadvertently be reduced below set minimum due to incorrect throttle stop adjustment." SB 20 requires operators to check the idle setting and adjust for backlash at the throttle stop in the cockpit. Compliance was mandatory, within two weeks or 10 hours time in service from receipt of the SB.

The pilot stated that while hovering the helicopter at about 50 ft over a dry riverbed, the engine note suddenly changed. He checked the instrument indications which confirmed that the engine RPM had increased. Almost immediately the indications returned to normal. He began to move the helicopter away from trees when the engine RPM again increased as drive to the main rotor system was lost.

During the attempted autorotational landing, the helicopter collided with a tree and landed heavily on the riverbank.

While vacating the helicopter, the pilot became aware of a "rumbling" noise and smoke coming from under the cabin. He attempted unsuccessfully to locate the source of the noise by isolating the electrical systems. He then switched off the battery.

Output from the helicopter engine is coupled through a V-belt drive system to the main transmission which drives the main rotor, and to the tail rotor drive system. The belt drive clutch control installation includes a linear actuator and electrical connections to a clutch control switch and warning light on the instrument panel. A cable and pulley interconnect the linear actuator to the clutch spring on the belt drive transmission. The clutch control switch, which is positioned on the lower left side of the instrument panel, has three positions, RELEASE, HOLD and ENGAGE (the normal operating position). With the switch in the ENGAGE position, the linear actuator retracts, applying tension through an idler pulley to the V-belts. The clutch warning light is on unless the clutch is fully engaged.

Examination of the helicopter rotor drive system did not identify any pre-existing defect which may have contributed to the accident.

The "rumbling" noise heard by the pilot was caused by the starter motor operating due to impact damage to the solenoid. There was no fire.

The linear actuator shaft was found to be extended by approximately 30 to 50% of its travel. It is normally fully retracted when the drive belts are correctly tensioned. The actuator, the clutch control switch, the clutch warning light and the associated wiring were tested and found to be serviceable. It was not possible to determine if the warning light was powered at the time of impact. The position of the linear actuator shaft may have changed following the accident when the pilot repositioned various switches and circuit breakers in an attempt to identify the "rumbling" noise.

The clutch control switch was fitted with a guard to prevent inadvertent operation. However, the retaining springs had weakened such that the guard was ineffective.

The circumstances of the accident are consistent with loss of rotor drive due to insufficient drive belt tension. However, the circumstances in which this occurred could not be determined.

The pilot was tasked with flying passengers from Port Augusta to Cook, via Ceduna. After arriving at Cook the aircraft was flown without passengers to Nullabor for an overnight stop. The following day, he was to return to Cook to collect a group of eight passengers and fly them to Port Augusta. The aircraft had been refuelled to full capacity at Ceduna on the flight out to Cook. During this refuelling the pilot did not note the amount of fuel used on the flight from Port Augusta. The pilot calculated that the fuel remaining at Cook for the return flight would be 439 litres. He also calculated that the fuel required to return to Port Augusta from Cook would be 452 litres. This fuel requirement was calculated using a fuel flow of 140 litres per hour and a true airspeed of 170 knots with a 15-knot headwind. The fuel requirement included a 100-litre fixed reserve. The pilot was aware that there was a 13-litre shortfall between the fuel on board and the fuel required. He did not eliminate this discrepancy by adding fuel at Nullarbor where it was available. The weight of the aircraft on departure from Cook was calculated by the pilot as being 35 kg below maximum take-off weight. However, this calculation was based on 423 litres of fuel, rather than 439 litres.

The aircraft departed Cook at 0007 UTC and the pilot passed an estimate with his departure report of abeam Ceduna at 0116. The aircraft reported abeam Ceduna at 0116 and gave an estimate to flight service for Port Augusta of 0238. This estimate gave a time interval which was 9 minutes longer than planned by the pilot. The pilot had submitted a flight plan to ATS that indicated a groundspeed of at least 165 knots would be achieved during the flight. The pilot had planned the flight expecting the true airspeed to be higher, and therefore the groundspeed to be higher, on the return flight. He also expected to obtain favourable winds at various altitudes and therefore did not plan to use the reserve fuel of 100 litres. He also assumed that he would obtain a better fuel flow than the 140 litres per hour that he had used during the planning for the flight. The flight progressed uneventfully until approximately 90 nm from Port Augusta when the pilot noticed that the fuel remaining on board had decreased to a lower level than expected. He elected to divert to Wudinna as he was now in some doubt as to whether the aircraft would make Port Augusta with the fuel remaining on board.

The decision to divert to Wudinna was based on the fact that the company held fuel stocks at this location. He advised flight service that he was diverting due to a higher-than-normal fuel burn. Flight service declared an alert phase at this time due to the pilot reporting 20 minutes endurance with a time interval to Wudinna of 10 minutes. En-route to Wudinna the pilot noticed that the airfield was situated beyond a heavily timbered area and he elected to carry out a precautionary search-and-landing in a cleared paddock whilst there was fuel remaining, rather than continue over the timbered area where he might suffer fuel exhaustion. The pilot advised flight service that he was carrying out a precautionary landing and they declared a distress phase due to the unsure fuel status of the aircraft. The pilot completed the precautionary search-and-landing without damage to the aircraft or injury to the passengers. The position of the precautionary landing was 19 nm north-east of Wudinna. The company was advised, and the aircraft was recovered to Wudinna later that day.

The estimated fuel remaining on board was less than 30 litres. In his report to the Bureau as part of the investigation, the pilot indicated that he operated the aircraft during both the outbound flight to Cook and the return flight to Port Augusta using fuel flow and exhaust gas temperature (EGT) settings from another company aircraft. These settings had resulted in an actual fuel flow of approximately 165 litres per hour.

Significant Factors

1. The pilot used a fuel flow setting during flight planning that was not representative of that which the aircraft would achieve during flight.

2. The pilot departed from an aerodrome where fuel was available knowing that the fuel on board was less than that required for the flight, with reserves.

During a flight involving stock spotting at 700 ft above ground level, the engine began to vibrate and then lost power. The pilot was able to restore power for about a minute but then was unable to maintain level flight. He landed the aircraft in a small clearing in the scrub. Shortly after touchdown, the nosewheel struck a log and the aircraft nosed over. The pilot reported that he crawled quickly away from the aircraft as fuel was leaking from the tanks.

A licenced aircraft maintenance engineer later reported that he had investigated the power loss and could not find any defects. He started and operated the engine for at least 30 minutes without fault. He ventured that carburettor icing may have been the cause of the power loss.

The Bureau of meteorology analysis of the temperatures indicate that the dewpoint was 6 degrees C and the estimate for the dry bulb temperature was 15 degrees C. These conditions with 20% relative humidity are well outside the range where carburettor icing could be considered a factor.

The reason for the power loss has not been determined.

The SAAB 340 departed normally from runway 16R, but shortly after becoming airborne the crew requested a return to land. When asked if operations were normal the crew reported the aircraft had suffered an engine failure. ATC activated the crash alarm and closed the airport. The crew were offered a circuit to either runway 07 or 34L/R but elected to continue for a right circuit to runway 16R.

The aircraft was flown on a visual low-level circuit to a safe landing. Preliminary investigation revealed major damage to the power turbine section of the left engine. The engine was subsequently returned to the manufacturer's engine overhaul facility for examination. Their engineering report revealed that the stage two turbine forward cooling plate had separated at the outer rim.

The report noted that the failure was consistent with other cooling plate separation events. The manufacturer had previously issued a service bulletin which introduces improvements to the cooling plate to preclude this type of failure. The operator had a program to incorporate this modification during planned hot section module changes. Incorporation of the modification to the failed engine was planned for the week following the incident.

The helicopter was spraying a melon crop when the main rotor came into contact with a steel rod in the crop. The subsequent rotor imbalance necessitated a forced landing. The helicopter subsequently landed heavily, and the main rotor blades cut off the tail boom resulting in substantial damage. This accident was not reported.