ATSB reminds pilots of Low Rotor RPM response time

ATSB reminds pilots of Low Rotor RPM response time 21 Oct, 20, Source: Australian Government

The Australian Transport Safety Bureau (ATSB) reminds all helicopter pilots to be prepared and respond immediately to low rotor RPM warnings, following the investigation into the collision with terrain of a Robinson R44 helicopter south of Ayers Rock Airport, Northern Territory, on 17 January 2018.

Shortly after departing the Yulara Town helipad for a 15-minute scenic flight with the pilot and three passengers onboard, the helicopter’s rotor RPM began to decay and the low rotor RPM warning activated. Following this warning, the pilot initially did not apply full throttle (for at least 5 seconds), and lower the collective lever sufficiently to stop the helicopter climbing.

The helicopter continued flight for about another 90 seconds, climbing to about 200 feet at a low airspeed. This resulted in the rotor RPM decaying further to a level from which the pilot could not recover. The pilot attempted a forced landing, but was unable to arrest the rate of descent, resulting in a hard landing. The pilot and two passengers were seriously injured, with the third passenger sustaining minor injuries. The helicopter was substantially damaged.

Transport Safety Investigators found that the take-off was conducted at a high density altitude at near maximum weight. Therefore, a high engine manifold pressure (MAP) would be expected for the take-off. However, passenger video evidence indicated the rotor RPM decay started at a relatively low MAP, and that the MAP increased slowly as the RPM steadily decayed.

The ATSB’s investigation concluded that the helicopter’s rotor RPM steadily decayed due to a likely limited opening of the engine throttle during take-off. Fine-tuning of the engine throttle is controlled automatically by the engine governor, but it can be manually overridden by the pilot. The reason for the limited opening of the throttle could not be determined.

“Low rotor RPM may develop in various flight conditions, but it is the low airspeed-low height condition that is most likely to result in an accident,” acting Director Transport Safety Kerri Hughes said.

“Helicopter pilots should ensure they are familiar with the power curve, the associated airspeeds for their particular helicopter, and be prepared to respond immediately to a low RPM warning.”

Further, the investigation established that the pilot had inadvertently adopted a practice of conducting rotors running turn-arounds (for passenger transfers) with the governor switched off. This was not in accordance with the manufacturer’s checklist requirement for the governor to remain on from start until shut-down, nor the operator’s procedure for the governor to be selected on for the engine run-up.

“Although the pilot reported that the governor was selected on and checked before lift-off, this practice increased the risk of an inoperative governor not being detected before take-off,” Ms Hughes said.

The operator, Professional Helicopter Services, used individual passenger weights for their loading calculations, which was considered best practice. However, it was found that the operator’s passenger scales were not calibrated and were under-reading the actual occupant weights.

“This resulted in the helicopter operating at a higher weight than planned, but less than the maximum weight,” Ms Hughes said.

“While the operating weight was within the published limits, the under-reading scales increased the risk of their helicopters not achieving their take-off performance.”

The ATSB has issued a safety recommendation to the Robinson Helicopter Company to review the R44 pilot’s operating handbook low rotor RPM recovery procedure for consideration to include a reference to the minimum power airspeed (Vy) for pilot awareness. Robinson reported that this will be reviewed by engineering staff for possible revision to the pilot operating handbook.

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