The Australian Transport Safety Bureau (ATSB) has issued a warning to helicopter pilots about the need for a quick response to low rotor speeds.
The alert comes after the ATSB completed an investigation report in to the crash of a tourist Robinson R44 at Yulara, NT, in January 2018.
According to the ATSB, the aircraft with four people on board descended into terrain shortly after take-off when the pilot took five seconds to respond to a low rotor RPM warning. The helicopter climbed to 200 feet at a low airspeed, but the pilot couldn't recover the situation, despite an attempt at a forced landing.
The pilot and two passengers were seriously injured in the resulting crash, and the remaining passenger experienced 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," the ATSB says. "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.”
The full report is on the ATSB website.