| Occurrence Briefs are concise reports that detail the facts surrounding a transport safety occurrence, as received in the initial notification and any follow-up enquiries. They provide an opportunity to share safety messages in the absence of an investigation. |
What happened
On 15 June 2018, at 1050 Eastern Standard Time, a Robinson R44 conducting a charter flight experienced an engine RPM governor[1] failure.
The charter was for a scenic tour of Brisbane, Queensland. On board the helicopter was a pilot and three passengers. Halfway through the flight at an altitude of 1,000 ft, the rotor RPM began to decay and the low rotor RPM horn[2] sounded. The pilot applied a low rotor RPM recovery technique[3] of lowering the collective and increasing the throttle. The aircraft descended to 800 ft before climbing back to 1,000 ft.
The pilot explained what was happening to the passengers who remained calm, then made a PAN[4] call to Air Traffic Control (ATC). ATC cleared a route for the aircraft to return direct to the airport. The pilot was not certain of the cause of the drop in RPM. Rather than turning off the governor as the flight manual instructs for governor failure, he elected to manually control engine RPM by overriding the clutch in the governor.
The aircraft’s magnetos had undergone a 500 hour service immediately prior to the scenic charter flight. An engineering inspection following the incident flight found that a problem with the tachometer points of the magneto[5] caused the governor to read a higher RPM than existed and wind down the throttle, subsequently slowing the rotor system.
Safety message
The right hand magneto provides a signal to the engine RPM governor. The tachometer points that provide the signal must be set precisely to avoid governor issues. In response to reports of governor malfunction between service intervals, Robinson issued service letter SL-62 stating, “Strict adherence to published magneto maintenance practices is essential for proper governor operation”. The service letter also refers to governor trouble-shooting advice in the aircraft maintenance manual.
In the normal course of operation, prior to the failure, the pilot had identified two potential forced landing areas, and during trouble-shooting the pilot steered the helicopter to maintain access to open ground. The pilot also alerted ATC and other aircraft in the vicinity to the problem by declaring PAN. This is another important element in managing an abnormal situation, which brings support to a pilot when they need it. On this occasion, the pilot had correctly determined that it was not necessary to land the helicopter as soon as possible. However, they had created options that allowed for doing so if required.
About this report
Decisions regarding whether to conduct an investigation, and the scope of an investigation, are based on many factors, including the level of safety benefit likely to be obtained from an investigation. For this occurrence, no investigation has been conducted and the ATSB did not verify the accuracy of the information. A brief description has been written using information supplied in the notification and any follow-up information in order to produce a short summary report, and allow for greater industry awareness of potential safety issues and possible safety actions.
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- Engine RPM Governor – This system senses engine RPM and makes adjustments to the throttle control to maintain a constant engine RPM, which leads to a constant rotor RPM in flight.
- Low rotor RPM horn – Maintaining correct rotor RPM is critical to sustained rotary wing flight. The low rotor RPM horn alerts the pilot to a reduction in rotor RPM. In the Robinson R44 it alerts the pilot if rotor RPM decays below 97%.
- Low rotor RPM recovery technique – For minor decay of rotor RPM, the pilot will lower the collective control, reducing aerodynamic drag on the rotor blades. Simultaneously they will open the throttle to increase the engine RPM providing increased drive to the rotor system.
- PAN call – Transmitted as “pan-pan” it is an internationally recognised distress call that alerts others to a problem aboard the aircraft that is currently less urgent than mayday.
- Magneto – A magneto is a device that provides a self-generated charge to the spark plugs of a piston engine. Two magnetos operate on the engine of an R44 and the engine’s right hand magneto provides the signal to the governor.