Propeller/rotor malfunction

What happened

On 21 February 2019, a Piper PA-32 6XT departed Orange, New South Wales (NSW) to conduct a private flight to Bankstown, NSW with two crew on board. During approach, at approximately 900 ft above mean sea level at Warwick Farm, the crew detected a vibration. The crew declared a PAN-PAN^[1] to Bankstown tower requesting priority landing. The tower cleared the crew for a straight in approach to runway 11L. The aircraft landed without incident and taxied to parking.

The post-flight inspection revealed the cause of the vibration was due to a 20 cm section missing from one of the propellers. Following the incident, the engineer suspects the likely cause of the propeller section breaking off was due to a stone chip, which developed into a crack as a result of engine operation, which caused stress on the propeller, which subsequently resulted in the propeller failing during flight.

Figure 1: Missing propeller section

Source: Pilot in Command

Safety message

It is important that pilots remain aware that despite conducting comprehensive pre-flight checks, unanticipated failures can still occur during flight. The crew, in this instance, took all possible precautions by following non-normal procedures, providing clear communications to ATC and landing the aircraft as soon as possible.

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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1. PAN-PAN - The radiotelephony message PAN-PAN is the international standard urgency signal that the crew on board an aircraft uses to declare that they have a situation that is urgent but, for the time being at least, does not pose an immediate danger to anyone's life or the aircraft itself.

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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1. 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.
2. 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%.
3. 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.
4. 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.
5. 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.

What happened

On 22 August 2018, at 1235 Eastern Standard Time, a Bombardier DHC-8 departed Cairns, Queensland (Qld) for a regular public transport flight to Moranbah, Qld.

During initial climb between 600 ft and 1,000 ft, the crew detected severe engine surging. The crew identified the no. 2 engine as the affected engine and shut it down. The crew completed the relevant checklists, declared a PAN PAN^[1] and continued climb to 7,000 ft. The crew then returned the aircraft for landing on runway 33 at Cairns.

Engineers replaced the propeller control unit and the over speed governor and returned the aircraft to service.

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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1. PAN PAN: an internationally recognised radio call announcing an urgency condition which concerns the safety of an aircraft or its occupants but where the flight crew does not require immediate assistance.

What happened

On 20 January 2018, a remotely piloted aircraft (RPA) was being operated on a training exercise over water off Wategos Beach near Byron Bay, New South Wales.

At about 1318 Eastern Daylight-saving Time (EDT),^[1] while maintaining 100 ft above ground level, the crew detected a propeller malfunction on one of the RPA’s six propellers. The crew elected to conduct a precautionary landing of the RPA into the water about 200 m offshore. The RPA operator indicated that this was in accordance with their emergency procedures for a propeller malfunction in order to reduce the risk to the beach-going public.

The RPA was recovered from the water and inspected by the operator. The initial inspection indicated that bolts fastening the propeller to the motor unit had failed. Figure 1 shows the failed bolts (left) along with a picture of a non-damaged propeller assembly (right). The RPA has been sent back to the manufacturer for further examination.

Figure 1: Failed bolts that secure the RPA propeller to motor (left) and example of an undamaged propeller and motor assembly (right)

Safety message

The ATSB’s research report A safety analysis of remotely piloted aircraft systems 2012 to 2016: A rapid growth and safety implications for traditional aviation found that, there has been rapid growth in the number of RPA systems in Australia. This incident highlights the importance of contingency planning for RPA operations. While the manufacturer indicated that, for this RPA configuration, control could be maintained in the event of a single propeller failure, a loss of a control surface would result in a degradation of performance. In this case, the crew identified the reduced performance and acted accordingly by landing the RPA offshore, thereby reducing the risk to third parties.

The Civil Aviation Safety Authority has published guidance on the operation of RPA’s in the advisory circular, Remotely piloted aircraft systems – licencing and operations. Section 4.6 specifically states that, procedures to be followed in the event of an engine/propeller failure should be in place and included in the RPA system mission plan.

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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1. Eastern Daylight-saving Time (EDT): Coordinated Universal Time (UTC) + 11 hours.