What happened

On 28 June 2015, at about 1500 Central Standard Time (CST), a Robinson R22 helicopter, registered VH-RBO, took off from a property near Daly Waters, Northern Territory, for a local flight, including inspection of bores on the property. The temperature was about 30°C with a south-easterly wind at 10-15 kt. After flying for about 20 minutes, the pilot, who was the sole occupant of the helicopter, landed to open a gate and put out a bucket of chain. The pilot selected the governor off, then exited the helicopter, leaving the engine running. The pilot then re-boarded the helicopter and took off.

After a further 5 to 10 minutes of flight, when at about 100 ft above ground level, and an airspeed of 40 kt, the pilot conducted a turn to the south. The low rotor revolutions per minute (RPM) warning horn sounded. The pilot immediately wound the throttle fully open, and lowered the collective^[1] to try to regain rotor RPM. The helicopter continued to descend. The pilot attempted to flare^[2] the helicopter when low to the ground. The main rotor blades collided with tree branches. The helicopter landed heavily, the skids dug in to the soil and the helicopter rolled onto its side.

The pilot sustained minor injuries and the helicopter was substantially damaged (Figure 1).

Figure 1: Accident site

Source: Aircraft engineer

Pilot comments

The pilot was unsure whether the engine was running normally when the helicopter touched down, and could only recall hearing the rotor RPM warning horn. The pilot was focused on looking outside the helicopter at an earth tank and a mob of cattle and not inside at the instruments. The pilot reported that it was their normal procedure to switch off the governor when exiting, and then select it back on when returning to the helicopter. However, on this occasion, the pilot could not recall specifically switching it back on.

Engineering report

A 100-hourly maintenance inspection and the replacement of two cylinders was completed on the morning of the accident flight. The pilot and an engineer then conducted a flight of about 20 minutes duration, during which the helicopter performance and all indications were normal.

An initial inspection of the helicopter following the accident found the following:

• No oil on the exterior of the engine or helicopter to indicate any oil line failure.
• Fresh oil droplets on the right skid and a smear on the right strut. Immediately adjacent to the helicopter there was a small oil spill on the ground, probably from impact damage.
• The fuel tanks still contained a substantial amount of fuel, which was leaking out down the mast.
• Fuel from the drum was checked with no contamination found. The helicopter was fuelled with premium unleaded petrol.
• Icing was found to have been unlikely.
• Drive belts were still on and intact.
• Clutch engagement position appeared normal.
• Main rotor blades were buckled and damaged, partly from falling onto a fence, but were still attached to the hub.
• Main rotor blades were evidently not turning fast when they hit the ground.
• The engine was running on impact.
• The governor switch was in the OFF position. The engineer turned on the master switch, and the governor light (GOV OFF) illuminated.

The engineer removed the main rotor blades and rolled the helicopter upright. The bottom spark plugs were removed and cleaned of oil, and then replaced. The engineer then started the engine and a positive oil pressure indicated. The engine was ground run for about 30 seconds and the magnetos, temperatures and pressure indicated normally.

Safety message

The Robinson Helicopter Company Safety Notice SN-24 states that rotor stall due to low RPM causes a very high percentage of helicopter accidents. These mostly occur close to the ground during take-off and landing. Safety Notice SN-10 reminds pilots to have their ‘reflexes conditioned so they will instantly add throttle and lower collective to maintain RPM in an emergency’.

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1. A primary helicopter flight control that simultaneously affects the pitch of all blades of a lifting rotor. Collective input is the main control for vertical velocity.
2. Final nose-up pitch of landing aircraft to reduce rate of descent to approximately zero at touchdown.