Aviation AO-2015-003

The cracked rotor blade
The cracked rotor blade
Robinson R44, Main rotor blade failure, Waikaia, 25 January 2015
Status: Current
Occurrence Date: 
25 January 2015
Report Publication Date: 
18 January 2018
Jurisdiction: NZ
While spraying gorse near Waikaia on 23 January 2015, the pilot of a Robinson R44 helicopter felt an unusual and significant vibration. He landed immediately and discovered a large crack in the lower skin of one of the main rotor blades.

The crack’s origin contained features characteristic of metal fatigue. The crack had started in a radius in the blade trailing edge, known as the ‘chord length transition’. Flight testing by the manufacturer found that the stress in this area was higher than had been thought.

As a result of this incident, the manufacturer developed a modification for main rotor blades in service, and made design changes to new main rotor blades for R44 (and R66) helicopters.

The helicopter had been operated primarily for agricultural flying, usually at or over the maximum power settings. The use of a ‘flick turn’ while operating the helicopter close to the maximum all up weight very likely subjected the main rotor blades to additional high stresses not envisaged by the manufacturer.

The helicopter was not designed specifically for agricultural flying. The manufacturer had therefore not been required to consider the increased loads and cycles of agricultural flying when calculating the service life of the rotor blades.

Two safety issues were identified during the inquiry. The first is that aircraft design organisations do not currently have to consider whether proposed major modifications to aircraft will significantly alter the use of the aircraft or will adversely affect the service life of any component. The second is the routine exceedance by operators and pilots of published limitations for aircraft, an issue that has been previously raised by the Commission.

The Commission recommended that the Director of Civil Aviation:
consult the original equipment manufacturer when considering a modification or supplemental type certificate, which, if approved, could result in any aircraft being used in a way that is significantly different from that which the manufacturer originally modelled and used as the basis for determining component fatigue lives and the aircraft maintenance programme.

The key lessons identified during the inquiry into this occurrence were:
- metal fatigue occurs continuously in dynamic components. A fatigue crack can lengthen very rapidly and the component lose its structural strength. If an unusual or severe vibration develops in flight, the pilot should land immediately and have the helicopter inspected before further flight
- the key to minimising stress in dynamic components is to fly conservatively, especially when operating close to the flight manual weight, speed and power limits. The Commission has noted in previous inquiry reports that operating an aircraft outside the flight manual limitations significantly erodes the safety margins factored into the service lives of components and can quickly lead to an early catastrophic failure
- aggressive and unusual helicopter manoeuvres, such as flick turns during agricultural operations, can prolong the period spent inside the ‘avoid curve’, where a safe autorotation may not be possible in the event of an engine failure. Operators should require their pilots to fly in accordance with established industry guidelines and to continually demonstrate their commitment to a strong safety culture
- operators and maintainers of aircraft that are subjected to cycles or flight profiles that are significantly different from those envisaged by the manufacturers when the aircraft were certificated should consider implementing reduced intervals for component inspections and earlier component replacement times.
Related Safety Recommendation(s):
Waikaia (-45.7167,168.8690) [may be approximate]