Propeller/rotor malfunction

Before take-off the pilot had been briefed on the operation of the aircraft by the owner. The owner had suggested the pilot make a short flight along the strip or that if he had the "feel" of the aircraft he could complete a circuit before landing. After becoming airborne the aircraft initially flew about 25 feet above the strip before entering a shallow climb. At a height of between 100 and 150 feet the engine noise suddenly ceased and pieces of the propeller were observed falling from the aircraft. The aircraft continued straight ahead for a short time before commencing a turn to the left. During the turn, the left wing and nose dropped and the aircraft struck the ground in a steep nose down attitude. Inspection of the wreckage revealed that one of the three propeller blades had disintegrated. The blade had failed due to the progressive propagation of spanwise splits along its length. Similar spanwise splits were also found in the other two blades. The splits were found to have originated from radial cracking at the hub of the blade which had been caused by local overstressing of the wood in the area of the blade retainers. Also, the widely spaced power strokes of the four stroke engine fitted to the aircraft would have contributed to the spanwise splitting of the propeller blades. The propeller manufacturer does not recommend the use of this type of propeller with the engine fitted to the aircraft. No other defects were found with the aircraft that could have contributed to the occurrence. Following the propeller failure, the pilot inadvertently stalled the aircraft while attempting to manoeuvre it for landing.

Following the fitment of shock absorbers to the landing gear system, the owner arranged for the designer of the aircraft to carry out a test flight. At the completion of the test flight, when the aircraft was at about 100 feet agl on the downwind leg of the circuit, the pilot heard a loud bang. He closed the throttle and landed the aircraft. The aircraft overran the strip and the nose gear failed when it contacted rough ground. Examination of the aircraft revealed that one blade of the propeller had failed in tension overload due to overspeeding. The overspeeding was caused by the inadequate pitch of the propeller blade for the type of engine used to power the aircraft.

The original propeller fitted to the aircraft had been damaged in a landing mishap, and the owner/pilot ordered a replacement from the aircraft manufacturer. The new propeller was of different dimensions and weight to the original, and the owner sought and obtained the manufacturer's assurance that it was suitable for the aircraft type. A number of static and taxy trials were completed without problem. However, on the first take-off, the aircraft had reached a height of about 50 feet when the propeller failed. The aircraft pitched down and struck the ground. A subsequent examination of the propeller revealed that it had failed as a result of the applied centrifugal forces. It had been constructed in four laminations, using two types of wood. One lamination contained a large knot in the hub area, and it was probable that another lamination was affected by brittleheart. This weakness is characterised by minute compression failures in the walls of the fibres, induced by growth stresses in the particular tree. When the propeller failed the aircraft was at a critical stage of flight, and stalled. There was insufficient height available for the pilot to regain control before impact with the ground.

The aircraft was being test flown following an engine top overhaul. Shortly after lift-off there was a sudden and substantial loss of power. The pilot landed straight ahead but was unable to prevent the aircraft from running off the end of the agricultural strip and colliding with a fence. An examination of the propellor revealed the failure of the pitch change actuating pin for the no. 3 blade. The aircraft logbooks indicated the no. 3 blade was damaged in a previous incident and subsequently repaired. The reason for the failure of the no. 3 pitch change actuating pin was not positively established. However, it is suspected the pin was damaged in the previous incident and the damage not detected during the repair procedure.

The pilot had been advised that the aircraft propeller was defective and he indicated to witnesses that he intended only to conduct taxying. However, the aircraft was seen to become airborne and start to fly a circuit. On base leg the engine noise grew louder and the aircraft fell to the ground with the rotor stationary. Wreckage examination revealed that part of the propeller had separated in flight and the remaining propeller section had impacted the rotor following failure of the engine mounts due to excessive vibration.

Circumstances:

The pilot had been spreading Graslan, a herbicide, about five kilometres from an agricultural airstrip. The aircraft was being flown back to the strip at about 300 feet above ground level when suddenly, the engine developed a severe vibration. The pilot diagnosed a propeller blade failure and shut the engine down before it shook itself out of its mounts. The pilot chose the clearest area available for the forced landing, a sloping paddock covered in windrows consisting of recently bulldozed trees. The aircraft encountered one such heap of timber during its 60-metre ground roll. Post flight inspection found a 21-centimetre tip missing off one of the two propeller blades. Specialist metallurgical examination of the fracture surface indicated that the loss of the propeller tip was due to metal fatigue. The fatigue failure was initiated from an improperly blended mark caused by previous foreign object damage.

Significant Factors:

The following factors were considered relevant to the development of the accident:

1. Fatigue fracture of propeller blade.

2. Improper maintenance by maintenance personnel.

3. Forced landing on unsuitable terrain.

The owner pilot had recently recovered his aircraft with new fabric, and is reported to have taxied it up and down the strip for approximately one hour before taking off. About 15 minutes after the aircraft became airborne, a witness reported that he heard a loud bang, followed by the engine noise stopping, and then observed the aircraft spiralling toward the ground from an estimated height of 600 feet. The aircraft impacted in an inverted attitude in a creek. The investigation revealed that the propeller had fractured in flight. It is considered likely, that due to the severe vibrations associated with the loss of part of the propeller blade, the pilot would have shut the engine down. The reason for the propeller failure could not be positively established. A modification to the engine installation had raised the aircraft thrust line and the pilot had carried out a number of additional modifications which moved the centre of gravity rearwards. It is considered probable that when the engine was shut down, a rapid pitch up occurred. Before the pilot could regain control, the aircraft stalled and fell into a spiral dive, from which there was insufficient height to recover.

The pilot reported that as full power was applied on the take-off run, he heard a loud bang and the aircraft yawed to the right. He shut down the right engine and brought the aircraft to a stop. Initial inspection revealed that the right propeller assembly had failed at the hub. One blade had separated from the hub and struck the nose section of the aircraft. It was determined that the hub had failed from fatigue which had originated at the root of the blade retention thread. It was estimated that the fatigue crack had propagated over about 900 engine start cycles, and had occurred at some time after a regular inspection to check for this type of cracking.

The pilot was competing in the Hinkler Bi-Centennial Air Race. Earlier in the day the aircraft had made a forced landing near Roma due to a propeller oil seal failure. Following repairs by the owner/pilot the aircraft departed Roma for Charleville. At approximately 1740 hours the pilot made a PAN call and advised he was shutting the engine down due to another propeller oil seal failure. During the subsequent forced landing the pilot had to swing the aircraft left to avoid a tree. The aircraft hit an anthill, causing the landing gear to collapse. The pilot received facial lacerations. Investigation revealed that the moulded leather propeller seal had failed in the area of the moulding. It could not be determined if the second propeller seal failure was associated with the change of the seal following the first propeller seal failure. This accident was not the subject of an on-scene investigation.

While conducting a survey flight at Ambalindum Station (approximately 135 km north-east of Alice Springs, NT), the pilot of a Robinson R22 Beta helicopter, registered VH-HZB, noticed severe vibration of the main rotor assembly and cyclic controls. The pilot landed the helicopter immediately, and a subsequent inspection revealed that a length of aerofoil skin had peeled back from the leading edge on the underside of one of the main rotor blades.

A review of the current information surrounding Robinson helicopter blade debonds found a number of previous incidents involving a similar failure mechanism. Additionally, the issue of main rotor debond had been addressed by a number of airworthiness directives (ADs) issued by the Civil Aviation Safety Authority (CASA) and the Federal Aviation Authority (FAA), along with a number of safety alerts and service letters issued by the manufacturer.

Debonding of the main rotor blade skin was considered to have been influenced by extensive surface erosion observed around the leading edges of the blade. Additionally, the investigation found no evidence to suggest that the actions contained within the current CASA Airworthiness Directive addressing blade debonding issues (AD/R22/54) had been integrated into the helicopter's maintenance routine. The logbooks and maintenance release documents for the helicopter have since been updated to include reference to AD/R22/54 Amdt 3.