History of the flight
At approximately 1715 on 29 August 2003, the crew of a Eurocopter AS332L 'Super Puma' helicopter, registration VH-BHY, being operated on an offshore commuter flight from Karratha, Western Australia, reported feeling a sudden airframe jolt, followed by a pitch up, roll, and a left yawing motion. Finding they had lost tail rotor control, the crew stabilised the aircraft using pitch and roll control inputs, before declaring a MAYDAY to air traffic services. After assessing the helicopter's condition and vibration levels, the crew elected to return to Karratha where a run-on landing could be performed. The MAYDAY condition was downgraded to a PAN and, after assessing the helicopter's performance during a precautionary approach, a safe run-on landing was conducted.
The aircraft was carrying a flight crew of two and six passengers who were uninjured.
Damage to the aircraft
Damage to the helicopter was limited to the tail rotor pitch change assembly and the tail boom lower keel fairing, which had pulled out several attachment screws. During the initial post-incident inspection, the operator's ground maintenance personnel found the nut and lock washer disconnected from the servo end of the pitch change rod, allowing the rod to move freely within the servo body. The nut and washer were subsequently found in the tail structure beneath the tail rotor drive shaft. The rod (P/N 332A33-0043-00) had sustained circumferential gouging and scoring around the surfaces adjacent to the inboard side of the pitch change spider bearing (P/N 330A33-9903-20). The bearing itself showed evidence of gross mechanical failure, with break-up of the ball cage and dislodgement of the outboard and inboard seals. The outboard bearing retention nut and lock washer remained in-place and secure (figure 2).
Manufacturer Aerospatiale (Eurocopter)
Model AS332L 'Super Puma'
Serial Number 2129
Year of manufacture 1984
Total airframe hours 13,525 (approx, at time of incident)
Tail rotor assembly information
The Super Puma helicopter tail rotor control was effected by a hydraulic servo-actuator that applies control force to the tail rotor blades via a central shaft and spider assembly. A locking nut and lock washer secured the actuator to the shaft, assembled to a nominal 266 - 443 pound-inches ( 30 - 50 Newton-metres) dry torque. At the spider end, the connection was similar, with a nominal dry torque of 115 - 266 pound-inches (13 - 30 Newton-metres). The Super Puma tail rotor turns in a counter-clockwise direction when viewed from the right side of the aircraft. The securing nut on the servo end of the pitch change shaft had a conventional thread, while the nut on the spider end of the rod had a left-hand thread. Figures 3 and 4 illustrate the tail rotor assembly and pitch change shaft location.
The failed tail rotor bearing was first fitted to VH-BHY in June 2000, as part of a complete replacement tail rotor gearbox (TRG) assembly. The gearbox, including bearing, had 199 hours time since overhaul (TSO) when installed. Replacement of the pitch change bearing is normally carried out during gearbox overhaul, however documentation to confirm that action was not available to the investigation.
In June 2003, maintenance action was carried out on the gearbox in response to elevated lateral vibration levels recorded by the helicopter's integrated health and usage monitoring system (IHUMS). Subsequently, on 7 August 2003, the tail servo was replaced after the discovery of leaked hydraulic fluid inside the boot between the tail rotor hub and the pitch change spider. It was evident that the fluid had travelled from the tail servo, through the tail rotor drive shaft and into the boot, bringing the fluid into close proximity with the inboard end of the tail rotor pitch change shaft bearing. The gearbox and assembly had accrued 1,888 hours TSO at that time. During the weeks following the hydraulic leak, the pitch change shaft bearing was inspected as required by service bulletin SB05-00-29 Rev. 3 and accepted for further service. At the time of failure on 29 August 2003, the TRG and pitch change shaft bearing had operated for 1,959 hours since overhaul.
The bearing fitted to the tail rotor pitch change assembly on VH-BHY was a single race, fully sealed ball bearing, manufactured by SNFA, France. The bearing carried the following identifying marks:
330A33990320 8020141 SNFA FRANCE V80I24K14
ATSB laboratory examination of the bearing confirmed the mechanical failure and break-up of the bearing cage, allowing the circumferential movement of the balls relative to each other and the resultant development of abnormal race loading and frictional conditions (figure 5). The bearing internal surfaces were dry and in most places covered with an adherent black compound (figure 6) that was sampled for later analysis. There was no evidence of any viscous bearing grease remaining within the bearing confines. All rolling contact surfaces of the bearing showed bruising and particle indentation damage (figure 7), however there was no indication of spalling or other rolling contact fatigue type breakdown. None of the bearing components showed evidence of gross overheating or frictional seizure. The bearing cage showed gross levels of wear and metal loss in areas exposed to contact with the rolling elements (figure 8 ). Several fracture surfaces showed evidence of fatigue cracking. The external surfaces of the bearing outer race showed fretting corrosion and wear to the extent of seating within the pitch change spider assembly ( figure 9). There was no evidence of circumferential scoring or other indications of race rotation within the housing or on the bearing seat. Traces of light oil were found on the bearing seat. The odour and appearance of the oil were typical of hydraulic fluid.
The ATSB examined the tail rotor pitch change bearings from two other AS332L helicopters maintained by the same operator. Both of those bearings and their integral seals were found to be in serviceable condition and showed none of the characteristic indications of failure presented by the bearing from VH-BHY. The service lives of both examined bearings were comparable to the failed unit from VH-BHY. A sample of grease from one of the serviceable bearings was subject to a solubility test with a small quantity of hydraulic fluid recovered from the tail rotor servo fitted to VH-BHY at the time of the incident. With a small amount of manual agitation, the grease proved miscible within the hydraulic fluid, producing a liquid with a characteristic viscosity not appreciably greater than the original hydraulic fluid. Weighing the bearing before and after cleaning found the unit carrying 1.88 grams of grease, which the aircraft manufacturer indicated was a nominal quantity.
Bearing contaminant analysis
Samples of the remnant lubricant from inside the failed bearing, the uncontaminated grease from a serviceable bearing and the hydraulic fluid from VH-BHY were forwarded to an analytical laboratory to determine whether any trace of the hydraulic fluid could be detected within the material from the failed bearing.
Results from that analysis confirmed the presence of characteristic spectral peaks from the hydraulic fluid to exist within the remnants of the grease from the failed bearing. These peaks did not exist within the sample of uncontaminated grease from the serviceable bearing.
The investigation found that the loss of tail rotor control reported by the flight crew of VH-BHY occurred as a result of the disconnection of the tail rotor pitch change servo from the control rod. That disconnection was a direct result of a breakdown in the anti-friction properties of the tail rotor pitch change shaft bearing, allowing the rotational torque along the pitch change shaft to overcome the assembly torque and locking assembly of the servo end shaft nut. The rotating shaft subsequently unscrewed the nut, allowing it to drop into the tail structure from where it was recovered.
The tail rotor pitch change shaft bearing failure occurred as a result of the contamination and dilution of the grease lubricant, leading to the internal mechanical breakdown of the bearing cage and the partial seizure of the assembly. Testing showed that the bearing grease was contaminated by hydraulic fluid, which likely released from a leaking tail rotor servo-actuator unit identified and replaced 22 days before the incident. The bearing was inspected at the time of the leak discovery and was found to be satisfactory for further service. At that time, there was no requirement to change the bearing in the event of the leakage of hydraulic fluid into the bearing space.
The following factors were identified as significant to the development of the incident.
- The tail rotor control servo unit developed a hydraulic fluid leak, with some of the lost fluid entering the pitch change shaft bearing space.
- Migration of hydraulic fluid into the bearing diluted the grease, affecting the lubricant efficacy and producing accelerated wear and break-up of the bearing cage.
- The bearing grease was soluble in the hydraulic fluid.
- The bearing was allowed to remain in service following the discovery and rectification of the hydraulic leak.
- The pitch change shaft inboard (servo end) nut was a conventional ( right-hand) thread, allowing it to be loosened and unscrewed by torque from the rotating tail rotor drive shaft.
- Disconnection of the pitch change shaft from the servo actuator caused control of the tail rotor to be lost.
Local safety action
Immediately following the incident, the helicopter operator changed the tail rotor pitch change shaft bearings in all of the AS332L helicopters in its fleet. Subsequently, the operator issued Alert Message AM/332/03/008 to all of its local and international facilities, instructing maintenance personnel to immediately change the pitch change shaft bearings should they have been contaminated by hydraulic fluid, or should any doubt exist as to the bearing's condition.
On 5 November 2003, the aircraft manufacturer issued Information Telex 00000151, alerting all operators of AS332, AS330 and AS532 aircraft of the subject incident and instructing maintenance personnel to check the tail rotor pitch change bearing if any fluid leak is discovered at the tail rotor servo actuator. Subsequently, Alert Telex 00000158 was issued on 8 December 2003 requiring 10-hourly inspections for hydraulic leaks at the tail rotor boot and detailing a new mandatory maintenance procedure to be applied should a leak be discovered. Due to an error in the original Alert Telex, an erratum document (Alert Telex 00000166) was issued on 19 December 2003.
On 26 February 2004, the Direction Gnrale de l'Aviation Civile France ( DGAC) issued airworthiness directive (AD) No. F-2004-031, mandating the 10-hourly inspection of the AS332 tail rotor hub boot for evidence of hydraulic fluid leakage. If fluid leakage is discovered, replacement of the pitch-change shaft bearing is required. The Australian Civil Aviation Safety Authority subsequently issued AD/S-PUMA/51 on 26 February 2004, mirroring the requirements of AD F-2004-031 for aircraft in the Australian fleet.
thread, clockwise rotation to tighten.
2 Counter-clockwise to tighten.
3 The aircraft manufacturer identified the bearing grease as Aeroshell-33 universal airframe grease (MIL-PRF-23827C Type 1).
4 Specified as Aeroshell fluid 41 (MIL-PRF-5606H).
5 Anexus Laboratories, Bulleen Victoria. Report No. C1110 "Assessment of Bearing Grease for Possible Contamination".
|Date:||29 August 2003||Investigation status:||Completed|
|Time:||1803 hours WST|
|Location:||North Rankin A Platform, (HLS)|
|State:||Western Australia||Occurrence type:||Propeller/rotor malfunction|
|Release date:||24 May 2004||Occurrence class:||Technical|
|Report status:||Final||Occurrence category:||Incident|
|Highest injury level:||None|
|Aircraft manufacturer||Aerospatiale Industries|
|Type of operation||Charter|
|Damage to aircraft||Minor|
|Departure point||Karratha, WA|
|Departure time||1630 hours WST|
|Destination||North Rankin A offshore platform|