On 3 July 2010, an Agusta Westland AW 139 (B-MHJ) being operated by East Asia Airlines, on a scheduled Visual Flight Rules (VFR) passenger flight (EA206A) from Hong Kong to Macao was climbing through approximately 350 feet amsl over Victoria Harbour in day Visual Meteorological Conditions (VMC) two minutes after take off when the tail rotor assembly fell off. The crew declared a MAYDAY and accomplished a successful ditching. Three of the 11 passengers sustained minor injuries, the others and the two pilots were uninjured.
The helicopter floating in Victoria Harbour (Reproduced from the Official Report)
An Investigation was carried out by the Civil Aviation Department of the Government of the Hong Kong SAR. Recorded data relevant to the Investigation was recovered from the Multi-Purpose Flight Recorder (MPFR) and the 2 hour Cockpit Voice Recorder (CVR).
It was noted that the two pilots both held ATPL(H) licences issued by the Civil Aviation Authority of the Macao SAR and had considerable helicopter experience although relatively little of this had been on the AW139, the 45 year old commander, who had been PF for the accident flight, having only 377 hours on type.
It was established that passing 350 feet, both pilots had heard "a loud bang" from the rear of the helicopter after which airframe vibration had occurred. The commander simultaneously found that he had no authority on the pedal controls and deduced that the tail rotor of the helicopter had failed. He immediately put the helicopter into autorotation and instructed the First officer to shut down both engines. A MAYDAY call was made on the Company frequency and a controlled ditching was achieved in a level attitude with a low forward speed at touchdown. On contact with the water, all four emergency floatation bags inflated automatically.
The helicopter initially remained floating in a slightly nose down attitude and both pilots exited the flight deck via the emergency exit windows on their respective external doors. The commander then opened the right side passenger door from the outside and both pilots assisted all the passengers to evacuate from the helicopter. Everyone was rescued from the water "within minutes of the accident" by the first two vessels on the scene, a nearby fishing boat and a commercial harbour pilot boat. All the passengers were taken to hospital for medical examination and six were then admitted to be treated for minor injuries such as bruising or dizziness before being discharged later the same day.
It was concluded that the evacuation had been "conducted in a timely manner without delay, allowing all the passengers to evacuate from the helicopter before it capsized" which occurred about 18 minutes after the ditching, a time confirmed to be compatible with certification requirements. The capsize was due to the failure of the right forward floatation bag. The buoyancy provided by the partially-deflated left forward floatation bag was sufficient to ensure that the overturned helicopter remained in a floating condition. It was concluded that it was the slightly out of balance condition of the helicopter on the water caused by the weight of the missing tail that had resulted in overload and damage to the two forward floatation bags.
It was noted that since the flight between Hong Kong and Macao on which the accident helicopter was used did not go more than 10 minutes flying time from land at normal cruise speed, the carriage of life jackets was not required under the prevailing regulations. It was observed that these regulations were in line with ICAO Annex 6 Part III as effective at the time. However, the Operator had chosen to equip the helicopter with both crew and passenger life jackets sufficient for the maximum occupancy of 14.
It was noted that the AW139 tail rotor had four glass-fibre composite blades designated in sequence in the direction of rotation as the Red, White, Blue and Yellow blades. Each of these blades was connected to the tail rotor hub via a spherical elastomeric bearing. The pitch of the tail rotor blades was adjusted through a linkage attached to a pitch control arm bonded to the root of each blade. An elastomeric bearing was used to transmit loads between the hub and each blade whilst providing some freedom for the blade to move in flap, in lead-lag, in pitch or any combination thereof. Blade damper attachment lugs allowed dampers to each blade. These features are shown in the diagram below.
It soon became evident that the loss of the tail rotor had been precipitated by functional failure of the assembly. As a result, "fracture analysis of vertical fin broken parts and (in particular) the separation of the White blade at the root area became the key elements" in the Investigation. However, when the broken portion of the White blade could not be recovered, it became clear that the evidence from analysis of the small portion of the White blade remained and other available wreckage parts would be critical.
A general view of tail rotor assembly (Reproduced from the Official Report)
During the Investigation, two similar AW139 helicopter accidents occurred:
- On 2 May 2011, an AW139 Helicopter in Qatar had just completed the Engine Power Assurance check and was about to taxy for a passenger flight when the commander heard a loud bang from the back of the helicopter and severe vibration was sensed through the pedals. It was found that the tail rotor had separated from the tail fin and dropped to the ground. One of the tail rotor blades was subsequently found detached and recovered from some distance away. The engine was shut down, a fire in the tail fin was promptly extinguished by ground personnel and the occupants left the helicopter.
- On 19 August 2011, an AW139 helicopter in Brazil was climbing through approximately 1800 feet when a hydraulic system failure was reported. The helicopter subsequently crashed into the sea and all on board were killed. Part of one of the tail rotor blades was found to have broken off between the blade root and the fork.
It was noted that AgustaWestland's initial response to the Hong Kong accident was that "no corrective actions…had been considered necessary" in respect of the AW139. But after the Qatar accident occurred 10 months later, that position began to be reconsidered.
By this time, examination of samples from the Blue and Yellow blades plus the remaining section of the White blade had indicated that these blades "did not comply fully with AgustaWestland's specifications in the root end area" with "manufacturing discrepancies including undersize and high level of voids of the upper and lower straps of the blade samples" were found. Reduction of torsion box thickness was also noted from the remaining portion of the White blade. Additionally, dis-bonding of the pitch control arm was found.
In the light of all these findings, the Investigation had concluded that the manufacturing process for AW139 tail rotor blades should be reviewed and that "further static, fatigue, dynamic and aerodynamic tests and analyses would be required to determine the cause of the breakage of the White blade and the subsequent detachment of the tail rotor and tail gearbox assembly". Since this action would clearly require reference to "proprietary and confidential manufacturing, design and certification data" held by AgustaWestland, ENAC (as the Competent Authority responsible for the Production Approval held by AgustaWestland) and European Aviation Safety Agency (EASA) (as the Type Certification Authority) which was not available to the Investigation, two corresponding Safety Recommendations (see below) were issued to ENAC and EASA respectively on 21 November 2011.
In responding to these Safety Recommendations and to later requests, it was noted that these agencies and AgustaWestland had subsequently provided the information necessary to complete the Investigation.
A joint review by EASA/ENAC of the interim findings of the Investigation and information from those into the other two accidents confirmed what were considered to be the main factors underlying the documented tail rotor failures:
- Manufacturing defects inside the straps (waviness, porosity and delaminations) at maximum acceptable values as a result of the assessment variability.
- Torsion box thickness reduction.
- Pitch control arm disbonding caused by over shimming of elastomeric bearing installation and manufacturing quality lapses which had the potential to cause an increase in the level of strain in the area of the failure.
This review also made the following significant findings:
- Other production issues (under dimensions of the strap cross sections, dislodging of the filler between the two straps), although representing non conformities to the drawings, had no direct effect on the failure pattern.
- In the production specification of the straps, there was no specific limitation for the presence of combined defects (combination of waviness, porosity and delaminations).
- The integrity of the torsion box could not be checked in an effective way after the production phase
At the conclusion of a meeting with AgustaWestland lasting from 8 - 13 September 2011, EASA identified the following actions as necessary to reducing the risk of further tail rotor failures:
- A review of the current contents of AgustaWestland Technical Specification STAP106, in relation to the maximum allowable defect and inspection criteria given the occurrence of in-service events.
- The establishment of an inspection procedure for in-service tail rotor blades.
- An internal review of the production inspection records of the in-service blade straps, aiming at identifying any blades that might have been delivered with strap defects in excess of those acceptable per AgustaWestland Technical Specification STAP106 as result of possible variability related to the comparative radiographic assessment.
- Taking into account the flight cycles accumulated by the blade failure in the Brazilian AW139 accident, an analysis of in-service occurrences, to confirm the installed service limit for the blades of the earlier of 1500 cycles / 600 hours introduced by an EASA AD issued on 25 August 2011 which mandated the provisions of AgustaWestland Alert Technical Bulletin BT139-265 of even date.
On other matters, the Investigation noted that:
- The prompt response of the commander and his skills in controlling the speed and attitude of the helicopter contributed to a controlled touchdown of the helicopter during ditching.
- The benign prevailing weather and water surface conditions had no bearing on what happened.
- In the light of the transmission of the MAYDAY call on Company Frequency rather than to ATC, it was noted that the Operator had since changed its procedures to require that both COM radios should be on the appropriate ATC frequency "during the significant phase of departure, between take-off and levelling off in the cruise".
The Conclusion of the Investigation was that the failure of the tail rotor White blade had led to tail rotor imbalance which had created static overload of vertical fin structure at a magnitude high enough to sever the tail rotor assembly from the fin so that directional control of the helicopter was no longer possible.
The Investigation formally documented the Causal Factors for the accident as:
- The failure of the white blade (which) was most probably caused by disbond of (the) pitch control arm and the reduction of torsion box stiffness at the blade root radii area which, when associated with manufacturing strap defects at maximum acceptable values in production specification, caused inter-laminar radial stress (ILRS) to exceed the maximum allowable limit, resulting in matrix delamination onset and then propagation under inter-laminar shear stress (ILSS) and complete failure of the blade.
- The reduction in torsion box stiffness at the white blade root radii area was most probably caused by manufacturing quality lapses and the lack of effective way for checking the integrity of the torsion box after production.
- Disbond of the pitch control arm could have been caused by over shimming of elastomeric bearing installation.
Safety Action taken during and as a result of the Investigation began after the Qatar accident and initially consisted of an EASA Emergency AD mandating an Alert SB issued by Augusta Westland relating to tail rotor blade inspections (36% of inspected blades were faulty) and the introduction of a new blade life limit. After the Brazilian accident, additional limits on blade life based on another Alert SB issued by AgustaWestland were mandated in a second EASA Emergency AD. Subsequently, AgustaWestland made a large number of changes to the Product Specification for manufacture of the tail rotor blades and finally introduced an improved blade which "fully restored the original tail rotor blade safe life".
It was noted that in respect of the blade failures which had occurred "the EASA Continuing Airworthiness Team concluded that there could be a possibility that tail rotor blades installed on AW139 fleet could contain defects and flaws in excess of those assumed during the static and fatigue tests performed for the certification".
Two Safety Recommendations were made during and as a result of the findings of the Investigation on 21 November 2011 as follows:
- that the Ente Nazionale per l'Aviazione Civile (ENAC) should, jointly with AgustaWestland, review the manufacturing process of the AW139 tail rotor blades to determine the causes of the discrepancies identified in the QinetiQ Report and evaluate their effects.
- that the European Aviation Safety Agency should require AgustaWestland to perform static, fatigue, dynamic and aerodynamic tests and analyses on AW139 tail rotor blades so as to minimise the possibilities of tail rotor blade failure which could have been caused by one or the combination of these effects.
It was noted that a number of "Useful or Effective Investigation Techniques" in the area of forensic engineering had been employed during the central airworthiness element of the Investigation in order to fully understand the failure characteristics of the blade which fractured and so precipitated the accident:
- Optical Microscope - A type of microscope which uses visible light and a system of lenses to magnify images of small samples.
- Scanning Electron Microscope (SEM) - A type of electron microscope that images the sample surface by scanning it with a high-energy beam of electrons in a raster scan pattern. The electrons interact with the atoms that make up the sample producing signals that contain information about the sample's surface topography, composition and other properties such as electrical conductivity.
- Image Analysis - The extraction of meaningful information from images; mainly from digital images by means of digital image processing techniques.
- 3-D X-ray Tomography - A quick and non-destructive method to produce three dimensional images that correspond closely to serial sections through an object. Sequential contiguous images are compiled to create three-dimensional representations that can be manipulated digitally to perform efficiently a large array of measurement and visualization tasks.
- Differential Scanning Calorimetry (DSC) - A thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference is measured as a function of temperature. Both the sample and reference are maintained at nearly the same temperature throughout the experiment. Generally, the temperature program for a DSC analysis is designed such that the sample holder temperature increases linearly as a function of time. The reference sample should have a well-defined heat capacity over the range of temperatures to be scanned.
- Burn-off Test - A method to determine the fibre volume of the composites being investigated.
The Final Report was completed on 2 June 2014 and published the following month. It confirmed that "ENAC and EASA, along with Agusta Westland, have taken necessary safety actions on Safety Recommendation 2011-3 and Safety Recommendation 2011-4 respectively" and that "all the safety actions have been accomplished and no further safety actions are envisaged."