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Recommendation issued to: United States Federal Aviation Administration

Recommendation details
Output No: R20030186
Date issued: 17 September 2003
Safety action status:
Background:

The Robinson Helicopter Company model 22 (R22) helicopter had been returned to service following maintenance during which the engine had been overhauled and a main rotor blade balance problem had been corrected. The first flight following this maintenance was planned as the second sequence in a training syllabus and was to consist of climbing and descending demonstrations and practice sequences. Witnesses reported observing and hearing the helicopter operating normally and it appeared to be flying straight and level. One witness reported what appeared to be a main rotor blade falling from the helicopter shortly before it descended in an out of control condition. The helicopter impacted the ground inverted and both occupants received fatal injuries.

Examination of the wreckage found that the debris trail was consistent with the loss of a main rotor blade in flight. The failed blade, the remaining blade and the main rotor head were recovered for laboratory examination.

The total time in service (TTIS) of the helicopter was recorded as 1986.2 hours at the time of the accident. Investigation to date has revealed some minor arithmetic errors in the recording of the time in service. The TTIS of the helicopter has been recalculated as 2009.5 hours. However, the investigation into establishing the history and verifying the actual operating hours of the helicopter is continuing.

The failed blade was examined in the ATSB laboratories and exhibited features consistent with a fatigue failure of the main rotor blade root fitting, in the counterbore of the bolt hole in the area of rotor station 10.35 (inboard bolt hole). This area was the same site as two previous in-flight rotor blade failures on R22 helicopters in Australia. It is also the same site that has been identified by the manufacturer as being the critical location for fatigue cracking during fatigue testing.

Microscopic examination of the failed surface revealed evidence of corrosion at the site of the fatigue crack initiation point. Significant areas of breakdown of the adhesive bonding between the skin, the end of the spar and the rotor blade root fitting were also observed. There was no evidence of cracking on either the upper or lower rotor blade skin in the area adjacent to the fatigue crack. Examination of the other blade from the accident helicopter revealed similar disbonding of the end of the spar, although to a lesser extent.

Six other R22 main rotor blades were obtained by the ATSB for the purposes of examining the adhesion between the end of the spar, the root fitting and upper and lower skins. A summary of examined blades by serial number and TTIS is listed below.

Blade Serial Number TTIS (approx)
S/N 11303A REV AH 1200 hrs
S/N 7481C REV AE 1693 hrs
S/N 11277B REV AH 200 hrs
S/N 4480 REV AC 1400 hrs
S/N 8411B REV AG 1700 hrs
S/N 8414B REV AG 1700 hrs
S/N 6294A REV AD 2009 hrs *
S/N 6283A REV AD 2009 hrs **


* Failed main rotor blade from accident helicopter
** Undamaged main rotor blade from accident helicopter


The six other blades exhibited similar disbonding (to varying extents) of the adhesive in the area of the end of the spar and the rotor blade root fitting. One example extended past the first and second bolt holes in the root fitting.

Incomplete adhesive filling, in the form of an elongated void of the gap that exists between the edge of the spar, the root fitting and the skin, was also observed in two blades.

The disbonding of the adhesive allows moisture, salts and other contaminants/debris to enter the area. Once corrosion forms in this critical area, initiation of fatigue cracking is likely to be accelerated.

The United States National Transportation Safety Board, the United States Federal Aviation Administration, the Australian Civil Aviation Safety Authority and the Robinson Helicopter Company have all been provided with information on these findings from the investigation to date.

Output text

Safety Recommendation

The Australian Transport Safety Bureau recommends that the United States Federal Aviation Administration, in conjunction with the manufacturer of the helicopter, the Robinson Helicopter Company, conduct a review of a representative sample of main rotor blade root fittings to establish the integrity of the adhesive bond in the spar to root fitting joint. The review should establish the extent of the loss of adhesion and the extent to which corrosion has infiltrated in the region of the inboard bolt hole of the blade root fitting. If possible, where disbonding is discovered, the operating history and in-service flight spectrum of the helicopter and the environmental conditions under which it operated should also be assessed.

When completed, the results of the review should be forwarded to the ATSB for analysis as part of the ongoing accident investigation.

Initial response
Date issued: 08 March 2004
Response from: United States Federal Aviation Administration
Action status: Monitor
Response text:

We offer the following response to the subject recommendation from the Australian Transport Safety Bureau (ATSB) regarding the failure of a main rotor blade (MRB) on a Robinson Helicopter Company (RHC) Model R22 helicopter:

The FAA released a Special Airworthiness Information Bulletin (SAIB) SW-04-36 on December 17, 2003, that recommends actions that can be taken by the operator to maintain the airworthiness of the MRBs. One of the recommended actions in the SAIB is to accomplish the inspections contained in RHC Service Letter SL-21 A, "Main rotor blade sealant and filler cracking," dated May 31, 2002. The service letter details inspections for cracks of the blade root area without damaging the root fitting. The FAA has determined that more in-depth inspections of the root fittings could result in damage to the blade and increase the possibility of cracks after the blade is returned to service.

The MRB inspections by the National Transportation Board (NTSB) have not resulted in an effective inspection or test to detect adhesive failure. The NTSB investigation is not complete, and additional steps remain to be taken. Metallurgical testing will be conducted on the unidentified substances found on the blades. The analysis will also look at those areas where the metal had an unusual corrosion like appearance. For the blades where the adhesive failure extended to the bolt hole and where the white crystalline deposits were found near the bore, the examination will include sectioning the root fittings through the inboard bolt hole to look for evidence of corrosion pitting and/or cracks. Blade root fitting specimens will also be sent to the adhesive manufacturer for evaluation of the adhesive.

RHC plans an on-going sampling of returned MRBs to look at the spar to root fitting joint so that a wider sampling of blades may be generated. Emphasis will be placed on examining the R44 blades (the problem area is completely enclosed on the R44 blade design) to determine if the design differences between the Model R44 and R22 blades impact this adhesive failure phenomenon. This is particularly important since the new R22 blade design for the Model R22, which has been FAA approved, incorporates the closed root fitting design from the R44 blade.

RHC has also modified the MRB to allow cracks to propagate more slowly, such that if the blade is flown beyond the service life and a crack develops, the operator will have more advanced warning prior to failure. The root fitting of the R22 MRB was redesigned to be more robust in the root fitting area. The new blades, part number (P/N) A016-4, will be the only new blades available and will eventually replace all of the older style blades. The FAA has also been working with RHC to determine if a new calendar life limit, in addition to the total in service time, for blades, P/N A016-1 and -2 is necessary.

Additionally, RHC has corrected the manufacturing procedure that allowed the mechanics to cause scratches during adhesive cleanup. Any R22 MRB produced after November 2001 should be free of these abrasion scratches. The depth of the shot peen layer i3 not in question, and the qualify control system at RHC has been reviewed and determined to be acceptable.

Also, the FAA has asked RHC to make a study of cattle mustering maneuvers to determine if they are causing high stresses that would cause premature fatigue failures. So far, it appears that these maneuvers should not cause excessive failures. There is interest in gathering information on different methods of mustering to determine if some operations are more severe than others.

Civil Aviation Safety Authority (CASA) has indicated previously that some operators have been under reporting MRB operational hours. We understand that the three accidents, which precipitated the CASA AD's, involved helicopters used in cattle mustering operations. All three aircraft experienced blade failure due to a fatigue crack in the first bolt hole outboard of the blade root, which is the location of a normal fatigue crack, due to operation after the 2200-hour retirement limit. Evidence shows that operators in Australia in the past have routinely flown beyond retirement life limits. Two of the three accident ships have records indicating they were operated past the life limit, and the third ship has circumstantial evidence that indicates that the blades were in service past the life limits. To help resolve the problem, RHC published Safety Notice SN-37 in the R22 Pilot Operating Handbook, which warns pilots not to exceed published service lives.

We do no plan any further action and recommend that this recommendation be closed.

ATSB response:

I refer to the recent FAA response to ATSB Recommendation R20030186, which was issued on 17 September 2003 following a main rotor blade failure on a Robinson Helicopter Company model R22 helicopter.  The FAA has assigned FAA recommendation number 03.233 to the response.

The background to the recommendation clearly indicates that the initiation point of the fatigue failure was from corrosion and not fatigue that would be induced by exceeding published retirement time-in-service.  While previous failures have been associated with exceedances of retirement times, and there may be industry rumours and anecdotal evidence that this is a widespread practice within the Australian Mustering industry, detailed investigation of this accident to date has not revealed any evidence of retirement life exceedance.

The Special Airworthiness Information Bulletin (SAIB) issued by the FAA on 17 December 2003 does not provide any additional information for operators to assist them in determining the airworthiness of the main rotor blades.  In the subject main rotor blade, which failed in this accident, none of the information provided in the SAIB, nor the RHC service letters referenced in it, would have allowed maintenance personnel to detect the crack.

The recent accident in Israel in which an R22 helicopter sustained what appears to be an identical failure of the main rotor blade, only highlights that the mechanism of failure is not confined to helicopters operated in a mustering environment.  The Israeli helicopter was previously operated in the USA before it was exported to Israel.  Preliminary information provided to the ATSB is that the main rotor blade had only been in service for approximately 2/3 of its published service life.

The common thread between the Israeli accident and the Australian accident is that immediately prior to the accident, both helicopters were inspected, by maintenance personnel, for main rotor blade vibration.  While RHC Safety Notice SN-39 addresses main rotor vibration, this vibration is now known to be at the end of the process of structural deterioration and may not provide sufficient time for a pilot to land the helicopter before the final fracture of the blade.

While the newly designed RHC main rotor blade may address the problem, it has only just entered production and it will take some time for all present A016-2 blades to be retired from the worldwide fleet.

The ATSB has classified the response to R20030186 as OPEN as it does not adequately address the problem highlighted in the recommendation.

The ATSB urges the FAA to again closely look at the problem of adhesive disbonding in the R22 main rotor blade root fitting and to what extent the disbonding permits corrosive compounds to enter this critical area.

The ATSB also urges the FAA to investigate methods of detecting cracks in the root fitting prior to failure.

The ATSB looks forward to your earliest reply.

Further correspondence
Date issued: 26 May 2006
Response from: United States Federal Aviation Administration
Response status: Closed - Accepted
Response text:

In response to the Australian Transport Safety Bureau (ATSB) letter of March 17, 2004, we have reviewed our memorandum of January 29, 2004, on the subject recommendation. The ATSB letter referred to the recent accident in Israel involving failure of a main rotor blade (MRB). The ATSB letter requested that we further look at adhesive disbonding in the MRB root fitting and methods for detecting cracks in the root fitting before failure.

Since our previous response, we have issued Emergency Airworthiness Directive (AD) 2004-06-52 on March 18, 2004 (copy attached). That AD was prompted by accidents in Australia and Israel involving failure of the MRB. The accident investigations revealed that cracked blades result in an increase in helicopter vibration. Following a track-and-balance of the blades, the vibrations return to normal for a short time and then slowly increase again until blade failure occurs. This prompted us to mandate replacing the blades if an abnormal increase in vibration occurs within 5 hours time-in­service (TIS) after the last track-and-balance. This AD also adds a retirement life of 10 years to the current 2,200 hours TIS retirement life.

Neither the FAA nor the manufacturer has identified an effective inspection or test to detect a crack or adhesive disbonding in the root fitting area before failure of the MRB. The FAA is taking part in investigating the failure of the MRB involved in the Israel accident. However, we believe the actions taken in the Emergency AD address the unsafe condition. If ATSB has suggestions for an inspection method, we would welcome any ideas and would evaluate the method with the manufacturer before putting it into practice.

 
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Last update 03 April 2012