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Thursday, June 1, 2006

Maintenance Safety


The two pilots and eight passengers were killed when a Sikorsky S-76A helicopter supporting offshore oil drilling crashed in the Gulf of Mexico. The National Transportation Board (NTSB) investigated and concluded that the pilots failed to arrest the helicopter's descent to the water, for unknown reasons. One of the factors complicating the investigation was the lack of a good cockpit voice recording (CVR), as evidenced by static on the recording and rather few and cryptic utterances from the pilots, such as, "I dunno we have enough (unintelligible words) ... `bout another hour of (flight or flying) to do (unintelligible words)."

Inadequate preflight and routine maintenance of recorder remain a concern, despite previous NTSB recommendations. The recently-released accident report says:

"The Safety Board concludes that preflight testing and maintenance checks of an aircraft's CVR are both necessary to ensure that audio data from CVR recordings are adequate. Therefore, the Safety Board believes that the FAA should require all operators of aircraft equipped with a CVR to (1) test the functionality of the CVR before the first flight of each day as part of an approved aircraft checklist and (2) perform a periodic maintenance check of the CVR as part of an approved maintenance check of the aircraft. ... The periodic maintenance check of the CVR should include an audio test followed by a download and review of each channel of recorded audio. The downloaded recording should be checked for overall quality, CVR functionality, and intelligibility." (NTSB accident report No. NTSB/AAR-06/02)


"The Board believes that this urgent recommendation, if acted upon quickly, will go a long way to prevent a catastrophic failure of the rudder," declared NTSB Acting Chairman Mark Rosenker. The occasion was the issuance March 24 of recommendations emanating from the composite rudder on a Federal Express airplane, an Airbus A300-600, that was damaged during maintenance on November 27, 2005.

In addition to the maintenance damage, examination found a "substantial area of disbonding" between the inner skin and the honeycombed core. Moreover, hydraulic fluid was found in the area, which can lead to "progressive disbonding," and that this disbonding can spread in flight.

The Board is concerned that the loss of the tailfin on an Air Transat A310, which has the same rudder/tailfin design, may be related. On March 6, 2005, that airplane lost almost all of its rudder and fin in flight, with only a nub of the tailfin remaining on the airplane. As a result of that incident, Airbus issued an All Operator Telex (AOT) ordering inspections, which was backed up by the force of an airworthiness directive (AD) from the FAA.

In this latest incident involving the Federal Express A300-600, Airbus also issued an AOT, calling for inspection and repair of the rudder within six months.

Not fast enough, and not thorough enough, says the NTSB. The inspections should be done "immediately," the NTSB told the FAA (see www.ntsb.gov/recs/letters/2006/A06_27_28.pdf). Furthermore, the tests called for in the AOT "did not include the areas in which the disbonds were found on the incident rudder," the NTSB said. "More recent examinations have disclosed that the hydraulic fluid can exist along the edges of the rudder's inner surface along with an accompanying area of substantial disbonding and that the inspection specified in the AOTs cannot detect the presence of the hydraulic fluid or the disbanding along the edges."

Moreover, the NTSB said, "Although the Safety Board concurs with the procedures outlined in [the] AOTs ... it is concerned that allowing an undetected hydraulic-fluid-induced disbond to exist for 500 flights, without supporting analysis or tests to better understand the safety risks, is unacceptable."

About 400 aircraft worldwide are affected.

The FAA issued AD 2006-07-13 on March 30, requiring that if a temporary repair is done in 2,500 flights that a permanent repair be made within 1,500 flights after that time. The AD is effective on receipt, but it seems to fall short of what the NTSB has called for (but, then, the AD makes no mention of the NTSB letter and the sense of urgency expressed therein).

Some experts believe poor bonding was also apparent on the rudder recovered in the 2001 loss of an American Airlines A300-600 after takeoff from New York City, although the NTSB attributed this crash to inappropriate rudder inputs from the pilot. The vertical stabilizer held to more than ultimate load before breaking away from the airplane. Without disputing this finding, some composite experts have observed bonding problems on the rudder skins from this accident are very similar to those observed in the Federal Express incident. They note that the skin separated cleanly from the core in many areas, indicating an "adhesive" failure mode. These experts claim that the correct failure mode is called "cohesive," where the bond is strong enough that the honeycomb core fails. While there is some cohesive failure on this rudder, 80-90 percent of the exposed failure region appears to be adhesive, these experts maintain. So the latest problems with the Air Transat A310 and the Federal Express A300-600 are likely to fuel the debate over the cause of the American Airlines A300-600 crash, despite the existence of a final investigative report pointing to pilot error that overloaded the structure.

Suffice to say, whenever something goes wrong, there are three issues - root cause, disposition and corrective action. Some thoughts:

  • Root cause: damage in the shop appears to have initiated delaminations in the Federal Express case, and actuator hydraulic leaks seem to have exacerbated the problem. The root cause of the Air Transat tailfin loss has yet to be determined.

    At the same time, delaminations (or disbonding) have affected other Airbus aircraft. For example, AD 2001-09-03 against all A330 models was issued "to prevent failure of the bonds of the spars, the rib, and the stringers of the vertical stabilizer spar box to the skin, which could lead to reduction in the structural integrity of the spar box."

    There are numerous ways bonding can go wrong (poor surface preparation, poor adhesive material, incorrect bonding tools, processes, maintenance, etc.). What's evident at this point is that what went wrong and how or if it's been fixed has not yet been told. The Air Transat tailfin loss is still under investigation by the Transportation Safety Board of Canada, and the FAA's actions stemming from the Federal Express case are described as interim in nature.

  • Disposition action: Airbus seems to be moving in the right direction but government prodding is still helpful. Consider that when the Air Transat incident occurred, Airbus called for fleet wide tap tests (a form of non-destructive inspection, or NDI) but only on external surfaces. At least the inspections were fleet wide and used a calibrated measurement method. On the other hand, the inspections were of the outer, not the inner skins. In a sandwich structure, experts say the inner skin is equally important to structural integrity as the outer skin. Now, a year later, Airbus and the NTSB are recommending the inner skin also be inspected fleet wide. Interestingly, the NTSB estimates the inner skin inspection only takes two hours. If so, this new inspection mandate raises a question: was the risk of having 400 aircraft flying in service for a year with possibly damaged rudders worth saving a few hours downtime?
  • Corrective action: according to composites experts, there are two aspects here:

    a) What procedures need to be changed, or have been changed, for better quality assurance of bonded structure?

    b) Given the "surprise" of undetected damaged aircraft parts, should NDI be performed on a recurring (versus emergency) basis, and more frequently, than is presently the case? The NTSB seems to be leaning in that direction.


    From the UK's Air Accidents Investigation Branch is this report, which indicates that the accident resulted from the failure to make service bulletin compliance mandatory. It concerns a Socata general aviation accident at Nottingham Airport:

    "During a touch and go landing, as power was applied a propeller blade detached. The resulting imbalance caused both the crankshaft to fracture (allowing the propeller to be released) and the engine to partly separate from the structure. Metallurgical examination indicated the presence of fatigue in the propeller hub. The location and nature of the fatigue was similar to that described in an existing Service Bulletin, however that document has not yet been classified as mandatory." (See www.aaib.gov.uk/publications/special_bulletins/s2_2006_socata_tb10_g_bnra.cfm)

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