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Friday, September 1, 2006

Safety Case Studies

The fuel tank explosion May 4 on a Malaysian-registered B727 freighter has added weight to the imperative to inert all fuel tanks, according to a National Transportation Safety Board (NTSB). A July 20 letter was sent to the Federal Aviation Administration to buttress the NTSB argument, expressed earlier in March, that all fuel tanks, not just those with heat sources like air conditioning packs nearby, need to be inerted. The FAA has proposed inerting only of tanks with nearby heat sources but has yet to issue a final ruling.

The NTSB noted that the Transmile Airlines B727 wing explosion occurred on the ground at Bangalore, India. "The similarity of the 727 wing tank structure to nearly all transport-category airplanes raises the concern that similar conditions could result in a catastrophic in-flight failure on any transport-category airplane," the NTSB intoned. The Board noted that the FAA has issued dozens of airworthiness directives (ADs) to mitigate ignition sources in fuel tanks for a variety of aircraft, but in this case the airworthiness directive failed to eliminate the risk:

"The ongoing investigation of the May 4, 2006, event revealed that ignition occurred where pump motor wires had melted through aluminum conduit, exposing the fuel vapors to potential ignition energy. Conduit burn-through was identified as a possible ignition source in 1998 following similar failures [on aging B737 jets], and, in this most recent incident, the Transmile Airlines 727 had been modified to prevent the problem. The investigation has not yet determined why the corrective design change was ineffective."

The design change involved the addition of chafe-resistant sleeving to wires in the conduit. That evidently did not work, and there are questions as to whether the original or replacement wiring was used in executing the AD.


The National Transportation Safety Board (NTSB) is "displeased" with the amount of time it took the Federal Aviation Administration (FAA) to issue aging airplane rules, which require inspections and maintenance reviews to ensure that old airplanes are safe to fly.

The rules the FAA finally enacted in 2005 are also shot through with loopholes. Only turbine-powered aircraft carrying 30 or more passengers, and type certificated after 1958, are subject to the new rules, which also changed the compliance date from December 5, 2007, to December 20, 2010. The subject of the Safety Board's ire was the Dec. 19, 2005, crash of Chalk's Ocean Airways flight 101. The airplane was a Grumman Mallard G73T seaplane, certificated in 1947 and modified in 1981 to increase passenger capacity from 10 to 17 and to fit turboprop engines, replacing the piston engines.

Since the airplane carries fewer than 30 passengers and was type certificated before 1958, it would not require any of the structural or maintenance inspections so belatedly mandated by the FAA. The airplane lost its right wing shortly after takeoff from Miami, killing all 20 passengers and crew. The cause of the crash has not yet been formally determined, but preliminary examination of the wreckage points to structural fatigue failure, most probably abetted by an improperly drilled out hole in the lower cap of the aft wing spar, and aggravated by corrosion in the salt water environment in which the seaplane operated (see AM, February 2006, p. 22).

In a July 25 letter to the FAA, the NTSB said it wants the new rules to apply to all airplanes that carry nine or fewer passengers or are in regular scheduled cargo service, irrespective of powerplant type. The NTSB let stand the 2010 deadline and did not urge that the FAA stick to its originally proposed 2007 date. (Full text of the NTSB letter is at www.ntsb.gov/recs/letters/2006/a06_52.pdf)


Of 27 exhibits in the docket, fully 9 deal with maintenance. The exhibits amount to hundreds of pages of source material recently released by the National Transportation Safety Board (NTSB) as a result of its investigation into the fatal December 19, 2005, crash of a Grumman G73T seaplane.

The airplane, operated and maintained by Chalk's Ocean Airways, crashed after a waterborne takeoff, when the right wing separated from the airplane during initial climb.

The investigation is far from complete, and the documents in the docket do not contain any interpretive analysis, but the cold reading of the factual reports and the extensive photographic evidence from the wrecked airplane, and from sister seaplanes operated by the company, point to metal fatigue as the likely cause of the structural failure. Contributing to the wing failure was corrosion and improperly drilled rivet holes.

To be sure, Chalk's seaplanes were operated in a saltwater environment that made corrosion a real threat, and the accident airplane was more than 55 years old (so corrosion had plenty of time to work its nefarious ways). As a consequence, the aircraft had undergone numerous repairs, to include the installation of straps and doublers, to strengthen wings eaten away by corrosion.

The effort was not sufficient to prevent catastrophe. As the NTSB documents noted, "All of the fracture surfaces that were examined exhibited evidence of overstress and fatigue."

Examination of surviving G73T seaplanes at Chalk's exhibited similar weakening. For example, on one of these airplanes, according to the NTSB, "Three pieces of the center wing box structure were removed and all three exhibited significant corrosion. The wing attach structure was also examined and exhibited extensive corrosion."

Cracks of 6 to 14 inches were recorded on various seaplanes operated by the carrier. The findings regarding the accident airplane were not atypical. Here is an extract from the NTSB's interview notes with a designated engineering representative (DER) who worked with Chalk's an another seaplane:

"An inspection revealed fuel leaking out of a lower skin crack on the right wing. Mr. [John] Patterson's analysis showed that the crack in the right wing reduced the load carrying capacity of the right wing box by about 50%. His analysis showed that skin and stringer carry about 70% of the load and the spar caps about 30% of the load. Based on his analysis, Mr. Patterson recommended that Chalk's remove the wing for repair, however Chalk's informed Mr. Patterson that they could not remove the wing to perform the repair. Mr. Patterson then decided to pull away the fuselage skin locally in the area of the skin crack and replace the lower wing skin from centerline to near the nacelle. ... Mr. Patterson also observed many repairs on the rest of the wing that he suspected were quite old ... The crack on the right wing was about 22 inches long ... He stated that the Chalk's mechanics were unwilling to do the repair due to its difficulty and recommended Mr. Jorge Sepulveda. During the repair process, Chalk's also installed a new belt frame on the right side due to corrosion. Mr. Patterson noted that the rear spar lower chord was also corroded and it also became part of the designed repair."


In other words, repair was done over corroded metal.


There's more about structural repair and the general state of maintenance at Chalk's buried in the hundreds of pages of NTSB documents. The litany acquires a certain power through repetition. The object lessons are manifest: (1) Maintenance of structure must be scrupulous as any aircraft ages in service, (2) while the Chalk's seaplanes represent an extreme case, corrosion requires constant vigilance, (3) proper repair is essential, which was not always the case at Chalk's, where numerous instances of improper drill holes and incorrect sanding techniques (e.g., taking away too much metal) were noted, and (4) maintenance records were incomplete.

Aging airplanes need scrupulous care, and the maintenance records need to be complete. If either is shortchanged, and there is an accident, the NTSB will document the embarrassing deficiency.

Crash Underscores Need for Flight Control Check After Maintenance

The aircraft's ailerons were apparently misrigged, such that a pilot command to bank left would have the opposite effect, causing the airplane to roll right. This is the tentative finding in the preliminary report issued by the National Transportation Safety Board (NTSB) regarding the July 25 fatal accident involving the prototype Spectrum 33 very light jet in Utah.

The two pilots aboard were killed. Crash witnesses said the aircraft entered a right roll almost immediately after takeoff. The roll continued to about 90 degrees right wing down when the right wingtip struck the ground.

According to the NTSB, "Examination of the translation linkage on the aft side of the [rear] pressure bulkhead revealed that it was connected in a manner that reversed the roll control."

"Specifically," the NTSB preliminary report goes on to say, "the linkage was connected such that left roll input from the side sticks would have deflected the ailerons to produce right roll of the airplane, and right roll from the side sticks would have deflected the ailerons to produce left roll of the airplane."

The NTSB reports that the airplane had undergone maintenance prior to the accident, during which time the main landing gear (MLG) was removed in order to stiffen the struts. When technicians reinstalled the MLG, they found that the modifications resulted in inadequate clearance between the left MLG strut and the aileron upper torque tube V-bracket. The bracket was removed and redesigned to allow proper clearance of the gear, which also required a portion of the aileron linkage to be removed, and subsequently reinstalled.

According to information provided by the operator, the airplane had accumulated about 44 hours of total flight time since its first flight in January. Prior to the accident flight, the airplane's most recent flight took place on June 30. After that flight, the airplane had been undergoing maintenance.

The accident underscores the importance of doing a full control surface check (using a mirror or direct line of sight) by maintenance personnel before the aircraft is returned to flight service, as follows:


"Stick left, left aileron UP, right aileron DOWN."



"Stick right, right aileron UP, left aileron DOWN."


Similarly for the rudder and elevator:


"Right boot, rudder RIGHT. Left boot, rudder LEFT."



"Stick back, elevator is UP. Stick forward, elevator is DOWN."


Recall that an A320 side stick was miswired during maintenance, and on March 21, 2001, only hasty action on the part of the flight crew saved the airplane, whose wing came within feet of striking the ground on takeoff (see AM, April, 2005, p. 20).

Any time maintenance work is performed on flight controls, it's just prudent to check their proper functioning before the airplane is returned to flight status.

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