Hidden Fatigue Cracks Suspected in C-130 Crash
MICHAEL A. DORNHEIM/LOS ANGELES
A 10-12-in. fatigue crack has been found in the center wing box of the firefighting Lockheed C-130A that crashed June 17, along the line where the right wing separated from the body. Other smaller fatigue cracks were also found along the parting line on the lower wing skin.
Lower skin of the C-130A center wing box is made of three spanwise 0.17-in.-thick planks stiffened by 12 hat-section stringers. Chord is 80 in. The planks are butt-joined using the extended base leg of a stringer as an internal splice plate, with a single row of rivets in each plate at the joint. Top left photo is looking inboard at the forward two stiffeners on the center plank. The surface facing the camera is where the right wing came off and is approximately 53 in. to the right of the fuselage centerline, which is about 8in. inside the fuselage. Circles show the areas covered by the enlargements at bottom, and these are the rivet holes from which fatigue cracks propagated and joined to make a 10-12-in. crack. Most of this area was hidden by an external doubler that is no longer present. Bottom photos highlight the fatigue growth rings, which also entered the stiffener. The smooth chevrons are interrupted by rough regions of rapid tearing.
The chordwise length of the wing box at that point is 80 in., meaning the crack itself covered 12-15% of the skin. Laboratory analysis of the crack confirms that it is due to fatigue (see photographs).
Last week the FAA was drafting an emergency airworthiness directive to C-130A operators based on the NTSB's discoveries. A preliminary idea is to ground the aircraft pending X-ray inspection.
The paint color of the outer surface around the crack is different from the rest of the wing box, and NTSB investigators suspect the crack was covered by the outer fringe of a 6 X 12-in.-chordwise 0.072-in.-thick doubler patch. The skin is 0.16-0.17 in. thick. There were no fastener holes or strong adhesive spanning the crack, so it appears the doubler was reinforcing structure immediately outboard of the crack. The doubler overhangs the fracture by 1-1.5 in. With this doubler in place, it would have been difficult or impossible to visually detect the growing crack, and there is a chance the extra stiffness of the doubler was actually causing the crack at its periphery.
X-ray inspection of the wing also shows cracks at areas away from fastener holes.
The completed NTSB investigation of a prior 1994 C-130A firefighter crash near Pearblossom, Calif., may be reexamined as a result of the recent crash near Walker, Calif. The NTSB concluded the 1994 accident, in which the right wing came off, was caused by the explosion of leaking fuel. An independent 1997 investigation of the Pearblossom accident by retired NTSB investigator Douglas R. Herlihy concluded that it was caused by structural failure from flying too fast in turbulent conditions, and not a fuel explosion (
www.iprr.org/COMPS/T82story.htm ).
The Pearblossom wreckage was located in somewhat difficult terrain, and not much was recovered by the NTSB. Herlihy and crew revisited the site and examined more than 200 pieces, not finding any sign of explosion or overpressure. Some of the NTSB's case was based on eyewitnesses who saw the wing "explode." Eyewitnesses said the same thing about the Walker crash, but a videotape of the crash shows the wing folds up first, then a fire starts a second later. In both crashes the aircraft shed upper wing skin in flight.
At Walker, the right wing broke off along a relatively straight line at approximately wing station 53 right (WS53R). Wing station is the length along the wingspan in inches, with the fuselage centerline being WS0. The fuselage intersects the wing at WS61, so the break point was about 8 in. inside the fuselage.
From studying the videotape, the right wing was the first to fail as the aircraft was pulling up from a firebombing run (AW&ST June 24, p. 56). About 0.3 sec. later, the left wing started to fold, and a fire started at the right wing root about 0.9 sec. after that wing started to fold.
The left wing broke at about the same spot--WS53L, along where the doublers are. NTSB investigator-in-charge George Petterson plans to carefully examine this failure point in mid-September. He also wants to recover the Pearblossom left wing lower skin for closer examination. The right wing that came off at Pearblossom was melted by fire and the left wing remained with the fuselage, but may have cracks. "We found cracks in the Walker wreckage where Lockheed wasn't even looking," Petterson said. Lockheed's lifetime studies of the lower wing skin concentrated around the WS61 fuselage joint and not in the WS53-59 area, where the wing failed in a fatigue test and in the Walker crash.
The C-130 wing has three main sections--the center wing box with 36.7-ft. span including the inboard engines, and the left and right outer wings, each with an outboard engine. The center box is continuous through the fuselage.
THERE WERE THREE doublers along the outboard side of the WS53R crack area, and this appears to have been a standard item on the C-130A, though the NTSB had not been able to determine as of late last week exactly what it was for. The aircraft went through a structural rehabilitation at Aero Corp. in Lake City, Fla., but the details are vague. "Every day I get different information on the rehab," Petterson said. "There are no records so far, just memories." Aero Corp. is now called Timco.
However, Lockheed's original fatigue tests of the C-130A wing circa 1956-58 show a failure similar to that of the Walker accident. At 13,203 cycles, the lower wing skin ripped open from the front to rear spar while at only 62.5% of limit load. The location was at about WS59R--just 6 in. outboard of where the Walker wing came apart, and in an area that also would have been covered by or very close to the doublers on C-130As.
Besides the rip, inspection of the test wing showed a large number of fatigue cracks around WS61 left and right, concentrated at the front and rear spars. Lockheed concluded in 1978 that the C-130A fleet should be inspected at 12,000 flight hours with recurrent inspections every 2,500 hr. It is not clear how to convert fatigue cycles into flight hours on aircraft that have had such a wide spectrum of use--but firefighting is about the worst duty imaginable (AW&ST Aug. 5, p. 51). Lockheed recommended that the wing be preloaded by jacking to spread cracks apart and make them more detectable. Paint may make it hard to find small cracks. The Walker aircraft, originally Air Force tail No. 56-538, was delivered to the service around 1957 and had 21,863 hr.
In the C-130A static test, the wing failed at 89% of ultimate load and was reinforced but not retested. But the failure location was not at the center box--it was the upper skin of the outer wing, 21 in. from the wing attach point.
The fatigue tests also highlighted the importance of appropriate materials. The prototype C-130A fuselage was made of 7075-T6 alloy aluminum. The T6 temper gives high strength, but the fuselage cyclic tests quickly showed it is susceptible to fatigue, leading to hundreds of failures, three of which would have been catastrophic. Lockheed changed the fuselage skin to fatigue-tolerant 2024-T3 aluminum and redesigned the structure to reduce stress.
But they did not change the wing center box alloy, which was 7075-T6. The C-130B and original C-130E center boxes remained 7075-T6, but poor results in the C-130E fatigue test, in which the center box lower skin suffered the most damage, caused a change of material to more fatigue-tolerant 7075-T7531, as well as a redesign to reduce stress by 20%. The wing center section continues to be a life-limiting part for newer C-130s.
A program was started to "rehabilitate" C-130A center and outer wings during depot maintenance, on an as-needed basis. Laboratory tests of the Walker wreckage show the center box is made of 7075-T6.
FAA orders inspections of tanker wing cracks
Associated Press
WASHINGTON (AP) – The Federal Aviation Administration ordered wing inspections on all C-130A air tankers Thursday after investigators determined that cracks in the wings caused the crash of a Wyoming-based tanker fighting a wildfire in California this summer.
The FAA also directed owners to inspect the wings of the P2V, another tanker commonly used in firefighting, because it said cracking is common.
The wings of the C-130A snapped off and the plane was engulfed in flames as it dropped retardant on a blaze on June 17. Pilot Steven Wass, 42, of Gardnerville, Nev.; co-pilot Craig Labare, 36, of Loomis, Calif.; and flight engineer Michael Davis, 59, of Bakersfield, Calif., were killed in the crash.
The FAA directive was issued “since an unsafe condition has been identified that is likely to exist or develop on other Lockheed C-130A airplanes.” The administration directed owners to inspect all C-130As for cracks within four days and conduct regular follow-up inspections.
“The Forest Service has some very serious decisions to make” as to whether an inspection regimen can be developed that would allow the C-130A to be used safely, said William Broadwell, executive director of the Aerial Firefighting Industry Association.
The FAA set regular intervals for inspections, beginning Oct. 1, of the P2V, also manufactured by Lockheed.
The National Transportation Safety Board has not released a final report on the investigation, although it said Tuesday that cracked wings caused the crash of the C-130A as well as the crash of a P4Y-2 in Colorado in July. That crash killed two.
Both planes were converted military aircraft operated by Hawkins and Powers Aviation, a Greybull, Wyo., company that contracts with the Forest Service to provide firefighting tankers.
“The cause of such fatigue cracking has been attributed to the age, time-in-service and flight cycles of the airplane,” the FAA’s C-130A directive said. “Such fatigue cracking, if not detected and corrected, could result in structural failure of the wings and consequent loss of control of the airplane.”
The C-130A was 46 years old and had 21,863 hours of flight time. The P4Y-2 was 57 years old but had just 8,200 flight hours.
Another C-130A crashed in 1994 in Pearblossom, Calif., killing three people aboard. The NTSB attributed that crash to a fuel leak that ignited, although Broadwell said that finding is under review.
This year the Forest Service contracted to use six C-130As and 14 P2Vs, but the agency has stopped using both planes in its firefighting operations. They remain grounded pending the outcome of the NTSB investigation.
There are 16 operational C-130As registered in the United States, said FAA spokesman Paul Takemoto said. All of them are former military aircraft that have been modified for other uses, primarily firefighting or aerial surveying.
The Forest Service does not own its own air tankers; instead, it leases the planes from private contractors. This year, the Forest Service signed contracts to lease 45 tankers.
But Broadwell stressed that any plane can be flown safely, regardless of its age, as long as it has been rigorously inspected and maintained.
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