Hercules Crash In Baghdad Points To Metal Fatigue In C130's Wing Center

Air Safety Week, Feb 21, 2005

It also stated that:

"... the US Secretary for Defence has declared: 'HCF is the number one readiness issue in the USAF.' It is known for example that galling and fretting can reduce the HCF strength of titanium alloys by 80 and 60% respectively. The two major concerns however are FOD (foreign object damage) and the complexity of the interactions between LCF and HCF." And, "The second technical challenge is to incorporate non-destructive evaluation as an element of fatigue management. The concern here will always be to characterize the largest defect that is not detected in large structures and complex systems where inspectability may be difficult." (Emphasis added)

This is the academic admission that fatigue management (and crack growth propagation) is an inexact science -- and that significant defects may remain hidden. It is indeed a dark black art, and more akin to necromancy than sorcery. You get to pick over the debris of your mistaken assumptions. Even though aircraft carry fatigue meters, and the positive and negative g spectrums ticking into those counting accelerometers can be interpreted by engineers to construct an FI (fatigue index), there are many intangibles. For instance, the fatigue meter is inhibited out of the picture on touchdown, so wing flexures in tactical hard landings just aren't factored in. The pilots just record the number of cycles (landings and takeoffs) and their fuel loads, and they're assumed to all be average "arrivals." But obviously, many are not.

The most significant factor is the amount of fuel being carried in outer wing tanks at any particular time because of stress relief upon the wing root. The fatigue meter has no way of recording that (or fuel taken onboard inflight), nor will a pilot or flight engineer always manage his fuel feed in the same way. This was a significant consideration for aerial fire-fighting C130s. They'd have little fuel in their wings and instantaneous water-load drops, plus rapid onset high-g pull-ups in thermal currents and turbulence, would all place large bending moments on wing-roots. Little wonder that a wing eventually fell off, even though it was an early A model Herc. Some C130s carry 130-gallon under-wing pylon tanks -- but only some of the time, another intangible.

It is known that the RAF's Hercules frequently carried operational war overloads. Operations over and above the 175,000-lb. design weight, whether necessary or not, carry a non-linear penalty. Another intangible is the extent of hidden corrosion and cracking. That is the reference above to "inspectability." In the 1998 salvage of "King 56," a C130 downed off the U.S. West Coast by a four-engine flameout due to fuel mismanagement, the investigating board was surprised at some revelations:

"...fleet-wide evidence suggests fuselage tanks are not being regularly drained of water, potentially leading to tank corrosion. The discovery of a coating of Corrosion Preventative Compound (CPC) in one tank is evidence of a nonstandard procedure resulting from unanticipated corrosion."