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Following the unfortunate crash of a C-130 aerial water tanker aircraft in Australia, a friend brought to my attention a 2002 C-130 tanker crash in the US. As can also be seen in this video, the wings fold upward and separate from the fuselage, which then rolls over and crashes.

According to the referenced Wikipedia-article, citing the NTSB (1, 2), the wing/fuselage-joint (likely) failed due to fatigue:

The NTSB investigated the crash and determined that the accident was caused by a structural failure that occurred at the wing-to-fuselage attach point, with the right wing failing just before the left. The investigation disclosed "evidence of fatigue cracks in the right wing's lower surface skin panel, with origins beneath the forward doubler. The origin points were determined to be in rivet holes which join the external doubler and the internal stringers to the lower skin panel. These cracks, which grew together to about a 12-inch (30 cm) length, were found to have propagated past the area where they would have been covered by the doubler and into the stringers beneath the doubler and across the lap joint between the middle skin panel and the forward skin panel."

This and another crash in the same year subsequently led to a fleet grounding, partially following concerns over training and maintenance.

Besides the incident referred to above, this got me thinking. The substantial and rapid change in weight when the tanker performs the drop must result in (temporary) large forces/moments on the wing/fuselage-joint. According to the summarised NTSB-report, the aircraft in the video carried some 3000 gallons of retardant (some 7000kgs at 9lbs/gallon), but does not specify volume flow. A more recent Modular Airbore FireFighting System that fits into a C-130 can disperse something under 3000 gallons in 5 seconds, yielding 600 gallons/sec - equivalent to 5400lbs or 2450kg/sec. Various comments on the YouTube-video (such as this and this) also point in that direction -- but unfortunately don't include sources. As such, I'm wondering:

  1. to what extent these changes in weight indeed introduce forces/moments that (might) go beyond the design specifications of the aircraft?
  2. what (technical or operational) measures are/were normally taken to prevent such problems (structural reinforcement during the conversion, limiting 'offloading speed', no dropping during climbing/manoeuvering, etc.)?
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    $\begingroup$ In theory, the stresses should be considerably less than (and certainly no worse than) for a normal aircraft taking-off and landing. (When an aircraft rotates to take-off, the entire weight of the aircraft is loaded onto the wings in a matter of seconds, and conversely when the aircraft lands, the entire weight is unloaded in a matter of seconds.) $\endgroup$ – Fiddlesticks Jan 23 at 13:11
  • $\begingroup$ @Fiddlesticks, I follow the analogy with landing (as also pointed out in John K's answer), but I'm not sure about take-off. It seems that load change is more gradual (lift is steadily increasing with increasing $V$ and $\alpha$, up to $L > W$) than in case of a landing (although take-off weight is often higher than landing weight) or mid-air drop operation. $\endgroup$ – Bram Jan 26 at 21:25
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The problem isn't the relief of stress from dumping of loads. If it was, you would have the same failures happening because of the act of landing, where the wing goes from being heavily loaded to unloaded in about the same amount of time. And it's once per flight in any case and is something that all airplanes have to cope with.

Fatigue is all about the number of load cycles, the beefiness of the structure designed to handle it, and identifying the "weak links in the chain" stucturally. The problem for tankers is they spend a large amount of time being subjected to very sharp and relatively numerous gust loads during drop operations. Working a fire, they are flying through extreme thermals and mechanical turbulence; it's a pretty rough ride.

So the airframes get "beaten up" you might say. Effectively it's like taking a car that could handle the odd pot hole with no problem and driving it exclusively on a road full of potholes all the time. Sooner or later something will break on the suspension, if you didn't anticipate the effects beforehand. And in the case of the C-130s that's about the long and short of it - they didn't anticipate the effects beforehand and the airplanes were already old.

So what happens is, the more extreme operating environment means the machine is living much closer to the margins of the original structural design analysis. This raises the risk that flaws in the original structural fatigue analysis will float to the surface, when they would normally stay dormant. Structural inspection programs that were working fine when the airplanes were being used in their normal role are suddenly inadequately frequent and/or comprehensive, and cracks form that were never a problem before, aren't being inspected for, and... ooops.

Unless you want to stop using them to fight fires, the answer is pretty simple. These are the options:

  1. Inspect more frequently, and or more invasively using NDT procedures.
  2. Add more meat, or redundant meat (damage tolerance) to the structure.
  3. Both (the probable result).
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    $\begingroup$ According to this article, for a BAe-146 air tanker, each flight cycle on a typical mission is equivalent in fatigue life to between four and seven normal flight cycles. $\endgroup$ – Fiddlesticks Jan 23 at 14:56
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    $\begingroup$ The analogy of extended periods on potholed roads is an apt one, & not new. When US & Soviet airforces changed the missions for some of their bombers in the 1960s & 70s from high altitude bombing runs (because of increasingly capable anti-air missiles) to extreme low level (to avoid radar), the aircraft suffered a huge reduction in their airframe life, found after post-training inspections. The atmosphere below 10,000ft (let alone 1000ft), even away from terrain-induced turbulence or fire/hot weather-caused updrafts & windshear, is like treacle, compared to the thin, smooth gas at 30-40,000ft. $\endgroup$ – Mackk Jan 25 at 3:34
  • $\begingroup$ Yeah I recall that when Cessna wanted to find out how the 172 stood up over time they wanted an absolute worst case. so they found a high time airframe, something like 12000 hrs, that had been used most of its life for pipeline patrol. Their specimen of the Chevy Impala of the skies passed with flying colours, only needing some hinge bearings. $\endgroup$ – John K Jan 25 at 4:00

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