“Whirl-Mode” Phenomenon May Also Be At Play
When a Royal Air Force (RAF) Hercules crashed about 19 miles northwest of Baghdad on Jan. 30, the natural assumption was that it had been brought down by enemy action. The brass weren’t so sure because it had spun down from an altitude that should have been above the range of a shoulder-fired missile. An opportunistic insurgent group released via al Jazeera a hastily cobbled-together video of flaming wreckage and a non-MANPADS (man-portable air defense system) missile being fired.
But that fraud quickly fizzled and the mystery deepened. Had a sapper or insurgent managed to place a device aboard? It seemed unlikely given where the aircraft had been and the security there. The inflight “explosion” had torn the right wing off near the root at medium altitude in the cruise, the right wing being found some considerable distance from the main wreckage. A senior RAF spokesman said, “sabotage is a distinct possibility but even though metal fatigue is another option, it’s considered much less likely.”
Less than two weeks later, the U.S. Air Force (USAF) announced that it was grounding 30 of its C130Es and placing another 60 C130Hs on restricted flight status. Some of the grounded aircraft were operating in Iraq, Afghanistan and Kuwait. The USAF’s Air Mobility Command (AMC) had been monitoring cracks in the planes’ wing box structure since 2001, but evidently the grounding decision hadn’t been premeditated — but was comparatively sudden. Inspections of the aircraft in the past four years have revealed that cracks “were greater in number and severity than originally expected,” the AMC spokesman at Robins Air Force Base, Lt. Dustin Hart, said. Replacing the older planes with new J models is “one of the options,” Hart said. Accordingly, on Feb. 10, Gen. John Jumper, the Air Force chief of staff, told U.S. senators the service was rethinking its plan to end purchases of C130Js, and he dismissed criticism of their performance, according to Reuters news service. The Pentagon has admitted that two other C130 crashes had been attributed to metal fatigue.
Stuff Of Legends Cracking
Air forces the world over would find it hard to do without the workhorse C130. Its presence in war zones and on humanitarian relief missions is stuff of legends. But the immediate reflexive response from the UK Ministry of Defense (MoD) was that RAF C130s would not be grounded, as they are similar “but not identical” models. The RAF C130K is indeed very similar to the E model, yet the distinctions have more to do with perceptions of fatigue as related to age than reality, according to some ex-RAF pilots. The RAF’s C130K, they say, is nothing more than a 1968-70 E model but with Dash 15 engines (as fitted to the later H model). Supposedly, the center wing-boxes are fabricated from the same type and strength of aluminum (as the C130E). The “K” model only designated UK specified avionics, i.e., it wasn’t a bona fide Lockheed model designation.
Unlike the C130E, the UK’s K model had no explosion suppressant foam in its tanks (which helps limit internal wing flexing to some extent). Most of the older RAF C130s were given new center-sections in the mid 70s to mid 80s at Marshalls of Cambridge, and the outer wings were rebuilt with new planks top and bottom. This was done specifically to address the fatigue problems with the older type wing. However, it has been the fatigue spectrum accumulated since the mid 80s that has aged the RAF fleet inordinately, particularly over the last five years. Over that same period, USAF Depot Level Maintenance revisit time was extended from a three-year cycle to a five-year cycle. Each airframe would take four to six months to refurbish. It was mostly at these teardowns that the picture of cumulative fatigue damage was realized.
Refocusing On The Metal
UK investigators are now focusing on metal fatigue because no evidence has been found of enemy action. An MoD source said: “It’s hard to believe that there might be a structural failure after all the close monitoring we do, but there is a history of surprises about metal fatigue.” British investigators are now examining evidence from a U.S. crash in 2002 in which metal fatigue caused a wing to break off in mid-air. The UK MoD has now taken the highly unusual step of calling in the civilian Air Accidents Investigation Branch (AAIB) to help find the cause of the crash of its 1965-built C130, number XV179, one of the six oldest in RAF service. In a Feb. 12 release, the RAF emphasized that the C130 “has an excellent safety record” and that its Hercules Fleet is “routinely and continuously monitored for metal fatigue.” An MoD spokesman said that the investigation had reached no conclusions and that “if the president of the board of inquiry had had any concerns, he would have already grounded the fleet. … If the U.S. thought there was a problem, they would have informed us.”
That last statement may have creased a few brows. Nevertheless, if the wing collapse was to be verified, at least 25 older RAF C130s would have to be grounded. That would leave the RAF with very little by way of strategic airlift assets, apart from a few unreliable C130Js and their four leased C-17 Globemasters. It is no wonder that UK MoD is in denial. Putting all your eggs in one basket is a phrase that comes to mind. The A400M next generation transport is still many years off.
Fatigue Should Have Come as No Surprise
No one should be surprised by the specter of metal fatigue when it comes to the C130. Recall that the Canadians grounded their C130s two years ago for cracks in spars joining the wings to the fuselage. The Australian Aircraft Research Laboratory (ARL) had to develop boron fiber patches to fix fatigue in stringers and stress corrosion in risers in the Royal Australian Air Force’s (RAAF) C130E models (many of those now being operated by the Pakistani Airforce after being traded in on the J model).
The RAF has been using its C130s in a number of different fatigue profiles. The RAF’s Special Forces infiltration birds do the NVG (night vision goggles) ground-hugging profiles — and that’s more than a little different fatigue-wise to Strategic Logistic route flying or tactical troop transport roles. In 2002, the C130 International Project Team (IPT) said that: “Due to operational demands placed upon the RAF’s CMk1 fleet … the cleared fatigue life of these aircraft has nearly been reached,” and revealed that the RAF had started a wing swap program on three aircraft (XV179 — the accident aircraft herein discussed — as well as XV196, and XV295) with K-model outer wing sets using H-model outer wings that were claimed to be “less affected by high FI (fatigue index) consumption rates.”
The fleet leader was then said to have amassed 29,300 flying hours, when the C-130K Fuselage Fatigue Test Programme (that was due to run from Feb. 3 to Feb. 6) was expected to clear the C-130K fuselage for 10,000 flight cycles, equivalent to 30,000 flying hours (with a further 9,000 hour option). Those “new” outer wings are assumed to be RAF assets — presumably taken from H-winged C1 and C3 aircraft withdrawn from use when the Js entered service (but may well have been E-model). They were looking to bridge the gap to the A400M transport’s introduction in 2010 by squeezing more service from the C1 batch. They conceded at that time that any unforeseen arisings, “such as centre wing cracks,” could ground the fleet. The MoD representative (Squadron Leader Saunders) admitted that the C130K was derived from the E model and that the main concern was the unmonitored nature of their center wing structure, particularly in the Special Forces support fleet (the Mk 1 mini-fleet). All attention had hitherto been on “the wing’s shoulders and armpits.” The RAF’s IPT presentation slides are on the Lockheed Martin Web site at http://www.lockheedmartin.com/data/assets/4157.pdf and …/4156.pdf
Monitoring and predicting is a form of passive observation. It’s neither reactive nor pro-active unless you have a contingency plan and a plan for activating that plan. It now looks like the UK MoD planners are center-stage — and need some cue-prompting on their lines.
Academically A Dark Black Art
In a Portsmouth University Paper (Report number: A405493, October 2000) entitled, “The Effects of LCF Loadings on HCF Crack Growth,” the authors write:
“… the increased use of aging aircraft has highlighted the limitations in the current technical and fundamental understanding of the fatigue integrity of engineering components. There is at present insufficient guidance to enable an engineer to account for the reduced high cycle fatigue (HCF) life consequent upon various forms and amounts of damage, such as low cycle fatigue (LCF).”
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.”
The bottom line is that workhorses do get thrashed in the field. Despite engineers’ best efforts, the actual condition of an airplane deployed into a remote theatre of operations may not be known until its next major overhaul. At that stage, any significant structural damage can be repaired, but who’s to know just when it happened? Aircraft might fly in an unrecognizably unairworthy state for months at a time. Cracks don’t propagate linearly in time or dimension. Military workhorses aren’t subject to the same strict scrutiny as airliners (hopefully) are. Inspections in the field cannot hope to be as thorough as they might be at home-plate, notwithstanding that all the fluids are checked and topped off, and visually the beast appears serviceable — and runs like a clock. No amount of optimistic monitoring can get things right when the first wing falls off (“Uh oh, we got that wrong”). Is this what we might hear when the actual fate of airframe XV179 is revealed?
Monitoring versus Action
No one’s saying that the RAF is unconcerned. There is an OLM (Operational Loads Measurement) exercise under way and there were inspections scheduled at 6FI increments. It is recognized that an air force cannot ground an aircraft type each time there is an accident. However, a wing falling off, plus the history, plus the USAF action has raised more than a red flag — it is now a veritable bunting of crimson pennants. Perhaps the RAF C130 fatigue spectrum has changed significantly. Center-section and rainbow fittings (wing joint attachment) may need more regular and robust checking. Perhaps variable-rate strain- gages, parameter transducers and data acquisition units should be fitted. Anti-corrosion aerosols (such as ACF-50) may need to be applied more liberally. The technology is available.
The inexact science of tracking, monitoring and detecting an incipient failure-mode can be fatiguing, particularly in the last quarter of an airframe’s life. That’s why eventually the MMH/FH (maintenance man-hours per flight hour) can eventually dictate an aircraft’s use-by date. Fatigue life consumption needs to be managed. Early trends may indicate positive hardware fixes and operating niceties that will extend fatigue life and make an airplane safer (“a gusset here and a gusset there” approach, as well as carbon/epoxy prepreg reinforcement). Deterministic fatigue life prediction methods and software tools are available.
The Whirling Dervish
But apart from undetected wing component cracking, what else might cause wing separation? Did re-winging the outer wings with stronger units shift the stresses and strains farther inboard to the center wing box? Did it change the natural bending harmonic of the wings? To explain, each wing has a natural frequency at which it will resonate. If you damage a glass-fiber sail-plane’s main spar, the glass-fiber man can tell that just by excitating the wing-tip and noting whether it still resonates span-wise at its natural harmonic. But why is the harmonic important? Ever heard of the “whirl-mode” phenomenon that destroyed two early L-188 Electras (N9705C and N121US) before Lockheed wised up to it? Whirl mode is a divergent oscillation of an engine and its mount about an axis parallel with the powerplant’s static thrustline. It occurs because of the positive feedback loop generated when a large, sudden gyroscopic moment is applied to an engine-mount structure that is insufficiently stiff.
The feedback loop occurs because a gyroscopic moment (produced by a pitch or yaw velocity) is applied in a plane that is 90 degrees to the excitation angular velocity. If the excitation mode couples with a pliant engine-mount or wing resonant frequency, the prop-end of the powerplant can begin to whirl around the static prop centerline, describing a cone of ever-increasing base diameter. The result, if the mode is not checked, is a wildly wobbling gyroscope that eventually begins to transmit its violent motion to a natural outlet: the wing. Think of what happens if you push a spinning top — it will wobble erratically.
For example, suppose that an aircraft has an engine mounting structure with insufficient STIFFNESS (as differentiated from strength). Suppose that aircraft encounters a violent gust that causes it to pitch-up rapidly. If the prop rotation is clockwise, the prop gyro-moment would try to bend the engine mount structure to the right. If the mount was sufficiently flexible, it would deflect (yaw) rapidly to the right, which would generate an upward gyroscopic moment on the engine mount. The flexible mount deflects upward, causing a yaw-left moment. The 90 degrees-out-of-phase excitation continues, and if these excitations and deflections occur at the natural frequency (or harmonic) of the engine/mount system, the deflections can develop into a whirling deflection of the engine structure of increasing amplitude — until something breaks.
When you consider the engine mount in the C130 context, you are also including the wing (and possibly one with changed frequency response characteristics because of concealed cracking and outer panel beef-ups). That was the Electra problem — its wing was “flutter” reacting with the outboard engines. What the investigators found, after the second crash, was that the engine mounts weren’t strong enough to dampen the whirl mode that originated in the outboard engine nacelles. The oscillation transmitted to the wings caused severe up-and-down vibration, which matched the wing’s harmonic and so the wing-flutter grew in amplitude until a wing tore off. Lockheed had to beef up that wing and the engine mounts (with diagonal braces). The C130 engines and the Allisons of the Electra are almost identical. Whirl-mode can be hard to detect. According to Don Keller of NASA Langley: “Some flutter modes are elusive. You can’t predict them until they happen.”
In Another Whirl
In 1999, the Air Line Pilots Association (ALPA) submitted a petition to the National Transportation Safety Board (NTSB) to have the cause of a Dec. 28, 1991, crash of a Beech 1900C reclassified as a whirl-mode breakup. N811BE of Business Express crashed off Rhode Island (NTSB NYC92FA053) and the NTSB had called it a loss of control/disorientation accident — despite one wing being found far from the main wreckage. Professor R.O. Stearman of the University of Texas later carried out a signal analysis of the CVR (cockpit voice recorder) and identified the structural acoustic signals of whirl-mode and the ensuing aircraft breakup. Evaluation of the Federal Aviation Administration (FAA) SDR (Service Difficulty Report) data revealed that six engine truss designs had been implemented in response to truss cracking over the 10-year history of the aircraft.
One commuter airline documented over 70 cases of cracked trusses, some detailing complete separation of truss tubes. The accident airline had 16 of these aircraft, all three years old or less. Even though airlines were encouraged to inspect the critical areas as frequently as every 100 hours, in two cases up to eight cracks were discovered in a single truss between inspections. The FAA Principal Aviation Safety Inspector for Airworthiness expressed his concerns about this matter in a November 1989 memorandum. Prior to the accident, a vehicle had struck N811BE (and the engine damaged) in 1987 (NYC87LA117). For Stearman’s Report see http://www.acoustics.org/press/133rd/2psa1.html
A Postulate Worth Positing?
So, could this C130 outer wing panel re-work program and/or center-section cracking have induced an aeroelastic instability of the whirl-mode variety? Lockheed engineers can check. But nevertheless, it is worth asking the question as a prompt. Flutter of any kind is an alarming oscillatory phenomenon that feeds upon itself and usually, like American Airlines [AMR] Flight 587 (A300-600 pilot induced oscillation), quickly ends in self-destruction (ASW, Jan. 31). First time around on both the Braniff Electra crash and the Beech 1900C’s, the investigators overlooked the whirl-mode phenomenon. As Theodore Karman, the famous aerodynamicist said: “Some fear flutter because they do not understand it. And some fear it — because they do.”
An authoritative source, found at http://www.fas.org/man/dod-101/sys/ac/c-130.htm, says: “A wing modification to correct fatigue and corrosion on USAF’s force of C-130Es has extended the life of the aircraft well into the next century.”
That would now appear to be an “inoperative” statement.
The RAF C-130 Fleet
The accident aircraft, number XV179, was built in 1965. It and its peers are listed here.
Note: The numbers 13021/13024 refer to the original U.S. Department of Defense serial numbers.
65-13021/13044 Lockheed C-130K Hercules C/N
4169, 4182, 4188, 4195/4196, 4198/4201, 4203/4207,
4210/4214, 4216/4220.
For RAF as Hercules C.1 XV176/XV199
- 13021 (XV176) returned to Lockheed, March 4, 2000.
- 13023 (XV178) returned to Lockheed, Nov. 26, 2000.
- 13024 (c/n 4195, XV179) crashed near Al Teji, Iraq, on Jan. 30, 2005. 10 onboard killed.
- 13025 (XV180) crashed at RAF Fairford on March 24, 1969, when engine went into reverse thrust during takeoff.
- 13026 (XV181) to be sold to Austrian AF
- 13027 (XV182) returned to Lockheed, Nov. 12, 2000.
- 13032 (XV187) returned to Lockheed, Jan. 28, 2001.
- 13034 (XV189) returned to Lockheed, Dec. 10, 2000.
- 13038 (XV193)crashed at Glen Tilt, Scotland, on May 27, 1993, when stalled after cargo drop.
- 13039 (XV194) ran off runway into ditch while landing at Tromso, Norway, on Sept. 12, 1972; damaged beyond repair.
- 13040 (XV195) returned to Lockheed, Jan. 28, 2001.
- 13043 (XV198) crashed at RAF Colerne, UK, on Sept. 10, 1973, when engine cut during touch and go.
Source: http://home.att.net/~jbaugher/1965.html (last revised Feb. 5)