How AFLC uses magnetic rubber, eddycurrents, X-rays, and other unusualtechniques to keep USAF's agingaircraft safe and battleworthy.

More Mileage fromOlder Warplanes

ONE need only watch routineflight operations to see the tri-

umphs and potential troubles of agraying Air Force fleet.

Aged fighters groan under thestress of six-G turns. Vietnam-eratransports rattle over primitivestrips. Thirty-year-old bombersshudder and shake on treetop runsto target.

Clearly, older airplanes still arecapable of stellar performance.Cracks and other old-age weak-nesses, monitored by USAF's Air-craft Structural Integrity Program(ASIP), are being held at bay.

But dangers are equally obvious.With many planes beyond theirplanned service lives and some fly-ing in ways never intended, con-cerns about structural failure areever-present.

Moreover, ASIP workers are fac-ing a fleet of 6,000 planes whoseaverage age, 15.8 years, is sure torise. This trend, made inevitable byyears of slack plane production, willcompel ASIP's inspectors to exer-cise even greater vigilance.

That reality is clearly recognizedby ASIP's corps of inspection andanalysis technicians, managed by

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A STAFF REPORT

Air Force Logistics Command,headquartered at Wright-PattersonAFB, Ohio.

The problem-"tired" metal-isbest explained by analogy. Just asbending a paper clip back and forthcreates microscopic cracks thatweaken it, so too does the stress offlying weaken aircraft parts.

Early discovery of weaknesses,whether to prevent disastrous fail-ure or to permit life-extending up-dates, is the essence of the thirty-year-old mission that ASIP per-forms with mounting sophistica-tion.

Once, says Bernie Nasal ofAFLC's Materiel Management Dep-utate, ASIP used a fairly primitiveanalytical model to predict whencracks would appear. This approachwas abandoned, he explains, whenASIP found "big differences" be-tween predicted failure times andwhen those failures actually oc-curred.

Since 1970, ASIP has been pursu-ing a new two-track effort-basedon safety and long-term durabil-ity-that employs more extensiveuse of reliable, hands-on inspectionas well as advanced predictive mod-

AIR FORCE Magazine / August 1989

els. The change has proven to be amajor success, if the record of air-craft longevity is any guide.

Still Going StrongHundreds of C-135 aircraft, the

first of which was delivered in 1955with a projected service life ofIO,OOO flying hours, are still goingstrong and are headed toward36,000hours. The fleet ofT-37B pri-mary trainers and T-38 advancedtrainers, introduced three decadesago, is expected to stay in servicepast the year 2000.

Long-range B-52 bombers-Gmodels dating from 1958 and Hmodels from 1961-long ago passedtheir original service-life goals of5,000 flying hours, and some willstay in action for another decade.Twenty-five-year-old C-141 trans-port craft continue to provide agreat chunk of US long-range airlift.F-lll fighters produced between1966and 1976 and F/FB-ll1 bomb-ers produced between 1967 and1970will remain in USAF's invento-ry well into the next decade.

The heart of the structural-integ-rity effort lies in ASIP's IndividualAircraft Tracking Program, a sys-tematic recording of flight historiesfor all USAF aircraft, whether theybe at the end of decades of service ornewly arrived on the flight line.

Even those in patently good

health are treated like patients in ahospital, with all vital signs moni-tored and scrutinized.

A multitude of specialized sen-sors, hooked to an aircraft likeemergency-room equipmenthooked to a patient, produces infor-mation on such invisible stress asdeflection of metal between twopoints, while other devices monitorand record factors such as airspeedsand G-force experience.

So important are the data to theprediction of structural problemsthat ASIP has devised a wide vari-ety of techniques to do the job:

• Counting accelerometers (usedon smaller aircraft) record each in-stance in which an aircraft exceedscertain gravitational limits.

• Mechanical strain recordersbolted onto all F-16A1B fighters anda few T-38s and F-5s identify un-usual structural movement betweentwo points by producing etchings onfoil-like tapes.

• Vgh (aircraft speed, vertical ac-celeration, and height) recorders,installed on ten percent of olderF-4s and T-38s, record these aspectsof a flight.

• Multichannel recorders, sam-pling aircraft performance up to 240times per second, keep track oftwenty-six individual flight param-eters.

• Specialized monitors on large

aircraft record fuel and cargoweights and numbers of takeoffsand landings.

Useful for all planes, tracking iscritical for air machines flown inways unforeseen by their designers.The B-52, to cite but one example,has been transformed over the yearsfrom a high-altitude to a very-low-altitude machine. The changeplaces unplanned-for stresses oncritical components throughout thebomber.

Hunting for FlawsCareful recording of stressful op-

erations to predict structural dam-age is but one facet of ASIP's work.The accuracy of these types of pro-jections is vigorously tested-andat times disproven-by frequent, di-rect examinations of actual aircraftcomponents.

In such "Stress Spectra Sur-veys," carried out at air logisticscenters across the US, maintenanceworkers use a variety of ASIP toolsand techniques to hunt for unantici-pated flaws.

Testing methods cover a widespectrum. They might be as simpleas tapping a coin on an aircraft'ssurface to detect changes of tone orusing the naked eye to inspect fas-teners. Most of the time, searchesare more technologically advanced.

One technique, known as the

C-141s undergo depotmaintenance and modifica-

tion at Warner-Robins AirLogistics Center, Ga. The

C-141 transport, which firstentered service in 1964, still

provides much of USAF'slong-range airlift.

AIR FORCE Magazine / August 1989 43

At the Sacramento Air Logistics Center, McClellan AFB, Calif., a quality assurancetechnician inspects a section of aircraft for structural weaknesses. AFLC "StressSpectra Surveys" monitor strain on aircraft through a variety of tests.

"eddy current," creates an electro-magnetic field within a specifiedpart; structural imperfection showsup as a glaring deviation in the pat-tern of the field.

To detect damage hidden inside acomponent, ultrasound devices areused in order to project soundwaves through material, paint a"picture" of the subsurface, and re-veal voids and gaps in the part.

X rays, N rays, and other ex-tremely-short-wavelength energyforms can be radiated through a sus-pect part to detect advanced corro-sion in metallic fabrications, welds,and castings.

The "Magnetic Rubber" MethodThen there is the so-called "mag-

netic rubber" method. In this pro-cess, liquefied rubber, laced withmetal filings, is applied to a magne-tized part. Attraction of the metal toa particular area indicates the pres-ence of a crack or fissure, one thatmight otherwise be too small to see.Application of fluorescent pene-trants, specialized liquids that glowunder black light, produces similarresults.

Armed with detailed data yieldedby the tracking and inspection pro-grams, analysts located at AFLC'sAircraft Structural Integrity Man-agement Information System beginthe laborious task of reviewing the

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results and putting them into usableform.

Delmar Teet manages the system,which is based at Oklahoma CityAir Logistics Center, Tinker AFB,Okla. He maintains that, after hiscomputers and analysts digest theraw data, they can provide logisticsmanagers with the kind of informa-tion they need both to determine theproper uses of older aircraft and toprescribe future maintenance pro-cedures to prolong the aircraft'slives.

Report summaries tell managershow a particular aircraft is beingused and, based on that record, howmuch damage the structure of theaircraft may have received. That in-formation is used to advise com-mands when airplanes are reachingcritical stress limits or when certainoperations could have dangerousconsequences.

For an idea of the importance ofthis service, one need only see theages of some workhorse USAF air-craft. They average twenty-eightyears for all 262 B-52s, 17.9 yearsfor all sixty-two F/FB-III s, fifteenyears for forty C-5As, 19.8 years forall 365 C-130s, and 17.9 years for all949 F-4s. Even among USAF's new-

er aircraft, there are pockets of age.Now at least nine years old are 116A-lOs, twenty E-3 Airborne Warn-ing and Control System aircraft,and 298 F-15 fighters.

Planning Aircraft ModificationsEqually useful is another product

of AFLC's computerized analyses:detailed advice on when, where,and how to modify older air vehiclesin order to maximize the number offlying hours the Air Force gets fromits original investment.

Projections are made about howmuch longer certain structures willlast under certain flight conditions.With this information, USAF offi-cers can decide when to replacewings and other key structures onentire fleets of aircraft. Such ASIPinformation has been used, for ex-ample, to plan the orderly wing-reskinning and reengining of theKC-135 aerial refueler, rewingingand structural strengthening of theB-52, and wing modifications to theC-141 airlifter.

USAF is not the only beneficiary.Friendly nations flying US-builtmilitary aircraft-Turkey, Greece,Norway, and Egypt, to name a few-also receive and use ASIP data.

Today, a structural integrity mas-ter plan is prepared for each aircraftthat enters the inventory. The planundergoes continuous refinementduring an aircraft's service life. Thefirst version of the master plan isincluded in manufacturers' bidpackages. From there, it is updatedas engineers develop durability anddamage-tolerance analysis proce-dures.

Early production models are sub-jected to rigorous vibration, flutter,durability, and damage-tolerancetests both on the ground and in theair. These test data are comparedagainst projections developed dur-ing the design phase of the aircraft.

For ASIP workers, the crucialtask for the last twenty years hasbeen to make certain that the agingaircraft on hand remain safe to flyand ready to perform their mission.While the program has enjoyed re-markable success, all signs are thatthere will be no early letup in thechallenge. I

The foregoing article is based on reporting by Kenneth Perrotte, Chief of MediaRelations at Air Force Logistics Command headquarters, Wright-Patterson AFB,Ohio, and on magazine staff reports.

AIR FORCE Magazine / August 1989