static electricity in flight threatens aircraft safety · pdf filestatic electricity in flight...

FLIGHT SAFETY FOUNDATION • AVIATION MECHANICS BULLETIN • JULY/AUGUST 1992 1

Static Electricity in FlightThreatens Aircraft Safety

byJohn F. O’Neill Jr.

JAYFON EnterprisesAerospace Consultants

Aircraft accumulate electrical chargescreated by static electricity under vari-ous flight conditions and dischargeof that electricity can generate radiofrequency interference (RFI). RFI canthreaten the safety of flight becauseit can interfere with aircraft commu-nications and navigation equipmentthat operate in very low frequency(VLF), high frequency (HF) and veryhigh frequency (VHF) ranges (Fig-ure 1, page 2) by increasing equip-ment ambient noise levels and makingreception difficult or impossible.

Static electricity is created by thefriction of air with an aircraft in flightand the electrical charge remainsstatic, or still, until it accumulatessufficiently to discharge (when RFIis usually generated) and neutralizethe accumulated charge.

When the atmosphere contains par-ticulates such as rain, snow, dust, sandor volcanic ash, more RFI (precipita-tion static) is generated as the mate-rials are struck by aircraft surfaces,especially antennas. Flying in rain

2 FLIGHT SAFETY FOUNDATION • AVIATION MECHANICS BULLETIN • JULY/AUGUST 1992

generally produces only moderate dis-charges, but heavy discharges can oc-cur when an aircraft is flying in clearair near or between electricallycharged clouds or while penetratingstorms with electrical activity.

Dry snow also leads to discharges;the colder and dryer the snow thegreater the discharge. At higher alti-tudes, the ice crystals found in cir-rus clouds can produce massivedischarges on aircraft, as can crys-tals in cold arctic air masses at loweraltitudes. Again, the colder and dryerthe crystals, the greater the potentialfor discharge.

Another source of RFI that oftengoes unrecognized is created by thedischarge of selectively ionized par-ticles in the engine exhaust. Whilenot as common or as strong as otherdischarges, it creates RFI that can

vary from aircraft to aircraft and en-gine type to engine type.

RFI is most commonly caused bythree types of discharges: corona,streamering and arcing.

A corona discharge is luminous andcan be audible. A corona — a faintluminous glow adjacent to the af-fected surface of an aircraft — iscreated by an electrical discharge thatcauses ionization of the air in thepresence of a strong electrical field.Ionization is caused when the atmo-sphere is broken down and normallybalanced atoms, with equal positiveand negative charges, become unbal-anced and carry a positive charge ora negative charge and become ca-pable of carrying an electrical cur-rent. The corona on nonmetalliccomposite radomes, winglets, wind-shields and propellers, for example,

Broadband Radio Frequency Interference (RFI)

Figure 1

0.001 0.01 0.1 1 10 100 1,000 10,000

Frequency (MHz)

Rel

ativ

e N

ois

e S

ign

al

Str

eng

th (

dec

ible

) 10

0

-10

-20

-30

-40

-50

Streamering

Corona

Arcing


is also known as St. Elmo’s fire.

When the charge on an aircraftreaches 100,000 to 200,000 volts,the electrical fields on the aircraftbecome concentrated on its extremi-ties — wing tips, tail surfaces, etc.(The voltage is not lethal becausethe current level is extremely low.)Air is ionized at the sharpest pointson these surfaces and current dis-charges into the surrounding air, re-sulting in RFI that causes a hissingnoise in a radio receiver.

Antennas, particularly those withsharp points, discharge currents thatpresent a strong signal to the VHFreceiver. This can cause the auto-matic gain control (AGC) to de-sen-sitize the receiver, which willremain quiet and be temporarily use-less for reception. This is likely togo unrecognized by the pilot untilthe charge level dissipates suffi-ciently to allow the receiver to oper-ate properly, and the flight crew maybe queried by air traffic control fornot responding to radio calls. Thissituation occurs more often thancommonly recognized and results inremoval of equipment for a suspectedintermittent receiver problem and a“could-not-duplicate” shop report.

Streamering is usually a brilliantelectrical discharge that may trailfrom the aircraft, including dielec-tric surfaces (an insulating materialthat can contain an electrical field

and undergo electrical polarization)such as composite radomes and com-posite winglets, as well as glass andceramics. The discharge may stream(jump) several inches from a com-posite material to a metal airframeor into the air. Or a charge can buildup on a painted surface before itstreams to a nearby unpainted screwor rivet head. Streamering can bereduced by coating nonconductiveareas with a high resistance paintthat allows the charge to be bled tothe airframe.

Arcing can create a brilliant lumi-nous glow, similar to streamering,but it is usually limited to jumpingpoint to point in distances of an inchor less. Often it is barely visible,but when observable it has the ap-pearance of a curved line. Arc dis-charges are almost always the resultof a part of the aircraft being electri-cally insulated from the main struc-tu re , so tha t when a chargeaccumulates on the isolated part, itarcs to the main structure.

Arcing is usually caused by the fail-ure of some device installed to re-duce precipitation static noise, suchas broken bonding straps at controlsurface hinge points, gear doors, ac-cess panels, fuselage cargo and en-trance doors. Cracked radomelightning diverter straps and static dis-charger bases cause arcing. (Bond-ing straps at control surface hingepoints are critical. The U.S. Federal


Aviation Administration (FAA) haspublished many reports of controlsurfaces with missing or broken bond-ing straps that were welded at hingepoints or blown completely off theaircraft by lightning strikes.)

RFI is reduced by preventing the un-controlled discharge of static elec-tricity from the aircraft to thesurrounding air; i.e., the charge isbled into the air mass around theaircraft at a controlled rate by usinga discharge device.

One of the earliest such devices wasthe carbon-impregnated cotton or ny-lon wick device that spawned thegeneric name “static wick.” The car-bon provided a high resistance paththrough the wick, and the individualcarbon crystals provided a multitudeof small, sharp discharge pointsthrough which small individual cur-rents could discharge into the air. Asair movement and weather erodedthe wick and depleted the carbon inthe exposed area, the core began toturn light gray and then white. Thenthe outer shield was cut back, ex-posing a new carbon surface, andthe faded portion of the wick wasremoved by a square cut.

Chelton Electrostatics Ltd., foundedin England in 1947, approached thedesign of aircraft static dischargersusing the same theories, but with aslightly different twist. In their ba-sic design, the discharger tips are

composed of a number of finenichrome wires with each providinga discharge path for static build-up.

In the late 1960s and early 1970s,Shaw Aero Devices developed a dis-charger composed of crystallizedcarbon, based on the theory that thecarbon would provide the requiredlocalized resistance and the pointsof the crystals would provide a mul-titude of discharge points that wouldbleed off low currents, resulting inless noise generation. These unitsworked very well and were adoptedfor use on several fleet types.

While all of the various dischargersprovided adequate quieting duringnormal electrostatic charging, air-frame damage continued to occurwhen charge levels accumulatedfaster than the dischargers could dis-sipate them or when an aircraft wasstruck by lightning. [A major U.S.airline’s recurrent training publica-tion recently reported that “commer-cial pilots experience an average ofone lightning strike per 3,000 flighthours and airlines average one hit peraircraft per year.” Editor.] The smallneedle points could not dissipate suchmassive currents, which often resultedin the destruction of the dischargerand damage to the aircraft.

A discharger device was developedthat would operate at low-charge lev-els, but could direct massive currentsinto the atmosphere. A metal sleeve


was placed around the carbon rod(similar to that on the top three dis-chargers in Figure 2), and it was testedin a high-voltage laboratory. Undernormal conditions, the device dis-charged with minimal RFI; whenmassive currents were applied, thecharge jumped the gap between thebase of the discharger and the bypasssleeve, traveled along the sleeve anddischarged into the atmosphere at theupper end of the sleeve. Instead ofaircraft metal being destroyed, thesleeve melted, ionized and evaporatedinto the air. Some devices have con-tinued to effectively discharge low-level charges even with the sleevecompletely destroyed.

Dischargers lower the aircraft staticcharge without creating RFI by elimi-nating uncontrolled discharges fromaircraft structure and preventing co-rona. However, the dischargers arevery intolerant of poor maintenanceand they must make a good electricalbond with the airframe.

They must be inspected frequently,not just to confirm their presence butto check them for cracks in the mount-ing bases, loose mounting bases, cor-rosion near or under the mountingbases, burned or missing needlepoints, and missing parts. If a sealantis used between the discharger mount-ing base and aircraft structure, it mustbe a conductive sealant recommendedor approved by the airframe and dis-charger manufacturers.

Static dischargers probably providethe best return on investment for ev-ery dollar spent on installation andmaintenance of any device on the air-craft. For example, a major airlinenoted an abnormally high usage ofquartz landing-light lamps. Failedunits were examined and it was notedthat the filaments were fractured andthat the normally sharp corners of

the small curved shields mountedwithin the lamps to eliminate glare inthe cockpit were melted. One theorywas that when the filament failed, thehot wire (which is coiled like a spring)flailed around and arced on contactwith the metal shields, which are con-nected to the ground terminal of thelamp assembly.

Further examination revealed that insome cases the remaining section offilament was connected to the groundside of the lamp and would not havearced on failure. It was also difficultto believe that a flailing filamentwould only contact the corners of

Figure 2

Photographnot available


the shield and nowhere else. Therehad to be another cause for theseeffects. Discussions with two differ-ent lamp manufacturers only raisedmore questions and a number offailed lamps were sent to them forexamination and evaluation.

When their reports came back, bothmanufacturers said that the filamentshad failed after only a few hoursrather than near their rated life of 50hours. (The hours a lamp has beenin actual use can be determined bymeasuring the depletion of tungstenfrom the filament.) In addition, bothmanufacturers reported that highvoltage apparently caused the fila-ments to fail.

The filament problem occurred dur-ing the same period of research onthe bypass sleeve project. The meltedcorners of the shields resembled themelted sleeves. Were they related?Could the high voltages present inthe static build-up on an aircraft bethe high voltages the lamp manufac-turers suspected were causing thefilament failures? The melted cor-ners of the shields could have beenthe result of high currents enteringor leaving these sharp points.

Several new lamps were forwardedto a high-voltage laboratory and weresubjected to high levels of static elec-tricity applied to the face of the lamps.The filaments failed and examina-tion showed that the corners of the

shields were melted. Field failureswere duplicated in the laboratory. Butwhat was the solution to the prob-lem? If the bypass sleeve functionedon a discharger, would some form ofbypass device function on these land-ing lights? A one-eighth-inch-thickby one-quarter-inch wide strip of alu-minum was installed diagonallyacross the face of the lamp and at-tached to the landing-light housingusing the same screws that held thelamp in place.

Identical tests were conducted in thelaboratory with this strap in placeand no lamp failures occurred. Flighttests showed that the proximity ofthe strip to the focal point of thelamp resulted in no shadow or mea-surable reduction in illumination.When all aircraft were modified withthis strip, landing-light lamp usagewas greatly reduced.

During modification of the landinglights, it was discovered that the glasslenses on aft-facing, white, high in-tensity wingtip lights appeared to bedelaminating and small chips of glasswere missing. Mechanics in the over-haul shop reported that this was acommon occurrence and lens replace-ment was high. Close examination ofremoved lenses showed evidence ofmelting and flaking.

The glass lenses of these lights werethe farthest point aft on the wingsand would therefore be subject to


discharge from static build-up. Analuminum strip similar to that usedon the landing light was installed onthe inboard side of the aft-facinglight lens, running from the basewhere it was attached to the airframe,just aft of where the lens began tocurve. The strip became the most aftpoint and higher discharges bypassedthe lens. After the changes weremade, lens replacement costs werereduced substantially.

Maintenance technicians need tomake sure that static electricity dis-charge devices on aircraft are in goodworking condition. The devices areimportant tools in controlling dis-charges that cause RFI disruption of

communication and navigationequipment functions and in prevent-ing physical damage to the aircraft.�

About the Author

John F. O’Neill Jr. is president ofJAYFON Enterprises, an aviationconsulting company. O’Neill wasemployed for 28 years at USAir,where he served in a variety of posi-tions, including director of develop-ment engineering and manager ofavionics, systems and equipment en-gineering. He is a frequent contribu-tor to technical aviation publicationsand is an FAA-designated engineer-ing representative.


NEWS & TIPS

Sioux Tools PublishesNew Aerospace

Brochure

Sioux Tools, one of the leading manu-facturers of air-driven power tools,recently published a four-page bro-chure of tools for the manufactureand repair of aircraft. Products suchas air drills, flat-head angle drills,45- and 90-degree drills, rivetinghammers, rivet shavers and skin-clamp runners are fully illustratedwith specifications for each item.

Contact Sioux Tools Inc., Box 507,Sioux City, IA 51102 U.S. Tele-phone (712) 252-0525.

Quiet UPS FreighterMakes First Flight

A United Parcel Service (UPS) Boe-ing 727 powered by three Rolls-Royce Tay 651 engines recentlymade its maiden flight following anengine retrofit by the Dee HowardCo. The flight lasted one hour andreached an altitude of 23,00 feet.

The Tay engine conversion is in-tended to allow the aircraft to easilymeet the U.S. Federal Aviation Ad-

ministration (FAA) Stage 3 noiseregulations. Additional benefits areexpected to be reduced fuel con-sumption and lower emissions. Fi-nal certification is targeted forNovember 1992. UPS has placed or-ders for modification of 40 aircraftwith options for an additional 40.The modification, which dubs thecompleted aircraft as the “B727QF”for quiet freighter, is expected togive the venerable Boeing 727s anextended service life.

SAE Aerotech ’92Conference Scheduled

The Society of Automotive Engi-neers (SAE) has announced theschedule for Aerotech ’92. The an-nua l mee t ing wi l l be a t theDisneyland Hotel in Anaheim, Ca-lif., Sept. 23-26, 1992. The confer-ence and exposition involves the totalspectrum of aerospace engineering,with the main focus on the commer-cial aerospace industry.

The conference agenda will includethe technologies applied to the de-sign, development, manufacturing,integration, testing, operation, main-tenance and support of aircraft, pow-erplants and systems.


Contact SAE Communications Di-vision, 400 Commonwealth Drive,Warrendale, PA 15096 U.S. Tele-phone (412) 772-7131.

Course Offered inRepair of Composite

Structural Components

Texas Aero Tech, an airframe andpowerplant maintenance technologyschool based at Love Field in Dallas,Texas, now offers an advanced courseon composite repair. The course isopen to any certified technician withan airframe mechanic rating.

The course, which is conducted overa five-day period, involves topics re-lated strictly to structural airframerepair of composite components.Among the topics covered are:

• A history of composites;

• Types of materials;

• Types of repairs and repairmethods; and,

• Fiber science, including car-bon and kevlar materials.

During the course, students willspend about 50 percent of the timein the classroom and the remainderof the time in laboratory/shop work

involving hands-on experience incomposite repair techniques.

Contact Randy Long, admissions di-rector, Texas Aero Tech. Telephone(214) 263-9780.

Ryder Airline ServicesTo Disassemble an

Airworthy Boeing 747

Ryder Airline Services in Ardmore,Okla. has announced that it will per-form what may be the first disas-sembly of an airworthy Boeing 747for parts. R. Frank Leftwich, Ryderexecutive vice president, said, “Thisis a case where the sum of the partsis valued greater than the whole.”

The aircraft was purchased throughPolaris Leasing from General Elec-tric Credit Corp. after returning froma lease to now-bankrupt Pan Ameri-can World Airways.

FAA Mandates GPWSFor Commuter Aircraft

The U.S. Federal Aviation Adminis-tration (FAA) on March 17, 1992, is-sued a final regulation that requires allturboprop airplanes with 10 or moreseats to be equipped with ground prox-imity warning systems (GPWS). Such


systems have been mandatory in theUnited States on large jet aircraft since1974 and on small jets since 1978.

The new rule was issued following astudy of accidents in which fullyqualified crews flew their aircraftinto the ground with no apparentawareness of impending disaster.

The study showed that 64 percent ofthe accidents between 1970 and 1988involving altitude control might havebeen averted if the aircraft had beenequipped with GPWS and the crewhad responded to a warning.

Under the rule, aircraft must beequipped with an approved warningdevice within two years of the effec-tive date. The regulation is expectedto affect more than 800 currentlyoperating aircraft.

ANSI Catalog ListsSafety and Health

Standards

Products and equipment standardsthat aim to help safeguard individu-als in the workplace are listed in theAmerican National StandardsInstitute’s (ANSI) 1992 Safety andHealth Catalog.

The nearly 1,200 standards listed pro-vide authoritative guides for indus-trial safety and individual protection.

They cover items ranging from flash-lights to fire alarms, and from ma-chine tools to mining equipment.

Founded in 1918, the institute is aprivate, not-for-profit membershiporganization that coordinates theU.S. system of voluntary consensusstandards. It is the only U.S. organi-zation with the authority to approveAmerican National Standards. Freecopies of the catalog can be obtainedby contacting ANSI’s PublicationsDepartment at (212) 642-4916.

Pratt & WhitneyBecomes TV Producer

Extensive training requirements as-sociated with being a major manu-facturer of turbine aircraft engineshave led the Pratt & Whitney Groupof United Technologies to install anextensive video production facility.The powerplant supplier for aircraftmanufacturers such as McDonnellDouglas, Boeing and Airbus is re-sponsible for developing and con-ducting technician training forengineers and service personnel whowork on its engines.

The complexity of the powerplantsand the technical nature of the train-ing demands high quality and cleardefinition video images. To achievethose standards, the company installeddigital production and duplicating


equipment. Use of digital technologyensures that the tapes suffer loss inaudio or visual quality when copied

repeatedly. The video department iscomprised of a 1,000-square-foot stu-dio and three editing suites. �

MAINTENANCE ALERTS

This information is intended to pro-vide an awareness of problem areasso that occurrences may be preventedin the future. Maintenance alerts arebased upon preliminary informationfrom government agencies, aviation or-ganizations, press information and othersources. The information may not beentirely accurate.

Older MagnetosContinue to Cause

Accidents

In a recent series of Safety Recom-mendations, the U.S. National Trans-portation Safety Board (NTSB) citeda number of fatal and serious acci-dents attributed to magneto failure.In one accident, a small amphibianaircraft suffered a total loss of enginepower about 10 minutes after takeoffand crashed while attempting anemergency landing. The airplanestruck a tree stump after touchdown,nosed over and was destroyed. Thepilot flying was seriously injured andthe instructor pilot was killed.

A subsequent investigation found

that both the left and right magnetoswere capable of producing adequateignition sparks at ambient tempera-ture but they would not operate whenheated to normal in-flight engine op-erating temperatures. The ignitioncoil in each unit was cracked, andthe insulation would break downwhen heated, resulting in faulty op-eration of the magneto.

This problem is not new. The U.S.Federal Aviation Administration(FAA) issued an Airworthiness Di-rective (AD) in 1973 (applicable toall Bendix S-20, S-200, S-600 andS-1200 series magnetos) that re-quired mandatory compliance withBendix Service Bulletin No. 560A.The AD calls for replacement of ro-tating magnets and ignition coilsprior to the accumulation of 2,000hours of flight time on the unit. Inthe instance cited above, however,the airplane (used primarily for sportand pleasure flying) had only accu-mulated a total of 1,295 hours. As aresult of this low utilization, it wasnot mandatory for the magnetos tobe modified even though they weremore than 35 years old.


Similar experiences have been docu-mented in many accidents involvingvarious types of airplanes. Since1985, the NTSB has cited magnetosas a cause or contributing factor in92 accidents involving 22 fatalitiesand 21 serious injuries. NumerousService Difficulty Reports (SDRs)have been submitted to the FAA, in-cluding 130 reports referencingcracking, burning, arcing, leaking orother deficiencies in magneto igni-tion coils.

Routine InspectionsMay Not Disclose

Defects

Normal procedures for annual and100-hour inspections usually con-sist only of a timing check and anoperational test to assure that eachmagneto is functioning properly. Themagneto is usually not removed un-less the operational check reveals aproblem. It is not uncommon, there-fore, for a unit to remain in servicefor 10, 20 or even 30 years withoutever having been subjected to ateardown inspection.

SDRs and accident investigationfindings continue to confirm that thislevel of inspection and maintenanceis inadequate to ensure that the mag-netos will continue to function asdesigned when subjected to the nor-mal effects of heating and vibration.

Magneto manufacturers have issuedservice bulletins and service instruc-tions prescribing specific overhauland inspection intervals, but most ofthese are not mandatory and there isno specific regulatory requirementfor routine, periodic removal, over-haul or comprehensive inspection ofmagnetos. One manufacturer has is-sued a service bulletin that recom-mends:

• A specific visual inspection ofthe magneto whenever it is foundnecessary to adjust timing;

• A 500-hour inspection of theimpulse coupling; and,

• A complete overhaul of themagnetos at each engine over-haul or at four-year intervalsregardless of accumulatedtime in service.

As a result of the high number ofserious accidents, the NTSB has is-sued recommendations to the FAAcalling for issuance of ADs thatwould:

• Require mandatory compliancewith Bendix Service BulletinNo. 560A at the next annualor 100-hour inspection, which-ever occurs first;

• Require that all Teledyne Con-tinental Motors, Bendix, andSlick Aircraft Products mag-


netos be subjected to a teardowninspection at the next annualor 100-hour inspection and atevery 500 flight hours there-after; and,

• Require mandatory compliancewith all manufacturer’s recom-mended overhaul intervals andinstructions.

Wear of PropellerControl Unit Cited asCause of Fatal Crash

On April 15, 1991, an Embraer EMB-120 commuter aircraft crashed dur-ing landing approach at Brunswick,Georgia. The airplane was destroyed.The two pilots, one flight attendantand all 20 passengers were killed.

The U.S. National TransportationSafety Board (NTSB) determinedthat the probable cause of this acci-dent was the loss of control in flightas a result of a malfunction of theleft engine propeller control unit(PCU), which allowed the propellerblade angles to go below the flightidle position. The circumstances ofthis accident indicated that a severeasymmetric lift and thrust conditioncaused a left roll that led to loss ofcontrol of the airplane.

The powerplant and propeller exami-nations indicated that the engines

were operating normally but that apropeller system malfunction oc-curred, causing abnormally low pro-peller blade angles and a high dragcondition with loss of lift on the leftside of the airplane.

Tests of the left propeller compo-nents indicated a propeller bladeangle of about 3 degrees at impact,while the PCU ballscrew positionwas consistent with a commandedblade angle of 79.2 degrees. The dis-crepancy between the ballscrew po-sition and the position of thepitchlock acme screw is a strong in-dication that a disconnect betweenthese two components occurred be-fore impact and that the left propel-ler had gone into an uncommandedlow blade angle below the normalflight range. The right propeller com-ponents were found to be at the ex-pected positions and properlyconnected.

Examination of the left PCU dis-closed extreme wear on the quillspline teeth that normally engagedthe titanium-nitrided splines of thepropeller transfer tube. It was foundthat the titanium-nitrided surface wasmuch harder and rougher than thenitrided surface of the quill. There-fore, the transfer tube splines actedlike a file and caused abnormal wearof the gear teeth on the quill. Usingmeasurements and the inspectionprocedures for the quill and transfertube that are in the manufacturer’s


service instructions, it was deter-mined that the left PCU quill splinewas worn to the extent that its gearteeth did not engage the transfer tubespline. Test cell and flight testingshowed that the propeller blade anglecould not be controlled by the PCUwith a disengaged transfer tube.

During the subsequent investigation,it was found that the original splinetube-quill components were nitridedfor protection against wear. Althoughthere had been no SDRs of adversewear, the manufacturer changed theprocess in June 1990 to call for theapplication of titanium-nitride coat-ing to further enhance the wear resis-tance. Subsequent test cell runs andcertification tests disclosed no prob-lems or any visible signs of wear.Because this spline-to-quill connec-tion is not subject to a high torqueload (only about 7 inch-pounds), wearwas not considered a factor.

The failure mode and effects analy-sis of all propeller components as-sign possible fai lures to twocategories. The first group includedfailures that had a predicted prob-ability of occurrence of less than10-9, and the second group includedfailures with a predicted probabilityof greater than 10-9. The transfer tubeand quill interface were listed in thefirst group and were assigned as an“on condition” component, i.e., in-spection is only required after a prob-lem is found during service.

During the investigation, the NTSBbecame aware of other incidents in-volving the PCU. On three occa-sions involving different airplanes,the operators found that a propellerwould not feather during groundtests. In these instances, it wasfound that the ballscrew teeth thatengage the quill were extremelyworn and would not engage the gearteeth on the quill. Although differ-ent from the situation found on theaccident airplane, these incidentswere found to be further instancesof wear conditions not consideredin certification.

As a result of these investigationfindings, the NTSB issued SafetyRecommendations to the U.S. Fed-eral Aviation Administration (FAA)which call for:

• A certification review of theHamilton Standard model 14RFpropeller system and modi-fication(s) to ensure that thesystem complies with the cer-tification standards;

• Examination of the certifica-tion basis of other model pro-peller systems that have thesame design characteristics asthe model 14RF and ensure thatthe fail-safe features of thosepropeller systems will func-tion properly in the event ofunforeseen wear of componentsin the system; and,


• Establishment of a periodic in-spection time requirement forthe transfer tube splines, servoballscrew and ballscrew quillon the Hamilton Standard model14RF and other propeller sys-tems of similar design.

NTSB Issues SafetyRecommendation onTeledyne ContinentalMotors 0-200 Engines

The U.S. National TransportationSafety Board (NTSB) recently is-sued two safety recommendationspertaining to the 0-200 series ofTeledyne Continental Motors (TCM)piston engines used on many lightaircraft. The investigation of a fatalCessna 150G crash disclosed thatthe rocker shaft bosses had failed onthe number three cylinder, resultingi n n o v a l v e a c t i o n a n d asevere loss of power. While attempt-ing an emergency landing, the stu-dent pilot was killed and theinstructor seriously injured.

Examination of the failed part dis-closed evidence of fatigue cracksthrough the rocker shaft bosses. Thecrack initiation was at the stampedcharacters on the top of the centerboss. After this boss failed, the outerportions quickly failed and resultedin complete separation and release

of the rocker shaft for both valves inthis cylinder.

These cylinder heads are made ofcast iron and the original installa-tion was without any bushings. Asthe cast iron boss wears, the manu-facturer's manuals permit reamingoversize for installation of a bush-ing. However, technicians are cau-tioned to ensure that the minimumwall thickness of the boss is no lessthan 0.25 inches prior to reamingand at least 0.18 inches after ream-ing. One portion of the rocker shaftboss on the failed engine was foundto have a wall thickness less than0.18 inches.

Laboratory examination further re-vealed that a portion of the fractureon the exhaust valve side boss con-tained several casting flaws. Baked-on oil residue was present in this area,which suggests that a portion of thisboss had been separated for sometime, but had gone undetected.

Other instances of similar failureswere investigated and the NTSBfound that fatigue cracking had ini-tiated from longitudinal scratchesthat occurred when bushings wereinstalled in reworked bosses. TheNTSB has not determined the num-ber of hours of operation needed topropagate a crack in a cylinder headboss from a detectable size to fail-ure, but it believes that these bossesshould, at least, be subjected to non-


destructive testing at each cylinderor engine overhaul.

As a result of these findings, theNTSB has sent recommendations tothe U.S. Federal Aviation Adminis-tration (FAA), suggesting that:

• An Airworthiness Directive (AD)be issued to require that the rockershaft bosses on P/N 641917 cyl-inders and earlier versions ofthe cylinder be subjected to pe-riodic nondestructive testing todetect the presence of cracksor other defects; and,

• An AD be issued to requirecompliance with the provisionsof TCM Service Bulletin M73-13 when bushings are installedin the rocker shaft bosses.

Pylon Cracks Result inEngine SeparationsFrom Boeing 707s

On April 25, 1992, the number 3engine and pylon separated from theright wing of a Boeing 707-324Ccargo aircraft during takeoff at Mi-ami, Fla. After separation from thewing, the number 3 engine struckthe number 4 engine nose cowling,impacted the runway and came torest on the grass adjacent to the run-way. The airplane was able to con-tinue the takeoff and safely returned

to the airport without incident.

On March 31, 1992, the number 3and number 4 engines separated froma Boeing 707-321C while climbingthrough 31,000 feet over southernFrance. The airplane landed safelyat a military airbase and the two en-gines were located about 90 milesfrom the landing site.

Both aircraft had relatively high timeand cycles: 53,257 hours/20,399cycles and 60,779 hours/17,873cycles, respectively. In each case,the investigation disclosed that thefailure initiated in a fatigue crack inthe number 3 engine inboard midsparpylon support fitting. The potentialfor cracks in these fittings wasknown and U.S. Federal Aviation Ad-ministration Airworthiness Directive(AD) 88-24-10, which incorporatesBoeing Service Bulletin No. 3183(Rev. 2), addresses the inspection offittings in the number 2 and 3 posi-tions of Boeing 707-300 series air-craft. Although the two aircraft hadpreviously been inspected in accor-dance with the AD, the presence of asmall fatigue crack had apparentlygone undetected and the crack con-tinued to grow to the point of even-tual failure.

As a result of these incidents, theU.S. National Transportation SafetyBoard (NTSB) issued a Safety Rec-ommendation calling for a revisionto AD 88-24-10 to:


• Significantly decrease the timesbetween inspection intervals;and,

• Require an improved means ofinspection to detect small cracks.

Open Baggage DoorResults in Fatal Crash

The Canadian Transportation SafetyBoard (TSB) recently issued an Avia-tion Occurrence Report detailing theinvestigation and findings followingthe crash of a Piper PA-23-250 air-craft.

The report stated that the Piper Az-tec had just become airborne whenthe aircraft’s nose baggage dooropened. When attempting to returnto the field, the aircraft nosed downand crashed, killing all five occu-pants on board.

The TSB determined that the nosebaggage door locking mechanismwas defective because of inadequatemaintenance, which allowed the doorto open in flight. The report indi-cates that the pilot may have beenpreoccupied with the open baggagedoor and the possibility of the bag-gage bin contents falling out and, asa result, allowed airspeed to decrease

to below stall speed, and he lost con-trol of the aircraft. The noise andvibration created by the open doormay have distracted the pilot andcould have masked the warnings ofthe approaching aerodynamic stall.

The nose compartment baggage dooron this type of aircraft opens up-ward and is located on the right-hand side of the nose forward of thecockpit. The door latch mechanismoperates two lock arms which, whenin the locked position, protrude fromthe fore and aft edges of the doorand fit into recesses in the frame.

Although the luggage door was tornfrom the aircraft in the crash, inves-tigators were able to determine thattwo screws were missing from theforward lock arm bracket and thatthe one remaining screw had beenloose. The aft lock arm mechanismwas severely worn and the entirelatching mechanism exhibited wearand lack of maintenance.

Although the aircraft should havebeen controllable and the open doorshould not have created aerodynamicdrag or control problems beyond theaircraft rudder authority, it was obvi-ous that the accident chain of eventswas initiated by a lack of mainte-nance on this simple, but critical, doorlatching mechanism. �


NEW PRODUCTS

Oxygen BoosterEnables More

Complete Use of FillerBottles

The complete use of all the oxygen ina high pressure bottle has always beena problem for technicians. When thebottle is depleted below the full sys-tem pressure, it is often useless, eventhough it may have had 1600 or 1700pounds per square inch (PSI) remain-ing pressure. A new unit introducedby Tronair is intended to solve thatproblem.

Tronair claims that this simple, light-weight booster unit operated by shopair pressure can allow the use of arefill bottle down to as little as 500PSI while refilling a system to 1800PSI.

Contact Tronair, 1740 Eber Road,

Holland, OH 43528 U.S. Telephone(419) 866-6301.

Product ReducesExposure to Stripped

Screw Heads

ND Industries has introduced a prod-uct called Drive Grip, which is saidto reduce tool wear and rounding orstripping of screw heads. The manu-facturer claims that a single drop ofthis product provides a positive gripand increases the grip strength be-tween the screwdriver and the screwby as much as 400 percent. The prod-uct can also be used on hex head/allen wrench screws as well as anytype screw head.

Drive Grip is said to reduce fatigueand potential technician injury byeliminating the need to hold exces-sive pressure on the tool to preventslipping. The product is odorless,nontoxic and noncorrosive. It is im-mediately usable without mixing andhas an unlimited shelf life. Accord-ing to the manufacturer, Drive Gripdoes not irritate normal skin andwipes clean with no staining or resi-due.

For a free sample and additional in-formation, contact ND Industries,



1893 Barrett Road, Troy, MI 48084U.S. Telephone (313) 362-1209.

LPS DevelopsCorrosion Control

Manual for AviationMaintenance

LPS Laboratories Inc. recently pub-lished Aviation Corrosion ProtectionManual. The comprehensive docu-ment details the company’s 30 yearsof experience in manufacturing cor-rosion preventive compounds andmethods of application. It is designedto help aviation maintenance techni-cians establish and implement cor-rosion prevention programs formodern as well as aging aircraft.However, the document does not ad-dress corrosion removal or reworkand is only intended to provide as-sistance in preventing corrosion.

The manual includes generally rec-ognized procedures obtained fromaircraft manufacturers, industry ex-perts, the U.S. Federal Aviation Ad-ministration (FAA), and maintenancepersonnel. It provides informationon each of the LPS corrosion pre-ventive compounds, cross-referencesspecifications, reviews applicationprocedures and presents recommen-dations for application equipmentand methods.

The document is divided into 10

sections referencing corrosion con-trol recommendations for commer-cial, corporate, light aircraft andhelicopters. Aviation maintenancetechnicians may obtain a free copyby submitting a request in writingto: LPS Aviation Corrosion Protec-tion Manual, 4647 Hugh HowellRoad, Tucker, GA 30084 U.S.

Paint Stripper Said toBe Environmentally

Safe

Research Chemicals Inc. has intro-duced a product that it claims is ef-fective in removing paint andcoatings from paint guns and equip-ment used in paint application, yetis safe to use and friendly to theenvironment.

RC 10 NC Stripper does not containmethylene chloride, acetone, methylethyl ketone, toluene or phenol andis said to be nonflammable and slowevaporating. The material has a highflash point of 195 degrees F. andwashes off with water. The manu-facturer claims that RC 10 NC safelyremoves gel-coat polyesters, elas-tomers, lacquers, enamels and someurethanes and epoxies, and is safe touse on aluminum.

Contact Research Chemicals Inc.,P.O. Box 1492, Fort Worth, TX 76101U.S. Telephone (817) 451-7565.


Multipurpose ToolHandy for Line

Technicians

Disstim Corp., a U.S. manufacturerof hand tools and other products, isnow offering a combination screw-driver and nutdriver that it claims isunlike anything else currently avail-able. The hand screwdriver-like toolhas adjustable jaws that the manu-facturer states can accommodatemetric size nuts from 3.5 mm up to6 mm, U.S. standard nuts for No. 4through 1/4 size fasteners, plus stan-dard slotted, Phillips-head and Torx-head screws.

The snap lock swivel cap allows allbits to be stored in the handle andthus provides technicians, such asline mechanics, with the ability toservice a number of fasteners with asingle tool. The materials and con-struction are claimed to be of excep-tional quality, and the manufacturerstates that testing proved the capa-bility to apply sufficient torque tostrip the threads of a 10-32 fastener.

Contact Disstim Corp., 217 S.Hurffville Road, Deptford, NJ 08096U.S. Telephone (609) 227-7904.

Battery ConditionMonitor Reported to

Eliminate Failures

A battery monitoring unit, the BatstatMonitor, has been developed byBatcon Inc. The unit is said to pro-long battery life and reduce mainte-n a n c e a n d w a t e r s e r v i c erequirements when installed onground service equipment.

The monitor can be installed on anyvehicle or machine that uses a bat-tery for primary power or engine start-ing. When permanently connectedto the battery, a light-emitting diode(LED) panel visible to the operatordisplays a green light under normaloperating conditions. Whenever thevoltage falls below a predeterminedlevel, even momentarily, a red warn-ing light illuminates and remains onuntil manually reset.

The manufacturer claims that thisunit automatically identifies a weakor defective battery, even in mul-tiple battery installations. Time con-suming hydrometer checks are saidto be eliminated, and load checksare no longer required. According toBatcon, the use of this monitor per-



mits maximum use of batteries andreduces downtime due to unexpectedbattery failures.

Contact Batcon Inc., 107 South Av-enue, Fort Walton Beach, FL 32547U.S. Telephone (904) 863-8828.

Primer Claimed Safer,Healthier Than Zinc

Chromate Paint

Tempo Products Co. has introduceda zinc oxide primer it says providesequivalent protection and bondingfor aluminum and steel with lesstoxic irritants than zinc chromateprimer paints.

The zinc oxide primer is supplied inpressurized cans containing no fluo-

rocarbon propellant gases. The prod-uct is said to have improved abra-sion resistance and to dry in aboutone-half the time required for thezinc chromate products.

Also available from Tempo Prod-ucts is an epoxy propeller coating.The coating is claimed to remainadhered to the blade for more flyinghours than standard lacquer andenamel products, thus reducing theeffects of corrosion and erosion onblade life. The coating is availablein a standard medium gray to matchthe original propeller color as wellas a bright red or yellow to providecontrast and visibility of the bladepath.

Contact Tempo Products Co., P.O.Box 39126, Cleveland, OH 44139U.S. Telephone (216) 248-4241. �

static electricity in flight threatens aircraft safety · pdf filestatic electricity in flight...

Documents