Aviation Mechanics Bulletin March-April 1999

FLIGHT SAFETY FOUNDATION Aviation Mechanics Bulletin MARCHÐAPRIL 1999

Monitoring Aircraft-tire Pressure Helps Prevent Hazardous Failures FLIGHT SAFETY FOUNDATION Aviation Mechanics Bulletin Dedicated to the aviation mechanic whose knowledge, craftsmanship and integrity form the core of air safety. Robert A. Feeler, editorial coordinator MarchÐApril 1999 Vol. 47 No. 2

Monitoring Aircraft-tire Pressure Helps Prevent Hazardous Failures ...... 1 Maintenance Alerts...... 14 News & Tips ...... 16

AVIATION MECHANICS BULLETIN Copyright © 1999 FLIGHT SAFETY FOUNDATION INC. ISSN 0005-2140 Suggestions and opinions expressed in FSF publications belong to the author(s) and are not necessarily endorsed by Flight Safety Foundation. Content is not intended to take the place of information in company policy handbooks and equipment manuals, or to supersede government regulations. Staff: Roger Rozelle, director of publications; Mark Lacagnina, senior editor; Wayne Rosenkrans, senior editor; John D. Green, copyeditor; Karen K. Ehrlich, production coordinator; Ann L. Mullikin, production designer; Susan D. Reed, production specialist; and David Grzelecki, librarian, Jerry Lederer Aviation Safety Library. Subscriptions: US$35 (U.S.-Canada-Mexico), US$40 Air Mail (all other countries), six issues yearly. ¥ Include old and new addresses when requesting address change. ¥ Flight Safety Foundation, 601 Madison Street, Suite 300, Alexandria, VA 22314 U.S. ¥ Telephone: +1 (703) 739-6700 ¥ Fax: +1 (703) 739-6708

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FSF Editorial Staff

The first lines of defense against cat- every effort should be expended to astrophic failures of aircraft tire/ prevent tire-pressure problems. wheel assemblies are tire-pressure measurement by maintenance techni- Tire/wheel assemblies for most cians and preflight inspections by transport-category aircraft provide a flight crews. Despite the increasing cushion of compressed nitrogen that reliability and accuracy of pressure- supports the weight of the aircraft on and-temperature sensors and warning the ground; a mechanism for control- systems installed on newer aircraft, ling the aircraft during taxi, takeoff such systems provide only safety re- and landing; and traction for braking. dundancy and early warning of prob- Correct tire-pressure maintenance is lems between routine checks. Aircraft a key factor that enables tire/wheel tires perform reliably so often that assemblies to perform reliably under complacency may develop in tire- high static loads and dynamic loads. pressure inspection and adherence to proper inflation/deflation procedures. These loads at takeoff speeds and land- Nevertheless, the potential conse- ing speeds generate high centrifugal quences of failure are such that forces that produce stress and wear in tires. Nevertheless, the most detrimen- multiplying the unloaded-pressure tal factor affecting tire performance is value by 1.04.2 excessive heat generation, according to tire manufacturers’ tests. Heat gen- Serious Consequences eration in aircraft tires typically ex- Linked to Incorrect ceeds all other types of tires. As tires Tire Pressure rotate, heat is generated by friction during tire deflection and by tread-sur- Deviations from approved tire- face oscillations (traction wave) pressure maintenance practices can caused by distortion of tires resuming have serious consequences. Low tire their normal shape after deflection. pressure has been cited as a causal fac- (Tire deflection is the change in height tor in some accident-investigation re- from the axle to the ground when a ports (in many incident reports, causal tire is installed on an aircraft. Aircraft factors have not been specified).3 Nev- tires typically are designed for a de- ertheless, consequences of inadequate flection of 32 percent — 35 percent tire maintenance have ranged from for some tires — approximately twice separation of tire/wheel assemblies as much as tires on cars and trucks.)1 from aircraft on the ground, sometimes damaging nearby buildings and vehi- Correct tire inflation is essential in cles, to wheel-well fires and in-flight assuring that tires can withstand the loss of aircraft control. Tire/wheel- centrifugal forces and heat of normal assembly failures on aircraft also have operations, with an adequate margin been reported during pushback, and of safety for unusual operating condi- tire-tread delaminations have occurred tions such as rejected takeoffs and on takeoff, with various degrees of hard landings. damage to aircraft engines, control surfaces and other components. The airframe manufacturer’s procedures, as adopted by the operator, The accident and incident reporting typically show the correct inflation system (ADREP) maintained by the pressure for each tire on a tire/wheel International Civil Aviation Organi- assembly when it is installed on the zation (ICAO) contained the follow- airplane, as well as correct removal/ ing events linked to tire pressure: installation procedures. Maintenance technicians should note whether ¥ The report of a July 1991 accident “loaded” or “unloaded” inflation involving failure of tires on a pressures have been specified. If un- McDonnell Douglas DC-8-61 dur- loaded pressure has been specified, ing takeoff from Jeddah, Saudi the pressure actually used for the tire Arabia, said, “The aircraft de- under load must be computed by parted with underinflated tires.

2 FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ MARCHÐAPRIL 1999 During takeoff, two tires failed and postflight cooling period of two ignited. After takeoff, the gear was hours to three hours. By comparison, retracted and fire was introduced manufacturers typically consider a into main gear wells. A series of “hot tire” to be any tire with a car- system failures followed. … The cass temperature that exceeds ambi- aircraft structural integrity or con- ent temperature by 30 degrees Celsius trol was lost on short final.” [C; 54 degrees Fahrenheit (F)]). The Fourteen crewmembers and 247 emphasis on adjusting only cold-tire passengers were killed in the ac- pressures is to ensure reliable results. cident; the aircraft was destroyed. Pressure and temperature of a hot tire can be measured for some safety or ¥ The report for a March 1990 U.S. maintenance purposes, but there are incident involving a tire failure no accurate ways to adjust pressure that destroyed two tires of a under these conditions using com- McDonnell Douglas DC-9-30 monly available equipment. during takeoff from Atlanta, Geor- gia, U.S., said, “The operator’s Some tire manufacturers recommend daily service inspection requires specific maintenance procedures for that tire pressure be checked with hot tire conditions. The procedures a tire gauge. Two additional blown call for comparing tire pressures on tires occurred during takeoff from wheels of like gear types (nose gear the same operator’s station with- or main gear) to verify that they are in three months. Evidence of tire in the same range and at least equal underinflation was discovered af- to the specified operational loaded ter the third incident.” pressure. At ambient temperatures, tire pressures should not exceed the Authorities Recommend specified operational pressure by Correct Tire Pressure more than 5 percent. For a hot tire, For Each Flight however, the overpressure amount can exceed 5 percent significantly. As a general rule, the major manu- Comparative pressures, not the indi- facturers of tires recommend a daily vidual pressures, are significant. cold-tire inflation-pressure check and The following actions typically are maintenance of a written record. This appropriate when evaluating hot tires: check should include measurement of pressure with a calibrated hand-held ¥ If comparison of pressures gauge, noting ambient temperature. on the same gear shows wide (“Cold tire” refers to a tire at approx- variation above the specified imately the same temperature as the operational loaded pressure, surrounding air, typically after a possible reasons are previously

FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ MARCHÐAPRIL 1999 3 inaccurate pressure adjustments procedures for adjusting tire pressures or brake malfunctions; based on changes in ambient temperature. The Rubber Manufacturers As- ¥ If tire pressure below the speci- sociation (RMA) of Washington, D.C., fied operational loaded pressure U.S. for example, recommends that is measured, further evaluation when a difference in ambient temper- of the problem should wait until ature greater than 50 degrees F (28 cold tire pressures can be degrees C) causes a lower tire pressure checked and leak tests can be than the specified operating pressure, performed; and, the cold-tire pressure should be adjust- ¥ When tire temperatures appear to ed to the specified operating pressure.5 be excessively high compared to The relationship between pressure and normal experience, measurement temperature is that a temperature of the temperature is appropriate change of 5 degrees F (3 degrees C) is to evaluate tire condition. One equivalent to a change in tire pressure manufacturer, for example, rec- of approximately 1 percent. ommended that tires be removed from service if the tire-surface Concerning ambient-temperature temperature exceeds 225 degrees change, tire manufacturers also gen- F (107 degrees C) or if brake heat erally recommend that before taking creates temperatures that exceed off for an airport where ground tem- 300 degrees F (149 degrees C) peratures are significantly lower com- at points where the tire is in con- pared to the airport of departure, tire tact with the wheel surface.4 Oth- pressure should be adjusted so tires erwise, pressure checks should will be inflated to the specified oper- be conducted only after the tire/ ating pressure at the colder airport.6 If wheel assembly has cooled to the specified operating pressure is ambient temperatures. 220 psi (15 bars), for example, a temperature drop of 50 degrees F at the By recording ambient temperature destination would require increasing with the cold tire pressure, mainte- tire pressure before departure by ap- nance technicians can make accurate proximately 10 percent to 242 psi comparisons over time, more easily (16.5 bars). This adjustment — super- identifying tire anomalies related to seding the normal tire overinflation temperature and pressure loss, and limit of 5 percent at constant ambient avoiding conditions that lead to tire temperature — is necessary because failures. the operating-inflation pressure re- mains constant to support the load of Maintenance technicians also should a specified aircraft configuration. follow aircraft-maintenance-manual Without this adjustment, a tire/wheel

4 FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ MARCHÐAPRIL 1999 assembly would be significantly un- from approximately 150 psi to 230 psi derinflated on arrival at the destination. (10 bars to 16 bars), with some pressures reaching nearly 300 psi (20 bars). Successful tire-pressure maintenance A typical transport-category aircraft requires not only adherence to the tire may be rated for a static load of regular-interval checks and preflight 13,000 pounds (5,897 kilograms) and inspections, but also thorough inspec- a maximum operating speed of 239 tions of demounted tires (and tubes, if knots (443 kilometers per hour). Engi- applicable). Policies and procedures neering standards require a new aircraft also should specify how tire/wheel tire to withstand at least four times the assemblies will be removed from air- rated inflation pressure for three sec- craft and inspected following abnor- onds, and a recapped tire (retread) to mal events such as rejected takeoffs, withstand at least three times the rated hard landings, excessive brake-heat inflation pressure for three seconds. conditions or failure of the other tire/wheel assembly on that axle. RMA has published the following Disposition of tire/wheel assembly operating-inflation-pressure main- components based on such inspections tenance criteria for aircraft tires. These also should be explicit and compliance 1994 criteria represent a consensus of should be documented. tire manufacturers and provide an over- view of safe tire-inflation practices: Because load ratings, ply ratings, physical dimensions and pressure ¥ Pressure never should be re- specifications for aircraft tires are el- leased from a hot tire for ements of safe operation, an essential adjustment to the recommended step in all tire-pressure maintenance service pressure; is to identify each tire correctly and ¥ Tire pressure should be checked to remove from service any tire for at least once every 24 hours, at which standard identification marks the start of operations, and/or are missing or unreadable. after operation (hot tires should cool for at least two hours before Specified operating pressures for air- the check [some tire manufac- craft tires are measured in pounds per turers recommend three hours]); square inch (psi) or bars (one bar equals one million dynes per square centime- ¥ Mounted tires should be inflated ter; a dyne is the force required to ac- with dry nitrogen where available celerate a free mass of one gram by one from 100 percent to 105 percent centimeter per second per second). of the minimum loaded service One bar is approximately 14.7 psi. pressure. Minimum loaded ser- Typical operating pressures range vice pressure is the inflation

FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ MARCHÐAPRIL 1999 5 pressure necessary to support the deflating and adjusting pressure in maximum operational load for aircraft tires vary. Nevertheless, that wheel position; and, common safety practices are being re-emphasized by aircraft and ¥ Reinflate any tire found between wheel-component manufacturers 100 percent and 95 percent of because of five incidents involving minimum loaded service pressure deaths or life-threatening injuries in to the correct pressure. Reinflate recent years among maintenance any tire found between 95 percent technicians servicing tire/wheel and 90 percent of minimum load- assemblies. ed service pressure to the correct pressure, and record this pressure Records of the U.S. National Trans- adjustment in the maintenance log portation Board (NTSB) provide the book to monitor leak rates. following example of such an accident in 1992 at Dayton, Ohio, U.S. Aircraft, wheel and tire manu- involving a McDonnell Douglas facturers also stress the importance DC-9-32: “The left nose-wheel tire of maintaining equal pressure in dual was damaged during pushback tire/wheel assemblies. When pres- when the tow bar failed. The tire was sures are unequal, the wheel with the replaced and while being serviced on higher pressure will bear a propor- the airplane, the tire rim failed, fatal- tionally greater part of the total load, ly injuring the mechanic. Exami- possibly exceeding the load for nation of the bottle used to service which the tire was designed. The goal the tire showed two pressure scales, is to discover any reduction of pres- bars on the outside and psi on the sure — often invisible when one tire inside. One bar equals 14.7 psi. The has correct pressure — before both high end of the scale was 10,000 psi tires fail. Logbook entries for pres- [680 bars] and the bottle was pressure differences greater than 5 psi surized to 4,000 psi [272 bars]. The should be kept at each pressure tire was to be serviced to 150 psi adjustment so that any trend can be [10 bars]. Examination of the fracture identified, and appropriate preventive surface found, ‘No … manufacturing or corrective actions can be taken. or material defects fracture surface is consistent with a tensile-load type Caution in Tire-pressure of failure.’ According to the company, Adjustment Helps no low-pressure regulators were Prevent Injuries available. They are kept under lock and key when not in use, and the re- Procedures followed by airlines sponsible person had already gone and repair stations for inflating, home for the day.”

6 FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ MARCHÐAPRIL 1999 NTSB said that the cause of this that use of safety cages outside of accident was, “A failed wheel rim tire shops is impractical for routine following an inadvertent overinflation servicing of tire/wheel assemblies by the mechanic. Additional causes already installed on an airplane. were the failure of the mechanic to Given the limited protection and the be aware of the pressure being used, possibility of human error, proper and his failure to follow procedures. tire-servicing procedures must be A factor related to the accident was followed with maximum care. the failure of the company to make available low-pressure regulators for The safest practice in demounting tire/ their mechanics during normal work wheel assemblies is to deflate tires hours.”7 before wheels are removed from the aircraft. Caution also is essential while Training and procedures for mainte- unscrewing valve cores for deflation; nance technicians have stressed tire pressure can eject the valve core the necessity of using appropriate with bullet-like speed and force. pressure regulators, even if a wheel has an overinflation pressure-relief RMA said that other tire bursts have (OPR) valve. Aircraft maintenance occurred because of operating con- manuals and component-assembly ditions such as overheating due to manuals also have included explicit excessive taxiing or operating with a warnings about the extreme risk of tire underinflated or overloaded; and attaching a high-pressure nitrogen excessive wheel heating due to high- bottle directly to a tire/wheel assem- energy braking, or a dragging brake. bly because the pressure can exceed Incident reports from several the design limits for the wheel and sources show that malfunctioning the wheel tie bolts. wheel bearings also may cause overheating and tire failure. A tire/wheel RMA further recommends that when assembly that has been damaged in inflating a tire/wheel assembly, the service should be deflated by a re- nitrogen-supply line should be mote means or — at a minimum — regulated to a pressure no more than allowed to cool for at least three hours 50 percent higher than the tire-service before the tire is deflated, said RMA. pressure. The pressure of nitrogen gas in high-pressure bottles typically used to service aircraft is 3,000 psi to 4,000 Nitrogen Reduces psi (204 bars to 272 bars) — possibly Risk of Fire 10 times to 25 times the required tire- service pressure. Aircraft manufactur- Since 1987 in the United States, dry ers and operators have acknowledged nitrogen gas has been mandatory for

FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ MARCHÐAPRIL 1999 7 inflation of aircraft tires mounted on available, aircraft maintenance man- braked wheels of specific transport- uals should be consulted for exact category aircraft. The use of nitrogen procedures and tables required to (an equivalent inert gas may be calculate and limit the oxygen approved) is intended to eliminate the content of a tire/wheel assembly. possibility that a chemical reaction (oxidation) between atmospheric Several Factors oxygen and volatile gases from the Affect Accuracy in tire inner liner will cause an explo- Tire-pressure sion of tire/wheel assemblies. Major Measurement tire manufacturers recommend that nitrogen be used at all times for all As a guide to safety decisions and tires for maximum safety and for maintenance procedures, aircraft-tire reduced degradation of casing plies pressure must be measured with a caused by oxidation. reasonable degree of repeatable accuracy. Appropriate gauges — wheth- Airworthiness Directive (AD) er equipped with a highly legible 87-08-09 of the U.S. Federal Aviation analog-dial pointer (with at least Administration (FAA) specifies that 5 psi increments) or a digital display aircraft tires mounted on braked — require regularly scheduled cali- wheels may not contain more than 5 bration and consistent procedures. percent oxygen by volume. The affect- ed aircraft also must have a placard Differences in inflation-gas pressures for maintenance technicians that says of the same tire often can be attributed “Inflate tires with nitrogen only.” The to the differences in gauges. Tire spe- AD explains various means of com- cialists discourage the practice of pliance under an FAA-approved main- keeping inaccurate gauges in service tenance program and permits the by adding a tag with notes about a substitution of air at remote locations known discrepancy; gauges that are that do not have nitrogen, provided that not within specified tolerances should the oxygen content does not exceed 5 be repaired or replaced. Attempts to percent by volume, or within the next repair gauges with unapproved lubri- 15 hours of time in service, the tire is cants, tools or techniques also may purged of air and inflated with dry reduce the gauge’s accuracy to an un- nitrogen so that the oxygen does not acceptable degree. For some critical exceed 5 percent by volume. measurements, such as pressure change in a new tire/wheel assembly Before using compressed air to over a 12-hour period or 24-hour inflate a low-pressure tire in a loca- period, the same gauge should be used tion where nitrogen is not readily for each measurement if possible.

8 FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ MARCHÐAPRIL 1999 Underinflation Overinflation Increases Produces Rapid Susceptibility to Damage Excessive Heat Aircraft tires should not be operated Tire/wheel assemblies operated at at pressures higher than the rated in- significantly less than the specified flation pressure. Although engineering pressure may experience any of the specifications require aircraft tires to following problems: withstand several times their rated pressure for a few seconds under test conditions, excessive inflation pres- ¥ Tires may creep or slip on the sure may cause the aircraft tire/wheel wheels during landing or when assembly to explode with lethal force brakes are applied, possibly and fragments. leading to shearing of a valve and destruction of the entire Aviation authorities also discourage assembly; the practice of compensating for excessive tire deflection on aircraft ¥ Rapid or uneven wear may loaded near maximum takeoff weight occur at or near the tread shoul- by increasing tire pressure. More than der or the edge of the tire’s tread 5 percent overinflation may cause the (providing evidence of chronic following effects: underinflation); ¥ Excessive strain on the cord body of the tire; ¥ Tire sidewalls or shoulders may be damaged or crushed by ¥ Excessive stress on the wheels; wheel-rim flanges on landing or ¥ Accelerated center-tread wear; while taxiing the aircraft; ¥ Reduced tire traction; and, ¥ Significantly increased tire/ ¥ Tires may flex over the wheel wheel-assembly susceptibility flange, with greater possibility of to cutting by foreign objects, damage to the bead and lower bruises and impact breaks. sidewall areas, resulting in a bruise break or rupture of the cord body; and, Interpretation of Pressure Changes ¥ Excessive deflection can cause Assures Safe Tire ply cords in the tire body to Condition loosen resulting in extreme heat, ply separation and destruction Aircraft maintenance manuals of the tire/wheel assembly. guide maintenance technicians in

FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ MARCHÐAPRIL 1999 9 tire-servicing and tire-disposition retains gas pressure by sealing the tire decisions under a wide range of nor- bead against the wheel bead seat. mal operations and unusual con- Aluminum or magnesium wheels for ditions. Tire-inflation procedures these aircraft typically are assembled ensure regulatory compliance, safety from split rims or one rim with a and full service life in the use of new demountable flange. Mated surfaces tires and remanufactured tires. The of the wheel are sealed against pres- following procedures are suggested: sure loss by an O-ring. The inflation gas is added or removed through a ¥ An aircraft tire that was in ser- tubeless-tire inflation valve mounted vice with pressure below 90 in the wheel. Tires, whether of con- percent of the minimum loaded ventional bias-ply design or more service pressure — or has lost recent radial design, are a composite more than 10 percent of pressure material of rubber, fabric and steel between scheduled daily components. pressure checks — should be demounted and returned for ex- Depending on their use, some air- amination by the manufacturer or craft wheels have fusible plugs (fuse by an authorized retreader to plugs) and/or OPR valves. Fuse determine whether the tire is ac- plugs have a metal core that melts ceptable for continued use; and, at a specific temperature to release ¥ Any tire/wheel assembly that gas pressure before a tire/wheel was in service with pressure be- assembly has an explosive failure. low 80 percent of minimum load- Such plugs mainly provide protec- ed service pressure similarly tion against explosive failure caused should be removed from service by excessive heat generation from along with its axle mate, except brakes. Tires should be scrapped if when the pressure loss is known extreme temperatures cause the fuse to have occurred while the air- plug to release the inflation gas, be- craft was parked and not moved. cause such temperatures may cause reversion of rubber (that is, change to the weaker uncured state) in the Wheel Components rim-contact area, but defective fuse Help Protect Tires plugs and leaks around fuse plugs From Overpressure also should be considered.8

In the tubeless-tire design used The various valves, valve caps and most commonly for transport- seals on a tire/wheel assembly can be category aircraft, the tire and wheel checked for leaks by wetting the both are part of one assembly that component with a leak-detection

10 FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ MARCHÐAPRIL 1999 solution, or by immersing the tire/ specified operating pressure. Tire wheel assembly in water. Repairs of- manufacturers also recommend pres- ten can be made and parts replaced sure checks more frequently than without demounting the tire from the normal for a few days after this sta- wheel. bilization period.

Another component — tire sidewall The minimum service pressure for vents — also has a safety function safe aircraft operation is the cold and should be checked carefully. unloaded inflation pressure specified Sidewall vents prevent pressure by the airframe manufacturer. A tol- buildup within the tire carcass body. erance of minus zero percent to plus Such pressure can cause tread, side- 5 percent of this minimum pressure wall or ply separation. Therefore, the is the recommended operating range. vents provide a path to bleed off a If pressure of a tire installed on an small amount of nitrogen — enough operating aircraft is checked with an to produce bubbles in leak-detection accurate gauge and found to be less solution but not enough to be felt by than the minimum pressure, the hand — trapped in the cord body. specific schedule of pressure interpre- The industry standard for maximum tation and actions adopted by the allowable diffusion after an initial operator should be followed carefully. 12-hour stabilization period is Some operators require a visual tire 5 percent pressure loss in any inspection — including checks with 24-hour period. Tire inspections a gauge — after every landing or at should include sidewall vents, which every turnaround. occasionally are sealed inadvertent- ly by spilled solvent, tire paint or Visual examination of aircraft-tire retreading. deflection is unreliable for detecting small, but significant, losses of tire Checklists Identify pressure. Thus, even the slowest Sources of Pressure Loss nitrogen leaks in an aircraft tire/ wheel assembly can lead to an un- The cord body of a typical new safe loss of pressure within two days aircraft tire has an initial 24-hour to three days, causing damage if not stretch period during which a nor- detected and repaired promptly. If mal pressure drop of 5 percent to pressure drops of 5 percent to 10 percent may occur because the 10 percent are not detected, and tires volume of the tire will increase. A operate for long periods in this con- new tire should not be placed in ser- dition, inadequate inflation weakens vice until at least 12 hours after be- the tires even though there appears ing mounted and inflated to the to be no damage. Internal structural

FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ MARCHÐAPRIL 1999 11 damage to tires after use of greater- ¥ Status of the initial stretch and than-normal braking energy also stabilization period for new may be invisible. Thus, checklists tires. should incorporate all the airframe manufacturer’s procedures and Identifying these underlying causes guidelines for determining air- of tire-pressure anomalies helps worthiness in these conditions. assure safe operation of aircraft. The complete maintenance process — Typical inspection checklists include including routine pressure checks items such as: of tire/wheel assemblies — increases the probability of timely problem ¥ Cuts or punctures; detection, prevention of serious damage and reaching the expected ¥ Evidence of damage from exces- tire service life. sive heat;

¥ Leaking valve cores, valve-hole Notes and References seals, valve caps, fuse plugs or 1. Deflection is a percentage based OPRs; on a series of calculations using simple measurements of tires ¥ Damage to O-ring seals and and wheels when a tire is other sealing surfaces, or im- not installed on an airplane (un- proper O-ring installation; loaded) and when a tire is installed on an airplane (loaded). ¥ Holes or cracks in wheel First, free height is calculated by assemblies; subtracting flange diameter from outside diameter and dividing ¥ Improperly torqued wheel tie the difference by two. (Flange bolts; diameter is measured at the ¥ Damage to tire beads (cracks or top of the wheel rim flange; out- scratches); side diameter is measured at the circumferential center line ¥ Poor seating or improper seating of an inflated tire.) Then, loaded in the bead area; free height is calculated by subtracting one-half the flange di- ¥ Damage to the tire inner liner (or ameter from the static loaded tube); radius. (Static loaded radius is the distance from the center of ¥ Ambient temperature change; the axle to the runway for a and, loaded tire.) Finally, deflection is

12 FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ MARCHÐAPRIL 1999 calculated by subtracting loaded 4. Bridgestone Corp. Bridgestone free height from free height and Aircraft Tires: Tire Specification dividing the difference by free & Maintenance Manual. Tokyo, height. Japan. November 1997. 2. The U.S. Federal Aviation Ad- 5. Tire Division, Rubber Manufac- ministration (FAA), the Rubber turers Association. “Aircraft Tire Manufacturers Association, Recommended Operating Infla- Michelin Aircraft Tire Division, tion Pressure Maintenance Crite- the Aviation Products Division of ria.” March 8, 1994. “Aircraft Tire the Goodyear Tire & Rubber Co. Bursts.” October 1989. Aircraft and the Aircraft Tire Sales Tire Information Service Bulletin. Department of Bridgestone Washington, D.C., U.S. Corp. said that loaded inflation 6. For example, the Aviation Prod- pressure is 1.04 times unloaded ucts Division of the Goodyear inflation pressure because of Tire & Rubber Co. recommends tire deflection. adjustment for an expected tem- 3. Examples in this article were perature drop that exceeds selected from three databases. 50 degrees Fahrenheit (F) and The data included air carrier the Aircraft Tires Sales Depart- incidents and accidents from ment of Bridgestone Corp. January 1983 to January 1999, recommends adjustment for an maintained by the U.S. Federal expected temperature drop Aviation Administration and the greater than or equal to 25 U.S. National Transportation degrees Celsius (77 degrees F). Safety Board, and the accident 7. Prepared from the official and incident report system records of the NTSB Aircraft (ADREP) maintained by the Accident/Incident Database by International Civil Aviation Air Data Research, 13438 Organization. Narrative infor- Bandera Road, Suite 106, mation did not provide causal Helotes, Texas 78023, U.S. factors consistently. Never- theless, studies by tire manu- 8. U.S. Federal Aviation Adminis- facturers have associated some tration. “Chapter 9: Landing of the types of tire/wheel-as- Gear Systems.” Airframe and sembly failures with tire pres- Powerplant Mechanics Airframe sure less than recommended Handbook. Advisory Circular values. AC65-15A. 1976.♦

FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ MARCHÐAPRIL 1999 13 MAINTENANCE ALERTS Cracked Case Causes crack that leaked high-temperature JT9D Engine Failure gas and damaged rotating engine On Takeoff components. The report said the CCOC was “an The U.K. Air Accidents Investigation early-version fabricated case with a Branch (AAIB) cited a Boeing welded axial seam and welded boro- 747-136 in-flight engine shutdown scope and drain bosses.” (The later in recommending action to prevent version is a one-piece case.) The re- cracking of the combustion-chamber port said that since 1991, three other outer case (CCOC) in Pratt & incidents involving extensive fractur- Whitney JT9D turbofan engines. ing of JT9D fabricated CCOCs have been reported. The B-747 was on takeoff May 27, 1997, from London (England) Heath- Based on these findings, the report row Airport when the flight crew saw recommended that the U.K. Civil a caution light indicating that the Aviation Authority and the U.S. Fed- no. 2 engine exhaust-gas temperature eral Aviation Administration “review was greater than 915 degrees Celsius the history and engineering analysis (1,679 degrees Fahrenheit). of the fabricated type of [CCOC] used on the JT9D engine and mandate The AAIB, in its incident report, said, measures aimed at preventing recur- “The flight engineer immediately rence of instances of extensive reduced the thrust lever setting, almost cracking of the case.” simultaneously noted that other no. 2 engine parameters had suffered a major decrease, and informed the other FAA: Chafed crewmembers that the engine had Fuel-float-switch failed.” The crew shut down the en- Wiring Could Cause gine, flew the airplane to 10,000 feet Explosion and jettisoned fuel over the English Channel to reduce airplane weight to The U.S. Federal Aviation Adminis- maximum allowable landing weight. tration (FAA) has ordered operators The crew then landed the airplane at of Boeing 737s to inspect and, if nec- Heathrow. essary, to replace center-fuel-tank float switches and wiring, and to in- The report said that the no. 2 engine stall Teflon sleeves over the float CCOC had a 69-inch (175-centimeter) switches and wiring.

14 FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ MARCHÐAPRIL 1999 The maintenance procedures are in NTSB is participating in the Trans- Airworthiness Directive (AD) portation Safety Board of Canada 99-05-12, which was prompted by (TSB) investigation of the accident, a report of chafed wiring in eight which occurred after the crew of the B-737s. FAA said that the airplanes MD-11, Swissair Flight 111, report- had between 32,000 flight hours and ed smoke in the cockpit and requested 85,000 flight hours, and that the clearance to divert the flight to a “con- chafing was caused by vibrational venient” airport. Air traffic control contact between the wiring and (ATC) cleared the crew to fly direct- wiring conduits. ly to Halifax International Airport in Nova Scotia. The crew declared an “The actions specified in this AD are emergency; communication between intended to detect and correct such ATC and the crew ceased shortly chafing and the resultant arcing thereafter. The airplane then struck from the wiring to the in-tank the water. All 229 occupants were conduit, which could present an killed. ignition source inside the fuel tank and consequent fire/explosion,” said NTSB said, “Numerous sections of FAA. wiring from the cockpit overhead area … exhibited heat damage and burned The AD is applicable to 100, 200, insulation, and several of the wires 300, 400 and 500 series B-737s, and from those sections showed evidence requires compliance with Boeing consistent with electrical arcing. Alert Service Bulletin 737-28A1132, Although some of the wires exhibit- issued on Dec. 2, 1998, and revised ing arcing characteristics are from the on Jan. 15, 1999. entertainment system that is unique to the Swissair MD-11 fleet, others have been identified as original NTSB Urges MD-11 wires.” Inspection of MD-11 Cockpit Wiring NTSB said that, as of January 1999, the accident investigation has not Evidence of electrical arcing in a determined that the electrical arcing McDonnell Douglas MD-11 that was caused a fire. Nevertheless, NTSB involved in an accident near Peggy’s recommended that the U.S. Federal Cove, Nova Scotia, Canada, on Sept. Aviation Administration require in- 2, 1998, has prompted the U.S. Na- spection of wiring in and around tional Transportation Safety Board MD-11 cockpit overhead circuit- (NTSB) to recommend inspection of breaker panels and avionics circuit- MD-11 cockpit wiring. breaker panels.

FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ MARCHÐAPRIL 1999 15 When Red Tape Is Good The maintenance technicians had made an entry in the aircraft flight log- For Your Aircraft book that the pitot-static system was being tested. Nevertheless, the possi- While participating in a safety audit bility existed that the black tape might of a large corporate-aircraft opera- be overlooked by the technicians tor, the maintenance auditor found a after the test was completed, or over- situation that could have led to a se- looked by pilots during a walk-around rious flight-instrument abnormality. preflight inspection of the helicopter. The discovery came as the auditor observed two maintenance techni- The auditor asked the technicians why cians troubleshooting a Sikorsky they did not use red tape, rather than S-76 helicopter air-data-system black tape, to cover the alternate static problem. The technicians were con- port. The auditor said that red tape (or ducting a leak test of the pitot-static another conspicuously different color) system. would be a conspicuous reminder that its removal was required after the test. The helicopter’s paint scheme fea- The maintenance technicians said that tured a one-inch (2.54-centimeter) they had not thought about using red black stripe along the tail boom. On tape, but agreed that it was a good idea. closer observation, the auditor saw that the alternate static port, installed The auditor then recommended that flush in the tail-boom skin within the the aviation-department manager pur- black stripe, had been covered with chase a few rolls of red tape and store black tape to seal the static system them with the pitot-static test equip- for the leak test. ment for future use.♦

NEWS & TIPS

Static-grounding Cable fueling, rescue operations, and Reels Designed for other situations where static electric- Hazardous Areas ity could cause a spark and an explosion, said the manufacturer.

The Hannay Reels HGR series The reels are offered in four different spring-rewind reels for grounding models and provide for the payout, cables are designed for use in retraction and storage of from 20 feet explosive atmospheres and hazardous to 100 feet (6.1 meters to 30.5 meters) conditions including aircraft of static grounding cable. The reels

16 FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ MARCHÐAPRIL 1999 feature spring rewind motors, lock The tester can measure in both in any position in a 270-degree arc clockwise and counterclockwise and require minimum cable pull to rotation, and the torque of already release the lock. tightened fasteners can be measured in first-peak mode to an accuracy For more information: Hannay of plus or minus 0.5 percent. The Reels, 553 State Route 143, P.O. device has automatic zero adjustment, Box 159, Westerlo, NY U.S. 12193- and printer and analog output jacks. 0159. Telephone +1 (518) 797-3791. For more information: ASG Division of Jergens, 19520 Nottingham Road, Torque Tester Cleveland, OH 44110 U.S. Telephone Stores Readings +1 (210) 486-6163.

ASG’s new HDM-100 digital torque tester can store torque data for up Portable Video to 450 screws or bolts. The device Microscope Is measures the torque of fasteners Self-contained and gives both audible and light signals when torque reaches a user- The Olympus PV10 video microscope selected tolerance range of plus or can be hand held and operated any- minus 3 percent, 5 percent or 10 where, said the manufacturer. The percent. Readings may be displayed device allows video inspection of com- in Newtons per meter, kilograms ponents that cannot be viewed with force per meter or pounds force conventional microscopes, and it per inch. features a variety of zoom and single-focus lenses for maximum depth of field, longer working dis- tances and magnification up to 1000X.

The PV10 accommodates lighting attachments and is compatible with Olympus borescopes and fiberscopes for inspecting hard-to-reach places.

For more information: Olympus America, Industrial Products Group, Two Corporate Center Drive, Melville, ASG HDM-100 NY 11747 U.S. Telephone (800) 446- Digital Torque Tester 5260 (U.S.); +1 (516) 844-5888.

FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ MARCHÐAPRIL 1999 17 Cabinets Provide Static-safe Storage

Electronic equipment sensitive to electrostatic discharge (ESD) can be stored safely in the StaticGard Sentry 100¨ modular drawer cabinets and work stations, said the manufacturer. The equipment meets test criteria of the ESD Association and the American National Standards Institute (ANSI) Accredited Test Standards and Advisories.

Sentry 100 products are shipped in static-neutral packing materials and covered with static-safe packaging. Drawer labels and interior partitions Simonds SR-100 and dividers are also treated to Pneumatic Snap Ring Tool control static.

For more information: Stanley ergonomic hand tool installs internal Storage Systems, 11 Grammes snap rings from one-half inch to Road, P.O. Box 1151, Allentown, 1-3/8 inches (1.27 centimeters [cm] PA 18105-1151 U.S. Telephone: to 3.49 cm) without repetitive (800) 523-9462 (U.S.); +1 (610) manual squeezing. 797-6600. The tool features interchangeable Pneumatic Tool Aids 0.094-inch (2.4 mm)-diameter tips for Ring Installation straight, 45-degree and 90-degree approaches. When the trigger is The Simonds SR-100 Pneumatic activated, the jaws close evenly with Snap Ring tool operates from shop 780 pounds (354 kg) of force. air at a pressure of 80 pounds per square inch (5.6 kilograms [kg] per For more information: Simonds, 248 square centimeter) and eliminates Elm St., P.O. Box 100, Southbridge, the need for repetitive manual MA 01550-9921 U.S. Telephone: squeezing, said the manufacturer. The +1 (508) 764-3235.

18 FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ MARCHÐAPRIL 1999 Portable Space Cooler air through moist cooling pads Provides Hot-weather (replaceable at four-year to five-year Comfort intervals). The unit weighs 75 pounds (34 kilograms [kg]) without water and approximately 100 pounds (45 kg) The Advanced Radiant Systems with water. The housing of the cooler COOLSPACE Portable Evaporative is polyethylene, and electrical termi- Cooler reduces effective air temper- nals and connections are watertight. ature as much as 26 degrees Fahrenheit (14 degrees Celsius) at For more information: Advanced low operating cost in outdoor or semi- Radiant Systems, 12910 Ford Drive, enclosed spaces and in high relative Fishers, IN 46038 U.S. Telephone humidity, said the manufacturer. (800) 557-5716 (U.S.); +1 (317) (Effective temperature is a measure 577-0417. of the physiological impact of air temperature, relative humidity and air speed.) The unit operates on 115-volt Wrenches Designed to power and uses water from a standard Reduce Hand Injuries, hose or optional portable tank. Fastener Damage

The cooler uses an 18-inch Knuckle Saver wrenches incorporate (0.45-meter) single-speed fan to move design features that prevent wrench slipping and reduce damage to fasteners, said the manufacturer. Mac Tools has incorporated a locking groove to help keep the wrench from slipping from the fastener, and an arched inner surface to improve contact between wrench and fastener. Increasing the contact area allows more force to be applied without rounding the fastener.

To reduce slippage of the wrench, a slight step in the jaw face inter locks the wrench with the fastener and reduces the probability that the Advanced Radiant Systems wrench will slide off the fastener COOLSPACE Portable when force is applied, said the Evaporative Cooler manufacturer.

FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ MARCHÐAPRIL 1999 19 For more information: Mac Tools, Data Display Documents 4635 Hilton Corporate Drive, Servicing of Large, Columbus, OH 43232 U.S. Tele- Vented NiCd Batteries phone: (800) 622-8665 (U.S.); +1 (614) 661-5300. The Christie Electric Corp. ProEase DATAFXª data display unit works New Grommet Receives with charger-analyzers to document FAA Approval servicing of batteries typically used in aircraft applications. Improve- ments include all-new circuitry and Device Technologies, a manufacturer software; a larger display that shows of composite grommet edging, has up to 22 cell voltages, amperage, received U.S. Federal Aviation Ad- total voltage and fault messages ministration endorsement for Spring- simultaneously; and a print function ¨ Fast Grommet Edging. The product to document displayed data, said the has a chafe-resistant surface that manufacturer. prevents fraying of wires in new or aging aircraft, said the manufacturer. The new DATAFX is available now, and retrofit options are available to The edging is a composite made owners of the original model. of polymer-encapsulated stainless steel with a sacrificial cushion that For more information: Christie provides a smooth, nonconductive Electric, 18120 South Broadway, surface to protect wire and cable Gardena, CA 90428 U.S. Telephone: from abrasion, according to the +1 (310) 715-1402.♦ manufacturer. The stainless steel core snaps on and locks into place without adhesives. Eight sizes are available, from 0.025 inch to 0.25 inch (0.64 millimeter [mm] to 6.4 mm).

For more information: Device Technologies, 3 Brigham Street, Marlborough, MA 01752 U.S. Tele- phone: (800) 669-9682 (U.S.); Christie Electric Corp. +1 (508) 229-2000 . ProEase DataFXª

20 FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ MARCHÐAPRIL 1999 BLANK INSIDE BACK COVER deRIO Janeiro,Brazil November 8–11, 1999 Enhancing Safety in the 21st Century

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International Federation International Air Transport of Airworthiness Flight Safety Foundation Association A Joint Meeting of 52nd FSF annual International Air Safety Seminar, 29th IFA International Conference and IATA For information, contact Ann Hill, tel. +1(703) 739-6700, ext. 105 or Ahlam Wahdan, ext. 102

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