Engineering and Metallographic Aspects of Gas Turbine Engine Failure Investigation: Identifying the Causes

by Robert E. Dundas, Principal Engineer Factory Mutual Engineering & Research

Technicians are seldom assigned the provide knowledge of how such responsibility of analyzing the cause failures can be avoided in the future. of a gas turbine engine failure, but it Knowing the features of cracks and is not uncommon for them to be in- fractures in materials used in gas tur- volved in the investigation of a pre- bine engines is important in associat- mature gas turbine engine ing them with failures. malfunction or failure. Table 1 (page 2) lists the most likely As a participant in the tear-down in- initiating failure mechanisms for the spection of the engine and review of major components of gas turbine en- the findings, the technician must un- gines. The list is derived from en- derstand the modes of metallic com- gine case histories and an ponent failures and how each understanding of the design and can be identified during the function of each component. The investigation. mechanisms in boldface represent the most common failures. The purpose of investigating a gas turbine engine failure is to identify a Cracks and fractures in gas turbine definitive cause of the failure to engine parts, both rotating and

FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ JANUARY/FEBRUARY 1994 1 Table 1 Failure Mechanisms of Gas Turbine Components

SectionComponents Problem Compressors Disks High-cycle fatigue Low-cycle fatigue Blades High-frequency fatigue because of resonant vibration High-frequency fatigue because of flutter Clashing and clanging because of surge* Stator vanes Clashing because of surge High-frequency fatigue because of resonant vibration or flutter

Combustors Liner Thermal fatigue Overheating

Turbines Disks Low-cycle fatigue, possibly aggravated by thermal gradients Fracture because of embrittlement Blades High-frequency fatigue because of resonant vibration Creep-rupture Nozzle vanes Thermal fatigue Overheating *Problems in boldface represent the most common failures.

Source: Robert E. Dundas stationary, occur as a result of four ¥ Creep; and, mechanisms: ¥ Embrittlement. ¥ Short-term loading; In nearly all cases of short-term ¥ Fatigue; loading, cracks progress almost in- stantly to complete fracture.

2 FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ JANUARY/FEBRUARY 1994 Fatigue, creep and embrittlement are is by rotor overspeed, and a rotating time-dependent phenomena, so cracks disk is usually the part with the least sometimes become evident before margin in overspeed limitations. complete fractures occur. Overspeed damage is usually easily identified without metallography Almost all materials of importance in analysis. gas turbine engine failure investigations are ductile, which means that the Ductile material fractures as a result materials can be stretched, drawn or of shearing flow of the material at hammer-forged without breaking. an angle approximately 45 degrees Nickel-based alloys used in turbine to the applied stress. This creates blades have very low ductility at room the characteristic appearance of the temperature, but the alloys become fracture surface, typified by a lip at more ductile at their operating tem- the edge, formed by shearing forces peratures. as the two parts of the component separate. The face of the fracture The features of cracks and of fracture surface tends to be rough and angu- surfaces provide indications about lar because of the shearing flow. which mechanism produced the crack Ductile fractures are usually or fracture. These can be studied by transgranular, in contrast with brittle fractography (macroscopic or micro- fractures, which are usually scopic) and metallography. intergranular.

Fractography is the examination of fracture surfaces by macroscopic (use Fractured Blades Become of the naked eye or up to 10x magnifi- Missiles cation) or microscopic means. Micro- scopic examination can be up to 60x magnification. Scanning electron mi- When a rotating blade fractures in a croscopy (SEM) also permits magni- gas turbine engine, it becomes a high- fications of 5,000x or greater. energy missile that can do extensive impact damage to other blades in Metallography is the study of the struc- the same stage and downstream tures and physical properties of met- stages of the engine. If a gas turbine als and alloys. failure involves the fractures of many blades, it is necessary to determine Short-time fracture occurs when a which blade failed first. Sometimes component is overloaded, as in a this is obvious (as in the case of tensile-testing machine. The only way high-cycle fatigue), but it is usually this can occur in a gas turbine engine less obvious (as in the case of creep

FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ JANUARY/FEBRUARY 1994 3 rupture). Thus, it is important to rec- vibration. This is usually resonant vi- ognize the characteristics of a short- bration, but, increasingly, there are ex- term impact fracture. amples of self-excited vibration (flutter). This type of failure can oc- Impact fracture of a rotating blade cur very early in the life of a gas involves a combination of bending turbine engine. and transverse shear. In the bending mode, the fibers on the side of Another category includes low-cycle impact are in tension, and the tension fatigue, thermal fatigue and corrosion field quickly moves across the blade fatigue. This type of failure occurs as the fracture progresses to comple- only after the gas turbine engine has tion. The macroscopic fracture sur- been in service for a long period. face is rough and angular, but there may be a distinct pattern of ridges The stress/cycle curve in Figure 2 roughly parallel to the direction of (page 5) shows in broad terms the fracture progression. cycle regimes of the various types of fatigue. Figure 1 shows the features of an impact fracture involving a thin section. The regime of low-cycle fatigue and This is typical of the impact fracture thermal fatigue extends up to 10,000 of a thin compressor blade or of a cycles of operation. A cycle includes thin-walled turbine vane or blade. startup, followed by operation and shutdown. The upper end of this range Fatigue failures might be or might not may never be realized, although air- be life-related. One category includes craft engines are typically designed high-frequency fatigue that is caused for fatigue-free periods of 20,000 by blade, stationary vane and disk cycles or more.

Graphic not available

Figure 1. Matching surfaces of a thin plate of medium-carbon alloy steel, fractured by impact. Origin is at top of upper face and at bottom of lower face.1

4 FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ JANUARY/FEBRUARY 1994 Low-Cycle Fatigue High-Cycle Fatigue Alternating Stress

10 2 10 3 10 4 10 5 10 6 10 7 Source: Robert E. Dundas. Cycles Figure 2. Stress/cycle (S/N) curve shows in broad terms the cycle regimes of various types of fatigue. The high-cycle or high-frequency fa- until failure, the crack might then be tigue regime is where the S/N (stress initiated at one of these minute flaws. against number of alternating load The cause of the failure is often incor- cycles to failure) curve is a horizontal rectly attributed to the flaw, when the line marking the fatigue strength or actual cause might be resonant vibra- endurance limit. For most materials tion, flutter, shaft misalignment or an used in gas turbine engines the S/N excessive number of operating cycles. curve “runs out,” and the fatigue life should be indefinite. The high-cycle regime is usually considered to start at 106 cycles. This re- The curve in Figure 2 represents the gime involves stresses that are usually number of cycles at which the first just above the fatigue strength after discernible (by usual nondestructive stress concentrations are included. testing) fatigue crack appears on the Such stress ranges are usually not surface. This is generally considered experienced unless the component to be a crack 0.03-0.06 inches (0.76- suffers damage that would increase 1.52 millimeters) long, which is no the stress concentration at the point of larger than a material flaw that can be maximum stress, or the fatigue reliably detected by nondestructive strength is reduced by corrosion. testing. Thus, a new part that has passed inspection for flaws can actu- If the misalignment of a gas turbine ally contain minute flaws. If the part engine’s bearings is sufficient to pro- is then subjected to alternating stress duce alternating stresses in the shaft

FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ JANUARY/FEBRUARY 1994 5 above the fatigue strength, the shaft in an elliptical pattern from an origin will eventually fail in high-cycle fa- on the blade surface until it covered tigue. Cycles of alternating stress build 40 percent or more of the blade cross at the rate of one per revolution. Thus, section. The crack surface is smooth, 107 cycles could be developed in ap- while the short-term final fracture sur- proximately 50 hours and, after initia- face is rough and characteristic of a tion of a fatigue crack, final fracture ductile material. The crack surface could occur in a few hours. contains distinctive lines or patterns concentric with the origin. These lines, High-frequency fatigue falls into the known as beachmarks, are associated category of high-cycle fatigue. How- with progression of the crack at dif- ever, the process of failure in high- ferent rates and stress levels as it ex- frequency fatigue is brief, after the tends through the blade cross section. excessive alternating vibratory stresses are established. For example, the ini- In all investigations of turbine blade tial crack can appear in 14 hours at a failure, microscopic examination of vibratory frequency of 200 Hz (hertz), 5,000x or greater helps identify stria- a typically low value, with final frac- tions whenever the crack surface is ture occurring no more than a few smooth and does not extend over hours later. the entire fractured section, even if beachmarks are not clearly evident Figure 3 shows the surfaces of a tur- at lesser magnification. SEM or bine blade that fractured in high- transmission electron microscopy frequency fatigue. The crack progressed (TEM) can be used for this

Graphic not available

Figure 3. High-cycle fatigue fracture of a turbine blade, showing typical beachmarks.2

6 FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ JANUARY/FEBRUARY 1994 examination. SEM is used when a compressor disk grows into a dis- small sample of the crack surface tinct crack. Further crack growth can be cut away from the failed part, then leads to a critical crack size, and TEM involves replication of the with subsequent unstable growth to crack surface and microscopic ex- complete rupture. The crack grows amination of the replica. Magnifica- an infinitesimal, but increasing, tions to 10,000x can be obtained with amount per cycle of operation. Each either of these techniques. cycle involves stressing of the disk to a maximum at operating speed, Microscopic examination of a surface followed by a deceleration to rest. cracked by fatigue almost always re- veals the phenomenon of fatigue stria- Low-cycle fatigue failure caused by tions. These are closely spaced parallel centrifugal stresses alone is rare in lines indicating the cyclic progression aircraft gas turbine engines. Never- of a high-cycle fatigue crack in a di- theless, thermal fatigue, in which the rection normal to them. Figure 4 shows centrifugal stresses are accompanied striations in a material used in gas by thermal strains caused by ther- turbine engines. Magnifications of mal gradients from bore to rim of 5,000x or greater should be used in the disk during startups and shut- the examination of cracked surfaces if downs, is a significant hazard to high-cycle fatigue is suspected.

Care is required in the identification of high-cycle fatigue striations. There are a number of microscopic features with which they can be confused. Fa- tigue striations characteristically never cross each other, as can be seen in Figure 4. Graphic not available

Low-cycle fatigue is a characteristic failure mode of components in which normal operating centrifugal stresses are close to the yield strength of the material and may be higher than the yield strength when stress concentrations are included. A minute (and undetect- able) flaw in some highly stressed Figure 4. High-cycle fatigue striations in surface on the gas turbine engine’s INCO 718 nickel-based alloy material.2

FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ JANUARY/FEBRUARY 1994 7 turbine disks. It occurs when the to- gradients developed throughout the tal strain (centrifugal and thermal) parts because of the constraints the includes a substantial plastic com- various sections of the vane struc- ponent, and involves cycles of strain- ture impose on each other. Ther- ing throughout the complete mal cracks in these vanes often operating range of the machine. develop after relatively few cycles of operation. Low-cycle fatigue striations tend to be broader than high-cycle fatigue After a crack begins, it tends to re- striations, and tend to be discontinu- lieve the thermal strains that produced ous, terminating in a distinctive lap it, and crack growth is slowed rather (Figure 5). than accelerated during subsequent cycles. Some cracking, therefore, can A purer form of thermal fatigue be tolerated, subject to cyclic limits occurs in turbine stationary nozzle developed by experience, as long as vanes. The vanes, especially in the the cooling flow in the nozzle vane is first stage, are exposed to the hot- not disrupted. Continued cycling can test temperatures in the gas tur- lead to breakup of the nozzle vanes bine engine at maximum power. with extensive downstream damage. The vanes are usually cooled to If a fuel nozzle deteriorates by being extend the thermal fatigue life of blocked, or if the combustor suffers the nozzle vanes as long as pos- serious damage, severe hot spots (more sible or economical. The mechani- severe than those anticipated in de- cal stresses involved are low, and sign) can develop in the flowpath at crack development is almost en- the nozzles. Continued cycling will tirely caused by the thermal cause breakup of the nozzles.

Combustor cracking is also often caused by this form of thermal fatigue. Thermal cracks develop after a relatively small number of cycles of Graphic not available operation with severe hot spots because of nonuniform combustion. The combustor cooling flow is badly disrupted as the cracks open, and the cracks can progress to complete dis- integration of the combustor.

Figure 5. Typical low-cycle fatigue fracture The crack surfaces of parts that have surface, shown at 2,000x magnification.2 failed in low-cycle or thermal fatigue

8 FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ JANUARY/FEBRUARY 1994 are not easily distinguishable from for creep lives of 60,000 hours, but those that have failed in high-cycle extrapolation of laboratory creep data fatigue. Because disks can, and do, to such long lives is uncertain, and fail in high-frequency fatigue, detailed premature failures have occurred. If microscopic examination of the frac- the blade cooling air is interrupted, ture surface should be undertaken failure may occur in a very short time. whenever fatigue is suspected. When a ductile material undergoes The phenomenon of creep occurs in a creep under stress at elevated component subject to stress and high temperature, voids develop in the temperature over time. The curve in grain boundaries as the grains move Figure 6 shows strain of a material at relative to one another. Thus, stress constant stress and temperature; it is rupture fractures in ductile materi- characterized by creep rate and time als are intergranular and are charac- to rupture. Failure is taken as the ini- terized by the intergranular voids tiation of tertiary creep, after which (Figure 7, page 10). the component stretches unstably to rupture. The rupture life is the number Surge occurs when a gas turbine en- of hours at which the rupture occurs. gine compressor is no longer able to compress the incoming air to exceed Turbine blades are the most likely gas the pressure in the combustion sec- turbine engine components to fail in tion. The air flow suddenly reverses stress rupture. Blades can be designed and large dynamic loads are

Constant tensile stress Constant temperature Rupture (fracture)

Stage III: Tertiary creep

Stage II: Secondary creep Stage I: Primary creep

Instantaneous strain Total Strain-in/in (cm/cm)

Time/hours Source: Robert E. Dundas. Figure 6. Creep vs. time for a material under constant stress and temperature.

FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ JANUARY/FEBRUARY 1994 9 usually catastrophic because complete blades are usually not broken off. The tip corners of the blades are, however, likely to be bent, torn or broken off.

Surge damage includes much impact Graphic not available damage, and is often confused with foreign object ingestion damage. Nevertheless, the impact during surge has a distinctive pattern, and unless a foreign object or loose part can be readily identified, caution should be Figure 7. Intergranular fracture in a exercised in dismissing the possibil- nickel-based alloy (Waspaloy) because of stress rupture.2 ity of surge. The operating condition at which the failure occurred is of generated on the pressure sides of the fundamental significance. Surge is blading. The blades deflect forward, most likely to occur at low-power set- generally in groups. Two types of dam- tings and particularly if gas turbine age may result from a severe surge: efficiency has deteriorated. Surge also occurs during startup, often involving ¥ Clashing occurs when the blade malfunction of the compressor bleed- deflection is great enough to valve system. impact the trailing edges of a previous row of stator vanes. Many failures in gas turbines involve The blades leave a triangular radial rubbing between rotating com- impact footprint on the vanes ponents and stationary blade rings. near the outer flowpath wall. Understanding these patterns is very The vanes are sometimes torn important in failure investigations. The or broken off completely by the impact.

¥ Clanging occurs when groups of blades deflect in a wave pat- Graphic not available tern. The blades do not all deflect the same amount, and the trailing edge tip edges impact the leading edges of the adjacent blades on the pressure sides Figure 8. Typical clanging damage (Figure 8). Clanging is not pattern.3

10 FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ JANUARY/FEBRUARY 1994 ADB C Stator

Rotor Ovalized Rotor

Centerline Centerline Rotor of Bearings of Bearings Orbit

Unrubbed Rubbed Source: Robert E. Dundas. Figure 9. Usual rubbing patterns associated with turbine engine failures. most common combinations of rubs A technician who understands the on the rotor and stator shown in Fig- causes behind metallic component failure 9 are the following: ure will be important during an investigation to determine the cause of a ¥ A 360-degree rub on both ro- gas turbine engine failure. tor and stator. The rotor and stator have bound radially at References some time during operation, probably during startup or shut- 1. American Society for Metals. down. There is insufficient ra- Fractography. Volume 12 of ASM dial clearance between rotor Metals Handbook. 9th ed. Ameri- and stator; can Society for Metals: 1987.

¥ A 360-degree rub on rotor, lo- 2. American Society for Metals. cal rub on stator. The stator is Fractography and Atlas of misaligned (offset) relative to Fractographs. Volume 9 of ASM the rotor bearings; Metals Handbook. 8th ed. Ameri- can Society for Metals: 1974. ¥ A 360-degree rub on stator, local rub on the rotor. This is 3. Walker, John L. “U.S. Naval caused by excessive rotor whirl; Marine Gas Turbine Mainte- and, nance.” In Gas Turbines/ Turbocompressors. Volume 1 of ¥ A 360-degree rub on stator, Sawyer’s Turbomachinery local rub at two diametrically Maintenance Handbook. opposite locations on the ro- Turbomachinery International tor. The rotor has become Publications: Norwalk, ovalized by creep. Connecticut, U.S., 1980.

FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ JANUARY/FEBRUARY 1994 11 NEWS & TIPS

Search Identifies Aviation engaged Taylor to build an engine Mechanic Pioneers that would produce 12 horsepower and “wouldn’t weigh too much.” Taylor's shop consisted of only a A few months ago, the Aviation drill press and a lathe, both powered Mechanic’s Bulletin began a search by belts driven by a stationary gaso- to identify the first A&P mechanic line engine. licensed by the United States. With assistance from the U.S. Federal Taylor said, “We didn’t make any Aviation Administration (FAA) and drawings. One of us would sketch the Professional Aviation Mainte- out the part we were talking about nance Association (PAMA), it was on a piece of scratch paper, and I’d determined that Frank G. Gardner spike it over my bench.” The crank- of Norfolk, Virginia, U.S., was is- shaft was made from a block of ma- sued the first aviation mechanic’s chine steel 6 by 31 inches (12 by license on July 1, 1927. 79 centimeters) and a little more than 1.5 inches (3.8 centimeters) Gardner was following closely behind thick. He traced the outline on this other aviation pioneers. slab of metal, drilled the basic outline with a series of holes and A 1948 Collier’s Weekly magazine knocked away the waste. The re- article relates the experiences of sulting blank was then chucked in Charley Taylor, a skilled machinist. his lathe and turned down to size. It weighed 19 pounds (8.6 kilograms) In 1902 the Wright brothers returned in its finished form and was per- from Kitty Hawk, North Carolina, fectly balanced. U.S., after spending the summer per- fecting their gliders. They were now The completed engine weighed 180 determined to build a powered air- pounds (81.6 kilograms) and devel- craft, but could not find an engine oped 12 horsepower at 1,025 rpm. to meet their requirements. They The entire engine was built in just went to Taylor. six weeks.

The Wrights were busy designing The engine was block tested using and building the airframe, so they a resistance fan with blades 1.5

12 FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ JANUARY/FEBRUARY 1994 inches wide and 5 feet 2 inches devoted to the fundamentals and (157 centimeters) long. It was then techniques of inspection by means of shipped to Kitty Hawk, where it visible light. was mounted on the aircraft for its historic flight. Subjects covered in detail include the physiology of vision, vision acuity, the physics of light, visual inspection ASNT Publishes New procedures and standards, borescopes, Visual Inspection image processing, machine vision, re- Handbook mote viewing and robotics. The book is available from ASNT and The American Society for Non- costs US$121.25 for non-ASNT mem- destructive Testing (ASNT) has re- bers and $97.25 for ASNT members. leased volume eight of the second For more information, contact: edition of the nondestructive testing ASNT’s Book Department, 1711 series Visual and Optical Testing. It Arlingate Lane, Columbus, OH is an extensively indexed, 380-page 43228-0518, Telephone (614) 274- book with more than 400 illustrations 6003 or fax (614) 274-6899.

FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ JANUARY/FEBRUARY 1994 13 MAINTENANCE ALERTS

This information is intended to pro- engine manufacturer issued a ser- vide an awareness of safety prob- vice bulletin calling for the installa- lems so that they may be prevented tion of a new type of engine-driven in the future. Maintenance alerts are fuel pump drive shaft. The new shaft based on preliminary information is somewhat longer (0.12 inches [.30 from government agencies, aviation centimeters]) and is designed to pro- organizations, press information and vide additional engagement of the other sources. The information may shaft with its driving recess. not be entirely accurate. The NTSB recovered the accident airplane’s drive shaft and found evi- Additional Factor Cited dence that it was “rounded off” or as Possible Cause of stripped to the extent that just one- thousandth of an inch (.002 centi- Cessna 402C Crash meters) more wear would have completely disengaged the drive A Cessna 402C crashed shortly af- shaft. In fact, there were circumfer- ter takeoff in the vicinity of the ential marks indicating that the shaft Grand Canyon in Arizona, U.S., in had disengaged to some extent in June 1992. The aircraft was return- the past. ing from a sightseeing flight over the canyon and carried nine passen- As a result of these findings, the gers in addition to the pilot. Every- NTSB issued a safety recommenda- one on board was killed, and the tion to the U.S. Federal Aviation Ad- airplane was destroyed by impact ministration (FAA) calling for the forces. Initial evidence indicated a issuance of an emergency Airwor- problem with wear on the wing fuel thiness Directive to require opera- tank inlet valve piston shafts. This tors of affected engines to comply condition was reported in the May/ with the manufacturer’s Service Bul- June 1993 issue of Aviation Mechan- letin #M93-9 Revision 1 within 30 ics Bulletin. flight hours.

The U.S. National Transportation This problem affects approximately Safety Board (NTSB) later became 12,454 units of various Teledyne aware of another possible factor in Continental motors such as the IO this fuel starvation accident. The 520, the IO 550, the TSIO 520 and

14 FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ JANUARY/FEBRUARY 1994 the TSIO 550. Technicians involved were also noted on the lower left side in the maintenance and inspection of the vertical stabilizer. In addition, of aircraft using any of these en- the push-pull rod attachment brackets gines should be alert for this prob- on both the left and right elevator lem while conducting their torque tube assemblies showed evi- inspections. dence of having impacted adjacent structure as a result of gross overtravel of the elevators. Both left and right Excessive Free Play in aileron inboard hinges, particularly the Aileron Tab Blamed in right inboard hinge showed evidence of contact with aileron cove structure, Fatal Twin Commander indicating aileron overtravel. 690 Accidents A rigging check of the airplane dis- In the past year, Twin Commander closed that the electrically actuated 690 series turboprop aircraft have trim tab on the left aileron had a been involved in three accidents, two free play of 0.230 inches (0.584 cen- of which resulted in fatalities. timeters). The manufacturer’s manual specifies a maximum allow- In August 1993, a 690A sustained able free play of 0.125 inches (0.317 an inflight loss of lateral control at centimeters). Right and left maxi- about 16,500 feet (5,032 meters) mum aileron control travel limits during a descent with the autopilot were also found to be incorrect. off. The pilot experienced a sudden There was no other evidence of air- uncommanded roll to the right that frame flutter or nodal vibration continued through a full 360 degrees signatures. before he could recover. The pilot made a successful landing, although The Twin Commander 690 aircraft he reported that full-up elevator was typically descends at close to the required to maintain control during maximum certificated operating air- the descent and approach. speed, making any encounter with turbulent air particularly severe. The A U.S. National Transportation Safety NTSB concluded that excessive free Board (NTSB) investigation deter- play in the aileron trim tab and/or mined that approximately five feet (1.5 improper aileron primary and bal- meters) of the outboard section of the ance cable tensions may result in right horizontal stabilizer and eleva- gross aerodynamic overbalance at tor had failed and folded upward, al- high speeds and loss of lateral con- though it remained attached to the trol. The Frise type ailerons installed airplane. Small compression buckles on these aircraft are susceptible to

FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ JANUARY/FEBRUARY 1994 15 overbalance as they are deflected, NTSB Warns destroying the normal static relation Some Fire-blocking of the two ailerons. Materials May Be In addition to the possible effect on Defective Frise aileron synchronization and aerodynamic balance, aileron rig- While investigating an inadvertent ging and trim tab free play limita- slat actuation incident on a tions must also be maintained in McDonnell Douglas MD-11 aircraft, order to avoid aileron/wing flutter the U.S. National Transportation at high speeds or in turbulence. Safety Board (NTSB) determined that fire-blocking material under the The aileron deflections that could dress covers of passenger seat cush- result might exceed the maximum ions had deteriorated to the extent wheel force that the pilot could exert. that the material no longer provided As a result of these findings, the NTSB fire protection. has recommended that the U.S. Fed- eral Aviation Administration (FAA) Samples of this fire-blocking mate- issue an Airworthiness Directive re- rial from the incident airplane, a quiring a check of the Twin Com- sample of the same material installed mander 690’s aileron control system in an ATR-42 commuter aircraft, and rigging and adjustments, in which par- a new sample supplied by the manu- ticular attention is given to maintain- facturer failed to meet the standards ing the tab free play within the set in U.S. Federal Aviation Regula- specified limits. tions (FAR) 25.853, the NTSB said.

The NTSB also recommended that the It was also found that the material FAA conduct a design review of the degraded under both normal and Frise aileron control systems as in- simulated wear-and-tear conditions stalled on 690-series airplanes to de- equal to two years in service. termine the adequacy of lateral control under all anticipated conditions. As a result of the findings, the NTSB recommended that any aircraft us- Technicians involved with the inspec- ing Testori-manufactured fire-block- tion and maintenance of Twin Com- ing material be retrofitted with new mander 690-series aircraft should material that meets the fire-retardant ensure that they are familiar with the requirements of FAR 25.853. The rigging, adjustment and maintenance NTSB identified the defective mate- of aileron control systems for the rial as Testori 0200-316 and aircraft. 0206-100.

16 FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ JANUARY/FEBRUARY 1994 Inadequate capability is reduced to about 20 per- Maintenance Manual cent of the intended capacity. Cited as Factor in Discussions with the aircraft’s tech- Landing Gear Failure nician and a review of maintenance and illustrated parts manuals detail- A Fokker F-28-MK-0100 operated ing this installation confirmed that by a U.S. air carrier experienced a the manual information was inad- landing gear collapse during land- equate. Although the spacer instal- ing in May 1993. Subsequent inves- lation is depicted in the maintenance tigation disclosed that the left main manual, it was not specific about gear strut had failed in the area of the dimensions of the two spacers. the upper torque link attach point. As a result of these findings, the Fokker had previously installed U.S. National Transportation Safety shimmy dampers to provide tor- Board (NTSB) recommended that sional/lateral stability to the landing the U.S. Federal Aviation Adminis- gears during the landing roll. Post- tration (FAA) issue an alert to af- incident examination of this aircraft fected operators and suggested that found that spacers on the upper and the manufacturer revise maintenance lower torque link attach points had manuals to include an improved de- been reversed during assembly re- scription and illustration of the cor- sulting in inadequate damping capa- rect installation of these critical bility of the assembly. The two spacers. The NTSB also suggested spacers are similar, but one is 1.6 that the manufacturer consider rede- inches (41.9 millimeters) long while signing the landing gear shimmy the other is 1.3 inches (33.3 milli- damper spacer assembly to prevent meters) long. With the two spacers the possibility of inadvertently in- switched, the total damping terchanging these parts.

FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ JANUARY/FEBRUARY 1994 17 SAFETY ALERT

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12345678901234567890123456789012123456789012345678901234567FAA Warns of 8

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12345678901234567890123456789012123456789012345678901234567Potential Fuel Dye 8

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12345678901234567890123456789012123456789012345678901234567Hazard 8

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12345678901234567890123456789012123456789012345678901234567The U.S. Federal Aviation Administration (FAA) has issued 8

123456789012345678901234567890121234567890123456789012345678 12345678901234567890123456789012123456789012345678901234567a safety alert following a recent decision by the U.S. 8 123456789012345678901234567890121234567890123456789012345678 12345678901234567890123456789012123456789012345678901234567Environmental Protection Agency and U.S. Internal Revenue 8 123456789012345678901234567890121234567890123456789012345678

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12345678901234567890123456789012123456789012345678901234567Service requiring that certain diesel and kerosene fuels be 8

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12345678901234567890123456789012123456789012345678901234567dyed in colors ranging from yellow to dark red. These 8

123456789012345678901234567890121234567890123456789012345678 12345678901234567890123456789012123456789012345678901234567colors also include the red, blue and green dyes used to 8 123456789012345678901234567890121234567890123456789012345678 12345678901234567890123456789012123456789012345678901234567identify aviation fuels. Officials say there is a danger that 8 123456789012345678901234567890121234567890123456789012345678

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12345678901234567890123456789012123456789012345678901234567nonaviation fuels could end up in aircraft fuel tanks because 8

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12345678901234567890123456789012123456789012345678901234567safety,” said Charles Huettner, acting FAA associate 8

123456789012345678901234567890121234567890123456789012345678 12345678901234567890123456789012123456789012345678901234567administrator for aviation safety. 8 123456789012345678901234567890121234567890123456789012345678

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12345678901234567890123456789012123456789012345678901234567The FAA alert said: “Aviation fuels are similarly dyed red 8

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12345678901234567890123456789012123456789012345678901234567(80/87), blue (100LL) and green (100/130) to assist in 8

123456789012345678901234567890121234567890123456789012345678 12345678901234567890123456789012123456789012345678901234567identifying one from another. As a result, the potential for a 8 123456789012345678901234567890121234567890123456789012345678 12345678901234567890123456789012123456789012345678901234567significant safety hazard exists. 8 123456789012345678901234567890121234567890123456789012345678

123456789012345678901234567890121234567890123456789012345678

12345678901234567890123456789012123456789012345678901234567“Pilots, fixed-base operators (FBOs), fuel vendors, air 8

123456789012345678901234567890121234567890123456789012345678 12345678901234567890123456789012123456789012345678901234567carriers and others engaged in aviation activities should be 8 123456789012345678901234567890121234567890123456789012345678

123456789012345678901234567890121234567890123456789012345678

12345678901234567890123456789012123456789012345678901234567especially alert to ensure that aircraft receive the appropriate 8

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12345678901234567890123456789012123456789012345678901234567fuel when serviced.” 8

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18 FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ JANUARY/FEBRUARY 1994 NEW PRODUCTS

Environmentally Safe to most sensitive plastics, according Contact Cleaner to the manufacturer. Available For more information, contact: CRC Industries, Inc., 885 Louis Drive, As a result of the international ban on Warminster, PA 18974, U.S. Tele- freon-based chemical products, new phone (215) 674-4300. products are being developed with al- ternative contents. CRC Industries re- cently introduced a contact cleaner that Hangar Floor Coating is alcohol-based and free of chlori- Offers Slip Resistance nated solvents and ozone-depleting substances. Crossfield Products Corp. has developed a urethane hangar floor coating product that is available with five dif- ferent slip-resistant profiles.

The manufacturer claims that the Dex- O-Tex hangar flooring system has ex- cellent wear and abrasion resistance, resists Skydrol and other chemicals encountered in aircraft maintenance activities and can be applied to a wide range of prepared surfaces.

The Dex-O-Tex Aero-Flor reflects light and is offered in 12 standard colors, in addition to clear and white. QD Contact Cleaner Selection of the slip-resistant profile to be used will depend on the degree CRC claims that its QD Contact of slip resistance and the cleaning char- Cleaner has outstanding cleaning acteristics desired. The coating is nor- power and effectively removes mally applied after the floor is contaminants from electrical compo- shotblasted, acid etched or mechani- nents and contacts. It evaporates rap- cally scarified to ensure proper bond- idly, leaves no residue, and is harmless ing. Application is by squeegee/roller

FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ JANUARY/FEBRUARY 1994 19 and involves a two-coat process. form bead of sealant to be applied.

For more information, contact: Dex- The sealants are available in several O-Tex Division, Crossfield Products colors, each with specific applications Corp., 3000 E. Harcourt Street, and benefits: Compton, CA 90221, U.S. Telephone (213) 636-0561. ¥ Blue — Best for fast gasketing in place. Fills gaps and forms flexible, leak-proof seals on in- Silicone Sealants in dustrial equipment. Pressurized Cans Stay ¥ Clear and Black — Best for Usable Longer waterproofing and bonding. Moisture-proof, ozone-resistant Permatex Industrial Corp. has intro- and authorized for applications duced a line of silicone sealants pack- in galleys, etc. aged in pressurized dispenser cans. This packaging/dispenser unit is said ¥ Red — Best for sealing heating to keep the product fresh with a self- elements and high-temperature sealing plug that is easy to remove. ductwork. Resists intermittent temperatures from 600 degrees The manufacturer claims that the vari- F (315 degrees C) to 650 de- ous silicone sealants can be applied grees F (343 degrees C). by simply pressing one finger against the nozzle, thus allowing one-hand For more information, contact: application in difficult locations. The Permatex Industrial Corp., 705 North packaging is also intended to elimi- Mountain Road, Newington, CT nate waste and allow a consistent, uni- 06111, U.S.

Silicone Sealant

20 FLIGHT SAFETY FOUNDATION ¥ AVIATION MECHANICS BULLETIN ¥ JANUARY/FEBRUARY 1994