a service publication of - lockheed martin...deterioration as engine operating hours accumulate....

A SERVICE PUBLICATION OFLOCKHEED-GEORGIA COMPANYA DIVISION OFLOCKHEED CORPORATION

EditorChar les I . Gale

Associate EditorsJames A. LoftinRobert J. R Rockwood

Vol. 14, No. 1, January-March 1987

CONTENTS

2 Focal PointGlenn M. GrayDirector

3 Conserving Turbine LifeA practical approach to extending the servicelife of 501/T56 engine turbines.

Cover: During the last quarter of 1986, the U.S.Marine Corps added two rather special Herculesaircraft to their KC.130 fleet. The advanced modelKC-130T tanker/transports are the 1799th (frontcover) and 1800th Hercules airlifters to be rolled outof Lockheed’s productIon line. The Marine CorpsKC-1305 have now accumulated the equivalent ofmore than 70 years of accident-free flying.

Cover photographs by John Rossino Engine compo-nent photographs on pages 4 through 13 courtesy ofAllison Gas Turbine Division, General Motors Cor-poration.

Published by Lockheed-Georgia Company. a Division ofLockheeed Corporation. Information contained in this issueis considered by Lockheed-Georgia Company to be accurateand authoritative; it should not be assumed. however. thatth is material has received approval from any governmentalagency or military service unless it i s specifically noted. Thispublication is for planning and information purposes only,and it is not t o be construed as authority for making changeson aircraft or equipment, or as superseding any establishedoperational or maintenance procedures or policies. Thefollowing marks are registered and owned by LockheedC o r p o r a t i o n : ” “Hercules: and

Written permission must be obtained fromLockheed.Gcorgia Company before republishing anymaterial in this periodical. Address all communications toEditor, Service News, Department 64-31, Zone 278.Lockheed-Georgia Company, Marietta. Georgia, 30063.Copyright 1987 Lockheed Corporation.

GLENN M. GRAY

The Difference That Counts

The Hercules aircraft has an amazing record. This air-plane has now been in continuous production for over 30years, the longest production run in aviation history. Per-haps even more significant from the standpoint of the Her-cules operator is the fact that of the more than 1800 thathave been built, nearly 86 percent are still in operation.

Statistics like these say a lot about an airplane, about theway it isdesigned and built, and aboutthefundamental rolethat quality has played in every phase of its manufacture.

We at Lockheed take pride in this record, but we know that the numbers alsohave another story to tell, That is a story about people, the people who fly theairplanes of the worldwide Hercules fleet, and the people who maintain them.

Flight crew and maintenance technicians are at the forefront of the effort tomaximize the service life of an airplane. It is they who translate inherent, latentqualities such as good design, mechanical ruggedness, and modern safety featuresinto the real �bottom line� in the world of aviation: performance and service life.

How do they do it? On the operational side, my area of specialization, it comesdown to making a habit of sound and conservative operational practices. Amongotherthings, this means reducing wear and tear on brakes and tires by using reversethrust for slowing the airplane whenever conditions permit; setting flaps at 50%during engine run-ups to best distribute stresses between flaps and empennage;using low-speed ground idle for taxiing as much as possible; extending engine lifeby limiting exposure to high TIT; and followingthe recommendations contained inthis issue of Service News.

In short, it is done through professionalism, dedication, and the everydayapplication of good sense. It is never by coincidence or happenstance that aproduct of modern technology such as the Hercules airlifter becomes preeminentin its field. We are proud of the quality we build into our products, but we areprouder still of the tradition of intelligent teamwork that has brought the Herculesfamily so much of its success. It�s the difference that really counts.

Sincerely,

Glenn M. GrayDirector of Flying Operations

M. M. HODNETT DIRECTOR

CUSTOMER INTEGRATED

SERVICE LOGISTICS SUPPORTAH McCRUM HT NlSSLEY

DIRECTOR DIRECTOR

CUSTOMERSUPPLY

H L BURNETTEDIRECTOR

The single most important cause of degradedperformance and premature failure in gas turbineengines is exposure to excessive heat.

It makes little difference from the standpoint of theef fect on the engine what the cause of theovertemperature condition might be. What is importantis only the degree of the overheating and how long itcontinues.

Each and every exposure to excessive operatingtemperatures takes its toll on engine life, decreasingoperational efficiency and increasing maintenance costs.

The important role that is played by the practicesof individual Hercules aircraft operators and theirmaintenance organizations in determining engine servicelife is not always fully appreciated.

The fuel system of the Allison 501/T56 engine thatpowers the Hercules aircraft is provided with effective,automatic controls that keep engine temperatures withinsafe limits over a wide variety of operating conditions.

Lockheed SERVICE NEWS V14Nl 3

As good as this system is, however, it was notdesigned to work alone. It cannot take the place ofcareful and conscientious management of the overalloperational environment. This is something only thehuman side of the equation can provide.

Life Cycle Factors

The normal maximum time between overhauls fora particular model of the Allison 501/T56 engine isestablished on the basis of engineering considerations,user experience, and a careful evaluation of any safetyfactors involved.

Within that time period, however, an engine’sindividual maintenance requirements are largely deter-mined by the effects of the operating environment uponmajor engine components.

For example, in areas subject to blowing sand orheavy burdens of dust in the atmosphere, the compressorsection may be seriously eroded long before other parts

of the engine require attention. More typically, the repairintervals will be determined by the condition of theturbine, combustion liners, and inlet guide vanes of the“hot” section.

The turbine section components are made of metalalloys designed to be as durable as possible in a high-temperature operating environment. Their service lifeis nevertheless rather dramatically affected by thetemperatures to which they are subjected. Metals donot “forget” incidents of exposure to overtemperatureconditions. Repeated exposure to high temperatureswill eventually result in changes in their physicalcharacteristics that can lead to material failure.

Let us look at some of the effects of high-temperature engine operation on turbine vanes andblades in a little more detail.

Sulfidation

Turbine blades and vanes are subject to gradualdeterioration as engine operating hours accumulate. Thisprocess is commonly referred to as sulfidation. Turbinesulfidation is caused by the accelerated oxidation ofmetals in the presence of sulfur ions, in particularsulfides and sulfates. The oxidized metal goes out thetailpipe, leaving eroded blade and vane surfaces behind.

To help prevent such damage to the metal surfacesand extend the service life of the turbine section com-ponents most often affected by sulfidation, diffusedcoatings of aluminum or aluminum and chromium(often referred to as Alpak and AEP, respectively) areapplied to new turbine blade and vane surfaces.

First stage turbine blades after service test.Top row: uncoated alloy.Bottom row: the same alloy protected withAlpak.

These coatings are effective while in place, but theyare eventually eroded away by the hot gases and abrasiveparticles passing through the turbine section. This meansthat sooner or later the base metal of turbine com-ponents will become exposed to the damaging effectsof sulfur ions.

First stage turbine vane sulfidation.

4 Lockheed SERVICE NEWS V14Nl

Air-cooled turbine rotor after accelerated sulfidation testing.

Close-up of turbineblade sulfidationdamage.

Controlling Sulfidation

The sulfur ions responsible for sulfidation comefrom a variety of sources and are not entirely avoidable.They may come from the sulfates in sea water, sulfurin jet fuel, the sodium sulfate in aircraft cleaning solu-tions, hydrogen sulfide and sulfur dioxide in the air, orsulfur-bearing particulates injected into the atmosphereby industrial processes.

Lockheed SERVICE NEWS V14Nl

While sulfidation is by far the most common andmost pervasive of the destructive processes affecting tur-bine components, it is also in some ways the most con-trollable. The key to reducing the damaging effects ofsulfidation lies in understanding the factors involved inthe process.

The rate at which sulfidation occurs in the enginesis dependent on three factors:

1. Hours of operation.

2. Concentration of sulfur ions.

3. Temperature of the metal components.

There are limits to what can be done about some of Sulfidation rates tend to be exponentially related tothese influences. It is obviously impractical to reducethe operating hours of an airplane just to avoid thesulfidation problem. It is also impossible to avoidencountering the sulfur ions in the atmosphere totally,although it is wise to avoid areas of heavy industrialpollution and unnecessary low-altitude exposure to seaair.

For all practical purposes, the first two of thesefactors can be regarded as constants, more or less beyondthe operator’s control. The situation is quite different,however, in the case of the third factor.

How rapidly protective coatings like Alpak and AEPare worn away, and how rapidly sulfidation proceedsthereafter, is strongly affected by the operatingtemperature. Sulfidation is fundamentally a chemicalreaction and, like most chemical reactions, it proceedsmore rapidly at higher temperatures. It is no coincidencethat this destructive process is often called simply “hotcorrosion. ”

the temperature of the metal, as shown in the chartabove. It is important to remember, however, thatturbine temperature is not a constant. The temperatureat which the turbine of the 501/T56 engine operates isa variable which is largely within the control of theoperator.

Reducing Sulfidation Effects

There are three principal ways in which flight crewsand maintenance specialists can help in reducing thetemperatures that turbine section components are ex-posed to, and thereby retard the sulfidation process.

1. Fly at reduced cruise TIT whenever possible:’

2. Pay careful attention to starting temperatures.

3. Ensure that the TIT sensing and indicating sys-tem is properly maintained.


Cruise Conservation

Consider a mission where all parameters are fixedexcept for cruise TIT. A reduction of only 30 or 40degrees metal temperature will reduce the sulfidationfactor by almost one-half. Even further increases inturbine life may be realized with further reductions incruise TIT. When operation of the aircraft can beaccomplished with a lower TIT, the life of the turbinesection is greatly increased.

The nature of aircraft operations requires that air-craft engines be subjected to high power outputs andhigh temperatures on many occasions during their timein service. However, experienced operators havediscovered that conservative engine temperature settingsin cruise and at other times when safety and operationalconsiderations allow it will yield worthwhile benefits interms of turbine section reliability and reduced operatingcosts.

Stress Rupture

Furthermore, the sacrifice in terms of airspeed thatwill result from a modest reduction in TIT is really quitesmall.

The charts on pages 8 and 9 provide a comparisonof power settings and their effect on cruise time or fuelconsumption over a given distance. These charts helppoint up the relatively minor time advantage that can beexpected from exposing the turbine sections of an air-craft’s engines to high cruise TIT.

Another heat-related factor which must be takeninto consideration in realizing maximum engine servicelife is stress rupture. Stress rupture is breakage of tur-bine blades as a result of exposure to physical stress.This characteristic is time-dependent and a result of thestresses applied by centrifugal force (turbine rpm) andtemperature in the turbine’s operating environment,normally expressed as TIT.

For 501-D22A, T56-A-15, T56-A-16, and T56-A-423engines, decreasing power from 1010 degrees C to 971degrees C increases block time only about 3.8 minutesper hour of block time, while a further reduction to 932degrees C increases block time by just 6 minutes perhour.

The relationship between temperature and turbineblade “stress rupture life,” a measure of resistance tobreakage of this type, is shown on the chart on page 10.Note how a 30-degree C increase above the normaloperating temperature will decrease the stress rupturelife by two-thirds. On the other hand, a 30-degree Cdecrease in blade metal temperature can triple stressrupture life.

Furthermore, it is possible to achieve fuel savings of Relatively few instances of actual turbine bladeapproximately 3 percent with a TIT reduction from 1010 breakage due to stress rupture have been recorded fordegrees C to 971 degrees C. A total of approximately the Allison 501/T56 engine, but in the majority of these5 percent savings may be realized by reducing engine cases inspection of the affected parts has revealedpower from 1010 degrees C to 932 degrees C. previous exposure to an overtemperature condition.

This points up two of the reasons why Lockheedrecommends that climbout TIT for this series of enginesbe set no higher than 971 degrees whenever possible, andcruise temperatures be set no higher than 932 degreesC whenever possible. Relatively little extra speed is tobe gained from the use of high TIT settings, and thesesame settings are also wasteful of fuel.

A Good Start on Conservation

Starting the engines is always a minor moment oftruth for an aircraft mission; the start can also besomething of a moment of truth as far as the engineturbines are concerned.

But most important, lower power settings will con-tribute greatly to a longer turbine life.

The same conservation philosophy can also be ap-plied to 501-D22, T56-A-9, and T56-A-7 engines withgood effect, although in these cases the temperature set-tings will be lower. A number of operators of Herculesaircraft equipped with these engines have found thatkeeping continuous cruise TIT at 900 degrees C or belowyields excellent results.

To obtain good starts with consistency, be sure tocomply with the checklist procedures closely. Payparticular attention to the following:

l TIT is the most important gage during starting.Watch this gage continuously during start.

l The throttle must be in the GROUND IDLE(military aircraft) or GROUND START(commerical aircraft) position.

Lockheed SERVICE NEWS V14N1 7

I Lockheed SERVICE NEWS V14Nl 9


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Blade Metal Tempersture. •c

The propeller should be at minimum torqueblade angle.

Aircraft boost pumps should be ON for the startto ensure a good fuel supply to the engine fuelsystems.

Be sure that the residual TIT in the engine is lessthan 200 degrees C before initiating a start.

The GTC or APU should be operating properlyand at maximum output. Always check for aminimum bleed air manifold pressure of at least35 psi before the start is initiated, and a minimumof 22 psi during the start itself.

Air conditioning must be off during engine start,and the ATM should be off unless absolutelyneeded. In cases where the ATM must be on forsome reason, be sure to monitor the bleed airpressure very closely while starting the firstengine.

On manually controlled starter systems, be surethat the engine reaches 60% rpm, then release thestarter switch or button promptly to ensure max-imum starter life.

Observe engine acceleration rate and the starttime specified in the applicable technical manualand operator handbook.

Do not use enrichment on normal ground startsunless the engine will not light off without it;however, all in-flight starts should beaccomplished using enrichment.

Closely monitor engine instruments throughoutthe start cycle.

Check for first rpm indication, which indicatesthat the start cycle has begun. Lightoff ordinarilyoccurs between 14% and 24% rpm, andacceleration should be continuous until theengine is on speed.

If enrichment is used, check for fuel flow andimmediate cutback after 16% rpm. Disregard forthe rest of the start.

The secondary fuel pump pressure light shouldbe steadily illuminated by the time the enginereaches 6 5 % rpm. It should go out above 65%rpm.

Lockheed SERVICE NEWS V14Nl

While the start cycle usually occurs withoutdifficulty, each start must be closely monitored SO thatit may be abandoned in case an unexpected problemarises. The start should be aborted for any of thefollowing reasons:

TIT exceeds temperature limits cited in theapplicable manuals.

Bleed air manifold pressure falls below theminimum acceptable value of 22 psi.

Hesitating or stagnating rpm.

Fuel is observed pouring from the nacelle drain.

Torching-visible burning of fuel in and aft ofthe tailpipe-is observed. Note that a momen-tary burst of flame, an “enrichment burst,” isnormal if enrichment is selected.

Excessive smoke is seen from the exhaust.

Compressor surging or stalling occurs.

Abnormal vibration is noted.

Permissible start time is exceeded.

The engine does not light off (no increase in TIT)by 35% or maximum engine rpm attainable withthe starter, whichever occurs first.

Problem Starts

The most desirable engine start is of relatively shortduration (less than 60 seconds), with peak TIT between780 degrees C and 810 degrees C. Starting temperaturescooler than 780 degrees C are not necessarily better ones.

Temperatures on the cool side may result in accelera-tion rates inadequate to complete the start within theprescribed time limit. Low starting temperatures may beevidence of a lean TD null orifice valve setting or a leanfuel control acceleration schedule.

These misadjustments can produce improper fuelnozzle flow patterns and result in such problems asdownstream burning of fuel, excessive blade/vane heatsoak temperatures, and a high incidence of abortedstarts.

On the other hand, starts with turbine inlettemperatures on the high side apply a great deal of

11

thermal stress to the turbine section of the engine. Therapid expansion forced upon turbine vanes, blades, andother critical components by the sudden application ofextreme heat can cause the parts to crack.

Such “hot starts” can do a lot of damage in a shortperiod of time and must be avoided. The following arethe start overtemperature limitations and the requiredaction if exceeded.

Over 830 degrees C

TIT over 830 degrees C, but excluding a momentaryovershoot at 65% and the peak normally occurring at94% rpm, requires the flight crew to record the extentof the overtemperature condit ion and cal l formaintenance at the next layover where maintenance isavailable.

The action by maintenance personnel will be tocheck the TD valve null setting and adjust it towarddecrease. If it is found on the subsequent engine startthat the previous adjustment did not reduce the star-ting temperature to within limits, it will be necessary formaintenance to perform the starting overtemperaturechecks described in the applicable maintenance manual.

Over 850 degrees C

If the TIT exceeds 850 degrees C, excluding the peakoccurring at 94% rpm, the flight crew should proceedas follows:

1. Shut the engine down by moving the conditionlever to the GROUND STOP position (notFEATHER), and record the maximum TIT.

2. Allow the engine to cool to below 200 degrees CTIT before attempting to restart. The engine may’be motored with the starter to cool it morerapidly, if necessary. When the engine has beencooled in this manner, wait one minute to makesure that the temperature has stabilized beforeinitiating another start. Remember not to exceedthe starter duty cycle.

3. If 850 degrees C is exceeded on the second start,shut down the engine and record the temperature,then call maintenance. Making another attemptto start the engine is not recommended.

Maintenance action will be to perform the startingovertemperature checks listed in the appropriatemaintenance manual.

Over 965 degrees C

If TIT exceeds 965 degrees C during a start, shutdown the engine, record the peak temperature and callfor maintenance. Make no attempt to restart the engine.Ma in tenance ac t i on w i l l be t o pe r f o rm anovertemperature inspection, consisting of a borescopevisual inspection of the turbine section components.

If no damage is found in the course of theseinspection procedures, all thermocouples must beremoved, visually inspected, and electrically tested. Ifthis inspection reveals no damage, maintenance canperform the starting overtemperature checks listed inthe appropriate maintenance manual as required tocorrect the condition.

Thermocouples

The proper operation of the TIT indicating systemhas a critical bearing on the overall operation and ser-vice life of the 501/T56 engine. Malfunctioning ordamaged thermocouples, or thermocouples of thewrong type, can lead to increased operating tempera-tures within the turbine and materially reduce enginelife.

This turbine vane burn-through was caused bythe use of thermocouples of the incorrect type.

12 Lockheed SERVICE NEWS V14N1

It is important not to become complacent about theaccuracy of the TIT indicating system. It is possible toexceed the maximum allowable takeoff TIT limitationwithout knowing it if the engine has several deterioratedthermocouples. Under such circumstances, the operatorwould experience significantly reduced engine service lifewhile mistakenly believing that the engine has beenoperated properly.

The engine TIT is measured by 18 thermocouplesinstalled in the turbine inlet casing. Each thermocouplehas two separate junctions that act as sensing elements.One junction of each of the 18 thermocouples isconnected in parallel to provide an averaged signal tothe TD amplifier.

The TD amplifier uses this signal to determine theactual TIT of the engine. The other junction of eachthermocouple is also connected in parallel. Its signalbecomes part of the averaged signal representing TITthat is displayed on the TIT indicator mounted on theengine instrument panel.

The Allison 501/T56 engine incorporates a systemof multiple combustion chambers, which causes thetemperatures measured at the turbine inlet to besomewhat nonuniform. A system of multiple ther-mocouples is used to compensate for these differencesin temperature by sampling both the hotter and coolerareas and supplying an average temperature signal.

Since the temperature signal represents a valueobtained by averaging the input from 18 differentlocations around the turbine inlet, it is very importantthat all of the thermocouples be functioningproperlyatall times. If any of the thermocouples fail, the signalbeing sent to the TD amplifier and the TIT indicator willbe affected.

Thermocouples fail for a variety of reasons, in-cluding high resistance, open circuits, and probe tipdamage. The failure of the thermocouple is an event thatcan initiate a whole series of further events, all of thembad.

Not surprisingly, thermocouples tend to fail mostfrequently at the hottest locations in an engine. The lossof a thermocouple in an engine hot spot will cause theaverage temperature signal sent to the TD amplifier andthe TIT indicator to decrease.

Since the reduced temperature signal to the TDamplifier no longer satisfies the reference signal set bythe throttle position, more fuel is directed to the fuelnozzles. The increased flow raises the TIT. and the fuel

flow will continue to increase until the reference signalis matched by the signal from the remainingthermocouples.

Once this has occurred, the TD system will regardthe situation as normal and the TIT indicator willdisplay a normal temperature reading. But the situationis not normal. The apparent restoration of normaloperation after the loss of a thermocouple has come atthe cost of increasing the actual engine TIT.

The change in the actual TIT values that can resultfrom having a defective thermocouple in the system isdependent upon its location, type, and the nature of thedamage it has suffered. One thermocouple with the aftwall of the probe tip eroded, for example, can cause thetrue operating TIT to increase by up to 7.5 degrees.

As if this were not bad enough, thermocouple pro-blems usually have cumulative effects. A sequence offailures involving five thermocouples with probe tip aftwall erosion can therefore raise the actual operatingtemperature of the engine by over 35 degrees at everypower setting above crossover.

Such exposure to excessive operating temperaturesis very destructive to engine components and will resultin shortened service life.

Thermocouple Maintenance

Since properly functioning thermocouples play sucha vital role in the engine control system, it is clear thata well-designed and conscientiously followed ther-mocouple maintenance program is of utmostimportance.

All thermocouples should be regularly removed, in-spected, and tested as described in T. 0. lC-130B-6 orSMP 515C, and the applicable Allison manual.

One of the most important things to rememberabout thermocouple inspections is to do it often enough.Finding more than three defective thermocouples in oneengine at inspection time is a good indication that theperiod between inspections needs to be shortened.

When thermocouples are removed for examination,be sure to tag them as they are removed and keep arecord of the positions where they were located. The con-dition of individual thermocouples often providesvaluable clues about underlying problems involvingother components.


For example, thermocouples with badly burnedprobe tips or unusual carbon deposits suggest malfunc-tioning fuel nozzles or defective combustion liners, asdoes repeated thermocouple failures at the samelocation.

Inadequate maintenance of fuel nozzles can con-siderably shorten engine life. Allison recommends thatall fuel nozzles be regularly removed, inspected, andfunctionally tested as part of an ongoing fuel nozzlemaintenance program. An inspection interval of 1200hours is suggested as a starting point for operatorsinitiating such a program.

For operators not equipped to accomplish fuel nozzleinspection at the operating facility, the most practicalapproach will be to remove the nozzles and replace themwith a set known to be serviceable every 1200 hours.

The removed units can then be sent to a properlyequipped repair facility. Those that meet specificationsafter inspection, testing, and any necessary repairs canbe returned to service in the next cycle.

Also, don’t overlook bad fuel as a possible sourceof thermocouple problems. Fuel contaminated by dirtor microorganisms can lead to thermocouple trouble byclogging fuel nozzles and altering fuel spray patterns.

Preventing Overtemperature Damage

Beyond strict adherence to the thermocouplemaintenance and inspection procedures set forth in theauthorized manuals, the best insurance againstovertemperature damage brought on by malfunc-tioning thermocouples is vigilance on the part of theflight crew. Higher than normal fuel flow abovecrossover for a given TIT is one possible sign ofmalfunctioning thermocouples.

Another is higher than normal torque for a givenTIT. Since fuel flow and torque are closely interrelated,an increase in fuel flow to raise the average TIT signalwill also cause an engine’s torque output to increase.Careful attention to the fuel flow and torque indicatorscan pay substantial dividends in terms of timelydetection of overtemperature conditions.

We have already seen that the TIT indicator willoften not be a reliable guide to the actual TIT when ther-mocouple trouble is present. Usually the indicated TITwill be lower, sometimes much lower, than the true value.

You can make use of this fact to perform a simple


test when problems in the thermocouple system aresuspected. With all engines running and above cross-over, and bleed air turned off, position the throttlelevers so that torque and fuel flow for each are aboutequal. An engine with thermocouple damage will showa noticeably lower TIT than the other three.

Finally, an important thing to remember is that tur-bine engines never get better with age. They can onlydeteriorate as the combined effects of heat, erosion,sulfidation, corrosion, and wear gradually reduce theirefficiency.

A 501/T56 engine that seems to be improving withage-producing more torque at the same indicatedTIT, for example-should immediately attract yourattention. Some things are too good to be true and thisis one of them. It is very likely that there are problemsin the TIT indicating system of this engine and promptaction is needed to prevent over temperature damage.

a service publication of - lockheed martin...deterioration as engine operating hours accumulate....

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