Mattingly J.D., Heiser W.H., Pratt D.T. Aircraft Engine Design

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APPENDIX N: TURBINE ENGINE LIFE MANAGEMENT 653

N.6.4 TASK IVmGround and Flight Tests

N. 6.4. I Environmental verification testing

1) Thermal survey: An instrumented engine shall determine the steady state

and transient temperature profiles for critical parts to correlate with thermal

analysis.

2) Ground vibration strain and flutter verification survey: An instrumented

engine will measure vibratory stress at all critical points in the flight envelope

with aircraft simulated inlet distortion in an altitude/ram facility with nominal and

off-nominal IGV schedules to ensure sufficient vibratory stress margins. Flutter

boundary exploration shall be conducted.

3) Rotor dynamic survey: A full-scale engine rotor dynamic test shall be con-

ducted on an instrumented engine with deliberate rotor unbalance. Bowed rotor

characteristics with clocked rotor out-of-balance shall be investigated.

4) Installed vibrations survey: An installed engine vibration survey shall be

conducted to ensure dynamic compatibility between the engine, its accessories,

and the airframe. The engine should have each rotor unbalanced to the maximum

allowable limit for this test.

5) External component resonance search test: A vibration survey shall be con-

ducted on all externally mounted components to identify resonance frequencies.

Resonance shall not occur at operating speeds.

6) Clearance control: Critical clearances (blade tip clearances and seal clear-

ances) shall be understood through the following test programs: a) high-energy

X-ray (HEX) testing, b) installed engine/nacelle thermal survey, c) flight test data

from clearance indicators, and d) optimized final green run cycle.

7) Individual engine vibration characteristics database: All development en-

gines shall have their vibration signatures characterized. Engine acceptance bands

shall be derived from this data.

N.6.4.2 Accelerated Mission Test spectrum derivation.

Accelerated

Mission Tests (AMT) shall be derived initially based on design mission profiles

and mix and continually updated based on real usage from the usage program in

Task V (lead the fleet with flight recorder data). When it is necessary, altitude

and/or ram testing shall be included. (See Sec. N.7.2 for definition of AMT.)

N.6.4.3 Durability tests

1) Flight readiness verification tests (to be completed before first flight):

a) overspeed and overtemperature test; b) resonance search and installed vibra-

tion survey; and c) a limited AMT test to verify engine durability for the length of

the planned flight test program plus incremental running.

2) Product verification AMT: Product verification (PV) engine run to one lifetime

prior to production release.

3) Production AMT#1: Test hard-tooled B/M engine to one lifetime using design

mission usage spectra.

4) Production AMT #2: Test hard-tooled B/M engine to a minimum of one

lifetime with spectra updated to real usage obtained from flight recorder data.

5) Overhauled engine AMT: Test random overhauled engine to remaining life-

line.

654 AIRCRAFT ENGINE DESIGN

N.6.4.4 Damage tolerance tests.

An instrumented PV configured engine

shall be employed for damage tolerance tests of safety critical structure:

1) Critical parts will be prefiawed at those critical points judged by analysis

and prior component tests to be critical and using the AMT test spectrum shall be

tested to one inspection period. Flaw growth will be monitored, fractographically

examined subsequent to testing, and correlated with the damage tolerance analysis

component tests.

2) FOD, ingestion, and containment tests will be tested to demonstrated com-

pliance with design criteria.

3) All repairs will be tested by AMT methods to two inspection periods.

N.6.4.5 Flight test strain survey.

Selected engine components known to

be sensitive to installation-induced vibratory stresses (for example, the fan) shall

be instrumented, and data shall be taken for ground and flight operational loading

conditions, such as gunfire or rocket exhaust gas ingestion, thrust reverse reinges-

tion, cross wind, and high angle of attack.

N.6.4.6 Update durability and damage tolerance control plan.

The re-

sults of Task III and IV shall be used to update.

N.6.4.7 Performance deterioration, structural impact assessment.

One of the most undesirable effects of deterioration is that combustion temperatures

must be increased in order to maintain engine thrust, thus reducing hot section parts

life.

N.6.4.8 Critical part update.

N.6.5 TASK V--Production Quality Control and Engine Life Manage-

ment

N.6.5. 1 Production engine analysis.

The Task II analysis shall be updated

to account for redesign components, changes in usage, material properties, and

results of TASK III and IV tests. The updated analysis shall form the basis for

preparation of the engine structural maintenance plan.

N.6.5.2 A summary report on safety and durability shall be prepared.

N.6.5.3 Engine structural maintenance plan.

The structuralmaintenance

plan shall include the inspection requirements for critical parts for both field and

depot. Critical part safety and economic life limits shall be supplied for use with

the individual engine tracking system. Diagnostic logic and procedures shall be

supplied.

N.6.5.4 Individual engine tracking.

Damaging events such as significant

power transients and time at and above intermediate power levels will be recorded

with exceedance-type counters so that inspection and part replacement can be

accomplished to preclude structural failures. Critical parts shall be serialized for

tracking purposes.

APPENDIX N: TURBINE ENGINE LIFE MANAGEMENT 655

N.6.5.5 Lead the force program (usage).

A "lead the force program"

shall be implemented and will provide early maintenance and operational usage

data. Usage data shall be recorded on the Service Loads Monitoring Program

(SLMP) recorder and shall include at least N1, N2, PLA, T2, and T4 vs time.

These data will be used to update analysis and AMT tests.

N.6.5.6 Durability and damage tolerance control plan implementation.

The updated durability and damage tolerance control plan shall be implemented

on the production program.

N.6.5. 7 Technical order update.

Technical orders are the principal means

by which changes in logistic and maintenance procedures are implemented in the

field. They are essential to solving field problems and improving availability and

life.

N.7 Development and Qualification Testing

N.7.1 Historical Perspective--The 150-H Test

From 1950 to 1970 turbine engine structural life qualification/verification was

mostly based on engine running tests that involved a 150-h formal qualification

test that was, in turn, the legacy of the qualification testing of piston engines. This

test, shown in Fig. N.18, was not mission oriented but did put emphasis on the

engine hot section by having appreciable time at high power settings to ferret out

high temperature-induced creep and stress rupture failure modes. Some partial

throttle transients were included but were far too few to drive a low-cycle fatigue-

type of failure mode in either the compressor or turbine sections of the engine.

Throttle stair steps were added to incite possible high cycle fatigue weakness, but

have proven inadequate. The major throttle cycle of "zero to max/mil power to

MIL -

IDLE

MANY ENGINE DEVELOPMENTS IN THE PAST DID NOT REVEAL

SERVICE RELATED PROBLEMS

TYPICAL 150 HOUR TEST

MQT TYPE

UVUU[/ IllU

6 HOURS- REPEAT 25 TIMES

CYCLE DEFINITION

150 HR 2000HR

TEST USAGE

IDLE-MIL-IDLE 300 20,000

0-MIL-0 25 1,900

Fig. N.18 Typical 15O-h qualification test spectrum.

656 AIRCRAFT ENGINE DESIGN

zero" with some cooldown, which is prevalent in every mission, does contribute

substantially to engine low-cycle fatigue damage and was virtually missing. Also

shown in Fig. N. 18 is a block showing a comparison between the qualification test

and the throttle usage of a typical modern fighter for 2000 flight hours.

Recognizing the shortcomings of the formal 150-h test requirement contained

in the coordinated Tri-Service Mil Spec 5007C (contains military requirements

for development of new turbine engines) written in 1966, the military revised

the specification to include additional structural component and engine tests in

1974. This revision was Mil Spec 5007D (Ref. 5), which still retained the 150-h

qualification test but also added an engine running mission related low-cycle fatigue

test requirement. In addition, this revision also contained a structural program

that required structural analysis and component testing along with a usage and

tracking system and contained many elements of the early ENSIP Program. This

revision, however, did not contain damage tolerance requirements. It was not until

1984 that damage tolerance requirements were introduced through ENSIP (Engine

Structural Integrity Program) in the Military Standard 1783. The mission-related

test is now known as the AMT, which is a formal engine qualification/verification

test to demonstrate full design engine life prior to full scale production.

N.7.2 AMT (Military)

The AMT 6 is a mission-related test that has all the nondamaging throttle cycles

and power levels, such as idle and cruise, removed from the mission flight profile as

shown in Fig. N. 19. This process uses severity analysis tools as shown in Fig. N.20

MAX

MIN

PART POWER

IDLE

FULL MISSION

i I

1.0 2.0

TIME (HOURS)

MAX

MIN

PART POWER

IDLE

0.5

TIME (HOURS)

Fig. N.19 Typical AMT for military

engine usage.

AMT

APPENDIX N: TURBINE ENGINE LIFE MANAGEMENT 657

Determination

~~j~ [~ of Performance

Parameters

% o RPM

P.L.A. II~ Time [l~

Pressures

• ° Temperatures

From Engine

Time Minutes Inlet Temp Cycle Deck

Definition of Mission

(35

Ambient Temp

Profines - P.L.A., AIt, Mach No. ~-J Distribution

(Power Level Angle)

Summation Calculation

of of Partial

Mission ~ll Cycle Life

Mix "~

N1 N 2 N 3 " " "

LCF

Life

N=~NIN2--

RPM

4~1 Stress ~[I Press Temp

Time Time

Calculation of

Definition of Simplified

Stresses Over Profile

Mission Profile

Fig. N.20 Use of severity analysis tools.

in order to reduce the test and calendar time required to reproduce the damage over

the life of the engine.

It is possible to balance or adjust the severity of an AMT cycle to better match

the most severe mission for each component by adjusting the magnitude of tran-

sients and hold time to control thermally induced stresses] This can be done

as shown in Fig. N.21 by adjusting hold time at maximum power, or if total

time at maximum power needs to be maintained, by altering hold time at idle or

subidle.

Certain qualification tests may require the use of ground facilities for run-

ning conditions to better simulate the flight envelope, such as duplicating high

Power

Setting

Time at A/B

for Exercise of

Augmentor, Etc.

to Impose Steady-

State Thermals on Disks

and to Expose Hot Parts

to Stress Rupture

Conditions (Blades/

Dovetails)

Adjustment of Cycle

Content to Exercise

Mechanical Stress Time at Flight Idle.

(Fan, Compressor) Idle or Sub-idle

to adjust thermals

and provide full

Strain Range on Slow

Responding Thermal Parts

Hold Time at Idle or Max

to Adjust Parts Responsive

to Both Thermal & Mechanical

Stress

Time

Fig. N.21 Test cycle design.

658 AIRCRAFT ENGINE DESIGN

Flight Envelope

/ Selected Flight /

, Max / C°ndiste° ~

Mach Number

35 kit

3 Se[ments for 1 Hr 45 Min

Max

Int

~~'~'- 35 Min

10 kit

24 Segments for 12 Hrs

Max

lnt

--~l I "~r- 30 Min

Sea Level

3 Segments for 3 Hrs

Int

-~'-I I Hr I~-

Fig. N.22 Low altitude high speed ram tests.

Mach number at altitude or low-altitude, high-speed ram tests as depicted in

Fig. N.22.

The RFP typically may require a series of AMT tests such as Mil Standard 1783

that involve a series of formal tests for flight clearance release points such as the

IFR, FFR, ISR, and OCR as shown and defined in Fig. N.23.

Component structural special engine testing has also become a formal qualifica-

tion requirement. The following verification tests and measurements are examples:

1) turbine and compressor over-speed engine running tests usually to 115 %; 2) disk

bust testing to a minimum of 122% of the maximum allowable steady-state speed;

3) overtemperature of the first stage turbine rotor by 8 I°F and engine static load

tests; 4) low-cycle fatigue and crack testing of critical engine components including

high-pressure cases, engine mounts, and spin pit testing of rotors and correlated

with analysis; and 5) engine AMT testing is usually accompanied by HCF stair-step

tests up and down.

A fighter engine typical AMT structural verification ground test is shown in

Fig. N.24. The mission mix is usually dictated by the customer or derived from a

usage survey.

N.7.3 1000 Simulated Service Cycle ("C" cycle) Test (Commercial)

Figure N.25 shows an example of two "C" type cycles used as a verification test

for engine integrity and reliability that must be passed for commercial engine cer-

tification. This test cycle is tailored to fit the expected usage such as time at takeoff,

APPENDIX N: TURBINE ENGINE LIFE MANAGEMENT 659

ENSIP Durability Requirement

ACCELERATED MISSION TEST (AMT) DURATION

INITIAL FULL INITIAL

FLIGHT FLIGHT SERVICE

RELEASE RELEASE RELEASE

(IFR) (FFR) (ISR)

AMTIFLIGHT CLEARANCE TEST

SERVICE LIFE [

(DSL)

.'~

X DSL

AMT II

DESIGN VERIFICATION

AMT III

DESIGN VERIFICATION

INCLUDING CHANGE

AMT IV

OPERATIONAL USAGE

OPERATIONAL

CAPABILITY

RELEASE

(OCR)

(lOG)

' 1-2 X DSL

Fig. N.23 Mil Standard 1783 formal test.

reverser throttle transient power levels expected, and cooldown times. A minimum

of 1000 cycles is to be generated prior to certification, and in many cases, many

more cycles are generated prior to initiation of revenue service. Additional tests

are also required for certification such as blade out, blade containment, overtem-

perature and rotor overspeed tests, bird ingestion tests, and rotor burst speed com-

ponent tests.

N.8 Summary

The current turbine engine lifing process has evolved over the past 50 years with

many of the factors that control life changing based on lessons learned from the

engine development programs and customer field problems. Some of the changes

were driven by state-of-the-art improvements such as finite element modeling,

improved material, engine controls, testing techniques, manufacturing, inspection

capability, surface treatments, usage monitoring, and tracking systems. Life man-

agement techniques and philosophies have changed, as well, with recent changes

involving the addition of damage tolerance lifing concepts. Lifing concepts such as

the safe life concept combined with damage tolerance requirements are now being

employed, which will ensure safety even in the event of a rogue flaw. In addition,

engine design development programs now contain improved design criteria and a

better understanding of intended usage, thorough understanding of material char-

acteristics, more specimen and component analysis and testing, and comprehensive

engine environmental and formal verification testing.

When taken together, these advances have had a very positive effect on our

ability to life manage turbine engine components, even in situations that involve

complex usage, which will, through better utilization of engine components, al-

low the customer to see benefits in the cost of ownership, improved service lives,

660

AIRCRAFT

ENGINE DESIGN

ACCELERATED MISSION TEST

AIR COMBAT

MAX-

INT-

CRUISE

IDLE

3OO

20 2020102020 30

o o ooon if

3 2.'

15 30

~ 180 []10 20 30 30

3ogo o T UUl

25 15 5 30 45 \

MAX"

INT-

CRUISE"

IDLE

300

TIME (SECONDS)

ACCELERATED MISSION TEST

AI R-TO-S U RFACE

20 15

R l

15 15 15 15 30 20 20 90 25 10 120 150 11151l 160

~Oll ~o~"o~o ~o~ I~oVO~ ~lfo~o ~ ~o"~

TIME (SECONDS)

100

80 - -- --

~9 60

~40 - -- --

I I I I I I I

0 I I I I I i I

0 6 12 18 24 30 36 42

HOURS

I HIGH ICYCLE iFATIGUEI TEST I

I I ~ I I

,. _l -f-,-~ - 4

Fig. N.24 Typical accelerated mission test profiles.

APPENDIX N: TURBINE ENGINE LIFE MANAGEMENT 661

Takeoff

and

Thrust Reverser

Climb

Power Setting

Descent

Approach

Flight

Idle

Ground

Idle

Motoring

Fig. N.25

More Time at T/O

, Engine A

.....

, 0 Engine B

I___ t

/ ~

Reverse

~', More Severe

--" t", ] Transient

_J : Longer

.....

-,' Cool-Down Time

, , , , , ; ,'., , ,

2 4 6 8 1 14 16 18 20

Time (Minutes)

Simulated service cycle ("C" cycle).

and on safety. Currently, there is a major, national high-cycle fatigue (HCF) ini-

tiative under way that will alter ENSIP so that more emphasis is placed on HCF

development.

References

]ASM International Handbook Committee,

Metals Handbook,

9th ed., Vol. 17, Non-

destructive Evaluation and Quality Control, ASM International, Meta]s Park, OH.

2Anderson, T. L., "Fracture Mechanics Fundamentals and Applications," CRC Press,

Boca Raton, FL, 1991, p. 793.

3Miner, M. A., "Cumulative Damage in Fatigue,"

Journal of Applied Mechanics,

Vol.

67, 1945, pp. 159-164.

4"Engine Structural Integrity Program," U.S. Air Force, MIL-STD- 1783, USAF, Wright-

Patterson AFB, OH, 30 Nov. 1984.

5,,Engine, Aircraft, Turbojet and Turbofan," MIL-E-5007D Military Specification, USAF,

Wright-Patterson AFB, OH, 15 Oct. 1973.

6Taylor, W. R., and Ogg, J. S., "Accelerated Mission Testing of Gas Turbine Engines,"

AIAA Paper 77-992, 1977.

7Tumbull, R. C., "Recent General Electric Engine Development Testing for Improved

Service Life," Society of Automotive Engineers, Paper 78-0990, Aerospace Meeting, San

Diego, CA, 27-30 Nov. 1978.

Appendix 0

Engine Controls

0.1 Control System Requirements

The operation of an aircraft gas turbine engine is certainly complex. It is a

mixture of aerodynamic, thermodynamic, chemical, and mechanical processes

functioning together as a single unit. The control of these processes can be quite

challenging, and the potential for unstable behavior abounds. As an example,

consider the operation of the high-pressure compressor, or the fan. In normal

operation for these components, the air discharges at a higher pressure. The func-

tion can be likened to pushing water uphill--it is forcing the fluid in a direc-

tion it does not want to naturally go. The potential for instability exists, and,

indeed, it is not uncommon for fan or compressor surges and stalls to occur in

operation.

For the purposes of this description of the engine control function, it will be

assumed that we are describing the control system for a current state-of-the-art

fighter aircraft engine, which is characterized as a low-bypass-ratio, twin-spool,

mixed flow, afterburning turbofan engine. The basic principles would also apply

to a high-bypass-ratio, nonafterburning engine, as used in a commercial airliner

or military transport, as well, with some minor differences.

The primary functions of the engine's control system are to 1) maintain con-

sistent, stable thrust levels; 2) maintain smooth, repeatable performance during

transient operation from one requested thrust level to another; 3) maintain stable

airflow, internal pressures and temperatures, and rotor speeds within safe operating

limits; and 4) avoid stalls and surges, and significant speed, pressure or temperature

variations.

In addition to these primary functions, there are secondary functions that the

engine control system must perform. These secondary functions include engine

startup and shutdown; engine bleed and power extraction; inlet anti-icing; hot gas

ingestion protection; and fan and compressor tip-clearance control, among others.

These secondary functions will not be described in detail, but it should be noted

that they are necessary functions and they can result in significant mechanical

complexity.

The first two primary functions of the engine control system focus on main-

taining and modulating engine thrust levels. To understand how the engine control

system accomplishes these functions, consider the equation for uninstalled engine

thrust:

F = (rh9g 9

rhoVo)/gc + (P9 - Po)A9

(4.1)

663