Jet Eng and Perf-Akg

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1

Introduction To Jet Engines



2

Purpose

• Demonstrate differences between piston

and jet aircraft engines

• Familiarize with jet engine operation



3

Objectives• Identify jet engine principles

• Identify the basic sections of a jet engine• Know the primary power setting method for the

737 NG

• Identify two components that control engines inthe 737 NG

• Define Maximum Takeoff and Go Around Thrust

• Define two methods of takeoff thrust reduction

• Be familiar with fuel mileage as a function of

altitude

• Identify some engine malfunctions



4

Jet Engine Principles

• The jet engine has the same four distinct cycles

that a reciprocating piston engine has: intake,

compression, power (combustion), and exhaust.

• In a jet engine these cycles occur simultaneously

and continuously. They are sequential in

reciprocating piston engine..

• The cycles in a jet engine occur in different

sections of the engine. In reciprocating piston

engines they are in the same location - thecombustion chamber (cylinder).

• The four sections of the jet engine are inlet,

compressor, combustor, and turbine.



5

Jet Engine Principles



6

• The turbofan engine has a fan on the front of the

engine and also primary airflow through the core

– Think of it as a fixed pitch propeller where the

air flow is diverted into an extra duct around

engine.

– Bypass ratio = mass airflow fan divided by

mass airflow core. A bypass ratio of 5 means

that five times more mass airflow goes

through the fan duct than goes through the

primary core.

– Approximately 75-80% of the total thrustcomes from the fan air in high bypass ratio

engines.

Turbofan Engine Principles



7

Turbofan Engine Profile



8

Commonly Encountered Jet Engine

Terms

EPR - Engine Pressure Ratio:

– Ratio of total pressure at the exhaust or turbine exit (e.g., PT7

or PT5 ) to total pressure at the front of the fan/compressor (PT2 )

– This is commonly used as a measure of engine thrust, and is

the primary thrust setting parameter on Pratt and Whitney andRolls Royce engines.

• N1 or %N1: – N1 is the rotation rate, in RPM, of the low-speed rotor of a two

or three-spool engine.

– N1 is usually expressed as %N1, a percentage of somenominal value.

– General Electric and CFMI engines use %N1 as the primary

thrust setting parameter.



9

Power Setting Methods

• Power is set and controlled on jetengines with N1 RPM (revolutions

per minute).

– N1 RPM is physical fan rotor speed, asindicated on a flight deck N1 RPM

guage (used for power setting on CFMI

engines).

– Manifold pressure is not used for jet

engine operation/control.



11

Power Setting Versus Outside Air

Temperature

• Thrust is limited by:

– Combustion section pressure in the pressure

limited region

– Exhaust Gas Temperature (EGT margins) in the

temperature limited region

Corrected

N1 (N1/√θ)

OAT

Pressure Limit

Temperature

(EGT) Limit

Thrust Break Point



12

Bleed Air Extraction System Effects

• When bleed air is extracted from the engine, less primary airflow ifavailable to generate thrust and a power setting reduction is

required

• Typical uses of bleed air are:

– Anti-ice systems

– Air conditioning

– Turbine clearance control



13

Electronic Engine Controls

• Jet engine thrust/power is controlled by theamount of fuel injected into the combustion

chamber (through fuel nozzles).

• The engine “hydro mechanical unit” (HMU)meters this fuel flow and receives control

signals from the electronic engine control or

power management control system.

• Earlier power management controlsschedule fuel flow based on power lever

angle, engine pressure and temperature,

and rpm.



14

• Modern electronic engine controls fine tuneengine performance and manage thrust,

schedule fuel flow, process information, and

control engine sub-systems with much

greater precision and accuracy.

• Electronic engine controls simplify thrust

management procedures. However, pilots

should still monitor engine parameters andverify that proper thrust is obtained.


(continued)



15


(continued)

• Some engine control terminology

– FADEC: Full Authority Digital Engine Control

– advanced technology, very accurate and

precise – EEC: Electronic Engine Control – is the main

component of the FADEC that has two basic

functions - information processing and engine

control



16

Thrust Versus Speed• Thrust available decreases as airplane

speed increases since thrust is a

function of mass airflow x velocity.

– As the airplane speed increases, the relative

velocity of the engine exhaust decreases.

90

60

70

80

100

0 0.1 0.2 0.3 0.4

Mach

KTAS

% of

Static

Takeoff

Thrust

Thrust versus Mach

(Sea Level ISA Day)

0 66 132 198 265



17

Thrust Versus Altitude

• Thrust available decreases as airplanealtitude increases.

– Takeoff and climb performance are

significantly reduced at higher elevation

airports.

90

60

70

80

100

0 5000 10000

Altitude (ft)

% of Sea

Level

Static

TakeoffThrust

Thrust versus Altitude(ISA Temperature)

15000



18

Takeoff Thrust Versus Altitude

• Thrust available decreases as airplanealtitude increases.

– Takeoff and climb performance are

significantly reduced at higher elevation

airports.

90

60

70

80

100

0 5000 10000

Altitude (ft)

% of Sea

Level

Static

TakeoffThrust

Thrust versus Altitude(ISA Temperature)

15000



19

Cruise Thrust Versus Altitude• Maximum cruise thrust available decreases with altitude.

• Thrust required, at a fixed true airspeed and gross weight, willdecrease with altitude up to the tropopause and then will increase

(pressure and temperature effects).

• Fuel flow will trend the same as thrust required.

Altitude

Thrust

or

Fuel Flow

Thrust versus Altitude (ISA Temperature)

Cruise thrust available

Cruise thrust required

Fuel flow



20

Jet Engine Spool-up Versus

Piston/Prop Spin-up

• Reciprocating piston engines spin up

• Jet engines spool up at a slower rate.

– FAR 25.119 : thrust in eight seconds

• So…Plan Ahead



21

Methods of Thrust Reduction

• Reduced thrust

– When actual takeoff weight < performance

limited takeoff weight

• Why?

– Reduces maintenance cost

– Increases engine reliability and life cycle



22

Methods of Thrust Reduction

• Two established methods of thrust

reduction

– Derate

– Assumed temperature

• Two different philosophies



23

• Derate

– Typically a fixed percent reduction of

takeoff rated thrust

– Programmed and selectable in the FMC.

Derate



24

• Assumed Temperature – A higher than actual ambient air

temperature

– Programmed and selectable in the FMC. – Maximum reduction is 25% of actual

thrust

Assumed Temperature



25

Assumed Temperature

MTOW

OATactual

Thrust

25% Maximum

Reduction

OAT

Pressure Limit

EGT Limit

Weight

Available thrust

Actual Required thrust

TassumedTbreak Tmax assumed



26

737NG FMC NI LIMIT Page

Derate selection in FMC.

N1 LIMIT 1 / 1 SEL / OAT

/ + 20 ° C

26K

< T O 24K DERATE

< TO - 1 <ACT> <SEL> 22K DERATE

< TO - 2

<PERF INIT

26K N 1

98.8/ 98. 8

CLB >

CL B - 1>

CL B - 2>

TAKEOFF> — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — —

— — — —



27

737NG FMC NI LIMIT Page

N1 LIMIT 1 / 1 SEL / OAT

/ + 20C 26K

< T O 24K DERATE

< TO -

<ACT> <SEL>

22K DERATE

< TO - 2

<PERF INIT

26K N 1

98.8/ 98. 8

CLB >

CL B - 1>

CL B - 2>

TAKEOFF> — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — —

Assumed temperature in FMC

Can be accomplished with derate also

+ 40C

1

737NG U C t



28

737NG Upper Center

Display Panel

Assumed temperature and target %N1 setting display

ENG 1

START VALVE OPEN

OIL FILTER BYPASS

LOW OIL PRESSURE

EGT

N 1

ENG 2

START VALVE OPEN

OIL FILTER BYPASS

LOW OIL PRESSURE

663 663

87.7 98.8

10

TAI

8

6 4 2

0 87.7 98.8

10

TAI

8

6 4 2

0

TAT +20C D - TO + 40C

FUEL

KG 1

900 3

CTR

2

900 3

600 7



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Fuel Mileage – Engine

Contributions

Thrust/δ

Mach

Constant W/δ

Min. Drag curve

W/δ Increasing

• Cruise fuel mileage = TAS/fuel flow = NAM/Lb of fuel• Thrust required equals drag (airframe effect)

• Fuel flow (engine effect)

• Power setting required (EPR or %N1)



30

Cruise Fuel Flow versus Altitude• Fuel flow, at a fixed true airspeed and gross weight, will

decrease with altitude up to the tropopause and then willincrease (pressure and temperature effects on thrust).

• Thrust required will trend the same as fuel flow.

Altitude

Thrust

or

Fuel Flow

Fuel Flow vs Altitude (ISA Temperature)

Cruise thrust available

Cruise thrust required

Fuel flow

Th t S ifi F l



31

Thrust Specific Fuel

Consumption versus Altitude

• Thrust specific fuel consumption (TSFC)for a given true airspeed and weight will

decrease with altitude.

Altitude

TSFC

Thrust Specific Fuel Consumption

(TSFC) versus Altitude (ISA Temperature)



32

Engine Malfunctions /

Anomalies

• A few anomalies on jet engines that

are different than what is

encountered on reciprocating piston

engine airplanes are: – Compressor surge

– Roll back or flameout

– Foreign Object Damage (FOD) from birdingestion into engine

– Tailpipe fire



33

Compressor Surge / Stall

• Compressor blades are airfoils that can stall if theairflow conditions in the engine are not correct.

– When compressor stall occurs, the airflow

through the jet engine becomes unstable to the

point where normal compression can no longertake place.

– There is a rapid reverse flow of higher pressure

air from a higher stage of compression.

• This rapid escape of high pressure air can cause aloud bang that may be accompanied by flames

shooting out the inlet and/or tailpipe.



34

Compressor Surge / Stall

• Compressor surge can be caused by:

– Malfunction of surge bleed control system

– Bird or foreign object ingestion

– Engine deterioration

– Other engine damage occurring

– Strong crosswinds at low airspeeds (during

takeoff) – Very rapid acceleration or deceleration of

the engine



35

Surge / Stall Engine Indications

• A loud bang, vibration, and/or airplane yaw may be

the first indication of a surge at high power settings

• High EGT and rapid EPR or rotor speed changes

are the main engine indications of a compressorstall

– These changes may be more rapid than can be

displayed on engine instruments

• Abnormal engine noises or no response to throttlemovement can also be indications of a surge



36

Engine Surge / Stall Recovery

• Compressor stalls are infrequent

events with modern electronic engine

controls. – The engine control system may perform a

self-recovery for single or multiple surges.

– Multiple or severe surges may beunrecoverable and the engine may flame

out (roll back) or require pilot action to

shutdown.



37

Surge / Stall Pilot Response

• The general pilot response is to: – Fly the airplane (maintain control)

– Disconnect the autothrottles

– Immediate power reduction on the surgingengine (throttles to idle)

– Refer to the appropriate non-normal

checklist

– Assess the engine condition and attempt

recovery if engine damage is not severe

• Partial thrust or idle thrust operation is better

than having an engine shut down



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• Flameout is a condition in which thecombustion process is stopped (fire or

flame in combustion section has gone

out).• A drop in EGT and rapid decrease in

EPR or rotor speed are the main

engine indications of a flameout.• Engine system malfunctions, such as

generators offline, may be the first

indication of a problem.

Flameout / Engine Rollback



39

• Flameout may be caused by:

– Unsteady airflow (e.g., strong

turbulence or severe weather)

– Engine control system malfunctions

– Improper fuel management / fuel

starvation – Volcanic ash ingestion

– Unstable engine operation (stall or

surge first)




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• The general pilot response is to:

– Fly the airplane (maintain control)

– Assess damage

– Attempt relight (airstart) if no engine

damage

– Refer to non-normal checklists



41

Engine Foreign Object Damage

• Jet engines are more susceptible to foreign objectdamage (FOD) due to:

– High inlet velocity and large inlet areas

– Lower engine to ground clearances

• Engine may pick up runway debris, tire fragments,animals in the vicinity…bird ingestion

• Be alert for engine damage malfunctions at high

power settings on the runway and / or taxiway due

to FOD (avoid high power during taxi)• Indication of FOD may be surge/stall, loud thud or

bang, high vibration, or even smell in bleed air

system from bird ingestion



42

Tailpipe Fire

• Tailpipe fires are caused by fuel puddling in the

turbine or exhaust section during start up or shut

down of the engine.

• A flame may shoot out the back of the engine and

it may startle or panic the passengers.

• If the tailpipe fire is not accompanied by any other

engine malfunctions, there may not be any

indications on the flight deck that something is

wrong.• Cabin crew, ground control, or passengers

usually notify the pilots of the problem.



43

Tailpipe Fire

• The general pilot response is to:

– Accomplish the tailpipe fire procedures

• This usually includes motoring the engine topurge the fuel from the exhaust

• A typical jet engine fire extinguishing

system does not inject flame retardant into

the tailpipe so pulling the fire handle anddischarging a fire bottle will not put out the

tailpipe fire



Summary

• There are significant differences

between piston and jet engines

• Thrust vs altitude and speed• Takeoff thrust reductions

• Engine malfunctions

Jet Eng and Perf-Akg

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