Jet Engine Theory

Question 1

Give examples of Low and High Pressure Bleed Air.

Answer 1

Low-Pressure Bleed Air: Engine Starting and Air Conditioning.
High-Pressure Bleed Air: Thrust reversers, cowl and wing anti-ice

Question 2

What phase of flight is a compressor stall most likely to happen?

Answer 2

Takeoff or a Go-Around / Missed Approach

Question 3

Where are the ignitors?

Answer 3

Combustion section

Question 4

How is ignition in the combustion chamber sustained?

Answer 4

Self Sustained after ignitors are used to start the engine

Question 5

Describe the path air will travel from start to finish in a Turbofan Engine?

Answer 5

In a Turbofan Engine the air is first separated into two pathways after encountering the fan. One is called the bypass air pathway and it is where 80% of the air travels, it aids in speed, noise reduction and fuel efficiency. The other possible path is through the engine core where it, immediately after the fan, encounters the low pressure and high pressure compressors. Next the air flows through a diffuser and then into a combuster where the fuel is injected and mixes fuel and air and allows it to combust continuously around 1700 degrees c. This build up of heat causes the gas to expand, escaping from the combuster at high energy to flow through the high and low pressure turbines causing the blades to rotate, this rotation is what drives the compressors and fan located int the front of the assembly.

Question 6

What is a reduction gearbox?

Answer 6

it decouples the fan and the low pressure turbine, allows all components to run at their optimum speed, the large fan slower, and the low pressure

Question 7

What are some common Turbofan Engine failures?

Answer 7

Compressor surge
Hot Start
Flameout
Hung Start
Bird/FOD
Fuel System Problems
Oil System Problems

Question 8

Explain what Flameout is?

Answer 8

More common in older systems
Caused by pilots moving throttle too quickly
Flame gets blown out by not enough air or too much air

Question 9

Explain what occurs in a Compressor Stall.

Answer 9

Disruption of airflow into engine usually caused by abrupt pitch changes from turbulence or Stall

Can happen on takeoff

Can turn on ignition system to prevent

Some aircraft do this automatically when stall warning or stick shaker activate

Reduce power, Reduce AOA

Question 10

What is TOT?

Answer 10

Turbine Outlet Temp
Similar to EGT

Question 11

What is TIT?

Answer 11

Turbine Inlet Temperature
- Temp of gasses from the combustion section as they enter the first stage of the turbine

• Highest temp inside a gas turbine
• Limiting factor of amount of power engine can produce
• Hard to measure, EGT more common and related to TIT

Question 12

What is ITT?

Answer 12

Inter-stage Turbine Temp
○ Temp of gasses between high-pressure and low-pressure turbine wheels

Question 13

What is N1?

Answer 13

Monitors low-pressure compressor section
● Fan speed
● Primary indication of thrust on most turbofans
● N1 Shaft includes Fan, LP compressor, and LP turbine

Question 14

What is N2?

Answer 14

Monitors gas generator section
● Used mainly for engine starting and some system functions
● N2 shaft includes HP compressor and HP turbine

Question 15

What is a basic explanation for how a Jet Engine works?

Answer 15

Develops thrust by accelerating a relatively small mass of air to a very high velocity

Question 16

What are the stages of a Jet Engine?

Answer 16

Induction(Intake), Compression, Combustion, Expansion, Exhaust

Question 17

Where are the two parts of Expansion taking place?

Answer 17

Part of expansion takes place in turbine section to drive compressor

Rest of expansion takes place in nozzle of tail pipe to accelerate gas to high velocity jet for thrust

Question 18

What is EPR?

Answer 18

Engine Pressure Ratio
Indicates how hard the engine is working
● Indication of Thrust
● Difference between turbine discharge pressure and engine inlet pressure
● Primary reference to establish power settings when equipped

Question 19

What is the Thrust to Thrust Lever Relationship?

Answer 19

Thrust is proportional to RPM (mass flow) and temp (fuel/air ratio)
● More thrust produced per increment of throttle movement near the top of the range than near the bottom

○ Substantial lever movements needed near bottom range (i.e. on go around or final)

Question 20

What is the Diffusor

Answer 20

Divergent section of the engine after the compressor section but before the combustion section

● Reduces high-velocity compressor discharge air to increased pressure at a slower velocity

● Allows air to enter combustion chamber at low enough speed to burn continuously and not extinguish the flame

● Also allows air to slow to be burned in a smoother, gradual process.

Question 21

What is a Jetpump?

Answer 21

A form of a fluid pump in an aircraft fuel system that produces a low pressure by moving fuel at a high-velocity through a venturi
● A jet pump helps remove vapors from fuel before it enters a fuel control unit
● Some of the fuel from the main pump is returned to the tank through a venturi or a jet pump
● The low pressure at the throat of the venturi pulls the vapor out of the chamber where vapor gets collected before returning fuel to the tank