19 Engine Instruments

Question 1

What is N1?

Answer 1

N1 is the RPM of the fans of a jet engine given as a percentage

Question 2

What is the formula for the Engine Pressure Ratio (EPR)?

Answer 2

EPR = Exhaust Pressure / Intake Pressure

Question 3

What sort of pressure instrument is used to measure engine pressure?

Answer 3

Bellows

Question 4

Describe a Mechanical RPM indicator:

• Aka
• Basic operating principle
• Require an external power supply

Answer 4

• Magnetic / Drag Cup indicator
• A cable is physically connected to the engine which spins a magnet. - The magnet sits within a ‘cup’ which is magnetised and try’s to spin with the magnet.
• The spinning cup applies a force to a spring which turns the needle
• The faster the engine, more force applied to the spring, further the needle moves
• No external power is required

Question 5

What is a Tacho-Generator also known as?

Answer 5

• Electric RPM indicator

- Induction RPM indicator

Question 6

Describe a Tacho-Generator RPM indicator which uses DC or 1 or 2 phase AC:

• Basic operating principle
• What is measured to give an indication on RPM
• Require an external power supply

Answer 6

• Engine is attached to a magnet which spins with it.
• The spinning magnet forms part of an electric generator
• The voltage from the generator is fed to an electric motor
• The motor applies a force to a spring which turns the dial
• The Induced Voltage supplied to the motor is measured to give RPM reading
• No external power supply is needed

Question 7

What is the downside of using a Tacho-Generator RPM indicator which uses DC or 1 or 2 phase AC?

Answer 7

• The induced voltage measured is affected by the resistance within the cables
• This can lead to inaccuracies in the reading

Question 8

Describe a Tacho-Generator RPM indicator which uses 3 phase AC:

• Basic operating principle
• What is measured to give an indication on RPM
• Require an external power supply

Answer 8

• Engine is attached to magnet which spins
• The magnet is surrounded by 3 sets of cables
• As the magnet spins it generates a current in each set in turn
• The frequency of these currents is measured to give an indication of the RPM
• No external power supply is required

Question 9

Describe a Tacho-Probe RPM indicator:

• Aka
• Basic operating principle
• What is measured to give an indication on RPM
• Require an external power supply

Answer 9

• Electronic / Induction Probe
• A probe supplied with AC current is positioned next to the fan blades of a jet engine
• As the blade passes by the probe is creates a magnetic distortion in the current in the probe
• Frequency of the distortions is measured to give an indication of RPM
• ## Does require an external power supply

Question 10

Name two engine performance indicators?

Answer 10

• Engine Pressure Ratio (EPR)

- N1

Question 11

Describe a Mechanical or Float Gauge Fuel Level indicator:

• What it measures
• How it measures
• Any associated errors

Answer 11

• Measure the volume of fuel
• A float is placed in the fuel tank and moves up and down according to the level of fuel
• The float is attached to a variable resistor and a DC current
• The difference between the DC current and that which reaches the indicator gives a reading of fuel level
• Only measures volume which is effected by temperature
• Measurement is effected by the attitude of the AC

Question 12

Describe a Electircal or Capacitance Fuel Level indicator:

• What it measures
• How it measures
• Any associated errors

Answer 12

• Measures the mass of the fuel
• Capacitors in the tanks read the capacitance at different locations
• The overall value derived from all capacitors indicates the mass of fuel
• This is compared to a reference capacitor which is always submerged
• No associated errors

Question 13

If the reference capacitor in fuel tank breaks, can the fuel amount be read in that tank?

Answer 13

No

Question 14

If the fuel quantity indicator for a tank is broken, can the AC still fly?

Answer 14

• Yes

- As long as the tank has a serviceable fuel flow indicator

Question 15

Describe a Mechanical Fuel Flow meter:

• What it reads
• How it reads it

Answer 15

• Measures the fuel flow volume (Volume/hr)
• A plate or propeller is placed in the fuel pipe
• How much this is displaced or how fast it is turned indicates the volume of fuel flowing

Question 16

Describe an Analogue Electrical Fuel Flow meter:

• What it reads
• How it reads it

Answer 16

• Measures the mass of fuel flow
• A motor or impeller causes the fuel to spin
• The fuel then flows into a torque meter
• The amount the torque meter turns gives an indication of the fuel flow mass

Question 17

What pressure probe is used to the measure the pressure of a low pressure fuel pump?

Answer 17

Aneroid Capsule

Question 18

What is the formula for Capacitance?

Answer 18

Capacitance = Dielectric Permittivity x (Area of Plates / Distance Between the Plates)

Question 19

What does a Tachometer measure?

Answer 19

Angular speed

Question 20

What is the purpose of a Micron Filter?

Answer 20

To remove small particles as a result of normal wear and tear of the engine

Question 21

What is the purpose of Magnetic Chip Detectors?

Answer 21

• Attract and retain small particles the the lubricating flow
• Particles are coming from a component which is wearing rapidly or starting to break up
• Gives an indication of impending failure

Question 22

How are vibration levels displayed?

Answer 22

• Displayed as a ‘Relative Amplitude’

- Relative to the amplitude measured by the manufacturer

Question 23

What is the engine pressure ratio?

Answer 23

Ratio of the turbine outlet total pressure and the compressor inlet total pressure

Question 24

As an AC climbs, what happens to N1?

Answer 24

• As pressure altitude increases, air density reduces meaning more thrust is required to maintain the same speed
• N1 is automatically adjusted accordingly

Question 25

What is the name of the unit used to measure capacitance?

Answer 25

Farad