Engine Instruments / Fuel

Question 1

What are the 2 categories of gas turbine indicators

Answer 1

Performance indicators

Condition indicators

Question 2

What is the SI symbol for rotation speed

Answer 2

N

Question 3

What is the first sign of a engine starting in a 3 spool engine

Answer 3

Rotation in N3

Question 4

What is the first sign of starting in a 2 spool engine

Answer 4

Rotation in N2

Question 5

What does the N1 gauge show

Answer 5

% of max rotational speed of low pressure (LP) shaft (fan)

Question 6

What is EPR

Answer 6

Engine pressure ration

P7/p1

Question 7

What is P1

Answer 7

Compressor inlet pressure

Question 8

What is P7

Answer 8

Exhaust outlet pressure

Question 9

What happens to EPR if P1 probe is blocked

Answer 9

EPR will overread

Question 10

What must be done to the EPR on takeoff

Answer 10

Set to a forward speed due to an apparent decrease in pressure at P1

Question 11

What does the N2 gauge show

Answer 11

High speed shaft (engine core) speed = shown as % of max value

Question 12

What does the P7 probe show

Answer 12

Situated at rear of engine gives exhaust pressure displayed at direct psi or %

Question 13

What are condition indicators

Answer 13

Oil temp / pressure 
Fuel temp / pressure 
Fuel flow 
Exhaust gas temperature (EGT)
Vibration

Question 14

How does the mechanical tachometer work

Answer 14

• flexible drive taken from rear of engine
• fed to rear of instrument where it drives a magnet within an aluminium drag cup
• rotating magnet sets up eddy currents
• turning force is restricted by hairspring

In engine close to cockpit and fairly low revving

Max length of drive = 2m

Question 15

How does a DC tachogenerator (electrical) work

Answer 15

Essentially a voltmeter

Output voltage varies with engine speed

Question 16

What is the 2 disadvantages of a DC tachogenerator (electrical)

Answer 16

Requires a commutator and carbon brushes = leads to wear = electrical sparking can cause radio interference

Voltage loss in transmission = indication error

Question 17

What is a single phase AC tachogenerator (electrical)

Answer 17

Same as a Dc tachogenerator except used AC and then rectifies it to Dc

Voltage is proportional to rpm

Still has indication errors caused by voltage losses in transmission

Question 18

What is a three phase AC tachogenerator (electrical)

Answer 18

3 phase ac current

Frequency varies with RPM

magnets designed to rotate at the same frequency as the generator = ‘asynchronous’ or ‘squirrel-cage’

Question 19

What is a squirrel- cage motor in the three phase AC tachogenerator

Answer 19

Rotates slightly slower than ac generator = rotor slip

Motor turns magnet on aluminium ‘drag-cup’

Drag cup turns against hairspring to move indicator needle

Question 20

What does a gas turbine need for the three phase Ac tachogenerator

Answer 20

Step- down gearing because of faster rpm

Question 21

Advantages of a three phase AC tachogenerator

Answer 21

Because frequency is senses not voltage then losses in transmission are negligible

Question 22

Disadvantages of a three phase AC tachogenerator

Answer 22

Because of extra wiring = heavier

Question 23

What is a induction tachometer (speed probe) ‘electronic’

Answer 23

For engine spoils that do not drive a gearbox

Probe adjacent to either a toothed wheel or the fan

Creates magnetic field as wheel turns

Charges induced are proportional to the rpm detected

Question 24

How is the signal for the induction tachometer processed

Answer 24

1) in older air craft = transformed to create high voltage / low current

signal loses little strengths in transmission over long cable runs

2) modern aircraft square off into digital signals

Not affected by voltage loss in transmission = suitable for engine management computer

Disadvantage is it required power

Question 25

How is engine vibration measured

Answer 25

Accelerometer or piezo electric element Number given in cockpit has no units

Question 26

What is a mechanical fuel quantity measure

Answer 26

Measured valine Affected by aircraft attitude May require power if sensor if variable resistance and for instrument light

Question 27

What is a capacitance type fuel quantity gauge

Answer 27

Dielectric property of fuel is 2 x that if air The charge the capacitor can hold is doubled in fuel Works by measuring the capacitance or the capacitor

Question 28

What do you do if the fuel quantity doesn’t quite match up

Answer 28

Use a drip stick

Question 29

What is a reference capacitor

Answer 29

Used for reference or the fuel in a capacitance type fuel quantity gauge Always submerged

Question 30

Is a capacitance fuel quantity gauge affected by attitude

Answer 30

Nope

Question 31

Fuel flow mechanical (vane) type

Answer 31

Measured volume Used with piston engines Ltr / hr

Question 32

Impeller type fuel flow system

Answer 32

F= ma Measured mass flow Kg/ hr Totaliser = fuel flow x time How much fuel has been used

Question 33

What is a synchro scope

Answer 33

To synchronise propeller RPM Tor mep Master sets reference of other engine(s) Can use voltage from alternator as measure Synchronise rpm for to reduce vibration and noise Doesn’t improve fuel burn

Question 34

What is synchrophase

Answer 34

Same as synchroscope Also adjusts prop position Further noise reduction