Engine Instruments / Fuel Flashcards
What are the 2 categories of gas turbine indicators
Performance indicators
Condition indicators
What is the SI symbol for rotation speed
N
What is the first sign of a engine starting in a 3 spool engine
Rotation in N3
What is the first sign of starting in a 2 spool engine
Rotation in N2
What does the N1 gauge show
% of max rotational speed of low pressure (LP) shaft (fan)
What is EPR
Engine pressure ration
P7/p1
What is P1
Compressor inlet pressure
What is P7
Exhaust outlet pressure
What happens to EPR if P1 probe is blocked
EPR will overread
What must be done to the EPR on takeoff
Set to a forward speed due to an apparent decrease in pressure at P1
What does the N2 gauge show
High speed shaft (engine core) speed = shown as % of max value
What does the P7 probe show
Situated at rear of engine gives exhaust pressure displayed at direct psi or %
What are condition indicators
Oil temp / pressure Fuel temp / pressure Fuel flow Exhaust gas temperature (EGT) Vibration
How does the mechanical tachometer work
- 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
How does a DC tachogenerator (electrical) work
Essentially a voltmeter
Output voltage varies with engine speed
What is the 2 disadvantages of a DC tachogenerator (electrical)
Requires a commutator and carbon brushes = leads to wear = electrical sparking can cause radio interference
Voltage loss in transmission = indication error
What is a single phase AC tachogenerator (electrical)
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
What is a three phase AC tachogenerator (electrical)
3 phase ac current
Frequency varies with RPM
magnets designed to rotate at the same frequency as the generator = ‘asynchronous’ or ‘squirrel-cage’
What is a squirrel- cage motor in the three phase AC tachogenerator
Rotates slightly slower than ac generator = rotor slip
Motor turns magnet on aluminium ‘drag-cup’
Drag cup turns against hairspring to move indicator needle
What does a gas turbine need for the three phase Ac tachogenerator
Step- down gearing because of faster rpm
Advantages of a three phase AC tachogenerator
Because frequency is senses not voltage then losses in transmission are negligible
Disadvantages of a three phase AC tachogenerator
Because of extra wiring = heavier
What is a induction tachometer (speed probe) ‘electronic’
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
How is the signal for the induction tachometer processed
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
How is engine vibration measured
Accelerometer or piezo electric element
Number given in cockpit has no units
What is a mechanical fuel quantity measure
Measured valine
Affected by aircraft attitude
May require power if sensor if variable resistance and for instrument light
What is a capacitance type fuel quantity gauge
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
What do you do if the fuel quantity doesn’t quite match up
Use a drip stick
What is a reference capacitor
Used for reference or the fuel in a capacitance type fuel quantity gauge
Always submerged
Is a capacitance fuel quantity gauge affected by attitude
Nope
Fuel flow mechanical (vane) type
Measured volume
Used with piston engines
Ltr / hr
Impeller type fuel flow system
F= ma
Measured mass flow
Kg/ hr
Totaliser = fuel flow x time
How much fuel has been used
What is a synchro scope
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
What is synchrophase
Same as synchroscope
Also adjusts prop position
Further noise reduction
