A - Engine instruments (06/11-06-18) Flashcards
What are the 2 categories of indications on an engine?
Performance
condition
What do performance indicators measure?
They measure power from pressure ratios or speed indications or temperature indications
What is an engine pressure ratio (EPR)?
the ratio of pressures between the compressor inlet and the turbine outlet/exhaust
What happens to EPR with a speed increase?
it decreases slightly
At what speed is the EPR usually set by?
60kts and not changed during TO unless there is an emergency
What is the function of the P1 probe with regards to engines?
Measures compressor inlet pressure
What happens to the indication of a blockage in the P1 probe?
it over indicates
causing EPR to increase,
causing fuel flow to decrease
causing less thrust
What happens to the indication of a leak in the exhaust probe of an engine?
EPR decreases
fuel flow increases
therefore higher thrust
What are the condition indicators in an engine?
Fuel pressure
Fuel temperature
Oil pressure
Oil temperature
Vibration
What can be measured to indicate power in a turboprop?
Torque
How is torque measured in a turbo prop with a hydraulic system?
axial load produced on helical gears is hydraulically measured when a torque is being transmitted to the prop
This pulls/pushes pistons which allow oil to flow easier or harder
more flow means higher pressure which means more power
How is torque measured in a turbo prop with an electric torquemeter?
2 exciter wheels, one reference one on the prop shaft, are mounted on the shafts which interact with magnetic pickups
when thrust is changed the exciter wheels become out of phase and this phase shift is measured as torque.
How is engine RPM measured on an engine?
electronically or mechanically by tachometer
What are the 3 types of tachometer?
mechanical (drag)
tacho-generator (electrical)
Induction probe (electronic)
How does a tacho generator measure RPM?
the engine turns the generator which creates a voltage
This voltage is sent to the instrument to be converted to an RPM measurement
How does an induction probe tachometer work?
a speed probe is mounted on the compressor casing near a fan blade/notched phonic wheel.
as notches/fan blades pass it a disturbance is measured by the probe and are counted to give RPM
How does a mechanical drag tachometer work?
A cable its rotated in a tube which is linked to a magnet
This rotates a drag cup on a spring which turns a needle
Why are engine indicator needles designed to point the same direction under normal operation?
to allow the pilot to easily see if there are any issues
What is a synchroscope?
A device that indicated if any engines are rotating at a slightly different speed to the master engine
thus allowing the pilot to make sure they are spinning at the same RPM
How is engine vibration indicated?
relative to a normal level of vibration.
What is the max pressure to be measured by a bourdon tube, its accuracy and where would one be used in an engine?
800psi
+/- 0.5 - 3%
engine and oil pressures
What is the temperature measurement range of a thermistor?
25 - 300degC
What is the max measurable temperature and +ve & -ve wire materials of thermocouples?
+ve wire, -ve wire, max Temp (application)
Copper, Constantan, 300degC (CHT)
Iron, Constantan, 410degC (CHT)
Chromel, Alumel, 1000degC (EGT)
Platinum, Rhodium-platinum, 1400degC
What is a pyrometer?
An instrument that measures EM waves emitted by an object to measure its temperature
How does a float fuel gauge work?
The float ball is attached to a variable resistor via a pivot
As the float rises the pivot changes the contact point on the variable resistor and thus reads a fuel volume.
How does a flat plate capacitance fuel gauge work?
The capacitance across the plates where the fuel is has a relative permittivity of 2 whereas that with air is 1.
Thus a measure of capacitance can be measured to provide a fuel level.
the plates are actually tubes and there’s many in a tank.
What is the equation for capacitance?
Capacitance = relative permittivity x area of plates / distance between plates
What are fuel flow sensors used for?
checking for fuel system leaks
estimate A/C/engine performance
compare fuel used with fuel remaining
What are the 3 types of fuel flow sensor?
Mechanical - fuel flow turns an impeller and rotation is measured as fuel flow
Electrical analogue - swirl from a motored impeller turns a torque reaction turbine. This torque is measured as fuel flow
Electronic - uses chemical make up of fuel
What does a fuel flow sensor require to measure mass flow from volume flow?
SG
What may excessive fuel flow indicate?
a fuel leak
Where is flow rate usually measured in an engine?
in the low pressure fuel supply
What is the schedule for engine maintenance based on?
Engine cycles
Hours of operation
Calendar intervals
