engine instruments

Question 1

resistance temp device

Answer 1

platinum used
used for ambient temp and cabin
varies positively with temp so measuring resistance you can deduce temp

Question 2

Wheatstone bridge temp measurement

Answer 2

ratio of resistance is measured
measured on ratio meter
typically used for OAT

Question 3

thermo couple temp measurement

Answer 3

2 dissimilar metals joined at two points make small voltage
voltage made directly proportional to temp
doesn’t need power supply
connected in series
hot junction : measures the temp
cold junction: maintained at a constant temp

used for: engines, turbine & exhaust temps

Question 4

radiation temp measurement

Answer 4

measures wavelength or frequency of emitted waves from an object to deduce the temp
used in turbine blade measurement and fire detection

Question 5

mechanical or magnetic tachometer

Answer 5

adv: cheap, simple, doesn’t need power
dis: wear and tear, flexible shaft can only work for low RPM, display has to be behind the engine so only good for engines in front of cockpit

Question 6

electrical tachometer(induction tachometer)

Answer 6

generator makes electricity from turning, motor turns a magnet inside a drag cup with spring and display like mechanical one.
DC tachometer- simple and ease of transmission, indication is voltage based on DC motor (disadvantage)
single phase tachometer(AC gen, DC motor)- simple, voltage based indication (disadvantage)
3 phase tachometer(both AC)- complex, frequency based indication(advantage)
commutator- type of DC generator or Moto- wears quick, creates electrical interference(spurious signals)

ADV: indépendant of aircraft power supply, absence of commutator is an ADV
DIS: wear and tear, voltage loss due to line resistance cause error indication(only DC motor), presence of a commutator is a DIS

Question 7

electronic tachometer

Answer 7

counts notches of a phonic wheel that pass a magnetic probe and determines RPM

DIS: requires AC power
ADV: most accurate, no RPM limit

Question 8

Exam Q:

what sort of GENERATOR tachometer does a modern jet a/c use?

Answer 8

3 phase AC-AC

Question 9

exam Q:

what sort of tachometer does a modern jet a/c use?

Answer 9

electronic

Question 10

exam Q:

what tachometer could be used?

Answer 10

any

Question 11

engine pressure ratio

Answer 11

= turbine outlet P / compresser inlet P
measured with an aneroid capsule

blockages during T/O: OUT/IN
normally both pressures increase the same
outlet blocked- under reads
inlet blocked- over reads

Question 12

torque meter

Answer 12

measure of power on turbo prop

mechanical:
uses helical gears
measures oil pressure and gives torque indication

electronic:
2 phonic wheels on same shaft
measures phase difference

Question 13

synchro scope

Answer 13

reduce noise by synchronising blade positions and RPM’s
usually done automatically
one engine acts as master and other engines synchronise to it
white symbol on display spins to show speed of engine compared to master, once stationary that means its achieved synchro phasing

Question 14

magnetic chip detector

Answer 14

magnets in oil line pickup any chips that have broken off the engine
if enough are collected a circuit is complete- warning light illuminates in cockpit
Question: it warns of an impending failure

Question 15

engine vibration indications

Answer 15

piezoelectric crystal:
uses voltage response to pressure within a piezoelectric crystal caused by vibration

electromechanical:
like a dashpot accelerometer

normal vibrations can be filtered out
number on display is dimensionless and scale differs between different a/c

Question 16

cylinder head temperature gauge

Answer 16

displays the hottest one

Question 17

fuel float gauge

Answer 17

measures fuel volume 
errors: 
volume change with temp
bank & pitch
acceleration 

mechanical version
electrical version: float connected to variable resistor, changes resistance in a circuit

indications are not distrusted by fluctuations in a/c power supply

Question 18

fuel capacitance gauge

Answer 18

gives mass indication
indication independant of temp, pitch, bank and acceleration

electrical output= D.C x (area of plates/ distance between them)
D.C- dielectric constant
electrical output can give volume

reference capacitor- works out D.C of fuel and determines density of fuel - system can then indicate mass

many plates so gives an average so no manoeuvre errors

Question 19

the dielectric constant(D.C) of fuel is?

Answer 19

approx twice that of air

Question 20

mechanical fuel flow meter

Answer 20

vane displacement converted to an electrical signal and indicates volume flow
mass flow obtained using a temp sensor to compensate for density changes
usually measured at engine intake- LP fuel supply

Question 21

electro mechanical fuel flow meter

Answer 21

single impeller
one of the blades has magnetic insert, magnetic probe field disturbed
measures rotation and from that fuel volume flow
can give mass flow if have temp sensor

Question 22

analogue electrical fuel flow meter

Answer 22

impeller turned by a motor
fuel swirled
swirling fuel passes through reaction turbine
torque created on reaction turbine is proportional to mass flow