engine instruments refined

Question 1

what is a tachometer?

Answer 1

an instrument that measures how fast the engine is rotated.

Question 2

what is the construction of a mechanical tachometer?

Answer 2

has a mechanical link between the accessory housing of the engine and the actual instrument.

Question 3

how does a mechanical tachometer work?

Answer 3

A cable is driven off the crankshaft drive gear and connects directly to the back of the tachometer.

As the engine turns, so does cable, turning the drive head on the back of the tachometer which drives the needle pointer

Question 4

what is the construction of a drag cup tachometer?

Answer 4

no physical connection between the engine and tachometer.

A cable connects to the tachometer and rotates as it’s connected to the engine and at the instrument end of the cable is a magnet that rotates.

Question 5

how does a drag cup tachometer work?

Answer 5

as the cable rotates it sets up magnetic eddy currents. rotates inside a cup and those resulting magnetic eddy currents drag the cup along.

cup is connected to a pointer which is calibrated into RPM.

Question 6

what is a manifold air pressure (MAP) gauge?

Answer 6

a guage that measures the air pressure in the intake manifold after carburettor or the fuel control unit.

Question 7

how does a MAP gauge work?

Answer 7

inside the gauge is a diaphragm which expands or contracts as more or less air enters it. this is connected to a needle on the front of the gauge.

Question 8

what is a boost gauge?

Answer 8

similar to MAP gauge but the boost refers to how many extra lots of atmospheric pressure are being delivered to the engine.

Question 9

if the boost gauge reads 1 what does this mean?

Answer 9

there would be an additional one atmosphere’s worth of pressure available to the engine.

Question 10

how is oil temperature measured?

Answer 10

using a vapour pressure thermometer.

Question 11

what is the construction of a vapour pressure thermometer?

Answer 11

uses a bourdon tube that is filled with a chemical that’s at normal temperatures part gas and part liquid. the probe is then immersed in the oil flow.

Question 12

how does a vapour pressure thermometer work?

Answer 12

as temperature increases, the liquid begins vaporising into gas and expands in volume. The tube is connected to the needle pointer which indicates the oil temperature on the gauge.

the tube is sealed so no gas escapes but instead puts pressure on the tube causing it to want to straighten.

Question 13

when is a vacuum gauge used?

Answer 13

for aircraft that have air-driven gyroscopic instruments.

Question 14

how does a vacuum gauge work?

Answer 14

a suction pump provides a vacuum of air in the instrument case which is then drawn into the case past the gyroscope rotor causing it to spin.

a bourdon tube arrangement is used to measure the vacuum being imparted into the system.

Question 15

what happens if the rotor in a vacuum gauge doesn’t spin fast enough?

Answer 15

the instrument cannot be relied on.

Question 16

what is the construction of a mechanical type outside air temperature gauge?

Answer 16

consists of a bi-metallic strip. two strips are bonded together forming a helical shape.

Question 17

what is the use of an OAT gauge?

Answer 17

t provides the information necessary to make true airspeed, density altitude, and other calculations. It also serves as an early warning of potential icing condition

Question 18

how does mechanical type outside air temperature gauge work?

Answer 18

the two metal strips have different qualities including that they expand and contract at different rates.

as they do this they move the pointer on the temperate gauge.

Question 19

how does the bourdon tube OAT arrangement gauge work?

Answer 19

the bourdon tube, house and probe assembly is filled with a part liquid, part gas chemical which causes the tube to expand and move the needle pointer.

Question 20

what is the most common type of fuel quantity indicated system used for aircraft?

Answer 20

a float in the tank which varies the resistance of an electrical circuit.

Question 21

how does the float system work?

Answer 21

as fuel is used and float is lowered, resulting change in resistance alters the current flow through the ratiometer type instrument causing movement of the pointer.

Question 22

more sophisticated aircraft use what to measure fuel quantity?

Answer 22

capacitance-type probe or series of probes in the tank to determine fuel quantity.

Question 23

when a potential difference or voltage is applied across two conducting surfaces called plates separated by a non-conducting medium called a dielectric they have the property of what?

Answer 23

storing an electric charge called capacitance which is used to measure the amount of fuel in a tank.

Question 24

what is a dielectric?

Answer 24

a non-conducting medium

Question 25

what is the potential difference or voltage across two conducting surfaces called?

Answer 25

plates

Question 26

the dielectric between the plates consist of what?

Answer 26

fuel toward the bottom of the probe and air toward the top.

Question 27

as fuel and air have different dielectric constants what happens?

Answer 27

they affect the amount of capacitance built up between the plates differently.

Question 28

how does a capacitance type probe gauge operate?

Answer 28

as fuel is consumed, the proportion of air in the dielectric increases and the capacitance of the probe changes accordingly.

then change is used to generate an electrical signal which is in turn used to position the pointer of the fuel quantity indicator

Question 29

what is the advantage of a capacitance type probe gauge?

Answer 29

that accurate readings can be obtained in large or unusually shaped objects.