Chapter 3 - Pitot/static instruments

Question 1

What are the three Q codes?

Answer 1

QNE - ICAO standard mean sea level pressure
QFE - Ambient static pressure existing at mean airfield level at any particular time of the day
QNH - Mean sea level pressure at any particular time of day

Question 2

How does a pilot obtain values for QFE and QNH?

Answer 2

ATC

Question 3

What will show on the instrument when QNH is input while the aircraft is on the ground?

Answer 3

Height of the airfield above mean sea level

Question 4

What should all aircraft instruments be set to above a transition altitude?

Answer 4

QNE

Question 5

How can an approximate true altitude be obtained locally?

Answer 5

By setting the altimeter to the local QNH

Question 6

What is the absolute altitude?

Answer 6

Height above ground level (AGL)

Question 7

What is true altitude?

Answer 7

Height above mean sea level (AMSL)

Question 8

How is absolute altitude calculated?

Answer 8

Absolute altitude = true altitude - elevation of the ground

Question 9

What is the only system capable of directly indicating the absolute altitude?

Answer 9

Radar altimeter

Question 10

What is indicated altitude?

Answer 10

Indicated reading which is seldom accurate as standard conditions are rarely met and errors may be present in the instruments and pitot/static systems

Question 11

What is calibrated altitude?

Answer 11

Indicated altitude correct for instrument error and pressure errors

Question 12

How will increases in temperature affect indicated altitude?

Answer 12

Drop in indicated altitude

Question 13

How will an increase in atmospheric pressure affect indicated altitude?

Answer 13

Shows a drop in indicated altitude

Question 14

What might cause a sensitive altimeter case to misread?

Answer 14

The instrument case is sealed, therefore leaks in a pressurised aircraft will cause the instrument to misread (altitude will read lower)

Question 15

Why is beryllium copper often used as the material of which the stack of three capsules is made within sensitive altimeter cases?

Answer 15

Natural springiness of the material

Question 16

How is capsule elasticity compensated for temperature changes?

Answer 16

Invar steel/brass bi-metallic ‘U’ shaped strip around the capsule stack

Question 17

How is reading of the altimeter dial simplified?

Answer 17

Number of different pointer lengths and shapes have been used

Question 18

What are servo altimeters?

Answer 18

Pointer and counter mechanism are driven using a servo (motor) system to overcome torque requirements of operating counter mechanisms

Question 19

Where is the input excitation supplied in a servo assisted altimeter?

Answer 19

AC supply is fed to the centre leg of the ‘E’ bar

Question 20

How are the secondary coils connected in the ‘E’ bar of a servo assisted altimeter?

Answer 20

The voltages in each leg are 180° out of phase and equal and opposite voltages

Question 21

Why should the pressure setting in a servo altimeter only be moved with electrical power supplied to the instrument?

Answer 21

The E&I bar might become out of balance by a large amount, so that when power is reapplied a large error voltage is created, and may drive the system very fast and damage internal components

Question 22

What scale of error is accepted in cabin altimeters?

Answer 22

± 500 ft

Question 23

How does the cabin altimeter sense cabin pressure?

Answer 23

The instrument’s case is vented directly to atmosphere

The vent is usually protected by a renewable filter

Question 24

How does a dual altimeter and differential pressure gauge display differential pressure?

Answer 24

Both atmosphere and cabin altitudes are displayed using different pointers

The difference between the two pointers is used to indicate pressure differential in psi

Question 25

What is the altitude capsule in the cabin differential pressure gauge connected to?

Answer 25

Aircraft static system

Question 26

What are the safety precautions to be taken when pressure testing the cabin with regards to the cabin altimeter and differential pressure gauge?

Answer 26

The unit is only designed to operate with less than atmospheric pressure, therefore damage to the aneroid capsules and the transmission could result It can be temporarily removed, or the cabin vent must be blanked

Question 27

What is the term used to describe the airspeed shown on the instrument?

Answer 27

Indicated airspeed

Question 28

How does indicated airspeed become calibrated airspeed?

Answer 28

Indicated airspeed has to be corrected for instrument error and aircraft pressure error, during calibration of by the pilot

Question 29

How does calibrated airspeed become true airspeed?

Answer 29

A correct for air density is carried out using outside air temperature to obtain TAS

Question 30

What are colour arcs and radial lines known as on an ASI?

Answer 30

Range markings

Question 31

What will red radial lines signify?

Answer 31

Maximum and minimum limits of airspeed (V_NE: velocity never exceed)

Question 32

What will a yellow arc signify?

Answer 32

Precautionary ranges (V_NO: velocity normal operating)

Question 33

What will a green arc signify?

Answer 33

Normal operating ranges

Question 34

What will a white arc signify?

Answer 34

Ranges in which the flaps may be lowered

Question 35

How is backlash or wear in the gearing absorbed in a typical ASI?

Answer 35

Hairspring on the pointer

Question 36

What are the typical uses of an airspeed switch?

Answer 36

To switch on electrical circuits which are only needed in flight To switch off electrical circuits which are only needed on the ground

Question 37

How is temperature compensation handled in an ASI?

Answer 37

Small strip of invar steel is laminated to the ranging bar The bar will be straight at 15°C but will bend when the temperature changes

Question 38

How does the capsule expansion change with airspeed?

Answer 38

Small at low speeds and large at high speeds

Question 39

What is the banana slot?

Answer 39

Slot in the sector gear which is so shaped so that there is some lost motion as the scale increases

Question 40

What is square law correction?

Answer 40

Taking allowances for the uneven capsule expansion due to the fact that pressure increases four times as speed doubles, to give a nearly linear scale

Question 41

What are fitted to British made VSIs?

Answer 41

Mechanical stops to prevent pointer movement exceeding the maximum scale reading They limit the push rod travel but allow the capsule to expand and contract beyond its normal range

Question 42

What are included in VSIs to compensate for density variations?

Answer 42

Metering unit

Question 43

What are the two types of metering units in common use in VSIs?

Answer 43

* Ceramic type * Capillary and hole (orifice)

Question 44

In the capillary and hole type of metering unit in a VSI, how does air density affect the component?

Answer 44

* A decrease in air density will increase turbulence through the capillary and therefore increase the pressure drop across it * An increase in air density will decrease the turbulence through the orifice and therefore decrease the pressure drop across it

Question 45

What are used instead of a tapered screw to achieve zero adjustment in some other VSIs?

Answer 45

Eccentric shaft

Question 46

At what value of percentage is the required zero adjustment of a VSI considered unserviceable?

Answer 46

More than 10% of the maximum reading

Question 47

What is a variometer?

Answer 47

Instantaneous vertical speed indicator Piston on a spring is affected by inertia, so pumps air out or draws air in at the moment a vertical change in displacement is effected, which gives an instantaneous reading on the VSI

Question 48

What is the speed of sound at sea level and at 40,000 feet?

Answer 48

Sea level - 760 mph 40,000 feet - 650 mph

Question 49

What is the simple expression for Mach number?

Answer 49

(Pitot - Static)/Static

Question 50

What will happen to the reading of a Mach meter when altitude is increased but airspeed is maintained constant?

Answer 50

The gauge will indicate a higher Mach number

Question 51

What is the 'Barber's pole' on a combined ASI and machmeter?

Answer 51

Indication of the onset of high-speed buffet due to the aircraft reaching its critical Mach number The 'Barber's pole' is linked to altitude and speed and is the setting of aircraft speed in knots which is the equivalent of the critical Mach number at the altitude the aircraft is flying at