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ATPL Theory - Module 1 > Instrumentation - Pressure Altimeters > Flashcards

Instrumentation - Pressure Altimeters Flashcards

1
Q

How is external static air pressure used in aircraft?

A

It is used to measure height with an instrument known as the pressure altimeter.

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2
Q

What is the ICAO Standard Atmosphere (ISA) definition for sea level pressure?

A

The sea level pressure is 1013.25 hPa.

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3
Q

What is the ICAO Standard Atmosphere (ISA) definition for sea level air density?

A

The sea level air density is 1225 g/m³.

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4
Q

What is the ICAO Standard Atmosphere (ISA) definition for sea level temperature?

A

The sea level temperature is +15°C.

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5
Q

How does temperature change with increasing altitude according to ISA?

A

The temperature reduces at 1.98°C per 1000 ft up to 36,090 ft at -56.5°C, staying constant to 65,617 ft, then rising at 0.3°C per 1000 ft.

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6
Q

What are pressure altimeters designed to indicate?

A

The altitude of the aircraft by detecting changes in the static air pressure.

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7
Q

What are the three mechanical types of altimeters?

A

Simple altimeters, sensitive altimeters, and servo-assisted altimeters.

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8
Q

How does a simple altimeter work?

A

Static pressure is fed to the instrument casing, causing an aneroid capsule to expand or compress, moving a needle on the instrument face.

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9
Q

Why are simple altimeters intended only for light aircraft?

A

The single capsule movement is the only force available to drive the needle, restricting the amount of gearing and the indicating range.

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10
Q

How are pressure altimeters calibrated?

A

They are calibrated to read correctly at all heights by making the linkage from capsule to display non-linear, correct only for ISA conditions.

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11
Q

What increases the sensitivity of a sensitive altimeter?

A

A stack of two or more capsules.

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12
Q

What feature do some sensitive altimeters have to overcome static friction?

A

Vibrators, to improve response to small altitude changes.

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13
Q

How do sensitive altimeters display altitude?

A

Through a more complex gearing system that allows the use of three pointers for tens of thousands of feet, thousands, and hundreds, or through a drum or digital display.

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14
Q

What additional feature do later sensitive altimeters have?

A

A digitizer pick-off to send flight level information to the ATC transponder.

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15
Q

What do both vibrators and digitizers in sensitive altimeters require?

A

An electrical supply.

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16
Q

What is the approximate conversion between pounds per square inch and bars?

A

One bar is approximately 14.5 PSI.

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17
Q

What is the unit of measurement for air pressure in aviation?

A

HectoPascal (hPa).

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18
Q

What does 1 bar equal in terms of hPa?

A

1000 hPa.

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19
Q

What does the older unit millibar (mb) equal in hPa?

A

1 mb = 1 hPa, 1000 mb = 1000 hPa.

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20
Q

What sensors are used to detect differential pressure?

A

Diaphragm, aneroid capsule, pressure capsule, bellows, Bourdon tube, pressure transducers.

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21
Q

What is the function of a diaphragm in pressure sensing?

A

It distorts under pressure to drive an indicator, typically for measuring small pressure changes.

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22
Q

How does an aneroid capsule measure pressure?

A

It uses a corrugated membrane shaped into a capsule, sealed with low internal pressure.

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23
Q

What is the difference between a pressure capsule and an aneroid capsule?

A

A pressure capsule is not sealed and detects differential pressure, while an aneroid capsule is sealed and measures absolute pressure.

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24
Q

How do bellows differ from simple pressure capsules?

A

Bellows are stacks of capsules that can be sealed or open to pressure feed and measure a medium range of pressures.

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25
Q

What is the application of a Bourdon tube?

A

Measuring high pressures, such as engine oil pressure.

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26
Q

How do pressure transducers convert pressure into an electrical signal?

A

By using a strain gauge that changes resistance with pressure.

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27
Q

What is dynamic pressure and how is it calculated?

A

Dynamic pressure is the pressure due to the speed of the aircraft, calculated as ½ ρV².

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28
Q

How does a pitot tube measure pressure on an aircraft?

A

It measures both dynamic and static pressure at the front of the aircraft.

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29
Q

What is Bernoulli’s equation for dynamic pressure?

A

Dynamic pressure = total pressure - static pressure.

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30
Q

How is static pressure typically sensed on an aircraft?

A

Through static ports or vents at right angles to the airflow.

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31
Q

What is a combination pressure head?

A

It combines sensors for pitot and static pressure, mounted to avoid boundary layer disturbances.

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32
Q

Why is position error significant in pressure measurement for aircraft?

A

It affects instrument indications due to changes in airspeed and aircraft configuration.

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33
Q

What is the role of angle of attack sensors?

A

To measure the angle between the wing chord line and the relative airflow, important for stall warning systems.

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34
Q

What was a significant issue with the B737 Max 8 Maneuver Characteristics Augmentation System (MCAS)?

A

Inaccurate alpha data from a defective measuring vane led to the aircraft stalling.

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35
Q

How do modern EFIS displays show altitude?

A

On a vertical tape scale, usually to the right of the PFD.

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36
Q

What is the ISA standard sea level pressure and temperature?

A

1013.25 hPa and +15°C.

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37
Q

What is the conversion factor between feet and meters for altitude measurement?

A

1 meter = 3.28 feet.

38
Q

What are the three mechanical types of altimeters?

A

Simple altimeters, sensitive altimeters, servo-assisted altimeters.

39
Q

What is the purpose of the subscale setting device in a simple altimeter?

A

To adjust the instrument to various datum pressure settings before flight.

40
Q

How do sensitive altimeters increase sensitivity?

A

By using a stack of two or more capsules and more complex gearing systems.

41
Q

What is the function of a digitiser in a sensitive altimeter?

A

To send flight level information to the ATC transponder.

42
Q

How do servo-assisted altimeters increase accuracy?

A

By using a pivoted I-bar and an electro-magnetic pick-off to transmit movement.

43
Q

What is the role of the Air Data Computer (ADC) in fourth-generation altimeter systems?

A

To calculate altitude using static pressure and send information to displays and transponders.

44
Q

What are the accuracy requirements for altimeters set by the ICAO PANS-OPS document?

A

±20 m or 60 ft for altimeters with a test range of 0 to 9000 m; ±25 m or 80 ft for a test range of 0 to 15,000 m.

45
Q

What are bellows used for in pressure sensing?

A

Bellows are used to increase movement by joining stacks of capsules, allowing measurement of a medium range of pressures.

46
Q

What are the two types of bellows mentioned?

A

Aneroid bellows and pressure bellows.

47
Q

What is a typical application of bellows in pressure sensing?

A

Sensing engine manifold pressure.

48
Q

Describe a Bourdon tube.

A

A Bourdon tube is curved like a ‘C’ and measures pressure by straightening as pressure increases.

49
Q

What is a typical application of a Bourdon tube?

A

Indication of engine oil pressure.

50
Q

What does a pressure transducer do?

A

Changes an input parameter like pressure into an electrical signal.

51
Q

What is a strain gauge in the context of a pressure transducer?

A

A resistive element that changes its resistance under strain.

52
Q

What is the formula for dynamic pressure?

A

Dynamic pressure = ½ ρV², where ρ is air density and V is true velocity.

53
Q

What does a pitot tube measure?

A

Measures total pressure, which includes both static and dynamic pressure.

54
Q

How is static pressure sensed on aircraft?

A

Through a static vent or port placed at right angles to the airflow.

55
Q

What is the combination pressure head?

A

A sensor combining pitot and static pressure measurements, often with heating and drainage provisions.

56
Q

What are errors in pitot or static pressure measurements called?

A

Position errors, which can be predictable configuration errors or less predictable maneuver errors.

57
Q

What is the angle of attack (alpha)?

A

The angle between the wing chord line and the relative airflow.

58
Q

What are the two types of alpha sensors?

A

Vane sensors and conical slotted probes.

59
Q

What causes a barometric error in altimeters?

A

Changes in sea level pressure from the datum set on the altimeter subscale.

60
Q

How can temperature error affect altimeter readings?

A

Temperature deviations from standard can cause altimeters to overread or underread.

61
Q

What should be done if a static line becomes blocked?

A

Use GPS altitude as an alternative, ensuring at least four satellites are in view for accuracy.

62
Q

What is the significance of the ICAO Standard Atmosphere (ISA)?

A

It sets the baseline for pressure instrument calibrations and comparisons.

63
Q

What is the definition of ISA sea level pressure?

A

1013.25 hPa.

64
Q

What types of altimeters are there?

A

Simple altimeters, sensitive altimeters, and servo-assisted altimeters.

65
Q

What is the main difference between simple and sensitive altimeters?

A

Sensitive altimeters use a stack of capsules to increase sensitivity and movement.

66
Q

What is a unique feature of servo-assisted altimeters?

A

They use an induction pick-off or electro-magnetic pick-off system for increased accuracy.

67
Q

How do ADC driven systems calculate altitude?

A

By using static pressure fed to and sensed by the Air Data Computer (ADC).

68
Q

What is the required accuracy for altimeters according to the ICAO PANS-OPS?

A

±20 m or ±60 ft for altimeters with a test range of 0 to 9000 m (0 to 30,000 ft).

69
Q

What is the formula for dynamic pressure using Bernoulli’s equation?

A

Dynamic pressure = total pressure - static pressure

70
Q

Why is it important to sense static pressure separately in aircraft?

A

To measure dynamic pressure and for other purposes such as indicating height, and rate of climb and descent.

71
Q

How is static pressure sensed in aircraft?

A

Through a static vent or port with holes placed at right angles to the airflow.

72
Q

What is the function of a combined pressure head in aircraft?

A

It combines sensors for pitot and static pressure and includes heating and drainage provisions to prevent icing and blockages.

73
Q

Why do large aircraft have multiple sets of instruments for measuring pitot and static pressure?

A

To safeguard against errors in displayed speed, height, vertical speed, and Mach number, providing independent feeds to the Captain and First Officer or Air Data Computers (ADCs).

74
Q

What is position error in aircraft altimeters?

A

Errors in the detected static pressure causing false instrument indications, influenced by air speed and configuration changes.

75
Q

What types of static pressure errors are associated with aircraft maneuvers?

A

Transient and unpredictable static pressure errors induced by rolling, pitching, or yawing, and random gusts.

76
Q

What is the angle of attack in aerodynamics?

A

The angle between the wing chord line and the relative airflow, critical for stall warning systems.

77
Q

What could be the consequence of incorrect alpha data being displayed in aircraft?

A

Catastrophic consequences including potential stalls and crashes, as seen in the B737 Max 8 MCAS incidents.

78
Q

What is the purpose of vane type alpha sensors in aircraft?

A

To measure the angle of attack by aligning with the relative airflow, protected against ice formation by internal heater elements.

79
Q

What is an ISA Standard Atmosphere?

A

A reference used for calibrations and pressure instrument calculations with sea level pressure of 1013.25 hPa, air density of 1225 g/m3, and temperature of +15°C.

80
Q

How does a simple altimeter work?

A

It uses an aneroid capsule to detect changes in static pressure, which moves a needle to indicate altitude, calibrated for ISA conditions.

81
Q

What is a sensitive altimeter?

A

An altimeter that increases sensitivity using multiple capsules and complex gearing to measure small altitude changes more accurately.

82
Q

What is the advantage of servo-assisted altimeters?

A

They provide more accurate measurements by using a servo motor and complex gearing instead of a direct mechanical link between the capsules and height pointers.

83
Q

What are ADC driven systems in aircraft altimeters?

A

Systems that use the Air Data Computer to calculate altitude, referencing static pressure, and display it on digital or mechanical readouts.

84
Q

What is the accuracy requirement for altimeters according to ICAO PANS-OPS?

A

Altimeters must show QFE or QNH accurately within plus or minus 20 m (60 ft) for altimeters with a test range of 0 to 9000 m (0 to 30,000 ft), or 25 m (80 ft) for altimeters with a test range of 0 to 15,000 m (0 to 50,000 ft).

85
Q

What is position error in aircraft altimeters?

A

Variations in altimeter readings caused by changes in true airspeed (TAS) and angle of attack, which can be minimized by good design.

86
Q

What is barometric error in aircraft altimeters?

A

Incorrect altitude readings due to changes in sea level pressure from the datum set on the altimeter subscale, correctable by adjusting the subscale.

87
Q

What is temperature error in aircraft altimeters?

A

Errors arising from deviations in atmospheric temperature from the standard atmosphere, causing the altimeter to over-read or under-read.

88
Q

How are temperature errors corrected in altimeters?

A

Using temperature correction tables to adjust decision heights and altitudes, especially in low temperatures near landing.

89
Q

What happens if there are blockages or leaks in the static line of an altimeter?

A

The pressure inside the instrument cannot change, leading to incorrect altitude indications. Cabin altitude may be displayed instead of aircraft altitude if the static line fractures.

90
Q

What are the three pressure datums for altimeter settings?

A

QFE, QNH, and standard setting of 1013.25 hPa or 29.92 inches of mercury.

91
Q

How is QNH different from QFE in altimeter settings?

A

QNH shows altitude above mean sea level (AMSL), while QFE shows height above the airfield or runway.

92
Q

What is density altitude and how is it calculated?

A

The altitude corrected for temperature, giving the same air density as the non-ISA atmosphere. Calculated using the formula: Density altitude = pressure altitude + (120 x ISA deviation).

ATPL Theory - Module 1 (17 decks)

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