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ATPL Theory - Module 1 > Instrumentation - Principles and Sensors > Flashcards

Instrumentation - Principles and Sensors Flashcards

1
Q

How has the quality and quantity of aircraft instrumentation changed over the last hundred years?

A

It has steadily improved.

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2
Q

What was the original aircraft instrumentation like at the beginning of the 20th century?

A

It was fairly basic, restricted to a few engine instruments, some form of airspeed indicator, and a compass.

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3
Q

What is a direct indicator in aircraft instrumentation?

A

It is a system where input data in the form of pressure or rotational speed is fed directly to the instrument.

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4
Q

Where are direct indicators still used?

A

In light aircraft, such as the Cessna cockpit.

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5
Q

What are servo-driven indicators?

A

Remote systems used since the 1960s where data is fed to a central computer, converted to electrical signals, and then used to drive needles or indicators in the cockpit.

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6
Q

Why did servo-driven indicators become necessary?

A

Due to the complex use of data and the increasing size of aircraft, making direct reading instruments impractical.

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7
Q

What are the four basic instruments required for safe flight in the ‘basic T’ arrangement?

A

Airspeed indicator, attitude indicator, altimeter, and compass or gyro-driven direction indicator.

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8
Q

What are primary flight instruments?

A

Essential instruments for safe flight; other instruments are considered useful extras or standby instruments.

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9
Q

What is the ‘six pack’ in modern aircraft instrumentation?

A

A classification that includes the turn coordinator and vertical speed indicator in addition to the primary instruments.

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10
Q

What are Electronic Flight Instrument Systems (EFIS)?

A

Fully electronic systems with LCD screens that allow selective display of flight parameters, replacing the basic T arrangement.

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11
Q

Why do modern aircraft still retain standby instruments?

A

In case the primary electronic systems fail, with standby instruments fed from discrete sensors and separately powered.

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12
Q

How is temperature described?

A

As a comparative measure of hot or cold, with absolute zero as the point where all molecular activity ceases.

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13
Q

What are the common temperature scales used?

A

Degrees Celsius (°C), degrees Fahrenheit (°F), and Kelvin (K).

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14
Q

What is the freezing point of water on the Celsius scale?

A

0°C

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15
Q

What is the boiling point of water on the Celsius scale?

A

100°C

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16
Q

What is absolute zero on the Celsius scale?

A

-273°C

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17
Q

What is the Kelvin scale used for?

A

The Kelvin scale is used to measure temperature and is based on absolute zero, which is -273°C.

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18
Q

How is a Kelvin different from a degree Celsius?

A

A Kelvin is the same size as a degree Celsius, but the Kelvin scale starts at absolute zero (-273°C).

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19
Q

How do you convert Celsius to Kelvin?

A

Add 273 to the Celsius temperature.

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20
Q

How do you convert Kelvin to Celsius?

A

Subtract 273 from the Kelvin temperature.

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21
Q

What is -55°C in Kelvin?

A

218 K

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22
Q

What was an old name for Kelvin?

A

Degrees absolute or °A.

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23
Q

What is the Fahrenheit scale?

A

The Fahrenheit scale is a temperature scale widely used in the US, with 0°F as the freezing point of salt water and 100°F as the average temperature of the human body.

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24
Q

At what temperature does fresh water freeze and boil on the Fahrenheit scale?

A

Fresh water freezes at 32°F and boils at 212°F.

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25
Q

How do you convert Celsius to Fahrenheit using a formula?

A

°F = (°C × 9/5) + 32

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26
Q

What is 9°C in Fahrenheit?

A

48°F

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27
Q

What are the four types of temperature sensors?

A

Bimetallic sensors, Resistance thermometers, Thermocouples, and Radiation pyrometers.

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28
Q

How does a bimetallic sensor work?

A

It relies on two different metals bound together. When heated, one metal expands more than the other, causing a strip to bend or a coil to uncoil.

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29
Q

How does a resistive sensor or RTD work?

A

It relies on the change of electrical resistance of a pure metal like platinum, nickel, or copper as temperature changes.

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30
Q

What is a Wheatstone bridge?

A

It is an arrangement of resistors used to measure small changes in resistance.

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31
Q

What is a thermocouple?

A

A sensor made of two dissimilar metals that creates an electrical potential at their junction proportional to temperature.

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32
Q

Where are thermocouples commonly used?

A

In environments with very high temperatures, such as the exhaust of a jet engine.

33
Q

How does a thermocouple measure temperature?

A

It uses the potential difference created at the junction of two dissimilar metals and measures it with a galvanometer or millivoltmeter.

34
Q

What is the formula to express the operation of a thermocouple?

A

E = K x Th, where E is electromotive force (emf), Th is hot junction temperature, and K is a constant.

35
Q

How are thermocouples wired?

A

They are wired in parallel so that the failure of one does not cause a total failure of the temperature monitoring system.

36
Q

What are the advantages of thermocouples?

A

They do not need to be powered by any external source of electricity and can sense temperatures between -200°C and 1250°C.

37
Q

What is the disadvantage of thermocouples?

A

They are not that accurate.

38
Q

What is a radiation pyrometer used for?

A

It measures the frequency of emitted radiation from the area to be examined and is used to measure turbine blade temperatures at the turbine inlet area of a jet engine.

39
Q

What are the two types of air temperature probes used in aircraft?

A

The expansion type (bimetallic strip) and the electrical wire/resistance type.

40
Q

What type of temperature sensor is normally used in light aircraft?

A

The bimetallic type, with a spring-like coil of bimetallic strip fixed internally to the probe.

41
Q

How is the probe mounted in light aircraft?

A

Through the windshield with the indicator inside and the sensor outside the aircraft.

42
Q

What are the limitations of the probe used in light aircraft?

A

It has a sunshield but no anti-ice protection and is only sufficiently accurate for low-speed aircraft.

43
Q

What is the problem of speed in temperature sensing for faster aircraft?

A

If air temperature is sensed as it flows past the aircraft, frictional heating will distort the result; if previously static air is brought up to speed, the extra energy will increase the sensed temperature.

44
Q

What terms are used to describe air temperature in faster aircraft?

A

Static Air Temperature (SAT) and Total Air Temperature (TAT).

45
Q

How is the increase in temperature caused by ‘ram rise’ calculated?

A

Using the formula: SAT (1 + 0.2M²) = TAT, where M is the Mach number.

46
Q

What is the constant in the SAT/TAT formula called?

A

The recovery factor, shown as K or Kr, with typical values around 0.9.

47
Q

How do you derive SAT from TAT in practical use?

A

SAT = TAT ÷ (1 + 0.2KM²).

48
Q

What is an alternative method for calculating temperature rise?

A

Using the aircraft’s True Air Speed (TAS) on the navigation computer.

49
Q

Give an example of temperature rise using TAS.

A

At 200 KT TAS, the temperature rise is 4.25°C; at 300 KT TAS, it is 9.5°C.

50
Q

What are some names for temperature sensors on faster aircraft?

A

They can be called ‘total head thermometer’, ‘Rosemount probe’, or just ‘temperature probe’.

51
Q

What feature does a Rosemount probe have to protect the sensor?

A

It has a heater to protect the sensor during flight in icing conditions.

52
Q

How does a temperature probe work?

A

Air is accelerated through a venturi, some of it is drawn off into a sensing chamber, where water and dust particles are separated. The temperature is measured by a resistive element.

53
Q

What is the role of the heating elements in a temperature probe?

A

They could affect the air temperature reading, so the sensing element is isolated from the effects of the de-icing heater.

54
Q

When will a temperature probe indicate correctly?

A

Only when the aircraft is moving because it relies on air being drawn into the sensing chamber.

55
Q

What happens to a temperature probe when stationary on the ground with the heater on?

A

The gauge may go to full-scale deflection, and the heated probe itself may reach temperatures in excess of 300°C.

56
Q

What are flat plate temperature sensors and how do they work?

A

They have the sensing element mounted flush with the aircraft skin, do not suffer ram rise, but are affected by frictional heating.

57
Q

What is displayed by temperature gauges?

A

They may display TAT (Total Air Temperature) and/or apply calculated corrections to display OAT (Outside Air Temperature).

58
Q

What are the three sources of error in air temperature measurements?

A

Instrument error, environmental error, and heating error.

59
Q

How can instrument error be compensated for?

A

By fine calibration of the instrument.

60
Q

How is environmental error caused and how is it managed?

A

Caused by solar heating or ice accretion; probes incorporate heaters and are mounted on the underside of the aircraft to stay in shadow.

61
Q

What causes heating error and how is it managed?

A

Caused by ram rise or frictional heating; can be compensated for by calculation, usually within aircraft computers.

62
Q

What is the time standard across aviation?

A

Coordinated Universal Time (UTC), virtually identical to Greenwich Mean Time (GMT).

63
Q

Why is time important in engine and systems maintenance?

A

Because some maintenance is carried out by date or by hours flown or engine running time.

64
Q

What is a system clock in modern digital aircraft?

A

An electronic clock providing ‘system time’ as the onboard reference for anything requiring a time reference.

65
Q

How can system clocks be updated?

A

Manually reset or updated from external time references, such as GPS data.

66
Q

How is time measurement accomplished on basic aircraft?

A

As part of a specific instrument or as a separate timer not feeding into any systems.

67
Q

What is a Hobbs meter?

A

A separate meter recording hours and tenths of an hour, either elapsed engine hours or flight time.

68
Q

How can a Hobbs meter be activated?

A

Electrically from the moment the aircraft is powered up, by oil pressure signifying engine running, by a weight-on-wheels switch, or by an airspeed sensing vane.

69
Q

What is a simple accelerometer used for in a cockpit?

A

It is used to display load factor and in some simple data recorders.

70
Q

How does a simple accelerometer work?

A

It uses a weight mounted on a spring; acceleration extends or compresses the spring, and a scale records the value of the acceleration.

71
Q

What do more sophisticated accelerometers consist of?

A

A weight suspended on a thin metal blade.

72
Q

How does a sophisticated accelerometer detect acceleration?

A

Acceleration in one axis moves the weight off-center, affecting the I-bar and E-bar system, generating a magnetic flux.

73
Q

What happens when the weight in an accelerometer is displaced?

A

The flux in the ends of the E-bar will no longer be equal, generating an output signal.

74
Q

How is the error signal in an accelerometer processed?

A

Through a feedback loop applied to electromagnets to push the weight back to the neutral position.

75
Q

What is the feedback signal in an accelerometer equivalent to?

A

It is a direct equivalent of the force felt by the weight and measures the acceleration along the input axis.

76
Q

What technology do modern Inertial Reference Systems (IRS) use for accelerometers?

A

They use Micro Electro Mechanical System (MEMS) accelerometers on chips.

77
Q

How do MEMS accelerometers work?

A

They sense the displacement of a small mass as either a change of resistance or a change of capacitance.

78
Q

How many accelerometers does an IRS have and why?

A

Three accelerometers mounted at right angles to each other to sense acceleration in all planes.

79
Q

What systems can the IRS provide acceleration outputs to?

A

Flight data recorder, autopilot, flight director system, yaw dampers, autobrake system, and any other system that requires acceleration information.

ATPL Theory - Module 1 (17 decks)

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