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

Instrumentation - Pressure Sensing Flashcards

1
Q

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

A

One bar is approximately 14.5 PSI.

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2
Q

What is a force measured over to define pressure?

A

A unit area.

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3
Q

What are the common units used to express pressure?

A

Pounds per square foot (lb/ft²), pounds per square inch (lb/in² or PSI), Newtons per square meter (N/m² or Pa), inches of mercury (inHg), and bars.

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4
Q

How is one bar defined?

A

One bar is an approximation to sea level pressure and is defined as 100,000 Pa.

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5
Q

What is the equivalent of 1 bar in hectoPascal (hPa)?

A

1 bar is equal to 1000 hPa.

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6
Q

What is the older unit for atmospheric pressure?

A

Millibar (mb), where 1 mb = 1 hPa, 1000 mb = 1000 hPa.

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7
Q

What is an absolute pressure?

A

The pressure exerted by the atmosphere, approximately 14.5 PSI or 100,000 Pa.

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8
Q

What is differential pressure?

A

The difference between the pressure being measured and the absolute pressure.

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9
Q

Name some sensors used to detect differential pressure.

A

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

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10
Q

What are diaphragms or membranes used for in pressure sensing?

A

They are used to indicate system pressures by distorting under pressure.

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11
Q

Why are simple diaphragms rarely used to indicate system pressures?

A

Because their maximum deflection is small, in the order of millimeters.

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12
Q

What is an aneroid capsule?

A

A sealed capsule with a corrugated membrane used to measure atmospheric pressure without fluid.

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13
Q

How do aneroid capsules work?

A

They have very low internal pressure and expand as external atmospheric pressure decreases.

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14
Q

What is the main limitation of aneroid sensors?

A

They measure a low range of pressures due to limited ability to expand and contract.

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15
Q

How do pressure capsules differ from aneroid capsules?

A

Pressure capsules are not sealed and measure the extra pressure over atmospheric pressure to detect differential pressure.

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16
Q

What is a bellows used for in pressure sensing?

A

To increase the movement, stacks of capsules can be joined together to make bellows, which can measure a medium range of pressures.

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17
Q

What are the two types of bellows?

A

Aneroid bellows and pressure bellows.

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18
Q

What is a typical application of bellows in pressure sensing?

A

To sense engine manifold pressure.

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19
Q

What is a Bourdon tube?

A

A Bourdon tube is curved like a ‘C’ fixed at one end and has an oval cross section. It is used to measure pressure and operates an indicating mechanism through movement proportional to the pressure.

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20
Q

What type of pressure measurements are Bourdon tubes typically used for?

A

High pressure measurements, such as engine oil pressure.

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21
Q

How does a Bourdon tube work?

A

The pressure is fed inside the tube at the fixed end, causing the tube to straighten and move proportionally to the pressure, which then operates the indicating mechanism.

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22
Q

What is a pressure transducer?

A

A device that changes an input parameter such as temperature or pressure into a different output parameter such as an electrical signal.

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23
Q

How does a pressure transducer sense pressure?

A

It uses a stainless steel diaphragm that presses on a resistive element, changing its resistance depending on the strain.

24
Q

What is an advantage of using transducers for pressure measurement?

A

They can indicate remotely without needing to pipe substances like oil from an engine to a gauge, reducing the risk of leaks and hazardous substances.

25
Q

What is the formula for dynamic pressure in an aircraft?

A

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

26
Q

Why is dynamic pressure important in aviation?

A

It represents the pressure of air flowing over the aircraft, relating to the lift it creates and the drag it experiences, which affects the flying characteristics.

27
Q

What does a Pitot tube measure?

A

The total pressure (static and dynamic) acting on the front of the aircraft.

28
Q

Who is the Pitot tube named after?

A

Henri Pitot.

29
Q

Where is a Pitot tube typically fitted on an aircraft?

A

Below the wing of a light aircraft.

30
Q

What is the formula for total pressure in relation to dynamic and static pressure?

A

Total pressure = dynamic pressure + static pressure.

31
Q

What is the formula for dynamic pressure using total and static pressure?

A

Dynamic pressure = total pressure - static pressure.

32
Q

What principle is used to find dynamic pressure?

A

Bernoulli’s equation or principle.

33
Q

Why do we need to sense static pressure separately in aviation?

A

Because it gives an indication of height and rate of climb and descent.

34
Q

How is static pressure sensed on an aircraft?

A

Through a static vent or port placed at right angles to the airflow to avoid detecting any dynamic pressure.

35
Q

What is a common method to sense static pressure on smaller aircraft?

A

Using a selectable standby static source inside the cabin.

36
Q

How are sensors for pitot and static pressure typically combined?

A

Into a single pressure head mounted on a mast to locate it outside the boundary layer, often with heating and drainage provisions.

37
Q

What is the purpose of a heater element in a combined pressure head?

A

To protect the sensor from blockage due to icing.

38
Q

How do large aircraft safeguard against errors in pitot or static pressure measurement?

A

By using more complex systems with independent feeds to the Captain and First Officer, Air Data Computers (ADCs), and standby instruments, including selectable reserve systems.

39
Q

What is position error in static pressure sensing?

A

Errors in detected static pressure that give false instrument indications, categorized as predictable configuration error and less predictable manoeuvre error.

40
Q

What factors can cause configuration errors in static pressure sensing?

A

Changes in aircraft’s true airspeed, whether flaps are retracted or extended, whether the undercarriage is up or down, and the angle of attack.

41
Q

Are standby instruments fed through the Air Data Computer (ADC) in modern systems?

A

No, even in modern systems standby instruments may have uncorrected position errors.

42
Q

What can cause transient and unpredictable static pressure errors during maneuvers?

A

Rolling, pitching, yawing, and random gusts.

43
Q

How can errors induced by yawing be minimized on transport aircraft?

A

By siting connected static ports on both sides of the fuselage.

44
Q

What does position error vary with?

A

True Air Speed (TAS) and angle of attack.

45
Q

What is angle of attack conventionally called and why?

A

It is called “alpha” from the Greek letter ‘α’ used in aerodynamic calculations.

46
Q

What is the angle of attack?

A

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

47
Q

Why is angle of attack important?

A

It is important when flying near stall and in emergencies, as it defines the stall in all conditions of flight and is used as an input to flight control computers and the aircraft flight data recorder.

48
Q

What are the two basic forms of alpha sensors?

A

A vane that lines up with the relative airflow and a conical probe with slots that measure relative pressure.

49
Q

What is the main input to stall warning systems?

A

Angle of attack.

50
Q

What can be the consequence of incorrect alpha being displayed?

A

It can be potentially catastrophic, leading to severe aircraft damage or crashes, as seen in incidents with the Citation jet and B737 Max 8.

51
Q

What is a vane type alpha sensor?

A

A small aerofoil that rotates to line up with the relative airflow, usually positioned on each side of the nose of the aircraft.

52
Q

How is the position of the sensor vane measured?

A

By a transducer (synchro-transmitter) and sent electronically to the flight systems.

53
Q

What protects the vane against ice formation?

A

An internal heater element.

54
Q

What is a pressure type alpha sensor?

A

A conical probe at right angles to the airflow with sensing holes or slots at 90° to each other, called a conical slotted probe.

55
Q

How does a conical slotted probe work?

A

It rotates in the airflow until the pressures are equal, with a potentiometer feeding the probe position to the Air Data Computer.

56
Q

What feature helps conical slotted probes prevent ice formation?

A

They are electrically heated.

ATPL Theory - Module 1 (16 decks)

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