Instrumentation - Pressure Altimeters Flashcards
How is external static air pressure used in aircraft?
It is used to measure height with an instrument known as the pressure altimeter.
What is the ICAO Standard Atmosphere (ISA) definition for sea level pressure?
The sea level pressure is 1013.25 hPa.
What is the ICAO Standard Atmosphere (ISA) definition for sea level air density?
The sea level air density is 1225 g/m³.
What is the ICAO Standard Atmosphere (ISA) definition for sea level temperature?
The sea level temperature is +15°C.
How does temperature change with increasing altitude according to ISA?
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.
What are pressure altimeters designed to indicate?
The altitude of the aircraft by detecting changes in the static air pressure.
What are the three mechanical types of altimeters?
Simple altimeters, sensitive altimeters, and servo-assisted altimeters.
How does a simple altimeter work?
Static pressure is fed to the instrument casing, causing an aneroid capsule to expand or compress, moving a needle on the instrument face.
Why are simple altimeters intended only for light aircraft?
The single capsule movement is the only force available to drive the needle, restricting the amount of gearing and the indicating range.
How are pressure altimeters calibrated?
They are calibrated to read correctly at all heights by making the linkage from capsule to display non-linear, correct only for ISA conditions.
What increases the sensitivity of a sensitive altimeter?
A stack of two or more capsules.
What feature do some sensitive altimeters have to overcome static friction?
Vibrators, to improve response to small altitude changes.
How do sensitive altimeters display altitude?
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.
What additional feature do later sensitive altimeters have?
A digitizer pick-off to send flight level information to the ATC transponder.
What do both vibrators and digitizers in sensitive altimeters require?
An electrical supply.
What is the approximate conversion between pounds per square inch and bars?
One bar is approximately 14.5 PSI.
What is the unit of measurement for air pressure in aviation?
HectoPascal (hPa).
What does 1 bar equal in terms of hPa?
1000 hPa.
What does the older unit millibar (mb) equal in hPa?
1 mb = 1 hPa, 1000 mb = 1000 hPa.
What sensors are used to detect differential pressure?
Diaphragm, aneroid capsule, pressure capsule, bellows, Bourdon tube, pressure transducers.
What is the function of a diaphragm in pressure sensing?
It distorts under pressure to drive an indicator, typically for measuring small pressure changes.
How does an aneroid capsule measure pressure?
It uses a corrugated membrane shaped into a capsule, sealed with low internal pressure.
What is the difference between a pressure capsule and an aneroid capsule?
A pressure capsule is not sealed and detects differential pressure, while an aneroid capsule is sealed and measures absolute pressure.
How do bellows differ from simple pressure capsules?
Bellows are stacks of capsules that can be sealed or open to pressure feed and measure a medium range of pressures.
What is the application of a Bourdon tube?
Measuring high pressures, such as engine oil pressure.
How do pressure transducers convert pressure into an electrical signal?
By using a strain gauge that changes resistance with pressure.
What is dynamic pressure and how is it calculated?
Dynamic pressure is the pressure due to the speed of the aircraft, calculated as ½ ρV².
How does a pitot tube measure pressure on an aircraft?
It measures both dynamic and static pressure at the front of the aircraft.
What is Bernoulli’s equation for dynamic pressure?
Dynamic pressure = total pressure - static pressure.
How is static pressure typically sensed on an aircraft?
Through static ports or vents at right angles to the airflow.
What is a combination pressure head?
It combines sensors for pitot and static pressure, mounted to avoid boundary layer disturbances.
Why is position error significant in pressure measurement for aircraft?
It affects instrument indications due to changes in airspeed and aircraft configuration.
What is the role of angle of attack sensors?
To measure the angle between the wing chord line and the relative airflow, important for stall warning systems.
What was a significant issue with the B737 Max 8 Maneuver Characteristics Augmentation System (MCAS)?
Inaccurate alpha data from a defective measuring vane led to the aircraft stalling.
How do modern EFIS displays show altitude?
On a vertical tape scale, usually to the right of the PFD.
What is the ISA standard sea level pressure and temperature?
1013.25 hPa and +15°C.
What is the conversion factor between feet and meters for altitude measurement?
1 meter = 3.28 feet.
What are the three mechanical types of altimeters?
Simple altimeters, sensitive altimeters, servo-assisted altimeters.
What is the purpose of the subscale setting device in a simple altimeter?
To adjust the instrument to various datum pressure settings before flight.
How do sensitive altimeters increase sensitivity?
By using a stack of two or more capsules and more complex gearing systems.
What is the function of a digitiser in a sensitive altimeter?
To send flight level information to the ATC transponder.
How do servo-assisted altimeters increase accuracy?
By using a pivoted I-bar and an electro-magnetic pick-off to transmit movement.
What is the role of the Air Data Computer (ADC) in fourth-generation altimeter systems?
To calculate altitude using static pressure and send information to displays and transponders.
What are the accuracy requirements for altimeters set by the ICAO PANS-OPS document?
±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.
What are bellows used for in pressure sensing?
Bellows are used to increase movement by joining stacks of capsules, allowing measurement of a medium range of pressures.
What are the two types of bellows mentioned?
Aneroid bellows and pressure bellows.
What is a typical application of bellows in pressure sensing?
Sensing engine manifold pressure.
Describe a Bourdon tube.
A Bourdon tube is curved like a ‘C’ and measures pressure by straightening as pressure increases.
What is a typical application of a Bourdon tube?
Indication of engine oil pressure.
What does a pressure transducer do?
Changes an input parameter like pressure into an electrical signal.
What is a strain gauge in the context of a pressure transducer?
A resistive element that changes its resistance under strain.
What is the formula for dynamic pressure?
Dynamic pressure = ½ ρV², where ρ is air density and V is true velocity.
What does a pitot tube measure?
Measures total pressure, which includes both static and dynamic pressure.
How is static pressure sensed on aircraft?
Through a static vent or port placed at right angles to the airflow.
What is the combination pressure head?
A sensor combining pitot and static pressure measurements, often with heating and drainage provisions.
What are errors in pitot or static pressure measurements called?
Position errors, which can be predictable configuration errors or less predictable maneuver errors.
What is the angle of attack (alpha)?
The angle between the wing chord line and the relative airflow.
What are the two types of alpha sensors?
Vane sensors and conical slotted probes.
What causes a barometric error in altimeters?
Changes in sea level pressure from the datum set on the altimeter subscale.
How can temperature error affect altimeter readings?
Temperature deviations from standard can cause altimeters to overread or underread.
What should be done if a static line becomes blocked?
Use GPS altitude as an alternative, ensuring at least four satellites are in view for accuracy.
What is the significance of the ICAO Standard Atmosphere (ISA)?
It sets the baseline for pressure instrument calibrations and comparisons.
What is the definition of ISA sea level pressure?
1013.25 hPa.
What types of altimeters are there?
Simple altimeters, sensitive altimeters, and servo-assisted altimeters.
What is the main difference between simple and sensitive altimeters?
Sensitive altimeters use a stack of capsules to increase sensitivity and movement.
What is a unique feature of servo-assisted altimeters?
They use an induction pick-off or electro-magnetic pick-off system for increased accuracy.
How do ADC driven systems calculate altitude?
By using static pressure fed to and sensed by the Air Data Computer (ADC).
What is the required accuracy for altimeters according to the ICAO PANS-OPS?
±20 m or ±60 ft for altimeters with a test range of 0 to 9000 m (0 to 30,000 ft).
What is the formula for dynamic pressure using Bernoulli’s equation?
Dynamic pressure = total pressure - static pressure
Why is it important to sense static pressure separately in aircraft?
To measure dynamic pressure and for other purposes such as indicating height, and rate of climb and descent.
How is static pressure sensed in aircraft?
Through a static vent or port with holes placed at right angles to the airflow.
What is the function of a combined pressure head in aircraft?
It combines sensors for pitot and static pressure and includes heating and drainage provisions to prevent icing and blockages.
Why do large aircraft have multiple sets of instruments for measuring pitot and static pressure?
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).
What is position error in aircraft altimeters?
Errors in the detected static pressure causing false instrument indications, influenced by air speed and configuration changes.
What types of static pressure errors are associated with aircraft maneuvers?
Transient and unpredictable static pressure errors induced by rolling, pitching, or yawing, and random gusts.
What is the angle of attack in aerodynamics?
The angle between the wing chord line and the relative airflow, critical for stall warning systems.
What could be the consequence of incorrect alpha data being displayed in aircraft?
Catastrophic consequences including potential stalls and crashes, as seen in the B737 Max 8 MCAS incidents.
What is the purpose of vane type alpha sensors in aircraft?
To measure the angle of attack by aligning with the relative airflow, protected against ice formation by internal heater elements.
What is an ISA Standard Atmosphere?
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.
How does a simple altimeter work?
It uses an aneroid capsule to detect changes in static pressure, which moves a needle to indicate altitude, calibrated for ISA conditions.
What is a sensitive altimeter?
An altimeter that increases sensitivity using multiple capsules and complex gearing to measure small altitude changes more accurately.
What is the advantage of servo-assisted altimeters?
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.
What are ADC driven systems in aircraft altimeters?
Systems that use the Air Data Computer to calculate altitude, referencing static pressure, and display it on digital or mechanical readouts.
What is the accuracy requirement for altimeters according to ICAO PANS-OPS?
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).
What is position error in aircraft altimeters?
Variations in altimeter readings caused by changes in true airspeed (TAS) and angle of attack, which can be minimized by good design.
What is barometric error in aircraft altimeters?
Incorrect altitude readings due to changes in sea level pressure from the datum set on the altimeter subscale, correctable by adjusting the subscale.
What is temperature error in aircraft altimeters?
Errors arising from deviations in atmospheric temperature from the standard atmosphere, causing the altimeter to over-read or under-read.
How are temperature errors corrected in altimeters?
Using temperature correction tables to adjust decision heights and altitudes, especially in low temperatures near landing.
What happens if there are blockages or leaks in the static line of an altimeter?
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.
What are the three pressure datums for altimeter settings?
QFE, QNH, and standard setting of 1013.25 hPa or 29.92 inches of mercury.
How is QNH different from QFE in altimeter settings?
QNH shows altitude above mean sea level (AMSL), while QFE shows height above the airfield or runway.
What is density altitude and how is it calculated?
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).
