Met: Atmosphere & Speed Flashcards
What is the aviation definition of height?
Height is the measured distance above the ground (i.e., QFE).
What is the aviation definition of altitude?
Altitude is the measured distance above the local pressure setting (i.e., QNH) or altitude above mean sea level (MSL).
What is the aviation definition of a flight level?
Flight level is the measured pressure level above the 29.92-in/1013-millibar datum.
What does ISA stand for?
ISA stands for International standard atmosphere.
What are the ICAO ISA conditions at sea level?
Temperature: + 15°C [environmental lapserate (ELR), -2°C per 1000 ft)
Pressure: 29.92 inches of mercury or 1013 hectopascals (millibars) (decreases with altitude)
Density: 1225 g/m3 (density is influenced by temperature and pressure)
What is air pressure?
Air pressure is the weight of a column of air or the gravity force of air molecules. It decreases with a rise in altitude.
How is air pressure measured?
Air pressure can be measured using a simple mercury barometer.
What units are used to express air pressure?
Air pressure can be expressed in inches of mercury or force per unit area (e.g., millibars or hectopascals).
What is the standard unit of pressure measurement recognized by ICAO?
Hectopascal is the standard unit of pressure measurement recognized by the International Civil Aviation Organization (ICAO).
How many feet are in a millibar (hectopascal)?
The number of feet in a millibar is variable and depends on the altitude. At sea level, 1 millibar = 27 ft.
What is the formula used to find the size of 1 millibar?
The formula is 96 * (absolute temperature (273K + actual temperature)) / altitude pressure (millibars).
How many inches of mercury equals a millibar?
0.0295 inch of mercury equals 1 hectopascal/millibar.
What is pressure altitude?
Pressure altitude is the height above the 29.92 inHg (1013 millibar) pressure datum, at which the pressure value experienced represents that of the level under consideration.
How do you calculate the pressure altitude’s actual height?
Calculate the difference between the regional QNH and the 29.92 inHg (1013 millibar) pressure datum, convert this pressure difference into height, and add or subtract from the pressure altitude based on whether the regional QNH is above or below the 29.92 inHg datum.
Why do you need to calculate the actual height of a pressure altitude?
To ensure ground clearance and calculate the performance capabilities of the aircraft.
What pressure altitude error is commonly experienced?
Barometric pressure error on the altimeter, which occurs when flying into an area with a different sea level pressure, leading to incorrect altitude readings. ie. High to low careful go ( aircraft overreads)
What is ISA temperature deviation?
ISA temperature deviation is the measurement of the actual temperature against the ISA temperature for the corresponding altitude, expressed as ISA ±6°C.
If you were at 33,000 ft with an outside air temperature (OAT) of -45°C, what is the temperature deviation from ISA?
The temperature deviation at 33,000 ft is ISA + 6°C.
What is density?
Density is defined as mass per unit volume of a substance.
What is density altitude?
Density altitude is the altitude above the 1013 millibar (29.92 inHg) datum, at which the air density value experienced represents that of the level under consideration.
What are the main influences on air density?
Air density is influenced by temperature, pressure, and humidity.
How does the variation of air density due to temperature affect an aircraft’s performance?
A warmer temperature decreases air density, leading to a decrease in the performance level of the aircraft’s engines and aerodynamic performance.
What density errors are commonly experienced?
Altitude error and air-speed error due to temperature deviations from ISA.
What is static pressure?
Static pressure is the equal weight of all the air molecules at any one point in the atmosphere.
What is dynamic pressure?
Dynamic pressure is the pressure of the air molecules impacting onto a surface caused by the movement of a body through the air or the air flowing over a stationary object.
Describe IAS.
Indicated airspeed (IAS) is a measure of dynamic pressure translated to speed and displayed on the airspeed indicator (ASI) usually as knots per hour.
Describe CAS/RAS.
Calibrated airspeed (CAS) is indicated airspeed corrected for instrument and pressure errors, while rectified airspeed (RAS) is the term used in the UK.
Describe EAS.
Equivalent airspeed (EAS) is rectified airspeed corrected for compressibility error.
Describe TAS.
True airspeed (TAS) is the actual speed of an aircraft through the air mass in which it is flying.
Describe ground speed.
Ground speed is the actual speed of an aircraft relative to the ground,1. a combination of true airspeed and wind effects. 2. Wind velocity (wind velocity represents the movement of an aIr
mass relative to the ground)
Therefore, ground speed can be either greater than true airspeed
(TAS) if the aircraft has a tailwind, less than the TAS if the aircraft
has a headwind, or equal to TAS in still air conditions. Ground speed
is used for navigation and flight-planning purposes
What is LSS?
LSS is the local speed of sound.
How is LSS influenced by temperature?
The speed of sound decreases with a decrease in temperature.
What is the formula to calculate LSS?
LSS = 38.94 * Vabsolute temperature.
What is the value of LSS at ISA mean sea level conditions?
At ISA mean sea level conditions, LSS is equal to 660 knots per hour.
How does altitude affect LSS?
LSS decreases with an increase in altitude.
What is Mach number?
Mach number is the true airspeed (TAS) of an aircraft, given as a percentage relative to the local speed of sound (LSS).
How is Mach number calculated?
Mach number (MN) is calculated as MN = TAS / LSS.
What happens to Mach number with an increase in altitude?
Mach number increases because TAS increases with altitude and LSS decreases.
What is the main influence on Mach number?
Temperature influences Mach number.
What happens to indicated Mach number in a long-range cruise as weight decreases?
It decreases.
What happens to CAS or IAS if you descend at a constant TAS?
The CAS (IAS) will increase.
What happens to TAS if you climb at a constant CAS or IAS?
The TAS will increase.
What happens to Mach number if you climb at a constant CAS or IAS?
The MN will increase.
What happens to CAS or IAS and TAS if you fly at a constant CAS or IAS into a warmer area?
The CAS (IAS) remains constant, while the TAS will increase.
If two aircraft are flying at different flight levels at the same Mach number, which has the higher TAS/CAS?
The aircraft at the lower altitude has a higher TAS/CAS.
What happens to Mach number and CAS or IAS when climbing through an isothermal layer at a constant TAS?
Mach number remains constant, and CAS (IAS) decreases.
What happens to CAS or IAS if an aircraft descends through an isothermal layer at a constant TAS?
CAS (IAS) increases.
What happens to CAS or IAS and Mach number if an aircraft climbs at a constant TAS through an inversion layer?
CAS (IAS) decreases more than normal, and MN decreases.
