Atmosphere and Speed Flashcards
What is the aviation definition of height?
Height is a measured distance above the ground (QFE)
What is the aviation definition of altitude?
Altitude is a measured distance above mean sea level (QNH)
What is the aviation definiton of flight level?
A surface of constant atmosphere pressure which is related to a specific pressure datum, 1013.2hPa above the transition altitude.
e.g. FL250 = 25,000 feet above mean sea level when the pressure at sea level is 1013.2 mb.
What are the ICAO ISA conditions at sea level?
- Temperature
- Pressure
- ELR
- Density
- Temperature = 15’C
- Pressure = 1013.25 hPa
- ELR = -1.98’C / 1000’ to 36,090 where it is said to say at -56.5’C
- Density = 1.225 kg/m3
What is air pressure?
Air pressure is the weigh of a column of air. Therefore the air pressure decreases with an increase of altitude.
What is the formula to work out how many feet there are in a milibar?
96 x absolute temp (in Kelvin 273 + temp)
Altitude Px in hPa
What is pressure altitude?
Pressure altitude is an atmospheric pressure expressed in terms of altitude which corresponds that of the international standard atmosphere.
How would you calculated the pressure altitudes actual height?
True Alt = Pressure Alt +/- barometric error
1 hPa = 27’ at msl
…. Otherwise:
96 x absolute temp (in Kelvin 273 + temp)
Altitude Px in hPa
What pressure error is commonly experienced?
Barometric error on the altimeter.
From high px to low px: Aircraft will be low and instrument will overread.
From low px to high px: Aircraft will be high and instrument will underread.
From high to low look out below!
What is denisty?
Density is the quanity of mass per unit volume
What are the main influences on density? (3)
Temperature (main influence. Low = more dense)
Pressure (High = more dense)
Humidity (Dry = more dense)
What denisty errors are commonly experienced?
Altitude and Air Speed
The altimiter is colibrated in accordance with ISA, which converts the static pressure into an altitude. The altimiter does not make any allowances for temperature deviation from ISA. Therefore, for a colder column of air that is more dense a lower flight level will cause an altimiter to over read and the aircraft to be lower than indicated.
The ASI is colibrated at ISA mean sea level and any divergence from this will cause the ASI reading to be incorrect.
What is static pressure?
Static pressure is the equal weight of all the air molecules at any one point in the atmosphere, such as the ambient pressure.
What is dynamic pressure?
Dynamic pressure is the pressure over and above the static pressure of the air molecules impacting on the surface of a moving mody.
The two main components of a moving body are:
1: Speed of the body relative to the air
2: The desnity of the air
Dynamic pressure (IAS) = 1/2 Rho V2
V2 = TAS
Describe airspeeds and their corrections? (4)
Indicated airspeed:
This is only correct at ISA MSL and is a measurement of Q (dynamic pressure).
Clibrated Air Speeds (aka RAS):
This is IAS corrected for Position and Instrument.
Equivalent Air Speed:
This is CAS, plus compressability error (>300KTAS).
True Air Speed (the only speed):
This is corrected for all of the above plus density error.
