Altimetry Flashcards
How do you calculate pressure lapse rate?
Pressure lapse rate = 96T / P
However, generally use 27ft/hPa or 8m/hPa
What is a regional QNH?
The lowest pressure forecast within a given area
It is issued 2 hours before it becomes valid for 1 hour
When moving from an area of high pressure to low pressure - what will be the effect on your altimeter?
Moving from HIGH - LOW
Reading will be HIGHER than you actually are (OVER-read)
When moving from an area of low pressure to high pressure - what will be the effect on your altimeter?
Moving from LOW - HIGH
Reading will be LOWER than you actually are (UNDER-read)
What is the equation to calculate Surface Pressure Variation?
TA = IA + 27(QNH - subscale)
What is the difference between Indicated and True altitude?
Indicated - what the altimeter reads
True - Actual distance above MSL
What is the equation to find Vertical Pressure Distribution?
True altitude within a pressure column
TA = IA + (4 x ISA dev. x Thickness) / 1000
What is the equation to use when there is a combined subscale and temperature error?
TA = IA + (4 x (IA/1000) x ISA dev.) + (27(QNH - subscale))
What is the basic way to correct for Temperature error?
For every 10°C deviation to the ISA temp - apply a correction of 4% to the IA
What effect does moving from warm to cold air have on the altimeter?
Moving from warm - cold
You’d be UNDERdressed (lower than you think)
What effect does moving from cold to warm air have to altimeter?
Moving from cold - warm
You’d be OVERdressed (higher than you think)
What pressure setting should you set on take off and the initial climb?
Airfield QNH should be set whilst climbing up to the transition altitude
What is the first point in flight that you change from QNH to S.P.S?
Set 1013 hPa at the transition altitude
When should you change back onto QNH after operating at flight levels?
As you descend through the transition level, you should set QNH
What pressure setting should you use during approach and landing?
Normally set QNH
But light aircraft may operate QFE
What is the transition layer and how it is adjusted?
This is the gap between transition altitude and level.
You should not fly in this level - only descend through it
They are normally 500ft thick, or less, but are adjusted to reflect the MSL pressure at that time.
If layer exceeds 500ft or becomes less 0ft, it is reset to minimum
When is QNH greater than QFF?
When the following are BOTH either correct or incorrect:
- Airfield is above MSL
- Temperature is greater than ISA temperature
What are the 3 factors that affect density?
Pressure (Increase = ρ increase)
Temperature (Increase = ρ decrease)
Humidity (Increase = ρ decrease)
What is the ideal gas equation?
Constant = PV/T
What are the density values in the hottest and coldest places on Earth?
Hottest: 1.1 kg/m^3
Coldest: 1.5 kg/m^3
What are the least favourable conditions for aircraft performance?
High altitude
High temperature
High humidity
Give the equation for calculating density altitude
DA = PA + 120 x ISA dev.
Define Density altitude
Density altitude is the ISA altitude corresponding to a measured density at a location.
It is also the Pressure density corrected for Temperature errors
