What is Altimetry?
Altimetry has been described as "Vertical Navigation". Basically, its the method and the rules by which we ensure that, at lower levels, we're not going to hit other aircraft or fly into the ground, and at higher levels, we're not going to hit other aircraft. So it's pretty important stuff!
Before we start, let's get some terminology straight!
Altitude refers to the height of an aircraft above mean sea level.
Elevation refers to the height of a location above mean sea level.
Height refers to the height of an aircraft or structure above ground level.
Flight Level refers to the height of an aircraft above the 1013 hPa pressure level, which is mean sea level in the International Standard Atmosphere.
QNH is the term used to denote mean sea level pressure at a location (which may be nowhere near the sea!) It is the only so-called Q code in common use. QFE and QNE shown in the diagram are rarely used.
Altitudes and Flight Levels
In Australia, below 10,000 feet, all aircraft are required to fly with their Altimeter showing ALTITUDE, which is the height above sea level. Because sea level pressure changes due to the constantly changing pressure systems moving across the country, we need a method of adjusting the altimeter to account for this. This is done by setting QNH, mean sea level pressure AT OUR LOCATION, in the subscale of the altimeter.
Since there are no mountains in Australia above 10,000 ft, if all aircraft in the one local area have the QNH for that area set in their altimeter subscale, the pilot can be confident that there won't be any rocks in the clouds. Assuming proper planning and flying at appropriate levels of course!.
However, above 10,000 ft, we only need to consider clearance from other aircraft. It is simpler and more accurate for all aircraft to have a common reference pressure set in their altimeter sub scale. The pressure chosen is 1013 hectopascals - mean sea level pressure in the International Standard Atmosphere (ISA MSL), and the altitude indicated is called a Flight Level. Flight Levels are Pressure Heights.
International Standard Atmosphere
Because the real world atmosphere is constantly changing, is is practically impossible to have a constant reference against which altimeters can be calibrated. So the International Civil Aviation Organisation - ICAO for short - has come up with a set of parameters called the International Standard Atmosphere, ISA. The parameters which concern us at basic piloting level are pressure and temperature, because together, these determine air density, and the density determines the aircraft's performance.
In the ISA, mean sea level pressure is 1013 hectopascals, hPa, (ignore the 0.25) and this pressure reduces by one hPa for every 30 feet gain in elevation.
In the ISA, mean sea level temperature is15 degrees Celcius, +15C, and this temperature reduces by 2C for every 1000 feet of elevation, up to 36,000 ft.
This is not the whole story, but it is all that concerns us at the basic pilot level, up to 10,000 ft altitude.
The Practical Application
You're already applying part of this when you set the altimeter during the after start checks in the Jabiru. The checklist requires you to set the airfield elevation of 3,100 feet on the altimeter, and check that the subscale reading is within 3 hPa of the QNH figure quoted on the AWIS. (The altimeter must read within 100 feet of the correct height to be regarded as serviceable, and this is a back-to-front but more convenient way of checking this.)
Pressure Height (also known as Pressure Altitude)
At any time, you can read the Pressure Height just by setting 1013 hPa in the altimeter sub scale!
The other way to work out pressure height is to calculate the difference between QNH and 1013, and multiply this figure by 30 to give the number of feet variation.
But do we add or subtract this figure?
Since we know that pressure reduces as the elevation increases, it follows that if QNH, mean sea level pressure is greater than 1013, then the 1013 level, known as ISAMSL or International Standard Atmosphere Mean Sea Level, must be higher than actual mean sea level, so we would subtract the variation to find pressure height. Similarly, if QNH is less than 1013 hPa, mean sea level is above the 1013 level (ISAMSL), and we would add the variation to find pressure height.
Let's work through an example:
On a given day, we're told that the QNH at Orange Airport is 1025. This is 12 hPa variation from 1013 (1025-1013=12), and if we multiply this by 30 we get 360 feet (12x30=360).
Since QNH is greater than 1013 hPa, then the 1013 hPa level must be above sea level, so we would subtract 360 ft from Orange Airfield elevation to get a pressure height of 2,740ft (3,100 - 360 = 2740). Answer: PH = 2740ft.
What about on a gloomy, cold, cloudy winter day when QNH is given as 1005 hPa? Same process - the variation is 8 hPa and the elevation difference is 8x30=240 feet.
Because QNH is less than 1013 hPa, then the 1013 hPa level must be below mean sea level, so we would add 240 ft to Orange Airfield elevation to get a pressure height of 3,340ft.
Answer: PH = 3,340 ft
Density height is pressure height corrected for temperature. It's the height the aircraft "thinks" it's at, and both the engine and airframe perform as though they were at that actual height. The density height is found by calculating the ISA temperature at the pressure height considered, then working out the difference between this ISA temperature and the actual temperature at our location, then applying a correction of 120 feet for every degree variation from the ISA temperature, to the pressure height.
So, in the ISA, the temperature at Orange Aerodrome would be ISA MSL temperature (+15C) less 2C for every 1,000 ft of elevation (round off to nearest 500ft). Orange's elevation is 3000 ft above MSL, so the ISA temperature would be 15-(2x3)=11C.
Referring to the first example above, where the pressure height is 2740ft, if the actual temperature is 28C, we can say it's ISA+17, that is, 17C above what it would be in the ISA!
Now the correction we have to apply to the pressure height because of the high temperature is 120 ft for every degree C variation from ISA temperature, and because the actual temperature is higher, we must add this correction.
So Density Height, DH=PH+(17x120) which equals 2740+2040=4780ft. The aircraft will perform as though it were at this altitude - considerably worse than we might have thought.
1. Calculate the density height at Alice Springs, elevation 1789ft if the QNH is 995hPa and the outside air temperature (OAT) is 42C. (Answer: 6169ft.)
2. Calculate the DH at Hobart, elevation 13ft, QNH 1031hPa, OAT -2C. (Answer -2687ft, that is, 2687ft below sea level.)