Indicated Altitude – The Altitude read on the Altimeter. The Altimeter should be set to the closest a available Altimeter Setting, or QNH. The QNH is the local Mean Sea Level pressure corrected for temperature so the altimeter will read airport elevation on the ground. With the Altimeter Setting unavailable, set ground elevation. Expressed as ASL, Above Sea Level
Absolute Altitude – This is the altitude the aircraft is above the terrain. Can be measured with the radar altimeter. Expressed as AGL, Above Ground Level. Also can be found by subtracting terrain elevation from True Altitude.
True Altitude – The Indicated Altitude corrected for non standard temperature and pressure conditions. Key thing with calculating True Altitude is that the Altimeter Setting is corrected for temperature already, so the error exists in the column of air above the reporting station. As you climb the error increases.
”From a High to a Low, Lookout Below” – the Altimeter is calibrated to read correctly in ISA (International Standard Atmosphere) conditions, with the standard ISA temperature lapse rate. If it’s warmer than standard, the altimeter with under read, and colder than standard it will over read. This could result in being too low. In Canada, when the temperature is below 0C, we apply cold weather corrections to all IFR procedure altitudes, unless under radar vectors or on an assigned ATC altitude. MSA, transition altitudes, initial, DH/MDA, and Missed Approach altitudes all need to be corrected based on the Height Above the Aerodrome, HAA, of the procedure altitude. Some approaches are surveyed up to a minimum temperature where only the MDA requires the correction. This will be in the approach chart notes. See chart below for cold weather corrections.
Height – Is the distance measured from a specific datum. QFE operations. When the altimeter is set to QFE it will read 0’ on the ground. Some military operations still use QFE in some procedures. It is less practicable as it only references your height above the station, and can be difficult to reference ground clearance from other terrain obstacles. It requires an altimeter capable of QFE operations. The risk here should be obvious. If you were given QNH thinking it was QFE.
Pressure Altitude (PA) – The Altitude read on the Altimeter when it is set to ISA pressure, 29.92” Hg or (1013.25 millibars in Europe). Referred to as QNE. When we reach the Transition Altitude we set QNE and fly a Pressure Altitude, referred to as a Flight Level in 100’s of feet. 30,000 becomes FL300. Typically in North America, the Transition Altitude is at 18,000’. It can be lower and is found on IFR charts. For example in Northern Domestic Airspace in Canada, the airports are few and far between to be able to obtain an Altimeter Setting, so all cruise is conducted using QNE and a Flight Level. That way everyone has the same Altimeter Setting making it easier to deconflict in the wide expanse of airspace where the pressure could vary greatly.
”From a High to a Low, Lookout Below” – The danger here is the same as for temperature as it is for pressure. Flying to a High pressure region to a Low pressure region *without resetting the altimeter* will result in the Altimeter reading too high. The same applies using QNE, if the local MSL pressure is lower than standard, you’re True Altitude will be lower with the opposite being the case of the pressure was higher. With a lower than standard pressure, the lowest useable FL is higher depending on the local Altimeter Setting. >29.91” FL180. 29.91” – 28.92” FL190. 28.91” – 27.92” FL200. The latter two are vary rare, and I only seen it go below 29.00” maybe once. ATC will not assign you an unusable FL, so you don’t really need to remember this but you should reference it when needed for flight planning. PA is useful for all performance calculations, take-off, landing, cruise, TAS, engine, etc. PA = (29.92” – Altimeter Setting) x 100’ + Indicated Altitude. Depending on design, performance charts may use different methods for applying PA but all factor in pressure variance.
Density Altitude – Pressure Altitude corrected for non-ISA conditions. Temperature (and humidity). It is calculated using a Flight Computer or onboard computer. Think of it as the Altitude the airplane feels like it is at. Higher the DA, the lower the density of the air. Some Aircraft performance charts use Density Altitude directly while other imply DA by incorporating temperature and Pressure Altitude in their graphs, charts, and/or performance tables. In a very hot and humid day, they airplane may not have the performance to take off!
It is also important to note the the Altimeter itself isn’t infallible as well. It can have an allowable error which varies with altitude. It’s common to see altimeters vary slightly from one another. They test the Altimeter during the pitot-static tests. RVSM(Reduced Vertical Separation Minimums) has strict requirements and we have to cross-check the two primary Altimeters prior to entering RVSM airspace. No greater than 200’ variance in flight, both altimeters with 75’ of ground elevation on the ground. Outside these limits I cannot enter RVSM, and I am limited to cruise below FL290 which could put a damper on my fuel calculations.
Other errors can include lag due to the mechanical and pneumatic action of the aneroid capsule. Sudden pitch applications can compress the air ahead of the static port causing a momentary reversal of the altimeter.