Understanding the fundamentals of altitudes and altitude reporting systems (altimeters) is important for flight planning, performance calculations, regulatory compliance, and in-flight problem solving. This subject is covered early in training for the Private Pilot Certificate, but I found it often needs review even up to the Certified Flight Instructor Certificate training level, thanks primarily to disuse.
The Five Altitudes We Need To Know
There are five unique altitudes that we must be familiar with in order to understand various other concepts.
- True Altitude
- Absolute Altitude
- Pressure Altitude
- Density Altitude
- Indicated Altitude
This is the aircraft’s actual height above Mean Sea Level (MSL). When we are planning for a cruising altitude anywhere in the United States below 18,000’, we are planning for True Altitude. When a controller tells us to “maintain 7,000,” we will maintain 7,000’ MSL. Our altimeter will be configured to read as close True Altitude as it can.
This is the actual height of the aircraft above ground level (AGL). This can be roughly determined by subtracting the known elevation of the ground below (found on aeronautical charts) from our true altitude indicated on the altimeter. Additionally, some GPS units will indicate approximate AGL, and certain helicopters are required to be equipped with a Radio Altimeter (RA) that provides AGL information.
This is the height of the aircraft above the standard datum plane, which is an atmospheric pressure of 29.92”Hg. Essentially, it is assumed that the standard pressure at sea level, on a standard day where the temperature is 15C, is 29.92”Hg. If we set our altimeter to 29.92” it will indicate our pressure altitude. This information is important for calculating performance, doing engine power checks, and flying above 18,000’ MSL (FL180), as altimeters are required to be set to indicate pressure altitude above said altitude (not a common altitude for helicopters, however).
This is pressure altitude corrected for non-standard temperature. Most of the time the temperature at sea level is not the standard 15C, and if the temperature is higher than the standard for any given altitude, then the density of the air is lower and performance capabilities are reduced. When calculating performance, we first find the pressure altitude and then correct that for the actual temperature at that altitude to find our limitations—thus determining density altitude.
Easy one: the altitude indicated on the altimeter. An acceptable amount of error can be found in altimeters, which you can read more about in 14 CFR 43 Appendix E if interested.
Altimeters work by measuring a calibrated aneroid container (imagine an accordion expanding/contracting) against the pressure of the air surrounding it from a static pressure source. The mechanical linkages are calibrated to read the correct altitudes from this contraction/expansion of the aneroid container, up to a certain point depending on the model.
Why Understand These?
We need to understand these different altitudes as they:
- Help us conceptualize what is happening to our performance as altitude/temperature changes;
- Help us plan our operation to optimize weight and balance limitations with regards to the altitudes we will be operating at (such as knowing the maximum patient weight we can pick up on a remote mountain scene at 8,000’ MSL after X minutes of fuel burn from startup);
- Help us comply with altitudes assigned by Air Traffic Control by being on the same altimeter setting as everyone else in the area;
- Help us comply with altitude minimums with regards to aircraft limitations, operational limitations, and regulatory limitations (such as the maximum 16,000’ pressure altitude limitation for an Airframe Fuel Filter-equipped Airbus EC130B4, minimum ceilings (AGL) of 800’ for local flights in our company’s operations manual, and minimum 300’ AGL enroute flight per 14 CFR 135 regulations); and
- Help us potentially identify errors in our pitot-static system through evaluating erroneous indicated altitudes vs known elevations.