Flight Theory and Aerodynamics. Joseph R. Badick
Altitude
Indicated altitude is the altitude that is read directly from the altimeter and is uncorrected for any errors. In the United States, below FL180 the altimeter is set to the current altimeter setting of the field you are departing from or arriving to, or is given by air traffic control for the current area you are flying in. In the United States, when flying at or above 18 000 ft, altitude is measured in Flight Levels (e.g. FL180 for 18 000 ft). At FL180, the indicated altitude will be equal to pressure altitude as the altimeter setting is set to 29.92″, standard pressure, or QNE. The altitude at which the crew changes to 29.92 is called the transition altitude (TA). When the crew descends for landing, the altitude at which they return the altimeter setting to local barometric pressure corrected to sea level (QNH) is called the transition level (TL). (Remember it this way: 29.92 is selected at the TA, and the “A” stands for aloft, as in climbing or cruise. When returning to land, the TL is set on descent, and “L” stands for low, or landing.)
When QNE is lower than 29.92, the lowest usable flight level is no longer FL180. The lowest usable FL is obtained from the aeronautical publications. For instance, in the United States, if the pressure in the area of operations is between 29.91 and 29.42″, the lowest usable enroute altitude is FL185. It should also be noted that the TA and TL outside the United States will not always be 18 000. ICAO members set their own values.
Incidentally, QFE is the reference pressure set in the altimeter if the pilot wishes to know the elevation above the airfield. When the aircraft is on the airfield, the altimeter reads zero. QFE is seldom used as it would be of limited value when away from the immediate vicinity of the airfield.
Calibrated Altitude
Calibrated altitude is indicated altitude corrected for instrument and installation errors.
True Altitude
True altitude is the actual altitude above mean sea level and is referenced as mean sea level (MSL). On most aeronautical charts, MSL altitudes are published for man‐made objects such as towers and buildings, as well as for terrain, since this is the altitude closest to the altitude read off the altimeter. An important note is that true altitude will only be the same as indicated altitude when flying in standard conditions, which is very rare. When flying in conditions colder than standard, the altimeter will indicate a higher altitude then you are flying, so true altitude will be lower than indicated altitude. The same dangerous situation can develop when you are flying from a high‐pressure area to a low‐pressure area and the altimeter is not corrected for the local altimeter setting. Your altimeter will interpret the lower pressure as a higher altitude and your true altitude will again be lower than your indicated altitude. From the variations in true altitude versus indicated altitude, the saying was developed “high to low, or hot to cold, look out below.” Of course, this assumes that the altimeter is never reset to local pressure for an entire flight covering a long distance with varying temperatures and pressures.
Absolute Altitude
Absolute altitude is the vertical altitude above the ground (AGL), and can be measured with devices like a radar altimeter. Of course, your absolute altitude is more critical the closer to the ground you are flying; so even when not equipped with a radar altimeter, a pilot should be aware of their AGL altitude. When conducting an instrument approach in inclement weather, knowledge of your AGL altitude is vital to the safe completion of the approach or execution of a missed approach. Your height above airport (HAA), height above touchdown zone (HAT), and decision height (DH) are all AGL altitudes and should be briefed before the approach.
Pressure Altitude
Regarding aircraft performance, two types of altitude are of most interest to a pilot: pressure altitude and density altitude.
Pressure altitude is that corrected altitude in the standard atmosphere corresponding to a certain static pressure. Pressure altitude is the vertical distance above a standard datum plane where atmospheric pressure is 29.92″. In the United States, at FL180 and above, the altimeter is always set to 29.92″ unless abnormally low pressure exists in the area. Pressure altitude is used in performance calculations to compute true airspeed, density altitude, and takeoff and landing data.
Figure 2.3 provides a convenient method to determine pressure altitude. For example, if a given airport elevation is 3500 ft and the automated weather observation states a pressure value of 30.10″, use the altitude correction column to determine that 165 needs to be subtracted from 3500 ft. The pressure altitude in the given condition is 3335 ft, which makes sense as it is lower. The pressure given is higher than standard pressure, thus the air density is higher, resulting in a lower pressure altitude.
Figure 2.3 Field elevation versus pressure altitude.
Source: U.S. Department of Transportation Federal Aviation Administration (2008a).
Density Altitude
Density altitude is the altitude used to calculate aircraft performance in most situations. Density altitude is found by correcting pressure altitude for nonstandard temperature conditions. Pressure altitude and density altitude are the same when conditions are standard. Once pressure altitude has been determined, the density altitude is calculated using outside air temperature. If the temperature is below standard, then the density altitude is lower than pressure altitude and aircraft performance is improved. If the outside air temperature is warmer than standard, the density altitude is higher than pressure altitude and aircraft performance is degraded.
Figure 2.4 is a commonly used chart for performance calculations to determine density altitude. As provided by the Pilot’s Handbook of Aeronautical Knowledge, using Figure 2.4, we will calculate the pressure altitude first, and then calculate the resulting density altitude based on temperature. With a given airport elevation of 5883 ft and an altimeter setting of 30.10″, as shown in Figure 2.3 the resulting pressure altitude is calculated to be 5718 ft. Since the density altitude is pressure altitude corrected for nonstandard temperature, we then use the provided 70 °C temperature and move from the X‐axis vertically until we reach the diagonal line (interpolated) for 5718 ft. Moving horizontally to the Y‐axis, the density altitude is determined to be 7700 ft, which makes sense as it is higher than the pressure altitude since the temperature is above standard for that altitude, resulting in lower air density (higher density altitude).
Figure 2.4 Pressure altitude conversion and density altitude chart.
Source: U.S. Department of Transportation Federal Aviation Administration (2016b).
EXAMPLE
Using Table 2.1 and several of the equations from earlier in this chapter, the density altitude can also be determined using ratios. An aircraft at an indicated altitude of 1800 ft has an altimeter setting of 29.70″ Hg (sea level) and an outside air temperature of 75 °F. Calculate