On the TC written exam, you may be asked to determine the terrain clearance of an aircraft given the temperature and altimeter setting. These questions can be somewhat difficult. First, let’s consider an example where a pilot has set an incorrect altimeter setting. A typical question you may have on the exam will be similar to the one shown below.
A pilot at Airport A, 500 feet ASL, sets the altimeter to the airport’s altimeter setting of 29.80 inches of mercury prior to departure for Airport B, 1,000 feet ASL, some 400 NM away. A flight altitude of 6,000 feet is selected for the westbound flight so as to clear a 4,800-foot mountain ridge lying across track about 40 NM from B. The pilot does not change the altimeter subscale reading until he makes radio contact with B when 25 NM out and receives an altimeter setting of 29.20 inches of mercury. Ignoring other possible errors, when the aircraft crossed the mountain ridge what was the actual ground clearance?
Initially, the correct answer may seem to be 1,200 feet (6,000 feet – 4,800 feet). However, we must consider the incorrect altimeter setting used by the pilot. In this case, the altimeter overread by (29.80 in. Hg – 29.20 in. Hg) x 1000 = 600 feet. Therefore the actual terrain clearance was 1,200 feet – 600 feet = 600 feet
This illustrates the importance of having the altimeter setting of the nearest airport along the route set on the instrument. However, in some instances, you may have to also consider the effect of non-standard temperature as well.
The amount of error will be approximately 4% of the indicated altitude for every 11°C that the average temperature of the air column between the aircraft and the “ground” differs from the average temperature of the Standard Atmosphere for the same air column. In practice, the average temperature of the air column is not known and “true” altitude is arrived at from knowledge of the outside air temperature (OAT) at flight level and use of a computer. The “true” altitude found by this method will be reasonably accurate when the actual lapse rate is, or is near, that of the Standard Atmosphere, i.e. 2°C per 1,000 feet. During the winter when “strong” inversions in the lower levels are likely and altimeters “habitually” over-read, in any situation where ground separation is marginal, a pilot would be well advised to increase the altimeter error found using flight level temperature by 50%. Consider the aircraft in the above example but now assume that the OAT at flight level in the vicinity of the mountain ridge was -20°C, what was the likely “true” altitude of the aircraft over the mountain ridge?
To calculate “true” altitude using a computer, the pressure altitude is required. In this case, the altimeter indicates 6,000 feet with 29.80 inches of mercury set on the subscale, therefore, if the pilot altered the subscale to 29.92 inches of mercury momentarily, the pilot would read a pressure altitude of 6,120 feet. Alternatively, the pilot can apply the following equation:
Pressure Altitude = Indicated Altitude + 1000 x (29.92 in. Hg – Altimeter Setting)
Although the indicated altitude is 6,000 feet, if the altimeter setting of the nearest airport (B) was set, the indicated altitude would be 5,400 feet. With 29.20 inches of mercury set on the altimeter subscale if the aircraft was on the ground at B, the altimeter would indicate the “true” altitude of 1,000 feet. Assuming no pressure difference, it can be taken that the altimeter set to 29.20 inches of mercury would indicate the 1,000-foot level at the mountain with no error due to temperature, therefore temperature error will occur only between the 1,000-foot level and the 5,400-foot level, i.e. 4,400 feet of airspace. With this information follow the steps below to determine the approximate true altitude
The margin of safety is now just over 200 feet, but this does not take into account variables that may prevail due to the mountain effect.
Winds which are deflected around large single mountain peaks or through the valleys of mountain ranges tend to increase speed which results in a local decrease in pressure (Bernoulli’s Principle). A pressure altimeter within such an airflow would be subject to an increased error in altitude indication by reason of this decrease in pressure. This error will be present until the airflow returns to “normal” speed some distance away from the mountain or mountain range. The “drop” in pressure associated with the increase in wind speeds extends throughout the mountain wave, that is downwind and to “heights” well above the mountains. Isolating the altimeter error caused solely by the mountain wave from error caused by non-standard temperatures would be of little value to a pilot. Of main importance is that the combination of mountain waves and non-standard temperature may result IN AN ALTIMETER OVERREADING BY AS MUCH AS 3,000 FT. If the aircraft in our example had been flying upwind on a windy day, the actual ground separation on passing over the crest of the ridge may well have been very small.