Lesson 11: Altitude Measurement of an Aircraft

How is the altitude measured for an Aircraft

Click here for the previous lessons, to learn about: Principle of Airframe; Principles of Aerodynamics; Airfoil Characteristics; Primary Flight Control Surfaces; Description and Operation of Helicopter; Miscellaneous Components of an Aircraft…

 They determine altitude by measuring air pressure. As altitude increases, air pressure decreases. This is because the density of air is lower (thinner) at high altitudes. It exerts less pressure on Earth below.


More details in this video


Subscribe here to our Youtube Channel ... And you will have the advantage of asking questions specific to you and you get quick answers to your situations ....


Click here for the previous lessons, to learn about: Principle of Airframe; Principles of Aerodynamics; Airfoil Characteristics; Primary Flight Control Surfaces; Description and Operation of Helicopter; Miscellaneous Components of an Aircraft…


The altimeter measures the height of an aircraft above a fixed level. The instrument senses this by taking the ambient air pressure from the static port. That air is plumbed through the back of the panel and into the back case of the altimeter. Inside the altimeter is a sealed disc called an aneroid, or bellows.


The pilot of an aircraft needs to know its altitude and there are several ways we could measure the altitude of an airplane. Radar might be used to measure the plane’s distance above the ground. Global Positioning System satellite signals can determine the plane’s position, including its altitude, in three dimensional space. These and some other possible methods of altitude determination depend on the operation of one or more electrical systems, and while we may want to have such an instrument on our airplane, we also are required to have an “altimeter” that does not depend on batteries or generators for its operation. Further, the altitude that the pilot needs to know is the height above sea level. The obvious solution is to use our knowledge that the pressure varies in a fairly dependable fashion with altitude.

If we know how pressure varies with altitude then we can measure that pressure and determine the altitude above a sea level reference point. In other words, if we measure the pressure as 836 pounds per square foot we can look in the standard atmosphere table and find that we should be at an altitude of 23,000 ft. So all we need to do is build a simple mechanical barometer and calibrate its dial so it reads in units of altitude rather than pressure. As the measured pressure decreases, the indicated altitude increases in accord with the standard atmosphere model. This is, in fact, how “simple” altimeters such as those sometimes used in cars or bikes or even “ultra-lite” aircraft. A barometer measures the air pressure and on some type of dial or scale, instead of pressure units, the equivalent altitudes are indicated.

The “simple” altimeter, however, might not be quite accurate enough for most flying because of the variations in atmospheric pressure with weather system changes. The simple altimeter would base its reading on the assumption that the pressure at sea level is 2116 psf . If, however, we are in an area of “high” pressure, the altimeter of an airplane sitting at sea level would sense the higher than standard pressure and indicate an altitude somewhat below sea level. Conversely, in the vicinity of a low pressure atmospheric system the altimeter would read an altitude higher than the actual value. If this error was only a few feet it might not matter, but in reality it could result in errors of several hundred feet in altitude readings. This could lead to disaster in bad weather when a pilot has to rely on the altimeter to ensure that the plane clears mountain peaks or approaches the runway at the right altitude. Hence, all aircraft today use “sensitive” altimeters that allow the pilot to adjust the instrument for changes in pressure due to atmospheric weather patterns.

The sensitive altimeter, shown in the next figure, has a knob that can be turned to adjust the readout of the instrument for non-standard sea level pressures. This can be used in two different ways in flight. When the aircraft is sitting on the ground at an airport the pilot can simply adjust the knob until the altimeter reads the known altitude of the airport. In flight, the pilot can listen to weather report updates from nearby airports, reports that will include the current sea level equivalent barometric pressure, and turn the knob until the numbers in a small window on the altimeter face agree with the stated pressure. These readings are usually given in units of millimeters of mercury where 29.92 is sea level standard. Adjusting the reading in the window to a higher pressure will result in a decrease in the altimeter reading and adjusting it lower will increase the altitude indication. With proper and timely use of this adjustment a good altimeter should be accurate within about 50 feet.

Figure 1.4: A Typical Mechanical Sensitive Altimeter

Figure 1.4: A Typical Mechanical Sensitive Altimeter


It should be noted that we could also use density to define our altitude and, in fact, this might prove more meaningful in terms of relating to changes in an airplane’s performance at various flight altitudes because engine thrust and power are known to be functions of density and the aerodynamic lift and drag are also functions of density. However, to “measure” density would require measurement of both pressure and temperature. This could introduce more error into our use of the standard atmosphere for altitude determination than the use of pressure alone because temperature variation is much more subject to non-standard behavior than that of pressure. On the other hand, we do sometimes find it valuable to calculate our “density altitude” when looking at a plane’s ability to take-off in a given ground distance.

If we are at an airport which is at an altitude of, lets say, 4000 ft and the temperature is higher than the 44.74oF predicted by the standard atmosphere (as it probably would be in the summer) we would find that the airplane behaves as if it is at a higher altitude and will take a longer distance to become airborne than it should at 4000 ft. Pilots use either a circular slide rule type calculator or a special electronic calculator to take the measured real temperature and combine it with the pressure altitude to find the “density altitude”, and this can be used to estimate the extra takeoff distance needed relative to standard conditions.


Altitude Measurement of Aircraft


Some may wonder why we can’t simply use temperature to find our altitude. After all, wasn’t one of our basic assumptions that in the Troposphere, temperature dropped linearly with altitude? Wouldn’t it be really easy to stick a thermometer out the window and compare its reading with a standard atmosphere chart to find our altitude?


Of course, once we are above the Troposphere this wouldn’t do any good since the temperature becomes constant over thousands of feet of altitude, but why wouldn’t it work in the Troposphere?




Best to know;

Is altitude same with height?


Height: Vertical distance from the point of observation on the Earth's surface to the point being measured. Altitude: Vertical distance from mean sea level to the point being measured.