The international standard atmosphere

The reader will have realised that there is liable to be considerable variation in those properties of the atmosphere with which we are concerned – namely, temperature, pressure and density. Since the performance of engine, aeroplane and propeller is dependent on these three factors, it will be obvious that the actual performance of an aeroplane does not give a true basis of comparison with other aeroplanes, and for this reason the International Standard Atmosphere has been adopted. The properties assumed for this standard atmosphere in temperate regions are those given in Fig. 2.2, earlier. If, now, the actual performance of an aeroplane is measured under certain conditions of temperature, pressure and density, it is possible to deduce what would have been the performance under the conditions of the Standard Atmosphere, and thus it can be compared with the performance of some other aeroplane which has been similarly reduced to standard conditions.

The altimeter

The instrument normally used for measuring height is the altimeter, which was traditionally merely an aneroid barometer graduated in feet or metres instead of millimetres of mercury or millibars. As a barometer it will record the pressure of the air, and since the pressure is dependent on the temperature as well as the height, it is only possible to graduate the altimeter to read the height if we assume certain definite conditions of temperature. If these con­ditions are not fulfilled in practice, then the altimeter cannot read the correct height. Altimeters were at one time graduated on the assumption that the tem­perature remained the same at all heights. We have already seen that such an assumption is very far from the truth, and the resulting error may be as much as 900 m at 9000 m, the altimeter reading too high owing to the drop in tem­perature. Altimeters are now calibrated on the assumption that the temperature drops in accordance with the International Standard Atmosphere; this method reduces the error considerably, although the reading will still be incorrect where standard conditions do not obtain.

As a barometer the altimeter will be affected by changes in the pressure of the atmosphere, and therefore an adjustment is provided, so that the scale can be set (either to zero or to the height of the airfield above sea-level) before the commencement of a flight, but in spite of this precaution, atmospheric con­ditions may change during the flight, and it is quite possible that on landing on the same airfield the altimeter will read too high if the pressure has dropped in the meantime, or too low if the pressure has risen.

Although it is convenient to use SI units for calculations and numerical examples, it should be remembered that knots and feet are still the inter­nationally approved units for speed and altitude respectively when dealing with aircraft operations. If you ever sit at the controls of an aircraft you will almost certainly find that the altimeter is calibrated in feet. We shall therefore use feet (ft) when referring to this instrument.

Modern altimeters are much more sensitive than the old types; some, instead of having one pointer, may have as many as three, and these are geared together like the hands of a clock so that the longest pointer makes one revol­ution in 1000 ft, the next one in 10 000 ft, and the smallest one in 100 000 ft. Unfortunately this has sometimes proved ‘too much of a good thing,’ and acci­dents have been caused by pilots mistaking one hand for another. The more modern tendency is to show the height level in actual figures in addition to one or more pointers. But, like a sensitive watch, the altimeter is of little use unless it can be made free from error, and can be read correctly. In the modern types, if the pilot sets the altimeter to read zero height, which he can do simply by turning a knob, a small opening on the face of the instrument discloses the pressure of the air at that height – in other words, the reading of the barom­eter. Conversely – and this is the important point – if, while in the air, he finds out by radio the barometric pressure at the airfield at which he wishes to land, he can adjust the instrument so that this pressure shows in the small opening, and he can then be sure that his altimeter is reading the correct height above that aerodrome, and that when he ‘touches down’ it will read zero. The altimeter may be used in this way for instrument flying and night flying, that is to say when the height above the ground in the vicinity of the aerodrome is of vital importance, but for ordinary cross-country flying during the day it may be preferable to set the sea-level atmospheric pressure in the opening. Then the pilot will always know his height above sea-level and can compare this with the height, as shown on the map, of the ground over which he is flying. If this method is used, instead of the altimeter reading zero on landing, it will give the height of the aerodrome above sea-level. There is, however, a snag in this method in that the sea-level atmospheric pressure varies from place to place and so different pilots may set their altimeters differently, thereby increasing the risk of collision; for this reason modern practice for flying above 3000 ft is to set the altimeter to standard sea-level pressure of 1013.2 mb, which means, in effect, that all the altimeters may be reading the incorrect height, but that only aircraft flying at the same height can have the same altimeter readings. Above 3000 ft heights are referred to in terms of flight levels (or hundreds of feet), e. g. FL 35 is 3500 ft, FL 40 is 4000 ft, then FL 45, 50, 55, and so on. Increases in flight levels are in fives because of the quadrantal system (in operation in the UK) which determines the height at which the pilot must fly for specific compass headings.

The question of altimeter setting has long been a matter for controversy among pilots – and even among nations.

In recent years there have been radical changes in aircraft instruments and displays. Instead of individual instruments there may now be a computer – screen type of display, but the altimeter display still looks quite similar to the traditional instrument. The information on which the display is based may also still be the external pressure, but there are now alternative, more accurate, height-reading devices such as radio or radar altimeters.

The reader who is particularly interested in altimeters and other instru­ments is referred to Aircraft Instruments by E. H. J. Pallett, a companion volume in the Introduction to Aeronautical Engineering Series.