elevation where the air is less dense. The density altitude is  higher that day. hold their assigned altitudes. Note that the true altitude or  actual height of an aircraft above sea level is only the same  as the pressure altitude when standard day conditions exist.  Otherwise, all aircraft with altimeters set to 29.92 "Hg could  have true altitudes higher or lower than the pressure altitude  indicated. This is due to the pressure within the air mass in  which they are flying being above or below standard day  pressure (29.92). The actual or true altitude is less important  than keeping aircraft from colliding, which is accomplished  by all aircraft above 18,000 feet referencing the same pressure  level (29.92 "Hg).  [Figure 10-35] Conversion factors and charts have been produced so pilots  can calculate the density altitude on any particular day.  Inclusion of nonstandard air pressure due to weather systems  and humidity can also be factored. So, while the effects of  temperature on aircraft performance do not cause an altimeter  to indicate falsely, an altimeter indication can be misleading  in terms of aircraft performance if these effects are not  considered.  [Figure 10-36] Other factors can cause an inaccurate altimeter indication.  Scale error is a mechanical error whereby the scale of the  instrument is not aligned so the altimeter pointers indicate  correctly. Periodic testing and adjustment by trained  technicians using calibrated equipment ensures scale error  is kept to a minimum.  Temperature also affects the accuracy of an altimeter. The  aneroid diaphragms used in altimeters are usually made of  metal. Their elasticity changes as their temperature changes.  This can lead to a false indication, especially at high altitudes  when the ambient air is very cold. A bimetallic compensating  device is built into many sensitive altimeters to correct for  varying temperature.  Figure 10-33  shows one such device  on a drum-type altimeter.  Temperature also affects air density, which has great impact  on the performance of an aircraft. Although this does not  cause the altimeter to produce an errant reading, flight crews  must be aware that performance changes with temperature  variations in the atmosphere. The term density altitude  describes altitude corrected for nonstandard temperature.  That is, the density altitude is the standard day altitude  (pressure altitude) at which an aircraft would experience  similar performance as it would on the non-standard day  currently being experienced. For example, on a very cold  day, the air is denser than on a standard day, so an aircraft  performs as though it is at a lower altitude. The density  altitude is lower that day. On a very hot day, the reverse is  true, and an aircraft performs as though it were at a higher  The pressure altimeter is connected to the pitot-static system  and must receive an accurate sample of ambient air pressure  to indicate the correct altitude. Position error, or installation  error, is that inaccuracy caused by the location of the static  vent that supplies the altimeter. While every effort is made  to place static vents in undisturbed air, airflow over the  airframe changes with the speed and attitude of the aircraft.  The amount of this air pressure collection error is measured in  test flights, and a correction table showing the variances can  be included with the altimeter for the pilot’s use. Normally,  location of the static vents is adjusted during these test flights  so that the position error is minimal.  [Figure 10-37]  Position  error can be removed by the ADC in modern aircraft, so the  pilot need not be concerned about this inaccuracy. 10-21 Figure 10-35.  Above 18,000 feet MSL, all aircraft are required to set 29.92 as the reference pressure in the Kollsman window. The  altimeter then reads pressure altitude. Depending on the atmospheric pressure that day, the true or actual altitude of the aircraft may  be above or below what is indicated (pressure altitude).  30.3 30.2 30.1 30.0 29.9 29.8 18,000 feet 18,000 feet High pressure Low pressure