setting. The altimeter should indicate the surveyed elevation  of the airport. If the indication is off by more than 75 feet from  the surveyed elevation, the instrument should be referred  to a certificated instrument repair station for recalibration.  Differences between ambient temperature and/or pressure  causes an erroneous indication on the altimeter.  When the aircraft is flying in air that is warmer than standard,  the air is less dense and the pressure levels are farther apart.  When the aircraft is flying at an indicated altitude of 5,000  feet, the pressure level for that altitude is higher than it would  be in air at standard temperature, and the aircraft is higher  than it would be if the air were cooler. If the air is colder  than standard, it is denser and the pressure levels are closer  together. When the aircraft is flying at an indicated altitude  of 5,000 feet, its true altitude is lower than it would be if the  air were warmer.  [Figure 5-6] A correctly calibrated pressure altimeter indicates true  altitude above mean sea level (MSL) when operating within  the International Standard Atmosphere (ISA) parameters of  pressure and temperature. Nonstandard pressure conditions are  corrected by applying the correct local area altimeter setting.  Temperature errors from ISA result in true altitude being  higher than indicated altitude whenever the temperature is  warmer than ISA and true altitude being lower than indicated  altitude whenever the temperature is colder than ISA.  True altitude variance under conditions of colder than ISA  temperatures poses the risk of inadequate obstacle clearance.  The cold temperature induced altimeter error may be  significant when considering obstacle clearances when  temperatures are well below standard. Pilots may wish to  increase their minimum terrain clearance altitudes with a  corresponding increase in ceiling from the normal minimum  when flying in extreme cold temperature conditions. Higher  altitudes may need to be selected when flying at low terrain  clearances. Most flight management systems (FMS) with  air data computers implement a capability to compensate  for cold temperature errors. Pilots flying with these systems  should ensure they are aware of the conditions under which  the system automatically compensates. If compensation is  applied by the FMS or manually, ATC must be informed  that the aircraft is not flying the assigned altitude. Otherwise,  vertical separation from other aircraft may be reduced creating  a potentially hazardous situation. The table in  Figure 5-7 ,  derived from International Civil Aviation Organization  Under extremely cold conditions, pilots may need to add an  appropriate temperature correction determined from the chart  in  Figure 5-7  to charted IFR altitudes to ensure terrain and  obstacle clearance with the following restrictions: Inherent Altimeter Error Cold Weather Altimeter Errors ICAO Cold Temperature Error Table •  Altitudes specifically assigned by Air Traffic Control  (ATC), such as “maintain 5,000 feet” shall not be  corrected. Assigned altitudes may be rejected if the  pilot decides that low temperatures pose a risk of  inadequate terrain or obstacle clearance. •  If temperature corrections are applied to charted  IFR altitudes (such as procedure turn altitudes, final  approach fix crossing altitudes, etc.), the pilot must  advise ATC of the applied correction. 5,000 foot pressure level  4,000 foot pressure level  3,000 foot pressure level  2,000 foot pressure level  1,000 foot pressure level  Sea level  30°C  15°C  0°C  Figure 3-6. Effects of nonstandard temperature on an altimeter. Figure 5-6.  The loss of altitude experienced when flying into an area where the air is colder (more dense) than standard. 5-5