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Altimeter Procedures

While flying an airplane, it is crucial to set the correct altimeter, since it varies from place to place. The altimeter setting depends on various factors, such as aircraft static pressure and non-standard temperature and pressure. If the altimeter setting is not carried out properly, the exact altitude of the aircraft will not be reflected in the altimeter--the altimeter will show a higher altitude. Therefore, it is essential to have a thorough knowledge of setting the altimeter correctly.

  1. QNH Setting Procedure

    • When passing the transition level, QNH is set. The transition level for the USA is 18,000 feet, for the UK it is 3,000 feet (London area, 6,000 feet), and in France, it depends upon the barometric pressure of the given day. QNH is set fixed at 4,000 feet followed by the takeoff and after passing the transition altitude. While descending, the pilot will be notified of the transition level of that particular day.

    Actual Procedure

    • The flight level of altitude is classified as 18,000 feet below sea level, and at or above 18,000 feet sea level. In the first category, the altimeter setting procedure will depend on the standard barometric pressure of 31 inches mercury or less. In this situation, the altimeter setting is done along the route and within 100 NM of the aircraft. When the barometric pressure is above 31 inches Hg, it is set and maintained at 31 inches Hg, until the aircraft touches down at the airport.

    Important Steps and Possible Errors

    • Barometric altimeters are checked to adjust altitude errors in an aircraft. There is no need for other restrictions if the aircraft is capable of setting the current altimeter itself. But for VFR (visual flight rules) aircraft, it is vital to put extra effort in flight planning and operating. Mechanical, temperature, elastic and installation errors may occur in altimeter settings. The variation between the known field elevation and the altimeter indication has to be carefully followed, and if this variation is plus or minus 75 feet, it has to be immediately rectified.

    Certain Restrictions for Aircrafts

    • There are certain restrictions for IFR (instrument flight rule) aircraft and the aircraft is not capable of setting the altimeter itself. The departure and destination airports must be determined by certain parameters, such as a minimum ceiling requirement of 100 feet and a minimum visibility requirement of one-fourth of a statute mile. For flights flying above 18,000 feet sea level, the lowest usable flight level is determined by the atmospheric pressure in the particular area of operation of the flight.

    Precautions Taken

    • It is crucial to frequently get the current altimeter settings, since an error of one inch in the altimeter settings is equal to 1,000 feet of altitude. Errors may also occur when there is a difference between the standard temperature and the temperature at altitude, called the ambient temperature. High barometric pressure is another factor, and when it exceeds 31.00 inches Hg, the actual altitude of the aircraft will be higher than the altimeter reading.

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