2.1.3 Where the minimum altitude, as prescribed in 14 CFR Sections 91.159 and 91.119, is above 18,000 feet MSL the lowest usable flight level shall be the flight level equivalent of the minimum altitude plus the number of feet specified in TBL ENR 1.7.2.
TBL ENR 1.7.2
Lowest Flight Level Correction Factor
Altimeter Setting Correction Factor
29.92 or higher none
29.91 to 29.42 500 feet
29.41 to 28.92 1000 feet
28.91 to 28.42 1500 feet
28.41 to 27.92 2000 feet
27.91 to 27.42 2500 feet
EXAMPLE.
The minimum safe altitude of a route is 19,000 feet MSL and the altimeter setting is reported between 29.92 and
29.42 inches of mercury. The lowest usable flight level will be 195, which is the flight level equivalent of 19,500 feet MSL (minimum altitude plus 500 feet).
2.1.4 Aircraft operating in an offshore CONTROL AREA should use altimeter setting procedures as described above, unless directed otherwise by ATC.
NOTE.
Aircraft exiting the oceanic CTA/FIR destined for the U.S. or transitioning through U.S. offshore control areas should use the current reported altimeter of a station nearest to the route being flown. When entering an oceanic CTA/FIR from U.S. offshore control areas, pilots should change to the standard altimeter setting 29.92.
3. Altimeter Errors
3.1 Most pressure altimeters are subject to mechani-cal, elastic, temperature, and installation errors. (Detailed information regarding the use of pressure altimeters is found in the Instrument Flying Handbook, Chapter IV.) Although manufacturing and installation specification, as well as the periodic test and inspections required by regulations (14 CFR Part 43, Appendix E), act to reduce these errors..any scale error may be observed in the following manner:
3.1.1 Set the current reported altimeter setting on the altimeter setting scale.
3.1.2 Altimeter should now read field elevation if you are located on the same reference level used to establish the altimeter setting.
3.1.3 Note the variation between the known field elevation and the altimeter indication. If this variation is in the order of plus or minus 75 feet, the accuracy of the altimeter is questionable and the problem should be referred to an appropriately rated repair station for evaluation and possible correction.
3.2 Once in flight, it is very important to obtain frequently current altimeter settings en route. If you do not reset your altimeter when flying from an area of high pressure into an area of low pressure, your aircraft will be closer to the surface than your altimeter indicates. An inch error in the altimeter setting equals 1,000 feet of altitude. To quote an old saying: “GOING FROM A HIGH TO A LOW, LOOK OUT BELOW.”
3.3 Temperature also has an effect on the accuracy of altimeters and your altitude. The crucial values to consider are standard temperature versus the ambient (at altitude) temperature. It is this “difference” that causes the error in indicated altitude. When the air is warmer than standard, you are higher than your altimeter indicates. Subsequently, when the air is colder than standard you are lower than indicated. It is the magnitude of this “difference” that determines the magnitude of the error. When flying into a cooler air mass while maintaining a constant indicated altitude, you are losing true altitude. However, flying into a cooler air mass does not necessarily mean you will be lower than indicated if the difference is still on the plus side. For example, while flying at 10,000 feet (where STANDARD temperature is .5 degrees Celsius (C)), the outside air temperature cools from +5 degrees C to 0 degrees C, the temperature error will nevertheless cause the aircraft to be HIGHER than indicated. It is the extreme “cold” difference that normally would be of concern to the pilot. Also, when flying in cold conditions over mountainous country, the pilot should exercise caution in flight planning both in regard to route and altitude to ensure adequate en route and terminal area terrain clearance.
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