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When twin-engine aircraft are flying across oceans, deserts, and the like,
the route must be carefully planned so that the aircraft can always reach an
airport, even if one engine fails. The applicable rules are known as ETOPS
(ExTended range OPerationS). The general reliability of the particular type
of aircraft and its engines and the maintenance quality of the airline are
taken into account when specifying how long such an aircraft may fly with
only one engine operating (typically 1–3 hours).
Flight planning systems must be able to cope with aircraft flying below sea level, which will
often result in a negative altitude. For example, Amsterdam Schiphol Airport has an
elevation of −3 metres. The surface of the Dead Sea is 417 metres below sea level, so low-
level flights in this vicinity can be well below sea level.
Units of measurement[
Flight plans use an unusual mixture of metric and non-metric units of measurement. The
particular units used may vary by aircraft, airline, and location (e.g., different height units may
be used at different points during a single flight).
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Distance units
Distances are always measured in nautical miles, as calculated at a height of 32,000 feet
(9,800 m), with due allowance for the fact that the earth is an oblate spheroid rather than a
perfect sphere.
Aviation charts always show distances as rounded to the nearest nautical mile, and these are
the distances that are shown on a flight plan. Flight planning systems may need to use the
unrounded values in their internal calculations for improved accuracy.
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Fuel units
There are a variety of ways in which fuel can be measured, depending mainly on the gauges
fitted to a particular aircraft. The most common unit of fuel measurement is kilograms; other
possible measures include pounds, UK gallons, US gallons, and litres. When fuel is measured
by weight, the specific gravity of the fuel must be taken into account when checking tank
capacity. Specific gravity may vary depending on the location and the supplier.
A - Z of Flight
July 2017
www.alliance-airways.net
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