Flight Planning - Units of Measurement

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, by airline, and by location (e.g. different height units may be used at different points during a single flight).

• 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 which are shown on a flight plan. Flight planning systems may need to use the unrounded values in their internal calculations for improved accuracy.

• 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.

There has been at least one occasion on which an aircraft ran out of fuel due to an error in converting between kilograms and pounds. In this particular case the flight crew managed to glide to a nearby runway and land safely (the runway was one of two at a former airport then being used as a dragstrip).

Many airlines request that fuel quantities be rounded to a multiple of 10 or 100 units. This can cause some interesting rounding problems, especially when subtotals are involved. Safety issues must also be considered when deciding whether to round up or down.

• Height units

The actual height of an aircraft is based on use of a pressure altimeter – see flight level for more detail. The heights quoted here are thus the nominal heights under standard conditions of temperature and pressure rather than the actual heights. All aircraft operating on flight levels calibrate altimeters to the same standard setting regardless of the actual sea level pressure, so little risk of collision arises.

In most areas, height is reported as a multiple of 100 feet (30 m), i.e. A025 is nominally 2,500 feet (760 m). When cruising at higher altitudes aircraft adopt flight levels (FLs). Flight levels are altitudes corrected and calibrated against the International Standard Atmosphere (ISA). These are expressed as a three figure group e.g. FL320 is 32,000 Feet (ISA).

In most areas vertical separation between aircraft is either 1000 or 2,000 feet (610 m).

In China and some neighbouring areas, height is handled using metres. Vertical separation between aircraft is either 300 metres or 600 metres (about 1.6% less than 1000 or 2000 feet).

Up until 1999, the vertical separation between aircraft flying at high altitudes on the same airway was 2,000 feet (610 m). Since then there has been a phased introduction around the world of reduced vertical separation minimum (RVSM). This cuts the vertical separation to 1,000 feet (300 m) between about 29,000 feet (8,800 m) and 41,000 feet (the exact limits vary slightly from place to place). Since most jet aircraft operate between these heights, this measure effectively doubles the available airway capacity. To use RVSM, aircraft must have certified altimeters, and autopilots must meet more accurate standards.

• Speed units

Aircraft cruising at lower altitudes normally use knots as the primary speed unit, while aircraft that are higher (above Mach Crossover Altitude) normally use Mach number as the primary speed unit, though flight plans often include the equivalent speed in knots as well (the conversion includes allowance for temperature and height). In a flight plan, a Mach number of "Point 82" means that the aircraft is travelling at 0.820 (82%) of the speed of sound.

The widespread use of global positioning systems (GPS) allows cockpit navigation systems to provide air speed and ground speed more or less directly.

Another method of obtaining speed and position is the inertial navigation system (INS), which keeps track a vehicle's acceleration using gyroscopes and linear accelerometers; this information can then be integrated in time to obtain speed and position, as long as the INS was properly calibrated before departure. INS has been present in civil aviation for a few decades and is mostly used in medium to large aircraft as the system is fairly complex.

If neither GPS or INS are used, the following steps are required to obtain speed information:

An airspeed indicator is used to measure indicated airspeed (IAS) in knots.

IAS is converted to calibrated airspeed (CAS) using an aircraft-specific correction table.

CAS is converted to equivalent airspeed (EAS) by allowing for compressibility effects.

EAS is converted to true airspeed (TAS) by allowing for density altitude, i.e. height and temperature.

TAS is converted to ground speed by allowing for any head or tail wind.

• Weight units

The weight of an aircraft is most commonly measured in kilograms, but may sometimes be measured in pounds, especially if the fuel gauges are calibrated in pounds or gallons. Many airlines request that weights be rounded to a multiple of 10 or 100 units. Great care is needed when rounding to ensure that physical constraints are not exceeded.

When chatting informally about a flight plan, approximate weights of fuel and/or aircraft may be referred to in tons. This 'ton' is generally either a metric tonne or a UK long ton, which differ by less than 2%, or a short ton, which is about 10% less.