Flight planning
It involves two safety-critical aspects: fuel calculation, to ensure that the aircraft can safely reach the destination, and compliance with air traffic control requirements, to minimise the risk of midair collision.
Within these airways, aircraft must maintain flight levels, specified altitudes usually separated vertically by 1,000 or 2,000 ft (300 or 610 m), depending on the route being flown and the direction of travel.
[1] Some commercial airlines have their own internal flight planning system, while others employ the services of external planners.
Aircraft must also carry some reserve fuel to allow for unforeseen circumstances, such as an inaccurate weather forecast, or air traffic control requiring an aircraft to fly at a lower-than-optimal altitude due to airway congestion, or the addition of last-minute passengers whose weight was not accounted for when the flight plan was prepared.
The way in which reserve fuel is determined varies greatly, depending on airline and locality.
Subject to safety requirements, commercial airlines generally wish to minimise costs by appropriate choice of route, speed, and height.
Flight planning systems must be able to cope with aircraft flying below sea level, which will often result in a negative altitude.
[7] While SI units technically are preferred, various non-SI units are still in widespread use within commercial aviation: Distances are nearly always measured in nautical miles[citation needed], as calculated at a height of 32,000 feet (9,800 m), compensated for the fact that the earth is an oblate spheroid rather than a perfect sphere.
Flight planning systems may need to use the unrounded values in their internal calculations for improved accuracy.
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.
[citation needed] The altitude of an aircraft is based on the use of a pressure altimeter (see flight level for more detail).
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.
When chatting informally about a flight plan, approximate weights of fuel and/or aircraft may be referred to in tons.
AIRAC (Aeronautical Information Regulation and Control) occurs every fourth Thursday, when every country publishes its changes, which are usually to airways.
Ocean tracks may start and finish about 100 miles offshore at named waypoints, to which a number of airways connect.
Tracks across northern oceans are suitable for east–west or west–east flights, which constitute the bulk of the traffic in these areas.
Flight planning systems organise this by inserting geographic waypoints at suitable intervals.
In free-flight areas, commercial aircraft normally follow a least-time-track so as to use as little time and fuel as possible.
A flight planning system may have to perform significant analysis to determine a good free-flight route.
There are three main factors that contribute to the cost: Different airlines have different views as to what constitutes a least-cost flight: For any given route, a flight planning system can reduce cost by finding the most economical speed at any given altitude and by finding the best altitude(s) to use based on the predicted weather.
The amount of calculation required to produce an accurate flight plan is so substantial that it is not feasible to examine every possible route in detail.
Techniques known variously as reclear, redispatch, or decision point procedure have been developed, which can greatly reduce the amount of reserve fuel needed while still maintaining all required safety standards.
These techniques are based on having some specified intermediate airport to which the flight can divert if necessary;[2] in practice such diversions are rare.
The use of such techniques can save several tons of fuel on long flights, or it can increase the payload carried by a similar amount.
[8] The original paper contains a lot of magic numbers relating to the optimum position of the reclear fix and so on.
In busy airspace with a number of competing aircraft, the optimum routes and preferred altitudes may be oversubscribed.
This problem can be worse in busy periods, such as when everyone wants to arrive at an airport as soon as it opens for the day.
To avoid this a suboptimal flight plan can be filed, asking for an inefficiently low altitude or a longer, less congested route.
The captain must prepare an alternate flight plan for when landing at the original destination is not possible.
"[10] Over and above the various cost-reduction measures mentioned above, flight planning systems may offer extra features to help attract and retain customers:
