CONSIDERATIONS AFFECTING CRUISE STRATEGIES
In addition to one of the overall strategic objectives listed above for cruise flight, pilots are often forced to deal with shorter term constraints that may require them to temporarily abandon their cruise strategy one or more times during a flight.These situations may include:
- Flying a fixed speed that is compatible with other traffic on a specified route segment.
- Flying a speed calculated to achieve a required time of arrival (i.e., RTA) at a fix.
- Flying a speed calculated to achieve minimum fuel flow while holding (i.e., maximum endurance).
- When directed to maintain a specific speed by air traffic control.
POSSIBLE CRUISE SCHEMES
There are two theoretical speed selections for the cruise phase of flight. The traditional speed is long-range cruise (LRC). LRC speed is interrelated with maximum-range cruise (MRC) speed, which is the speed that will provide the furthest distance traveled for a given amount of fuel burned and the minimum fuel burned for a given cruise distance.
LRC has been historically defined as the speed above MRC that will result in a 1 percent decrease in fuel mileage in terms of nautical miles per kilogram or pound of fuel burned. The classic text, Aerodynamics for Naval Aviators, revised in 1965, states: "Most long-range cruise operation is conducted at the flight condition which provides 99 percent of the absolute maximum specific range. The advantage is that 1 percent of range is traded for 3 to 5 percent higher cruise velocity. Since higher cruise speed has a great number of advantages, the small sacrifice of range is a fair bargain." This concept is graphically illustrated in figure 1.
Because fuel is not the only direct cost associated with a flight, a further refinement in the speed for most economical operation is ECON speed, based on the entered CI. This speed, which includes some tradeoffs between trip time and trip fuel, is based on an estimation of the time-related operating expenses that are specific to each airline's operation. CI is defined as the ratio of time-dependent costs to fuel costs.