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AP4ATCO - Factors Affecting Aircraft Performance During Cruise
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- The following SKYbrary Articles:
a) article description Factors affecting performance during cruise (range, endurance, speed). Engine failure during cruise.
b) source (IANS) 6.3 6.6
c) additional sources d) SKYbrary source FAA Airplane Flying Handbook (FAA-H-8083-3A) – chapter 12
High Altitude Flight Operations
The cruise phase of flight starts after aircraft has leveled off from the climb and it ends when the descent for landing is initiated by the crew. This phase involves level flight most of the time and very few level changes. There are two main options for cruise: maximum possible range (also known as best range) and maximum possible speed (also known as best speed). Sometimes due to operational reasons (holding for example) best endurance cruise is used instead of the two previous. Some of the factors that affect the cruise phase and their relations are already explained in the flight envelope; however the following paragraphs are written from a different (operational) point of view.
Factors affecting range
Range is the distance traveled with the fuel available. It is usually required to fly so that the maximum range is achieved, that is, to cover the greatest distance for the fuel carried, or to use the least fuel for the distance which is required to travel. The maximum range will depend on the amount of fuel carried and the number of miles flown per kilogram of fuel (also known as specific range). The following factors will have effect on the maximum possible range:
Increased aircraft mass increases the drag due to increased induced drag (greater lift required) and increased profile drag (higher speed at the same angle of attack). This requires greater thrust to balance the drag, which increases the fuel flow and reduces the specific range (number of miles flown per kilogram of fuel).
Approximately 10% increase in mass will require 10% increase in thrust and fuel flow and 5% decrease in range.
Air density (altitude)
Increasing altitude (decreasing air density) increases the range up to an optimum altitude, and then decreases again. The range is increased with altitude because of increased jet engine efficiency. However, above the optimum altitude, the effects of air compressibility cause the drag to increase resulting with reduced specific range.
This optimum altitude for best range increases as weight decreases. The procedure to give maximum range would therefore be to allow the aircraft to climb as the weight decreases during the flight. This is achieved with few step climbs during the cruise phase of the flight.
The speed which gives the maximum range for a given aircraft weight and altitude is called best range speed. Flying at higher speeds than the best range speed increases the drag and the fuel flow, and therefore reduces the range. Lower speeds than the best range speed reduce the drag and the fuel flow, but they also reduce the distance traveled per time which is more dominant, and therefore reduce the range.
Best range speed is considered in reference to the air. If the air mass is moving (wind), the speed in reference to the ground is different. The best range will be reduced in a headwind condition and the best range speed would be higher. The opposite occurs in a tailwind condition, the best range will be increased and the best range speed would be lower.
The required change of speed in headwind conditions is relatively small unless the wind is very strong (jetstream), and in this case a change of altitude where the wind strength is less would be preferable. The change of altitude is desirable if the gain for more favorable wind exceeds the loss from the change of altitude. This is also known as wind-altitude trade.
Factors limiting the speed (TAS)
The second cruise concept (maximum possible speed) is usually used on short or mid range flights when airlines are running behind schedule. It allows greatest distance travelled per given time or use minimum time to travel the required distance. The maximum speed is achieved when the maximum thrust available is balanced by the drag and it depends on the thrust available and the maneuver capability.
The speed achieved with maximum cruise thrust will vary with aircraft mass, altitude and temperature: − increased aircraft mass increases the drag, and therefore reduce the excess thrust required for acceleration; − thrust decreases with altitude, however the speed (TAS) will increase until a certain altitude and then decrease. For a given weight there is an optimum altitude that gives the best speed. This altitude is different from the optimum altitude for the best range; − increased temperature decreases the maximum thrust that a jet engine can produce;
As an aircraft increases speed (TAS) the airflow over some part of the fuselage or wings may be accelerated up to the speed of sound and a shock wave will form. These shock waves cause more drag, less lift, turbulent flow, and a reduction in effectiveness or even a reversal of control reactions. So, the maximum speed is limited by the critical Mach number.
Factors affecting endurance
Endurance is the time that aircraft can remain airborne with the fuel available. It will be greatest when the fuel is used at the lowest possible rate, that is, the fuel flow is minimum. The fuel flow depends on the thrust. Minimum thrust is required when an aircraft is flying with minimum possible drag. The following factors will have effect on the maximum possible endurance:
Increased aircraft mass increases the drag due to increased induced drag (greater lift required) and increased profile drag (higher speed at the same angle of attack). This requires greater thrust to balance the drag, which increases the fuel flow and reduces the endurance.
Air density (altitude)
Increasing altitude (decreasing air density) increases the endurance due to the increase in jet engine efficiency. However, at a very high altitude, compressibility will increase the drag, causing increased fuel flow, and therefore reducing the endurance.
At the optimum altitude, operating costs will be minimum when operating in the most economical (ECON) mode; it is also the cruise altitude for minimum fuel burn when in the Long Range Cruise (LRC) mode. In both cases, optimum altitude increases with reducing aircraft weight. In addition, in ECON Mode the optimum altitude increases with a reduction in cost index; in LRC Mode, it increases as speed reduces. Speed
The speed which gives the minimum drag for a given aircraft weight and altitude is called best endurance speed. Flying at higher speeds then the best endurance speed increases the drag and the fuel flow, and therefore reduces the endurance.
Also slower cruising speeds, often used as a means to save fuel, but may mean routinely flying closer to L/D max (Lift/Drag maximum); this gives less time to recognise and respond to any speed loss and eventual risk of a stalled wing condition. Small changes in either ‘External Factors’, such as variable winds, increased drag in turns, turbulence from any source, ice accretion or ‘Internal Factors’ such as use of anti-icing, un-commanded thrust rollback or engine malfunction can lead to loss of airspeed. Heavily damped autothrottles, designed for passenger comfort, may not always apply thrust aggressively enough to prevent a slowdown below L/D max. Close monitoring is essential.
Temperature (jet engine)
The jet engine efficiency reduces with increase of temperature, giving increased fuel flow and reduced endurance.
To achieve the best possible range it is necessary to fly at an optimum altitude and at an optimum speed for the given aircraft mass. However, small deviations from these optimum conditions do not make great difference to the range. On the other hand large deviations from the optimum, may cause significant loss in range and may even require aircraft to divert to an enroute alternative airport for an intermediate stop and refueling, which is time consuming and expensive. The following table gives an overview to the loss in range depending on the deviation in cruising level:
Deviation from optimum % in loss in range 2000 feet above 1 Optimum 0 2000 feet below 1 4000 feet below 4 8000 feet below 10 12000 feet below 15
To extract the maximum cruise performance from any airplane, the power setting tables provided by the manufacturer should be closely followed (either calculated by the pilot or by aircraft systems).
Flying at higher speeds than optimum requires more thrust and burns more fuel; while flying at lower speeds than optimum requires longer flying time and again more fuel. Therefore speed control (speed assigned to an aircraft) has an adverse effect to the range. It shall be used when absolutely necessary for long periods of time and the deviations from the optimum shall be as small as possible.
1. [Question type: true or false, based on AirQuestions FACT-ENR/076]
Q: Aircraft's mass has an effect on maximum range achieved. The higher the aircraft's mass the lower the distance that can be covered with the fuel available. A1:True A2: False
Correct answers: A1
2. [Question type: multiple response, based on AirQuestions FACT-ENR/080]
Q: Maximum range is increasing with altitude because of A1: increased air compressibility A2: positive effect of jet streams at high altitude A3: increased efficiency of jet engine A4: increased ground speed A5: increased thrust created by jet engines A6: reduced drag (lower air density)
Correct answers: A3, A4, A6.
3. [Question type: multiple choice]
Q: An altitude deviation of 4000 feet from optimal cruising level will reduce airplane’s range by A1: 1 % A2: 4 % A3: 8 % Correct answer: A2.
27. Factors affecting aircraft performance during descent and initial approach