Cruise

The cruising altitude should be selected based on a consideration of trip length, winds aloft, and the airplane’s performance. The power selection for cruise is determined based on several considerations. These include the cruise performance characteristics, the range profile and endurance profile. The relationship between power and range is illustrated by the Range Profile Chart shown below. Considerable fuel savings and longer range results when lower power settings are used.

The Range Profile chart shown below is typical of those used in general aviation. It is useful in determining what power setting is required to achieve any desired range. When reading any chart it is important to note the conditions for which the chart is valid. In this particular case, the range profile is valid if the aircraft weighs 2,550 pounds, is using the recommended lean mixture for cruise and flying in calm winds. It is also important to take into consideration the note at the bottom of the chart which advises the pilot that fuel used for engine start, taxi, takeoff and climb is taken into consideration. Also note that beneath the title, the chart takes into consideration a forty five minute fuel reserve. That being said, a pilot that plans to fly at 8,000 feet to an airport 500 miles away may cruise at 65% power, as this would provide a range of approximately 550 miles. If a pilot wants to further increase the range of their aircraft a lower power setting is preferable which may sound counterintuitive, however, the reduced burn rate offsets the increased flying time.

Now that a 65% power setting has been decided, a pilot may then make use of the Cruise Performance Chart after noting the cruise temperature, for the sake of this example let’s assume the temperature is 20°C above standard. The cruise performance chart can then be entered at 8,000 feet pressure altitude (a close approximation to the indicated altitude) and 20°C above standard. In order to achieve the desired %BHP an engine speed of 2,600 RPM is selected which results in the following:

  • Power: 64%
  • True Airspeed: 117 knots
  • Cruise Fuel Flow: 8.9 GPH

Note that the cruise temperature will rarely be standard, as is assumed on the range profile chart. For this reason, always err on the side of conservatism and select a power setting lower than indicated on the chart.

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