Takeoff

The minimum takeoff distance is of primary interest in the operation of any aircraft because it defines the runway requirements. The minimum takeoff distance is obtained by taking off at some minimum safe speed that allows sufficient margin above stall and provides satisfactory control and initial climb rate. Generally, the lift-off speed is some fixed percentage of the stall speed or minimum control speed for the aircraft in the takeoff configuration. As such, the lift-off is accomplished at some particular value of lift coefficient and angle of attack. Depending on the aircraft characteristics, the lift-off speed is anywhere from 1.05 to 1.25 times the stall speed or minimum control speed.

To obtain minimum takeoff distance at the specific lift-off speed, the forces that act on the aircraft must provide the maximum acceleration during the takeoff roll. The various forces acting on the aircraft may or may not be under the control of the pilot, and various procedures may be necessary in certain aircraft to maintain takeoff acceleration at the highest value.

The powerplant thrust is the principal force to provide the acceleration and, for minimum takeoff distance, the output thrust should be at a maximum. Lift and drag are produced as soon as the aircraft has speed, and the values of lift and drag depend on the angle of attack and dynamic pressure.

In addition to the important factors of proper procedures, many other variables affect the takeoff performance of an aircraft. Any item that alters the takeoff speed or acceleration rate during the takeoff roll affects the takeoff distance. For example, the effect of gross weight on takeoff distance is significant, and proper consideration of this item must be made in predicting the aircraft’s takeoff distance.

Increased gross weight can be considered to produce a threefold effect on takeoff performance:

Higher lift-off speed
Greater mass to accelerate
Increased retarding force (drag and ground friction)

The effect of wind on takeoff distance is large, and proper consideration must also be provided when predicting takeoff distance. The effect of a headwind is to allow the aircraft to reach the lift-off speed at a lower groundspeed, while the effect of a tailwind is to require the aircraft to achieve a greater groundspeed to attain the lift-off speed.

The effect of proper takeoff speed is especially important when runway lengths and takeoff distances are critical. The takeoff speeds specified in the AFM/POH are generally the minimum safe speeds at which the aircraft can become airborne. Any attempt to take off below the recommended speed means that the aircraft could stall, be difficult to control, or have a very low initial ROC. In some cases, an excessive AOA may not allow the aircraft to climb out of ground effect. On the other hand, an excessive airspeed at takeoff may improve the initial ROC and “feel” of the aircraft but produces an undesirable increase in takeoff distance. Assuming that the acceleration is essentially unaffected, the takeoff distance varies with the square of the takeoff velocity.

The effect of pressure altitude and ambient temperature is to define the density altitude and its effect on takeoff performance. While subsequent corrections are appropriate for the effect of temperature on certain items of powerplant performance, density altitude defines specific effects on takeoff performance.

An increase in density altitude can produce a twofold effect on takeoff performance:

Greater takeoff speed
Decreased thrust and reduced net accelerating force

If an aircraft of given weight and configuration is operated at greater heights above standard sea level, the aircraft requires the same dynamic pressure to become airborne at the takeoff lift coefficient. Thus, the aircraft at altitude takes off at the same indicated airspeed (IAS) as at sea level, but because of the reduced air density, the TAS is greater.

The effect of density altitude on powerplant thrust depends much on the type of powerplant. An increase in altitude above standard sea level brings an immediate decrease in power output for the non-supercharged reciprocating engine. However, an increase in altitude above standard sea level does not cause a decrease in power output for the supercharged reciprocating engine until the altitude exceeds the critical operating altitude. For those powerplants that experience a decay in thrust with an increase in altitude, the effect on the net accelerating force and acceleration rate can be approximated by assuming a direct variation with density. Actually, this assumed variation would closely approximate the effect on aircraft with high thrust-to-weight ratios.

Proper accounting of pressure altitude and temperature is mandatory for accurate prediction of takeoff roll distance.

The most critical conditions of takeoff performance are the result of some combination of high gross weight, altitude, temperature, and unfavorable wind.

In all cases, the pilot must make an accurate prediction of takeoff distance from the performance data of the AFM/POH, regardless of the runway available, and strive for a polished, professional takeoff procedure.

In the prediction of takeoff distance from the AFM/POH data, the following primary considerations must be given:

Pressure altitude and temperature – to define the effect of density altitude on distance
Gross weight – a large effect on distance
Wind – a large effect due to the wind or wind component along the runway
Runway slope and condition – the effect of an incline and retarding effect of factors such as snow or ice

Example Calculation

The takeoff distance chart shown below is typical of those used in general aviation aircraft. It should be noted that in this particular example distances shown are based on the short field technique. Conservative distances can be established by reading the chart at the next higher value of weight, altitude, and temperature. For example, when determining the takeoff performance of an airplane weighing 2,550 pounds departing from an aerodrome in which the temperature is 28°C and field pressure alttidue is 1,500 feet, the takeoff distance information presented for a weight of 2,550 ponds, pressure altitude of 2,000 feet and a temperature of 30°C should be used and results in the following:

Ground Roll: 1,285 feet
Total distance to clear a 50-foot obstacle: 2,190 feet

NOTE: While using more conservative values is easier and safer, on your transport Canada written exam you will be expected to determine exact values.

A correction for the effect of wind must be made based on Note 3 of the takeoff chart. For example, the correction for a 12 knot head wind is:

12/9 x 10% = 13% decrease

This results in the following distances, corrected for wind:

Ground Roll: 1285 – 1285 x 0.13 = 1118
Total distance to clear a 50-foot
obstacle: 2,190 – 2,190 x 0.13 = 1905

Also note that if takeoffs are conducted from a dry, grass runway distances must be increased by 15% of the ground roll figure.