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Flight Performance - Level 3

Airplane Performance-Section 2

LANDINGS AND TAKE-OFFS AT UNIMPROVED AIRSTRIPS

The data published in your Airplane Flight Manual generally applies to dry paved runways. It is almost impossible to produce accurate figures applicable to turf runways because of the number of variables involved. The conditions change depending on whether the turf is wet, dry, frozen, snow or ice covered and change still further depending on how wet, how much snow, etc. The weight of the airplane is a factor since a heavier airplane tends to sink into the surface. The size of tires, tire pressure and tire tread all have an effect both on take-off roll and on braking action. New tires and new brakes can produce greater braking action. In addition, take-off and landing performance vary with the condition of the airplane and the powerplant.

For the most part, a pilot has to learn by experience what his airplane will do by way of performance at unimproved airstrips.

There are, however, some rules of thumb that will supplement the approved take-off and landing performance charts and increase the safety of operations at unimproved airstrips. They should not be used at variance with data published by the manufacturer.

Density Altitude: For every 1000 foot increase in the density altitude, add l0% to the ground roll and to the distance to clear a 50 foot obstacle up to 3000 feet density altitude and add 20% above 3000 feet density altitude.

Weight: For a 10% decrease in the weight of an airplane, there is a 10% decrease in the take-off ground roll and in the distance to clear a 50 foot obstacle. For a 10% increase in the weight, there is a 20% increase in the ground roll distance (a ratio of 1:2).

Surface: On a firm turf runway, add 7% to the ground roll distance.

On a rough or rocky runway or on short grass (up to 4 inches), add 10%

On long grass (more than 4 inches), add 30%.

On a soft surface, such as mud or snow, add 75%. In some situations, it may be impossible to achieve take-off, if the conditions are bad enough.

Headwind: Pilots should always pre-determine the full effects of headwind on both take-off and landing procedures, to ensure maximum safety and maximum performance in such operations. A variety of methods, ranging from formula, graphs, airplane manufacturer's charts/tables, monographs or even proven and tested "rule-of-thumbs", may be appropriately used, to make such pre-determination. Usually though, the formula method should never be used for a headwind component of less than 10 knots.

Tailwind:  Pilots should always pre-determine the full effects of tailwind on both take-off and landing procedures, to ensure maximum safety and maximum performance in such operations. A variety of methods, ranging from formula, graphs, airplane manufacturer's charts/tables, monographs or even proven and tested "rule-of-thumbs", may be appropriately used, to make such pre-determination. Usually though, the formula method should never be used for a tailwind component greater than 5 knots. Take-offs with a tailwind component greater than 10 knots is not recommended, under any circumstances.

Sloped Runway: For every l of upward slope in the runway, add 10% to the take-off roll. An upslope of 2 will add quite significantly to take-off distances. Sloping runways provide a different visual perspective than a level surface and landings and take-offs demand more planning and closer attention to flying techniques. When attempting to take off from an upsloping runway, it is important to know if the airplane can climb safely away from the rising terrain. Uphill take-off with tailwinds should be avoided.

A 1 downward slope in the runway reduces the take-off roll by only 5%. If a take-off is commenced downhill with a tailwind, the airplane will accelerate more rapidly. Avoid the temptation to lift off prematurely because the groundspeed seems high. Wait for a safe indicated airspeed.

Combined Effect. The combined effect of a change in aircraft weight, runway surfaces, runway slope and/or density altitude, in one or more combinations, can dramatically alter take-off distances.

Aborting a Take-Off: At 25% of the ground roll to take-off, the airplane should have achieved 50% of its lift-off speed.

At 50% of the ground roll, it should have achieved 70% of its lift-off speed.

At 80% of the ground roll, it should have achieved 90% of its lift-off speed.

Lift-off speed should be reached within the first 75% of the usable runway. If lift-off has not been achieved in this distance, the take-off should be aborted. For example, with a ground roll to take-off of 1200 feet, the minimum runway length must be 1600 feet.

CROSS WIND LANDINGS AND TAKE-OFFS

The amount of headwind is an important factor in determining the take-off and landing distance that will be required in any given situation. The wind, however, is rarely a direct headwind, especially at single strip airports. It is more likely to be blowing from some angle off the runway and will, therefore, be a headwind component.

The headwind component can be quickly calculated using the wind side of a flight computer that has a squared graph section.

Strong cross winds that are blowing at angles approaching 90 degrees to the runway can cause a problem in directional control for the pilot. Any pilot who has attempted to land a light airplane at a small airport with a single strip that happened that day to be directly 90 to a heavy wind knows the value of good cross wind technique.

There are essentially two kinds of contrary wind conditions that affect light planes on landing. There is the fairly steady wind at a pronounced angle to the runway which can be determined. There is also the gusty changeable wind. This second cross wind condition requires instinctively correct responses from the pilot, who must use aileron for keeping the airplane over the runway and opposite rudder to line it up with the runway, and at the same time be prepared to deal with sudden changes in airspeed and altitude due to the gusty conditions.

When the wind is gusting, a shift in almost any direction, even to 180 , can occur. The pilot is well advised to pay special attention to the behavior of trees, bushes and dust and not just the windsock.

Cross wind considerations are of considerable importance for take-off as well as for landing. In take-off, the effect of torque must be remembered. Since torque will tend to push the airplane's nose to the left, a cross wind from the right will not be too difficult to handle. In a cross wind from the left, however, it could be possible to run out of right rudder. If the cross wind is near maximum permissible speed, a left-hand cross wind calls for a decision to stay at home, unless the airplane has counter-rotating propellers.

In a stiff cross wind with full upwind aileron held, it is possible to end up making the final segment of the take-off on the upwind wheel only. This makes the airplane difficult to handle but the pilot should avoid the temptation to haul the airplane off prematurely. It is advisable to break ground at a slightly higher than normal speed, even if it is necessary to force the airplane to remain on the runway. Once in the air, the pilot wants to have the positive control responses that come with good flying speed.

On the other hand, it is inadvisable to carry too much airspeed when landing in a cross wind. The greater the angle of the cross wind, the less head wind component there is, so that in a 90 cross wind there is no headwind component at all. Although in extremely gusty conditions, some extra speed is advisable, too much may complicate the job of bringing the airplane to a stop before the end of the runway.

 

CLICK HERE for a more detailed and advanced review of Take-off and Landing performance of airplanes on unsloped and sloping runways.


The material for this section is reproduced from the publication, FROM THE GROUND UP, with the permission of its copyright owner, Aviation Publishers Co. Ltd. No further reproduction is authorized, in any print, electronic or other form of media, without the prior consent of the publisher at http://www.aviationpublishers.com . Any questions regarding this portion of the website should be directed to Dr. Claudius Carnegie. Questions regarding the publication, FROM THE GROUND UP, should be directed to the publisher at info@aviationpublishers.com.

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Updated: May 03, 2008