Factoring A Safe Landing Distance
When evaluating aircraft runway requirements- both take-off and landing distances must be considered. As with take-off distance there are several factors that affect the landing distance of an aircraft- explains David Wyndham.
Aircraft represent a beautiful balance among competing elements such as weight to be carried- wing area and planform- installed power- the environment- and the desired performance the designer wishes to achieve. If you want to carry a lot of weight at high cruising speeds- you should select an aircraft with large sweepback wings and lots of power - but you can expect the runway required for takeoff and landing to be long- particularly if you want to operate from airports located at high elevations in hot climates.
If you want to operate from airports with limited runway length- you’ll need an aircraft with large wings without sweep- and fitted with special devices such as high-lift flaps and leading edge slats. Anticipate limited cruising speed- however- since the wing shape that allows short take-off and landing distances does not favor high-speed cruise- and flaps and slats must retract into the wing when not in use to limit their drag.
The following all influence the minimum runway length for safe operation of a business aircraft:
• Aircraft Weight
• Runway Elevation
• Runway Air Temperature
• Runway Slope
• Runway Condition
• Regulatory Statutes.
THE BASIC PHYSICS OF LANDING
Landing distances are commonly calculated from 50 feet above the ground. The amount of distance required from touchdown to a full stop is referred to as landing ground roll. Harkening back to Physics 101- the momentum of an object increases with the square of its speed: doubling the speed quadruples the momentum. To stop in the allowable runway distance- the aircraft’s momentum must be fully dissipated at the conclusion of the ground roll.
When landing- aircraft have what are called high-lift devices that change the aerodynamic characteristics of the wing. Flaps on the rear of the wing and slats on the leading edge change the curvature of the wing to allow the aircraft to be flown at slower speeds. These are deployed for take-off and landing and retracted at other times.
The heavier an aircraft weighs- the more runway it requires for take-off and landing. A heavier aircraft requires more airspeed in order to stay airborne. This increased speed equates to a higher amount of energy needed to be dissipated upon landing. At higher elevations- the air is thinner. As a result- an aircraft needs more airflow over the wings to generate the lift needed to counteract its weight. So as the airport elevation increases- the speed the aircraft needs to fly increases. This increased speed during the approach and landing increases the length of the runway needed.
Higher air temperature also reduces air density. So a warmer day has the same effect as if the runway were at a higher elevation: it forces the aircraft to use more distance for landing. Landing uphill shortens the landing distance and landing downhill lengthens it - and both are connected with the ability to decelerate the aircraft. This is the opposite of a runway slope’s effect on take-off distance. Pilots are required to calculate the influence of runway slope in their determination of required runway length.
Stopping your car on a wet or snowy road requires more distance. Landing on a slippery runway decreases the ability of the aircraft to stop the same as for your car.
Headwinds slow the aircraft’s speed over the ground- so when landing- the aircraft touches down at a slower speed due to the headwind. Crosswinds and gusty winds make the handling of the aircraft more difficult as it transitions from air to ground. Thus in gusty crosswinds- the approach speed of the aircraft is increased to compensate and maintain a high level of safety.
THE BASIC REGULATIONS OF LANDING
To this point all of our discussion has centered around physics. Our last point centers around the regulatory requirements for calculating how much runway is needed for a safe landing. While the wise pilot adds for the unexpected- aviation regulations also can require additional margins for error. Under a not-for-hire operation (FAR Part 91)- the aircraft owner can decide how much added runway they wish to have as a safety margin.
If the flight is conducted under for-hire regulations (FAR Part 135 for on-demand charter or Part 121 for the airlines)- additional safety margins are specified. They state that the aircraft must be able to land within 60% of the available runway.
If your aircraft- for its weight and operating conditions- needs 4-000 feet to land from 50 feet above the runway- that distance allows for no added room in case things are not exactly as predicted. Under FAR 91- the pilot decides how much extra runway is needed for a safe landing. Under a for-hire operation- the FARs dictate that the 4-000 foot landing distance be accomplished on a runway no shorter than 6-667 feet. Under European Aviation Safety Regulations (EASA)- for a slippery runway an additional 15% of length must be added- bringing our required runway length to 7-667 feet.
Regardless of what authorities require- many pilots operate to the for-hire regulations when evaluating runway lengths. In addition- the pilot may add further to the safety margin in reduced visibility- at night- or when landing at a strange airport after a long duty day! Never assume that because you landed somewhere last week you can land there safely the next!