The best landings are usually the result of well-planned and executed approaches. Every airplane’s pilot operating handbook (POH) recommends the speeds to use on approach, with corresponding settings of flap extension. These approach speeds are important, and should always be applied.
The standard rule-of-thumb for determining a final approach speed for any conventional general aviation airplane is to use 1.3 times the aircraft’s Vso (ie. its stall speed in landing configuration) as the approach speed. This calculation should use the calibrated airspeed of the aircraft which is the airspeed upon which POH data is normally based.
Weight is an important consideration in determining landing speed. When they’re light, airplanes stall at slower airspeeds. A lightly loaded airplane, if landing at the same airspeed as is used when it’s heavy, will float before touchdown to dissipate excess energy. This will result in a longer landing distance. Therefore, when using the 1.3Vso rule-of-thumb, the stalling speed for the actual weight of the aircraft – that is, its fuel-burned, end-of-flight weight – should be used, not the maximum landing weight.
If the POH doesn’t provide a table of approach speeds as a function of reduced weight, a rule-of-thumb is to reduce the calibrated approach airspeed for the maximum weight of the airplane by one-half of the percentage of the weight decrease. For example, if the airplane is 20 percent below maximum weight at the end of the flight, the calibrated approach airspeed should be reduced by 10 percent.
It is important to note that some airplane manufacturers may stipulate one specific approach speed regardless of the aircraft’s weight. This is often the consequence of certification flight testing during which it may have been found that, for greatest stability and safety, one fixed airspeed proved optimum. Whatever the case, always do what the POH recommends that you do on approach.
Information about landing performance is published in charts provided by every manufacturer for every airplane they produce. These charts, found in every aircraft’s POH, provide data particular to a specific aircraft configuration: for instance, a specific flap setting, engine power off, no wind, and maximum braking performance on a level, paved and dry runway. The chart will provide the expected ground roll, and distance to clear a 50 foot obstacle, when landing at airports of various elevations, at various ambient temperatures. They’ll also stipulate by how much the landing distance will be reduced for specific headwind speeds encountered.
When an airplane is landing on a grass runway, the ground roll after landing can be expected to be longer. In some case, much longer. On dry grass, a ground roll can increase by as much as 45 percent. On wet grass, deceleration capacity will be even worse. Again, the aircraft’s POH will provide the data required to ensure a safe termination of a flight.
Density altitude affects landing performance as much as it affects take-off. High temperatures and elevation will increase the landing roll because the true airspeed is higher than the indicated airspeed. (For a given indicated airspeed, the true airspeed is about 2 percent higher than the indicated airspeed per 1,000 ft of altitude above sea level.) Therefore, when using the same indicated airspeed for approach and landing as is used at sea level, the true airspeed is faster, leading to a faster groundspeed. That faster groundspeed will increase the landing distance. (If the field is short, there could be problems.)
If an airplane’s POH contains performance charts that relate landing distances to density altitude, pilots who operate from high elevation airports should pay heed to their data. By developing the habit of studying these charts, a botched landing can best be avoided.
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