Page 22 - Volumne 18 Number 5

The configuration of the airport has a great influence on whether a takeoff or landing should be attempted. A wind blowing straight down the runway might impart little hazard, while one aligned directly across the runway presents a real challenge. All airplanes have crosswind component limitations, published or otherwise. For takeoff, one might have to consider whether it’s possible to keep the aircraft on the ground until aerodynamic control is sufficiently enhanced to manage the wind. Not all airplanes tolerate prolonged takeoff runs, because of their designed-in angle of incidence sitting on the landing gear.How Much Is Too Much?A crosswind landing is evaluated on the basis of how much crosswind component exists, and that’s most easily calculated with rules of thumb, such as “60 degrees of angle to the runway or more means it’s all crosswind” and “a 30-degree angle means half the total wind speed is crosswind”. If and when you decide to attempt a landing, be prepared to divert to another airport with a more favorable runway alignment, and determine this option long before you need it. Having an alternate airport ready for use eases the decision to divert.The final evaluation of your ability to make the landing becomes apparent on a stabilized final approach, as you observe the crab angle and/or amount of sideslip required to maintain runway alignment. Most aircraft are limited by the rudder’s ability to generate a sideslip that will negate wind drift. When you’ve bottomed out the downwind rudder pedal, you’re no longer able prevent the aircraft from moving laterally across the runway. Other limitations can be keeping enough clearance with the lowered wing to accommodate extended wing flaps or underslung engines. When you’re up against the airplane’s physical limits, it’s time to abandon the approach and gosomewhere else. Bringing in power on a twin’s upwind wing-mounted engine can add the necessary yaw, but at the cost of increased runway used. And, at some point, you’ll have to get all three landing gears planted firmly to gain traction for the rollout.For both takeoff and landing, it’s important to understand the local topography and how it affects low-level turbulence generated by the wind. When buildings, trees or hills are upwind of the runway, the turbulence behind them may be both surprising and intense as you fly through it to take off and land. In most cases, the landing configuration sacrifices more ability to control the aircraft in a crosswind, given the large flap deflection, drag and spooled-down engine thrust that leaves you vulnerable to the wind. On the other hand, the aircraft is usually heavier during takeoff and can’t accelerate out of a wind-shear sink as quickly as when at landing weight.Beware of landing approaches that cross lower terrain when a strong wind is blowing down the runway. The sinking flow of air descending into a ravine just off the end of the runway will take an airplane on short final down with it. Don’t try to spike your wheels onto the first foot of pavement under these conditions; you might leave your maingear in the rough, short of the threshold.Beware The ShearWindshear is a modern term for what old-timers called a “sinker”, encountered while flying into a subsiding wind during descent on the approach. Strong winds above 1,000 feet AGL will likely decrease at ground level, from the surface friction effect, and the aircraft will be left without sufficient lift as it momentarily loses airspeed. The cure, of course, is to carry more speed into the approach, but that can set up an overshoot if not managed carefully.20 • TWIN & TURBINEMAY 2014