cross wind techniques

Because of restricted or limited areas of operation, it is not always possible to take off or land the seaplane directly into the wind. Such restricted areas may be canals or narrow rivers. Therefore, skill must be acquired in crosswind techniques to enhance the safety of seaplane operation. The forces developed by crosswinds during takeoffs or landings on water are almost the same as those developed during similar operations on land. Directional control is more difficult because of the more yielding properties of water, less surface friction, and lack of nosewheel, tailwheel, or brakes.

Though a water surface is more yielding than solid land, a seaplane has no shock absorbing capability, so all the shock is absorbed by the hull or floats and transmitted to the aircraft structure. As shown in Figure 10, a crosswind tends to push the seaplane sideways. The drifting force, acting through the seaplane's centre of gravity, is opposed by the water reacting on the area of the floats or hull in contact with the water. This results in a tendency to weathervane into the wind. Once this weathervaning has started, the turn continues and is further aggravated by the addition of centrifugal force acting outward from the turn, which again is opposed by the water reaction on the floats or hull. If strong enough, the combination of the wind and centrifugal force may tip the seaplane to the point where the downwind float will submerge and subsequently the wingtip may strike the water and capsize the seaplane. This is known as a "waterloop" similar to a "groundloop" on land.

Figure 10: Crosswind landing technique

Because of the lack of clear reference lines for directional guidance, such as are found on airport runways, it is difficult to quickly detect sidedrift on water. Fortunately, early detection of sidedrift is not really essential, because the seaplane takeoff and landing can be made without maintaining a straight line while in contact with the water. A turn should be made toward the downwind side after landing. This will allow the seaplane to dissipate its forward speed prior to its weathervaning into the wind. By doing this, centrifugal force while weathervaning will be kept to a minimum and better aircraft control will result with less turnover tendency. One technique sometimes used to compensate for crosswinds during water operations is the same as that used on land; that is, by lowering the upwind wing while holding a straight course with rudder. This creates a slip into the wind to offset the drifting tendency.

The upwind wing is held in the lowered position throughout the touchdown and until completion of the landing. Another technique used to compensate for crosswinds (preferred by many seaplane pilots) is the downwind arc method. Using this method, the pilot creates a sideward force (centrifugal force) that will offset the crosswind force. This is accomplished by steering the seaplane in a downwind arc as shown in Figure 10. The pilot merely plans an arced path and follows this arc to produce sufficient centrifugal force so that the seaplane will tend to lean outward against the wind force. During the run, the pilot can adjust the rate of turn by varying rudder pressure, thereby increasing or decreasing the centrifugal force to compensate for a changing wind force. In practice, it is quite simple to plan sufficient curvature of the takeoff path to cancel out strong crosswinds, even on very narrow rivers.

As illustrated in Figure 11, the takeoff is started at the lee side of the river with the seaplane heading slightly into the wind. The takeoff path is then gradually made in an arc away from the wind and the liftoff accomplished on the downwind edge of the river. This pattern also allows for more climbout space into the wind.

Figure 11: Crosswind takeoff and landing technique

It should be noted that the greatest degree of the downwind arc is during the time the seaplane is travelling at the slower speeds of takeoff or landing. At the faster speeds, the crosswind effect lessens considerably, and at very slow speeds the seaplane can weathervane into the wind with no ill effect. Unless the current is extremely swift, crosswind or calm wind takeoffs and landings in rivers or tidal flows should be made in the same direction as the current. This reduces the water forces on the floats or hull of the seaplanes. Again, experience will play an important part in successful operation during crosswinds. It is essential that all seaplane pilots have thorough knowledge and skill in these manoeuvres.