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CHAPTER 8—Emergency Open Sea Operations
OPERATIONS IN OPEN SEAS Open sea operations are very risky and should be avoided if possible. If an open sea landing cannot be avoided, a thorough reconnaissance and evaluation of the conditions must be performed to ensure safety. The sea usually heaves in a complicated crisscross pattern of swells of various magnitudes, overlaid by whatever chop the wind is producing. A relatively smooth spot may be found where the cross swells are less turbulent. Both a high and a low reconnaissance are necessary for accurate evaluation of the swell systems, winds, and surface conditions. DEFINITIONS When performing open sea operations, it is important to know and understand some basic ocean terms. A thorough knowledge of these definitions allows the pilot to receive and understand sea condition reports from other aircraft, surface vessels, and weather services. Fetch—An area where wind is generating waves on the water surface. Also the distance the waves have been driven by the wind blowing in a constant direction without obstruction. Sea—Waves generated by the existing winds in the area. These wind waves are typically a chaotic mix of heights, periods, and wavelengths. Sometimes the term refers to the condition of the surface resulting from both wind waves and swells. Swell—Waves that persist outside the fetch or in the absence of the force that generated them. The waves have a uniform and orderly appearance characterized by smooth, regularly spaced wave crests. Primary Swell—The swell system having the greatest height from trough to crest. Secondary Swells—Swell systems of less height than the primary swell. Swell Direction—The direction from which a swell is moving. This direction is not necessarily the result of the wind present at the scene. The swell encountered may be moving into or across the local wind. A swell tends to maintain its original direction for as long as it continues in deep water, regardless of changes in wind direction. Swell Face—The side of the swell toward the observer. The back is the side away from the observer. Swell Length—The horizontal distance between successive crests. Swell Period—The time interval between the passage of two successive crests at the same spot in the water, measured in seconds. Swell Velocity—The velocity with which the swell advances in relation to a fixed reference point, measured in knots. (There is little movement of water in the horizontal direction. Each water particle transmits energy to its neighbor, resulting primarily in a vertical motion, similar to the motion observed when shaking out a carpet.) Chop—A roughened condition of the water surface caused by local winds. It is characterized by its irregularity, short distance between crests, and whitecaps. Downswell—Motion in the same direction the swell is moving. Upswell—Motion opposite the direction the swell is moving. If the swell is moving from north to south, a seaplane going from south to north is moving upswell. SEA STATE EVALUATION Wind is the primary cause of ocean waves and there is a direct relationship between speed of the wind and the state of the sea in the immediate vicinity. Windspeed forecasts can help the pilot anticipate sea conditions. Conversely, the condition of the sea can be useful in determining the speed of the wind. Figure 8-1 on the next page illustrates the Beaufort wind scale with the corresponding sea state condition number.
While the height of the waves is important, it is often less of a consideration than the wavelength, or the distance between swells. Closely spaced swells can be very violent, and can destroy a seaplane even though the wave height is relatively small. On the other hand, the same seaplane might be able to handle much higher waves if the swells are several thousand feet apart. The relationship between the swell length and the height of the waves is the height-to-length ratio [Figure 8-2]. This ratio is an indication of the amount of motion a seaplane experiences on the water and the threat to capsizing. For example, a body of water with 20-foot waves and a swell length of 400 feet has a height-tolength ratio of 1:20, which may not put the seaplane at risk of capsizing, depending on the crosswinds.
However, 15-foot waves with a length of 150 feet produce a height-to-length ratio of 1:10, which greatly increases the risk of capsizing, especially if the wave is breaking abeam of the seaplane. As the swell length decreases, swell height becomes increasingly critical to capsizing. Thus, when a high swell height-to-length ratio exists, a crosswind takeoff or landing should not be attempted. Downwind takeoff and landing may be made downswell in light and moderate wind; however, a downwind landing should never be attempted when wind velocities are high regardless of swell direction. When two swell systems are in phase, the swells act together and result in higher swells. However, when two swell systems are in opposition, the swells tend to cancel each other or “fill in the troughs.” This provides a relatively flat area that appears as a lesser concentration of whitecaps and shadows. This flat area is a good touchdown spot for landing. [Figure 8-3]
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