Aeronautical Terms beginning with S

Severe Icing

The rate of ice accumulation is such that ice protection systems fail to remove the accumulation of ice and accumulation occurs in areas not normally prone to icing, such as aft of protected surfaces and other areas identified by the manufacturer. A representative accretion rate for reference purposes is more than 3 inches (7.5 cm) per hour6 on the outer wing. Immediate exit is required by many Airworthiness Directives (AD), flight manuals, and operations under part 91, §§ 91.13(a) and 91.527; part 121, § 121.341; part 125, § 125.221; and part 135, § 135.227.7

Significant Meteorological Information (SIGMET)

Information about in-flight weather of operational significance to the safety of all aircraft. SIGMETs may include severe icing. (See CWA and AIRMET.)

Stagnation Point

The point on a surface where the local air velocity is zero. The region of maximum icing collection efficiency is near this point.

Stationary Front

A front that has little or no movement because the opposing forces of the two air masses are relatively balanced.

Stratus Clouds.

Clouds that form layers with a uniform base. Stratus clouds can appear in ragged patches and may produce drizzle, rain, or snow.

Sublimation

A process in which ice turns directly into water vapor without passing through a liquid state.

Supercooled Large Drops (SLD)

Water drops with a diameter greater than 50 micrometers (0.05 mm) that exist in a liquid form at air temperatures below 0 °C. SLD conditions include freezing drizzle drops and freezing raindrops.