Weather Theory

Hail Engine Water Ingestion Hail competes with turbulence as the greatest thunderstorm Turbine engines have a limit on the amount of water they hazard to aircraft. Supercooled drops above the freezing level can ingest. Updrafts are present in many thunderstorms, begin to freeze. Once a drop has frozen, other drops latch on particularly those in the developing stages. If the updraft and freeze to it, so the hailstone grows—sometimes into a velocity in the thunderstorm approaches or exceeds the huge ice ball. Large hail occurs with severe thunderstorms terminal velocity of the falling raindrops, very high with strong updrafts that have built to great heights. concentrations of water may occur. It is possible that these Eventually, the hailstones fall, possibly some distance from concentrations can be in excess of the quantity of water the storm core. Hail may be encountered in clear air several turbine engines are designed to ingest. Therefore, severe miles from thunderstorm clouds. thunderstorms may contain areas of high water concentration, which could result in flameout and/or structural failure of As hailstones fall through air whose temperature is above 0 one or more engines. °C, they begin to melt and precipitation may reach the ground as either hail or rain. Rain at the surface does not mean the Chapter Summary absence of hail aloft. Possible hail should be anticipated Knowledge of the atmosphere and the forces acting within with any thunderstorm, especially beneath the anvil of a it to create weather is essential to understand how weather large cumulonimbus. Hailstones larger than one-half inch affects a flight. By understanding basic weather theories, a in diameter can significantly damage an aircraft in a few pilot can make sound decisions during flight planning after seconds. receiving weather briefings. For additional information on the topics discussed in this chapter, see the following publications Ceiling and Visibility as amended: AC 00-6, Aviation Weather For Pilots and Flight Generally, visibility is near zero within a thunderstorm Operations Personnel; AC 00-24, Thunderstorms; AC 00-45, cloud. Ceiling and visibility also may be restricted in Aviation Weather Services; AC 91-74, Pilot Guide: Flight in precipitation and dust between the cloud base and the ground. Icing Conditions; and chapter 7, section 2 of the Aeronautical The restrictions create the same problem as all ceiling and Information Manual (AIM). visibility restrictions; but the hazards are multiplied when associated with the other thunderstorm hazards of turbulence, hail, and lightning. Effect on Altimeters Pressure usually falls rapidly with the approach of a thunderstorm, rises sharply with the onset of the first gust and arrival of the cold downdraft and heavy rain showers, and then falls back to normal as the storm moves on. This cycle of pressure change may occur in 15 minutes. If the pilot does not receive a corrected altimeter setting, the altimeter may be more than 100 feet in error. Lightning A lightning strike can puncture the skin of an aircraft and damage communications and electronic navigational equipment. Although lightning has been suspected of igniting fuel vapors and causing an explosion, serious accidents due to lightning strikes are rare. Nearby lightning can blind the pilot, rendering him or her momentarily unable to navigate either by instrument or by visual reference. Nearby lightning can also induce permanent errors in the magnetic compass. Lightning discharges, even distant ones, can disrupt radio communications on low and medium frequencies. Though lightning intensity and frequency have no simple relationship to other storm parameters, severe storms, as a rule, have a high frequency of lightning. 12-25