The problems of aircraft icing for this type of aircraft combine those of conventional aircraft and turbojet aircraft. Engine icing problems are similar to those encountered by turbojet aircraft, while propeller icing is similar to that encountered by conventional aircraft.

Propeller icing is a very dangerous form of icing because of the potential for a tremendous loss of power and vibrations. Propeller icing varies along the blade due to the differential velocity of the blade, causing a temperature increase from the hub to the propeller tip. Todays turboprop aircraft have deicers on the propellers. However, these deicers are curative, not preventive, and the danger remains.

When helicopters encounter icing conditions, the icing threat is similar, but potentially more hazardous than with fixed-wing aircraft. When icing forms on the rotor blades while hovering, conditions become hazardous because the helicopter is operating near peak operational limits. Icing also affects the tail rotor, control rods and links, and air intakes and filters.

The first phase in the preparation of an aircraft icing forecast consists of making certain preliminary determinations. These are essential regardless of the technique employed in making the forecast.

Determine the present and forecast future distribution, type, and vertical extent of clouds along the flight path. The influence of local effects, such as terrain features, should not be overlooked.

Determine those legs of the proposed flight path that will be in clouds colder than 0C. A reasonable estimate of the freezing level can be made from the data contained on freezing level charts, constant-pressure charts, rawinsonde observations, and airways reports (AIREP) observations, or by extrapolation from surface temperatures.

Check surface reports for precipitation along the proposed flight path, and forecast the precipitation character and pattern during the flight. Special consideration should be given to the possibility of freezing precipitation.

Note that each of the following methods and forecast rules assumes that two basic conditions must exist for the formation of icing. These assumptions are

1. the surface of the aircraft must be colder than 0C, and

2. supercooled liquid-water droplets, clouds, or precipitation must be present along the fight path.

The following text discusses icing intensity forecasts by using upper air data, surface data, and precipitation data, as well as icing formed due to orographic and frontal lifting.

Intensity Forecasts From Upper Air Data

Check upper air charts, pilot reports, and rawinsonde reports for the dewpoint spread at the flight level. Also check the upper air charts for the type of temperature advection along the route. One study, which considered only the dewpoint spread aloft, found that there was an 84 percent probability that there would be no icing if the spread were greater than 3C, and an 80 percent probability that there would be icing if the spread were less than 3C.

When the dewpoint spread was 3C or less in areas of warm air advection at flight level, there was a 67 percent probability of no icing and a 33 percent probability of light or moderate icing. However, with a dewpoint spread of 3C or less in a cold frontal zone, the probability of icing reached 100 percent. There was also a 100-percent probability y of icing in building cumuliform clouds when the dewpoint spread was 3C or less. With the spread greater than 3C, light icing was probable in about 40 percent of the region of cold air advection with a 100-percent probability of no icing in regions of warm or neutral advection. However, it appears on the basis of further experience that a more realistic spread of 4C at temperatures near 10 to -15C should be indicative of probable clouds, and that spreads of about 2 or 3C should be indicative of probable icing. At other temperatures use the values in the following rules:

0 to 7C and the dewpoint spread is greater than 2C, there is an 80-percent probability of no icing. 8 to 15C and the dewpoint spread is greater than 3C, forecast no icing with an 80-percent chance of success.

16 to 22C and the dewpoint spread is  greater than 1C, a forecast of no icing would have a 90-percent chance of success.

Colder than 22C, forecast no icing regardless of what the dewpoint spread is with 90-percent probability of success.

l If the dewpoint spread is 2C or less at temperatures of 0 to 7C, or is 3C or less at 8 to 15C:

In zones of neutral, or weak cold air advection, forecast light icing with a 75-percent probability of success.

In zones of strong cold air advection, forecast moderate icing with an 80-percent probability of success.

In areas with vigorous cumulus buildups due to insolational surface heating, forecast moderate icing.

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