Impact Icing Can Be Hazardous

NTSB NYC02LA043 - On December 26, 2001, a Cessna 182 experienced a total loss of engine power, and was substantially damaged during a forced landing. The commercial pilot sustained minor injuries, and the passenger was not injured. Night instrument meteorological conditions (IMC) prevailed for the flight. An instrument flight rules flight plan was filed for the personal flight.

A recording of the pilot's weather briefing forecasted light snow along the entire route of flight. The FSS specialist provided an AIRMET for occasional moderate rime or mixed ice from 6,000 to 12,000 feet msl. The specialist advised of several broken cloud layers along the route, with variable cloud tops from 8,000 to 10,000 feet msl. A PIREP from a Piper Cherokee flying 25 miles west of LOU reported light rime ice at 5,000 feet msl.

The Cincinnati weather at 1908, revealed light to moderate precipitation between LOU and TZR. The reported weather 20 miles northwest of the accident site, at 1851, was: wind from 270 degrees at 16 knots, gusting to 20 knots; visibility 10 miles; ceiling 9,000 feet overcast; temperature 25 degrees F; dew point 16 degrees F; altimeter 29.81 inches hg.

The flight departed LOU about 1830, and after climbing to 5,000 feet msl, was in IMC. According to the pilot, about 10 minutes later, he noticed "snow that was not forecasted, with no structural icing." About 1900, while all engine gauges were indicating "normal," the airplane experienced a total loss of engine power. The pilot added that there was no sputtering or partial power loss prior to the total loss of engine power.

The pilot advised air traffic control that he had a total loss of engine power, and believed it was due to carburetor ice. He then performed a restart procedure from memory, which included activating the carburetor heat. After completing the checklist from memory, the engine ran for 2 seconds, but then stopped again. ATC provided vectors to the nearest airport, but the airplane struck trees and came to rest.

During the accident investigation it was noted that the right fuel tank contained fuel, but the left wing fuel crossover line had ruptured during the accident, and all fuel leaked from the left tank. The engine was removed and subsequently tested, where it started with no difficulty, and ran continuously.

FAA Advisory Circular AC 20-113 states:

Red arrow is pointing to the engine air filter.

"Impact Ice - Impact ice is formed by moisture laden air at temperatures below freezing, striking and freezing on elements of the induction system which are at temperatures of 32 degrees F° or below. Under these conditions, ice may build up on such components as the air scoops, heat or alternate air valves, intake screens, and protrusions in the carburetor. Pilots should be particularly alert for such icing when flying in snow, sleet, rain, or clouds, especially when they see ice forming on the windshield or leading edge of the wings. The ambient temperature at which impact ice can be expected to build most rapidly is about 25 degrees F°, when the super cooled moisture in the air is still in a semi liquid state. This type of icing affects an engine with fuel injection, as well as carbureted engines. It is usually preferable to use carburetor heat or alternate air as an ice prevention means, rather than as a deicer, because fast forming ice which is not immediately recognized by the pilot may significantly lower the amount of heat available from the carburetor heating system..."

The NTSB determined that this accident resulted from the pilot's flight into known adverse weather conditions. ASF believes that the engine stoppage may have been caused by blockage of the engine air filter due to snow and ice. Although this model of C182 does not have an alternate air system, the carburetor heat does bypass the air filter, which is located in the front of the cowling. Additionally, carburetor heat must be used as a preventative or anti-ice measure. In general, it is recommended that full carburetor heat be used when flying in heavy rain and snow to avoid the possibility of engine stoppage due to excessive water ingestion or carburetor ice. Once the engine stops, it's too late.

This accident report as well as others can be found in ASI's Online Database.

More information on icing can be found in ASI's Safety Advisor "Aircraft Icing".