A cool, moist air mass is a perfect recipe for induction ice to form, even if there is no visible moisture present. Itï¿½s important to be aware of how those conditions can adversely affect airplane performance, even if youï¿½re not doing the flying and are sitting in the backseat as a passenger.
On January 1, 2004, a Piper Seminole crashed during an emergency landing at the Illinois Valley Airport in Cave Junction, Oregon, following a partial loss of power in both engines. The pilot in command was in the left seat and the copilot, who was working the radios, was in the right seat. The pilot's former instructor was in the back seat with another passenger. The PIC and both passengers suffered serious injuries and the copilot was killed.
Before the flight, the copilot called Oakland Flight Service to file an IFR flight plan and request a standard weather briefing. He was told of en route weather, which included widespread moderate to heavy rain, a cool, moist air mass, and airmets for icing above the freezing level to 18,000 feet.
The flight departed Oakland International Airport at 4:11 p.m. Pacific Time, destined for North Bend, Oregon. Between 4:35 p.m. and 5:37 p.m., several requests were made to change altitudes. At 6:12 p.m., the copilot contacted Seattle Flight Watch and was told about airmets for "occasional moderate rime or mixed icing in clouds."
At 6:24 p.m., the Seminole asked ATC for a descent and vectors for obstacle clearance. The aircraft was cleared to descend to 7,300 feet. At 6:26 p.m., the Seminole declared an emergency and requested vectors to an airport immediately because of "engine problems relating to induction ice." Radar contact was lost one minute later.
After the accident, the PIC reported that he had noticed a reduction in manifold pressure and airspeed. During his attempt to regain power, he applied carburetor heat, checked that the mixtures were rich, turned the auxiliary fuel pumps on, tried cross-feeding the engines, and opened the cowl flaps. He said that during the descent, he cycled the throttles. Although he lost power, both engines kept running.
The general meteorological conditions in the vicinity of the accident site included cloud bases at 2,750 feet msl, extending up to 9,000 feet, with relative humidity at 75 percent. Cloud layers continued above 9,000 feet with mixed rime and clear icing conditions from 3,000 feet up to 9,000 feet. The Seminole was not certified for flight in known icing conditions and, aside from carburetor and pitot heat, had no anti/deice systems.
The PIC held a commercial pilot certificate with airplane single- and multiengine land ratings as well as an instrument airplane rating. He also held a current flight instructor certificate with 390 hours of flight experience. The copilot also held a commercial pilot certificate with airplane single- and multiengine land ratings, an instrument airplane rating, and a total 440 hours of experience. The former instructor held an airline transport pilot certificate with instrument and multiengine ratings. Additionally, she held a current flight instructor certificate and 2,681 total hours of flight time.
The NTSB determined the probable causes of this accident was both front seat pilots allowing the aircraft to enter an area of adverse weather, resulting in induction icing, induction filter blockage, carburetor icing, and the subsequent partial loss of power in both engines.
Induction icing can occur at the engine air intake filter. If carburetor heat (which draws air from within the engine cowling) is not selected, or is ineffective, power loss will ensue. When air is near freezing, movement of water molecules over an object such as the air filter may sometimes cause instantaneous freezing. Ice may also form on the cowling intakes and cause engine overheating.
When obtaining a weather briefing for a cross-country flight, pilots should pay close attention to cool, moist air masses and airmets for icing, especially if your aircraft is not certified for flight into icing conditions.
To learn more about winter flying conditions, visit the AOPA Air Safety Foundation's current Safety Hotspot, Winter Flying .
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