A cool, moist air mass is a perfect recipe for ice to form, even if there is no visible moisture present. Pilots should be aware of the weather conditions and how those conditions can adversely affect airplane performance.
Before the flight, the right-seat pilot called Oakland Flight Service to file an IFR flight plan. He received a standard weather briefing, 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. PST, destined for North Bend, Oregon. Between 4:35 and 5:37 p.m., several requests were made to change altitudes between 12,000 and 13,000 feet. At 6:12 p.m., the right-seat pilot 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 pilot in command 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, cycling the throttles changed the amount of power, and he never completely lost power in either of the engines.
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 a relative humidity of 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-ice/deice systems.
The pilot in command held a commercial pilot certificate with airplane single- and multiengine land, and instrument airplane ratings. He also held a current flight instructor certificate, with 390 hours of flight experience. The other pilot also held a commercial pilot certificate with airplane single- and multiengine land and instrument airplane ratings, and had 440 hours of experience.
The NTSB determined the probable causes of this accident was that the pilots allowed 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 their aircraft is not certified for flight into icing conditions.
To learn more about conditions that can affect your flying during the winter, visit the AOPA Air Safety Foundation's Winter Weather Hot Spot.
Kristen Hummel manages the GA accident database for the AOPA Air Safety Foundation. She holds a commercial pilot certificate with multiengine and instrument ratings.
Related Articles
