Wing icing seems to be the main focus of investigators seeking the cause of a single-engine cargo aircraft's crash south of Bellevue on Dec. 6, 2004, in which the pilot and a passenger perished.
Pilot Fred Villaneuva, 60, of Farmington, Utah, was still in a thick, cold overcast when the Friedman Memorial Airport control tower lost radio contact with the Cessna 208B Caravan within minutes of the plane's landing.
Villaneuva's last radio transmission indicated he did not have the Hailey airport in sight and was flying on instruments.
Also aboard and killed was a passenger, Raymond Ingram 32, of Salmon, Idaho, who like Villaneuva was employed by Salmon Air, which had chartered the aircraft to UPS to deliver parcels and cargo in Idaho and Utah.
In a long, preliminary factual history covering details of the flight, the aircraft, the pilot's flying history and weather conditions at the time, the National Transportation Safety Board reported that the aircraft's engine and forward section hit the ground at a 75-degree nose-down angle, indicating a steep dive. The plane and cargo was consumed by fire.
The NTSB wrote that at the time of the accident, the pilot of a Cessna Citation jet departing Friedman Memorial a few minutes earlier reported accumulation of "light to occasional moderate rime ice" on the aircraft's wings. Rime ice "may seriously distort airfoil shape and thereby diminish the lift" of wings, according to a meteorological technical manual.
The Caravan aircraft, which has had a record of more than 100 accidents since being introduced in the 1980s, has a weakness for ground and in-flight icing, the NTSB report noted.
Another factor may also have entered into the last moments of the flight.
The NTSB found that Salmon Air's training manual warns Caravan pilots, "Never cycle boots below 130 KIAS (knots indicated air speed)—preferably higher. The higher the speed the more effective the boots work."
"Boots" are a pneumatically pressurized rubber-like apparatus on the leading edge of aircraft wings that are operated to de-ice aircraft in flight. But when operated, they can interfere with wing lift. If aircraft speed is too low, the result could be a stall.
Furthermore, the NTSB reported, the Caravan's manual stresses, "Minimum speed during flight in icing conditions with the flaps up is 105 KIAS." Investigation found that "the flap jackscrew mechanism indicated that the flaps were in the retracted position."
The NTSB seems to be trying to determine if icing, slow speed and cycling of the wing de-icing boots might have combined simultaneously to create an unrecoverable stall.
As a result of nine "incidents" involving Caravan models in recent months, mechanical and operating modifications in de-icing boots is now being proposed by the Federal Aviation Administration, which is accepting comments from pilots and aircraft owners until Aug. 22.
The NTSB reported that Villaneuva had a total of 9,757 hours, mostly as pilot-in-command, in various aircraft, with about 202 hours in the Caravan model. He had an air transport pilot rating as well as a flight instructor's rating.
A final "probable cause" report from the NTSB is several months away.