A Wasilla man who died when his plane crashed into Prince William Sound last year reported icing conditions but continued flying into them, the National Transportation Safety Board said this week.
According to the NTSB report, 53-year-old Dale Carlson's Cessna 180 crashed on April 14, 2015 due to his decision "to continue flight into known icing conditions, contrary to the airplane's limitations."
NTSB investigator Shaun Williams said he concluded that built-up ice cut off air flow to the plane's engine before it plunged into the waters of the sound roughly 18 miles east of Whittier.
"We haven't determined whether it was carburetor icing or induction icing," Williams said. "We don't know, and we were not able to make that determination."
Carlson took off from Wasilla at noon on the day of the crash, and flew toward Valdez in instrument meteorological conditions (often abbreviated IMC) that included "low ceilings and visibility in snow showers," the final report said.
"Moderate icing was forecast between 4,000 and 10,000 ft above ground level," NTSB officials wrote. "It is likely that the airplane encountered IMC and potential icing conditions below 10,000 feet."
Carlson radioed air traffic controllers from the vicinity of Perry Island at 1:21 p.m., reporting icing and intermittent power from his engine. He said he was trying to land on an island in the area.
"I have a problem with my engine," Carlson said. "I think I've been through the light stuff."
Two minutes later, contact with Carlson's plane was lost; an emergency beacon from the plane transmitted for about 20 seconds before falling silent. The day after the crash, searchers found Carlson's body and his plane's left landing gear near Culross Island.
According to the NTSB, Carlson's plane wasn't rated for flight into known icing conditions. Aircraft approved for such flight, Williams said, typically have additional equipment similar to that found on commercial passenger planes -- heated wings, propellers and deicing boots.
Another pilot in the area, flying about six minutes behind Carlson, reported icing on his left engine cowling at an altitude of about 9,500 feet.
"Given this statement, the weather reports, and the accident pilot's report that he was having engine trouble, it is likely that the airplane flew through an area favorable for the formation of induction or carburetor icing, which resulted in a total loss of engine power," NTSB staff wrote.
The consistent accounts of icing from both pilots led investigators to believe Carlson was aware of the dangerous conditions, Williams said.
"The statement from the aircraft that was about six minutes behind the accident plane was that he was flying through suspended ice crystals," Williams said. "These airplanes are not certified for flight into known icing -- the idea is to get out once you notice it."
Williams said that once Carlson noticed the icing, he could have turned around or attempted a descent through non-cloudy skies -- something which could have at least prevented induction icing, if not carburetor icing.
"For structural icing, you need clouds; you need visible moisture," Williams said. "For carburetor icing, you do not -- that's a temperature dew point."
Investigators spoke with Carlson's mechanic, who said Carlson reported that the Cessna's controls felt "mushy" when he activated the carburetor's heating system, which drew in hot air from the plane's muffler. The mechanic told Carlson he should inspect the carburetor's control arm for damage, because "it could prevent the carburetor heat from operating correctly."
"Before the accident flight, the mechanic sent a text message to (Carlson) asking if he had checked the carburetor heat control, and the pilot replied that he had not but would do it that day," NTSB staff wrote.
But because more than one type of icing could've been involved in the crash, Williams cautioned against assigning too much weight to the mechanic's account.
"That note from the mechanic may not even be a contributing factor to the accident," Williams said.
Although the NTSB said damage to the carburetor control arm could have kept the deicing function from working properly, investigators weren't able to examine it because Carlson's plane was never recovered.