Amateur-built NORTHERN EXPLORER N907BT accident near Chickaloon, Alaska

Probable cause

A partial loss of engine power due to carburetor icing, which resulted from the pilot’s failure to use carburetor heat.

NTSB analysis

The pilot reported that he was conducting a low reconnaissance in his tailwheel-equipped airplane of a potential grass landing site in remote mountainous terrain during the late evening hours. While conducting the low reconnaissance, he engaged the carburetor heat, descended over the landing site, disengaged the carburetor heat, applied full throttle, and then departed from the landing site. As the airplane departed from the landing site, he reported that the engine revolutions per minute (rpm) were "very low" due to carburetor ice. He stated that due to the loss of power the airplane was capable of level flight, but unable to climb. As the airplane approached rising terrain, he turned about 180 degrees, and landed in an area of thick vegetation about 20 miles per hour. During the landing sequence, the propeller and left wing impacted terrain. The airplane sustained substantial damage to both wings and the fuselage.

The pilot further reported that the airplane did not have a carburetor temperature gauge installed.

The pilot verified that there were no preimpact mechanical failures or malfunctions with the airframe or engine that would have precluded normal operation.

In the recommendation section of the National Transportation Safety Board Accident/Incident Reporting Form 6120.1, the pilot stated that the accident may have been avoided if he did not utilize the landing site at that time of the day.

The Federal Aviation Administration has published Special Airworthiness Information Bulletin CE-09-35 Carburetor Icing Prevention (2009). This document discusses carburetor icing and states in part:

Pilots should be aware that carburetor icing doesn't just occur in freezing conditions, it can occur at temperatures well above freezing temperatures when there is visible moisture or high humidity. Icing can occur in the carburetor at temperatures above freezing because vaporization of fuel, combined with the expansion of air as it flows through the carburetor, (Venturi Effect) causes sudden cooling, sometimes by a significant amount within a fraction of a second. Carburetor ice can be detected by a drop in rpm in fixed pitch propeller airplanes and a drop in manifold pressure in constant speed propeller airplanes. In both types, usually there will be a roughness in engine operation.

The pilot should respond to carburetor icing by applying full carburetor heat immediately. The engine may run rough initially for a short time while ice melts.

Text from the NTSB. Open the NTSB docket for ANC16CA056 for the full record.