On October 6, 1997, at 1830 central daylight time, a Beech G18S airplane, N9312Y, owned and operated by May Air Express of New Boston, Texas, was substantially damaged during a forced landing following a loss of engine power, near Crosbyton, Texas. The airline transport rated pilot, the sole occupant, sustained serious injuries. Instrument meteorological conditions prevailed in the vicinity of the accident and an IFR flight plan was filed for the Title 14 CFR Part 135 air cargo flight. The flight originated from Lubbock Municipal Airport, Lubbock, Texas, at 1730.

The pilot reported the following information in the Pilot/Operator Aircraft Accident Report(NTSB Form 6120.1/2) and during an interview with a FAA inspector. He stated that, while in cruise flight at 9,000 feet MSL, the left engine began to "run rough and lose power," which he suspected to be due to "carburetor ice." He further stated that he "deployed the manifold heat" but "encountered mechanical trouble moving the lever all the way down (heat on)." After " a few minutes" the he secured the left engine and feathered the propeller to "let the ice melt." He "repeatedly" attempted to restart the engine, but the propeller "would not unfeather." During the "whole ordeal" the pilot was unable to maintain altitude or a descent rate less than "200 feet per minute," and the aircraft kept buffeting on the edge of a stall. He stated that, after the airplane descended through a scattered cloud layer, he realized that he would not make it to the Crosbyton Airport. Subsequently, he executed a forced landing to grass covered terrain in a canyon about 5 miles northeast of the airport. He reported that the touch down was "hard but smooth [and then the airplane] struck uneven ground."

According to recorded transmissions between the pilot and Lubbock Approach Control, the pilot notified the controller at Lubbock Approach that the airplane was experiencing a problem involving "manifold icing," and he would be landing at Crosbyton Municipal Airport, Crosbyton, Texas.

Examination of the aircraft by an FAA airworthiness inspector found that the front of the airplane, from the instrument panel forward, was separated from the fuselage. Both engine nacelles were found separated from the wings and the empennage and tail remained intact. He reported that the hinge for the alternate air door on the left engine was attached, but the alternate air door(part number, 18-970011-25) was missing. He stated that the hinge did not appear to be damaged or distorted. The alternate air door was not found in the wreckage. Examination of the right engine revealed that the alternate air door was also missing from its hinge (which was not damaged). According to the FAA airworthiness inspector, a missing alternate air door on the carburetor air box would allow ambient air to enter the carburetor, rendering the carburetor heat system ineffective.

At the time of the accident the reported weather conditions at the Lubbock Airport, located 32 nautical miles west of the accident site were; winds from 140 degrees at 15 knots, a scattered cloud layer at 800 feet, a broken cloud layer at 1,600 feet, and an overcast cloud layer at 11,000 feet. The temperature was 64 degrees and the dew point was 63 degrees. According to carburetor icing probability charts, the reported temperature and dew point values would be favorable to the formation of induction system icing.

According to the Pilot/Operator Aircraft Accident Report(NTSB form 6120.1/2) the airline transport rated pilot had accumulated a total of 3,222 hours, of which 1,328 hours were in the Beech 18. The pilot had accumulated 650 hours of night flight time, of which 180 hours were in the Beech 18.

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