Incident
The airplane, a Beech King Air 200, was being operated by a company pilot. A noncompany pilot, who had not completed training or checkout for the aircraft, accompanied him to accumulate flight time. The flight required only one pilot. While in cruise flight at 27,000 feet mean sea level, the cockpit voice recorder (CVR) recorded the sound of the windshield fracturing. The CVR transcript indicated that the company pilot was not in the cockpit at that time, as he was emptying trash in the cabin. The windshield stayed in place.
Cockpit Voice Recorder
The company pilot stated that within seconds after the fracture, he depressurized the airplane because he was unsure about the windshield's integrity. However, the Beech King Air Airplane Flight Manual (AFM) states to maintain cabin pressurization in the event of a fractured windshield and that the airplane can continue flight for up to 25 hours with the fracture. An unapproved document containing several checklists was found on the airplane; it did not include a checklist for a cracked or shattered windshield. The company pilot used this document and stated it "came with the airplane." He most likely was not aware that depressurization was not recommended nor that the airplane could operate for 25 hours with the fracture.
Oxygen System
After depressurizing, the pilots attempted to use oxygen masks but were unable to receive any oxygen. They most likely did not turn the oxygen on once needed, either forgetting due to the emergency or lacking time. During preflight, the company pilot had turned the oxygen system ready switch to the OFF position to "save" oxygen, contrary to the AFM's Before Start checklist. Post-accident testing showed normal operation. The unapproved checklist did not include the instruction to leave the oxygen on, but the pilot stated he knew the approved checklist required it and chose to turn it off anyway.
Uncontrolled Descent
About one minute after the pilots tried to get oxygen, the CVR recorded the last comment. For the next approximately seven minutes, it recorded increased engine propeller noise, landing gear and overspeed warning horns, and altitude alerts, indicating an uncontrolled descent. The CVR's 4-g impact switch was found in the open position. Radar data showed the airplane descended from 25,400 feet to 7,800 feet in five minutes. The pilots eventually regained consciousness and recovered from the descent. The airplane was substantially damaged by the acceleration forces incurred.
Windshield Examination
Examination revealed a dense network of fractures on the inner glass ply, but the windshield maintained structural integrity and did not lose significant glass. The fractures did not preclude safe continued flight. Post-accident examinations indicated the fracture initiated due to a design deficiency. The manufacturer redesigned the windshield in 2001; the accident airplane was manufactured in 1998. No known similar fractures have occurred in the new design. The manufacturer chose not to issue a service bulletin for retrofit because a single-pane fracture is not a safety-of-flight issue.
Probable Cause (Official Findings)
The company pilot's poor judgment before and during the flight, including turning the oxygen system ready switch to the OFF position after the preflight inspection and using an unapproved checklist, which did not provide guidance for a fractured windshield and resulted in his depressurizing the airplane. Members Hersman and Sumwalt did not approve this probable cause. Member Hersman filed a dissenting statement, with which Member Sumwalt concurred.