N34633Cessna 177B · 1973

On November 11, 2020, at 1144 Pacific standard time, a Cessna 177B airplane, N34633, was substantially damaged when it was involved in an accident near Whidbey Air Park (W10), Langley, Washington. The private pilot and flight instructor were fatally injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

Radar and air traffic control data indicated that, earlier in the day, the airplane flew from Boeing field (BFI), Seattle, Washington, to Bellingham International Airport (BLI) Bellingham, Washington. About 1110, the airplane departed BLI and flew south over Whidbey Island toward BFI. The airplane climbed to about 6,500 ft mean sea level (msl) for about 16 minutes when the airplane’s groundspeed decreased, and the airplane started to descend. Shortly thereafter the flight instructor reported to air traffic control that they were declaring an emergency, and the airplane’s transponder code changed to 7700. He further reported that they had “full power and RPM setting, and about to lose the engine.” Air traffic control told the flight instructor to maintain current heading, but he informed them the airplane was “unable to maintain altitude.” The flight instructor reported they were at “1100 rpm unable to hold altitude with max power, about 75.”

The controller directed the pilot toward W10 and the radar track showed a left turn toward the east then southeast. Shortly thereafter, the flight instructor reported they “had a little bit of power but are still descending at 5,000 [ft].” The controller provided the location of Snohomish County Airport; Paine Field (PAE), which was 12 miles away, and asked if they could make it there; the flight instructor replied, “we could try, the engine does not stay on for very long, we are back at idle.” The controller asked if he had carburetor heat on and he replied “affirmative.” The controller continued to provide positions for PAE and W10.

After temporarily loosing radio contact, the flight instructor reported that they were unable to make PAE with no engine; they are going directly to W10. The airplane crossed over highway 525 and flew over it for about 44 seconds then turned northbound toward W10. The flight instructor stated they were diverting off-airport and the airplane track turned right. However, about 15 seconds later, he reported the airport insight and the airplane’s track changed north toward the airport. There was no further communication from the accident airplane. The last radar point was over the runway at an altitude of about 300 ft.

A witness who landed at W10 about 1 hour and 15 minutes before the accident reported that mist rising from the trees made it harder than normal to find the airport. He was standing at the northwest corner of the airport when he observed the accident airplane about pattern altitude passing from west to east. The accident airplane continued slightly east of W10 before it made a left turn to a northwest heading. It appeared as if the airplane was “porpoising,” continuously nosing up and down. He noted that the propeller was not turning and there were no audible engine sounds. As the airplane passed over the runway the left wing dropped, and the airplane spun out of sight behind the trees and hangars. Shortly thereafter the witness heard the impact.

The airplane’s engine monitoring system revealed the carburetor temperature had been steadily decreasing from 31°F during the accident flight. When the carburetor temperature reached 17°F there was an abrupt decrease in fuel flow and a corresponding reduction in exhaust gas temperatures. For the remainder of the flight the exhaust and cylinder head temperatures slowly decreased, and the carburetor temperature slowly increased. The fuel flow spiked several times, but immediately decreased.

The nearest weather reporting station was about 9 nautical miles southeast of the accident site. At 1153, the temperature was reported as 6°C and dewpoint, 2°C. When plotted on the Federal Aviation Administration (FAA) carburetor ice chart, this temperature dewpoint spread is consistent with serious icing at cruise power.

FAA Special Airworthiness Information Bulletin (CE-09-35) – Carburetor Icing Prevention, stated that:

…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 airplane came to rest about 153 ft west of the runway surface against trees that were about 65 ft tall. The bark was scraped off one side of a tree about mid height extending down to the airplane, which came to rest nose low at the base of the tree. Both the left and right wings remained attached to the fuselage and their leading edges exhibited aft crush damage. A tree shaped indent was also noted along the top of the right wing. Both the left and right fuel tanks were breached. The cabin area sustained extensive impact damage. The empennage remained intact but exhibited impact related damage.

During a postaccident examination, flight control continuity was established throughout the airframe. The fuel strainer bowl exhibited some minor debris, and the screen was clear. The airframe fuel pump was functionally tested with no anomalies were noted. The fuel flow transducer was removed and clear of debris. Compressed air was applied to each fuel line and vent lines; all were clear of debris or blockages. The wing fuel tank finger screens were examined and clear of debris.

The engine did not exhibit any visual indications of catastrophic failure. The engine remained attached to the firewall. Both the induction and exhaust systems sustained crush damage, consistent with impact. The exhaust was removed and there was no evidence of any internal damage or obstructions. The propeller, upper spark plugs, and fuel pump were removed. The crankshaft was rotated using a hand tool attached to an accessory mounting pad. Thumb compression was obtained on all cylinders. Movement of all intake and exhaust rocker arms was observed and no abnormal coloration was noted. The single-drive dual magneto was removed from the engine and rotated by hand; spark was obtained on all eight ignition leads. The ignition harness sustained impact damage and was unable to be functionally tested.

The lower spark plugs were removed. All eight spark plugs exhibited ‘severe worn-out’ signatures. All four cylinders were borescoped and all exhibited normal operating signatures. The carburetor sustained impact damage. The fuel inlet screen was clear of debris. The throttle remained secured to the throttle arm, and it moved freely from stop to stop. The mixture arm was separated from the mixture control shaft. The mixture control shaft was bent, consistent with impact damage. The carburetor was disassembled; the internal floats and needle valve were intact and undamaged. The fuel pump, and propeller governor were disassembled; no anomalies were noted. One propeller blade remained attached to the propeller hub and was mostly straight. The second blade was partially separated and bent aft at the root; it exhibited various striations on the chamber side of the blade.

The single-drive dual magneto was removed from the engine, and it functionally tested normal. The throttle cable was removed from the airplane and sent to the National Transportation Safety Board laboratory. The cable fracture features of the sheath and strands were consistent with overstress fracture. No other features consistent with other fracture modes were observed. All other features detailed were consistent with post-fracture damage.

The Island County Corner’s Officer located in Coupeville, Washington, performed an autopsy of the flight instructor. The flight instructor’s cause of death was multiple traumatic injuries. The Federal Aviation Administration (FAA) Bioaeronautical Sciences Research Laboratory, Oklahoma City, Oklahoma, performed forensic toxicology on specimens from the flight instructor with positive results for azacyclonol and fexofenadine, both of which are approved for flight per the FAA.

The Island County Corner’s Officer located in Coupeville, Washington, performed an autopsy of the pilot. The pilot’s cause of death was multiple traumatic injuries. Toxicology on specimens from the pilot were positive for Irbesartan, which his approved for flight per the FAA.