By Robert Hedrick and Kerry Kovarik
As we predicted shortly after this accident, the NTSB preliminary report supports our suspicion that an engine issue was in fact involved in this accident. The planned flight route for the Cessna 421, N245T, a light twin-engine aircraft, was from Longmont, Colorado to Ogden, Utah. When near Yampa, Colorado the pilot reported a cylinder temperature issue with one of the engines, and also requested to divert to Steamboat Springs Airport (SBS). After the airplane flew over that airport, the pilot changed his mind and requested to divert to FNL – Fort Collins/Loveland airport. En route to FNL the pilot reported that the left engine had just failed, and so air traffic control cleared the pilot to SBS for landing. SBS itself does not have a control tower.
The pilot apparently first planned to land on Runway 16 at SBS and reported to area traffic that he was on a 4-mile final approach to that runway. Shortly after, the pilot reported that he was on the base leg for runway 32 (which is the same runway as runway 16 but landing in the opposite direction). It was on this base leg that things went wrong.
The airplane was observed entering into a flat spiraling descent until it struck the ground, and the post-impact fire broke out. The accident occurred 0.4 mile from the approach line of runway 32. On inspection, the propellor blades of the left engine were found to be in the feathered position. This means that the pitch of the propellors for that engine were turned leading edge into the wind, likely by the pilot, which is designed to minimize adverse drag from windmilling propellors after an engine failure. Feathering usually stops the propellor from turning. Pilots typically feather an engine when the engine either fails or is having problems and the pilot shuts it down.
Cylinder head temperatures on air-cooled aircraft engines are typically measured by a thermocouple probe inserted underneath one or more of the engine’s spark plugs. The signal is then converted to a temperature which is indicated on a gauge on the aircraft’s instrument panel. On engines that use only a single probe, the hottest cylinder is usually chosen, though many aircraft are equipped with aftermarket gauges that indicate the individual temperatures of each of the engine’s cylinders. Cylinder head temperature is used by pilots to adjust the fuel/air mixture for best power and economy, to ensure smooth running at altitude, and to monitor the health of the engine.
High cylinder head temperature indications could indicate a number of issues from an excessively lean air/fuel mixture, engine detonation/pre-ignition, spark timing issues, valve issues, disruption in airflow over the engine, broken piston rings, a seizing piston, a cracked cylinder head, or a cracked exhaust riser resulting in an exhaust leak. While all of these problems could result in significant engine damage if not addressed, the complete power-loss suffered by the accident aircraft shortly after the rise in cylinder head temperature suggests that the aircraft had suffered a significant mechanical failure.
At this point, it is unclear if the high cylinder temperature indication was accompanied by some other indication, such as high oil temperature, but assuming it was not, it is likely that the NTSB will focus on determining the condition of the engine’s fuel delivery system, pistons, rings, valves, magnetos, cylinder heads, cylinder barrels, and exhaust.
Total power loss would have been likely caused by either seizure of the engine, some kind of mechanical interference, or significant disruption of combustion timing or fuel flow.
ALG attorneys are continuing to monitor developments related to this most unfortunate accident.
The full text of NTSB Preliminary Report is found below: