Engine Roughness on Base — Venice Municipal

Departing Venice Municipal Airport (KVNC), Venice, FL — Runway 22, a local instructional flight in a Cessna 172M. Elevation 18 ft MSL; the field sits on the Gulf Coast. You are a Private pilot with roughly 180 hours total time, current and proficient. This is your third flight in the 172M; you have been flying a 172N at your home field and are getting familiar with the older, lower-powered 150 hp Lycoming O-320 in the M-model.

It is a warm, humid Florida afternoon in late spring: OAT 24°C, dew point 18°C, altimeter 29.94. Scattered clouds at 2,800 ft, light rain shower two miles to the south. Visibility 10 SM. Classic Gulf Coast conditions — warm, moist, and exactly the environment the FAA icing probability chart marks as 'serious icing at glide power, moderate icing at cruise power.' You did not apply carburetor heat during the run-up because the engine ran smoothly. You did not apply it after takeoff because the climb was normal and you were heads-down on the instruments.

You have completed a local flight, descended into the pattern for Runway 22 (heading 225°), and are now on base leg at 800 ft AGL, 1.2 nm from the runway. Airspeed is 70 KIAS, descent rate 300 fpm. The engine is running smoothly. The runway is in sight. You are about to turn final. The 172M is a fixed-gear, fixed-pitch airplane — no gear to raise, no prop to manage. Fuel selector is on BOTH. You have roughly 15 gallons remaining.

Then, at 750 ft AGL on base, the engine begins to run rough. The tachometer drops 100 RPM. The descent rate increases slightly. You are still well above the runway, but the power loss is unmistakable. The off-field environment off Runway 22's approach end (heading 225°) is open water — the Gulf of Mexico. The off-field environment off Runway 04's approach end (heading 45°) is open water as well. The only dry land off-field options are to the north and east — dense residential and commercial development. KVNC is non-towered; you are on CTAF 122.8.

Aircraft: Cessna 172M, solo, 15 gallons fuel, within limits. Carbureted Lycoming O-320-E2D, 150 hp, fixed-pitch prop, steam panel (vacuum-driven attitude and heading indicators), fuel selector BOTH. Nothing was written up; the airplane was airworthy at departure.

Pilot: you — a Private pilot, current, 180 hours total, familiar with the 172N but new to the 172M's lower power and marginal climb performance. You are on base leg at 750 ft AGL, 1.2 nm from the runway, with a rough engine and dropping power.

NTSB ERA09LA379 (2009): A Cessna 172M student pilot on a solo instructional flight experienced engine power loss during the base-to-final turn in the traffic pattern. The ambient conditions (75°F OAT, 55°F dew point) were conducive to serious icing per the FAA icing probability chart. The pilot made a forced landing in a field. The probable cause was carburetor icing at glide power, with the engine power loss occurring at exactly the altitude and phase of flight where recovery options are most limited.

NTSB DFW05CA237 (2005): A Cessna 172M lost engine power during initial climb due to carburetor icing in serious icing conditions (high density altitude, warm moist air). The pilot attempted a go-around and stalled while maneuvering to avoid a fence. The probable cause was the pilot's failure to maintain airspeed during the recovery attempt, with contributing factors including the loss of engine power and high density altitude. The 172M's marginal climb performance meant that a go-around with partial power was not viable.

NTSB CEN24LA168 (2024): A Cessna 172M on an IFR flight experienced engine power loss due to carburetor icing during descent in night IMC. The pilot touched down on a building roof and impacted a retaining wall and ground. The probable cause was the pilot's delayed use of carburetor heat, which resulted in ice accumulation beyond the point where heat could restore full engine power. The delay was fatal.

NTSB CEN22LA181 (2022): A Cessna 172M on a personal flight experienced partial engine power loss during a go-around attempt from a low approach to an upsloping turf runway. The probable cause was the pilot's failure to use carburetor heat during the approach and an unsuitable flight profile for the runway configuration. The 172M's 150 hp was insufficient to climb out of the situation.

The real accidents cited above occurred at other airports and in other aircraft types — NOT at Venice Municipal Airport. KVNC has its own accident history (see field dominant patterns: LOSS_OF_CONTROL_INFLIGHT 24.4%, FORCED_LANDING 12.2%, SPATIAL_DISORIENTATION 12.2%). The scenario is localized to KVNC to make the off-field environment real and consequential for you as a student here: Runway 22's approach end is open water — the Gulf of Mexico. An engine-out approach there is a ditching, not a field landing.

The consistent thread across all these events: carburetor ice in the C172M is insidious. It builds gradually, the first symptom is roughness and a dropping tachometer (not a dramatic power cut), and by the time it is obvious, it may be too late for a comfortable recovery. The fix — full carburetor heat, immediately, at the first sign of roughness in conducive conditions — is simple. The failure is always a delay. The 172M's 150 hp means you cannot afford to waste time diagnosing on approach.