Rough Running
Carburetor ice over Clearwater — a partial power loss, a bad neighborhood, and the clock running out
The scenario
Field: Clearwater Air Park (KCLW), Clearwater, FL — elevation 71 ft MSL, single runway 16/34, 4,108 ft of asphalt. You departed Runway 34 thirty minutes ago on a local afternoon flight and are now maneuvering in the practice area roughly 5 miles northeast of the field at 2,500 ft MSL.
Aircraft: Cessna 172N, N-number yours, solo, fuel selector on BOTH, within all limits. Lycoming O-320, carbureted. Steam panel, vacuum-driven attitude and heading indicators.
Weather: KCLW ASOS reported 30 minutes ago — OVC025, temperature 26°C, dewpoint 23°C, winds 340° at 8 knots. A thin overcast has lowered since departure; you're in and out of light drizzle. The spread between temperature and dewpoint is 3°C. You did not apply carburetor heat during the climb or since leveling off.
Pilot: you — a Private pilot, 180 hours total, 60 in type. You've flown out of KCLW a dozen times. The engine has been running smoothly. Until right now.
The scenario begins: the engine begins to run rough. RPM drops from 2,350 to 2,100 and continues to decay. The airplane is 5 miles northeast of KCLW at 2,500 ft MSL, heading 270°. You are not over any open water. The terrain below and in every direction toward the field is dense suburban development — the same dense development that lines both ends of Runway 16 and Runway 34 at KCLW.
- {'label': 'Field', 'value': 'KCLW · Clearwater Air Park'}
- {'label': 'Runways', 'value': '16/34'}
- {'label': 'Elevation', 'value': '71 ft'}
- {'label': 'Aircraft', 'value': 'C172N'}
- {'label': 'Dominant phase', 'value': 'Landing / Approach'}
The decision
Before the scenario runs — which of these are already in your head? (Pick all that apply; this records your baseline.)
What the record shows
What the NTSB files show
NTSB CEN24LA362 (2024): A Cessna 172N on a local personal flight encountered light rain and carburetor ice at 1,800 ft AGL. The pilot experienced engine roughness and power loss. The probable cause was carburetor ice formation in conditions conducive to serious icing, with insufficient time and altitude for the carburetor heat to clear the accumulated ice load. This event occurred at another location — not at KCLW.
NTSB CEN14LA276 (2014): A Cessna 172N on a cross-country flight experienced engine roughness and power loss at cruise altitude in conditions conducive to carburetor icing. The pilot made a forced landing on an island where the aircraft nosed over in soft sand. The probable cause could not be determined due to premature aircraft release, but the conditions were consistent with carburetor ice. This event occurred at another location.
The pattern across C172N carburetor-ice events is consistent: the Lycoming O-320 is a carbureted engine with a well-documented susceptibility to carburetor ice in conditions that VFR pilots routinely fly — temperatures between 0°C and 30°C with high relative humidity, visible moisture, or even clear air with a narrow temperature/dewpoint spread. The FAA carburetor icing probability chart shows 'serious icing at glide power' is possible at 26°C OAT with a 3°C spread.
The failure mode is insidious: the ice builds gradually, the roughness is easy to rationalize, and by the time the pilot acts, the ice load may be too heavy for the carb heat to clear before altitude runs out.
At Clearwater Air Park specifically, the forced-landing environment is uniformly poor. USGS land-cover data shows dense development off both ends of Runway 16/34 and throughout the surrounding area. There are no open fields, no parks large enough for a safe off-airport landing, and no water within glide range of the practice area. This makes early recognition and early carb heat application — before the ice load becomes unmanageable — the only reliable defense. Once the engine quits near KCLW, the outcome depends entirely on how much altitude you have left and how well you fly 65 KIAS to the best available surface.
Key lesson — Carburetor ice in the C172N's O-320 builds silently in conditions VFR pilots fly routinely. The correct response to engine roughness in a carbureted airplane is carburetor heat FULL ON immediately — expect a brief further RPM drop as hot air hits the ice, then recovery. Applying it late, or not at all, converts a manageable nuisance into an engine-out forced landing over terrain that, at KCLW, offers no good options.
Debrief — teaching points
Know the carb-ice envelope — and the O-320 lives in it.
The FAA carburetor icing probability chart shows serious icing at glide power is possible from roughly 0°C to 30°C OAT with relative humidity above 50%. At 26°C OAT and a 3°C temperature/dewpoint spread — a routine Florida afternoon — the C172N's O-320 is squarely in the serious-icing zone. This is not an edge case. Preflight weather review should include a deliberate carb-ice assessment, not just ceiling and visibility.
Carb heat is the first response to engine roughness — not the last.
In a carbureted engine, unexplained roughness or RPM decay means carburetor heat FULL ON, immediately. The correct sequence: carb heat on → expect a brief further RPM drop (hot air melting ice through the venturi) → wait for recovery → if no recovery in 30–60 seconds with heat on, the problem may be something else. Reaching for mixture, throttle, or 'wait and see' before carb heat wastes the only resource you have — altitude — and allows the ice to accumulate further.
Carb heat timing is everything — late application may not work.
CEN24LA362 is explicit: 'insufficient time and altitude for the carburetor heat to clear the accumulated ice.' Carb heat melts ice by raising the temperature of the induction air — but a large ice mass requires sustained heat to melt. If you apply carb heat at the first hint of roughness, the ice load is small and recovery is fast. If you wait until RPM has decayed to 1,600 and the engine is shaking, the ice load may be too heavy to clear before you run out of altitude. Apply it early.
Best glide is 65 KIAS — fly it precisely, especially over bad terrain.
In the C172N, best glide is 65 KIAS at gross weight. This is not a target range — it is a specific number that maximizes glide distance. Flying above 65 KIAS increases drag and shortens the glide. Flying below 65 KIAS increases the descent rate and approaches the 48 KIAS clean stall speed. Over the suburban development surrounding KCLW, where every foot of altitude translates directly into landing options, precise airspeed control at 65 KIAS is the difference between reaching a road and stalling into a neighborhood.
At KCLW, the forced-landing environment is uniformly poor — altitude is your margin.
Both ends of KCLW's Runway 16/34 are bordered by dense development. The surrounding area is suburban Clearwater — residential neighborhoods, commercial strips, and arterial roads. There are no open fields, no parks large enough for a safe off-airport landing, and no water within normal glide range. This means that at KCLW, the only way to avoid a forced landing in a built-up area is to prevent the engine failure in the first place — through proactive carb heat use, early recognition, and immediate response. Once the engine quits, the best available outcome is a controlled, slow, full-flap landing on the best available road surface.
Built from the real accident record
Scenario built from NTSB CEN24LA362 (2024 C172N carburetor ice in light rain, engine roughness and power loss at 1,800 ft AGL) and CEN14LA276 (2014 C172N engine roughness/power loss in carb-ice conditions, forced landing). Localized to Clearwater Air Park (KCLW). Real events occurred at other locations.
NTSB reports: CEN24LA362 · CEN14LA276 · ERA09LA517 · WPR24LA167 · GAA19CA534
ACS tasks: PA.I.F — Weather Information · PA.I.H — Human Factors · PA.IX.C — Emergency Approach and Landing · PA.II.B — Engine Starting / Systems Knowledge · PA.XII.B — Aeromedical Factors / Situational Awareness
Relevant FARs: §91.3 · §91.13 · §91.185
Step through the full decision tree, make the calls, and see where each choice leads — then debrief it with your CFI.
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