Rough Running
Carburetor ice over coastal Florida — a slow trap that closes fast
The scenario
Field: Sarasota Bradenton International Airport (KSRQ), elevation 30 ft MSL. You departed Runway 04 twenty minutes ago on a local VFR flight, climbed to 3,500 ft MSL, and are now cruising southbound over the coastal flatlands west of the field. You plan to return and land Runway 22.
Aircraft: Cessna 172N, solo, full fuel, within limits. Carbureted Lycoming O-320, 160 hp. Steam gauges, vacuum-driven. Fixed gear, fixed-pitch prop. Fuel selector on BOTH — confirmed at runup.
Weather: A classic southwest Florida summer afternoon. OAT 28°C at altitude, dew point 22°C — spread of only 6°C. Scattered cumulus at 2,500 ft, bases ragged. Visibility good in haze. Light rain showers in the area; you've been threading between them. The ATIS reports winds 220° at 8 knots. No PIREPs of significance.
Pilot: You — a Private pilot with 120 hours, comfortable in the C172N, current. You flew this same area two weeks ago without incident. Today feels routine.
The trap: Florida's warm, humid air is among the most carburetor-ice-conducive environments in the country. The carb-ice probability chart for these conditions — OAT near 28°C, high humidity, visible moisture — puts you squarely in the 'serious icing at glide power' zone. You have been at reduced power for the last five minutes, descending gently toward the field.
- {'label': 'Field', 'value': 'KSRQ · Sarasota Bradenton'}
- {'label': 'Runways', 'value': '4/22 · 14/32'}
- {'label': 'Elevation', 'value': '30 ft'}
- {'label': 'Aircraft', 'value': 'C172N'}
- {'label': 'Dominant phase', 'value': 'Takeoff / Landing'}
The decision
Before the scenario begins — which of these are actually in your head right now? (Pick all that apply; this records your baseline awareness.)
What the record shows
What the NTSB files show
NTSB CEN24LA362 (2024): A Cessna 172N encountered light rain and carburetor ice at 1,800 ft AGL. Engine roughness and power loss developed in conditions the NTSB described as 'conducive to serious icing.' The pilot had insufficient time and altitude for carburetor heat to clear the accumulated ice before the forced landing. This accident did not occur at KSRQ.
NTSB CEN14LA276 (2014): A Cessna 172N on a personal cross-country experienced engine roughness and power loss at cruise altitude in carburetor-icing-conducive conditions. The pilot made a forced landing on an island; the aircraft nosed over in soft sand. The probable cause could not be determined due to premature aircraft release — but the conditions and presentation are consistent with carburetor ice. This accident did not occur at KSRQ.
The common thread: the Lycoming O-320 in the C172N is a carbureted engine. The carburetor venturi creates a pressure drop that can lower local temperature by 20–70°F — enough to form ice even when the OAT is well above freezing. Florida's warm, humid coastal air is among the most ice-conducive environments in the country. The FAA carburetor icing probability chart places OAT near 28°C with a 6°C spread squarely in the 'serious icing at glide power' zone.
The mechanism is consistent across accidents: pilot reduces power for descent or approach; carburetor icing begins; pilot attributes the RPM drop to turbulence, fuel mixture, or normal variation; by the time the roughness is undeniable, significant ice has accumulated; carb heat applied late may not clear the ice before altitude runs out.
The fix is simple and costs nothing: apply carburetor heat proactively in conducive conditions — before the roughness starts — and routinely during any extended reduced-power operation. When roughness does appear, apply full carb heat immediately and hold it in. Expect a brief worsening as the ice melts and ingests; do not pull the heat off because it 'made it worse.'
Key lesson — Carburetor ice in the C172N forms silently and accumulates fast in Florida's warm, humid air — especially at reduced power. The correct response to any unexplained RPM drop or roughness in these conditions is immediate, full carburetor heat, held in until the engine fully recovers. Proactive application before the roughness starts is better still. Waiting to 'see if it gets worse' is how a manageable situation becomes a forced landing.
Debrief — teaching points
Florida's air is a carburetor-ice factory.
The FAA carburetor icing probability chart is not theoretical — at OAT 28°C with a dew point spread of 6°C and visible moisture, you are in the 'serious icing at glide power' zone. The carburetor venturi drops local temperature 20–70°F through pressure reduction alone; ice forms even when the OAT is well above freezing. Coastal Florida, with its persistent high humidity and frequent visible moisture, is one of the highest-risk environments for carburetor ice in the country. Treat every flight in these conditions as a carb-ice flight.
Apply carb heat before the roughness — not after.
The C172N POH recommends carburetor heat application during extended reduced-power operation and any time icing conditions are suspected. 'Suspected' means the conditions are present — not that the engine is already rough. Applying carb heat proactively during descent, at reduced power, in humid or visible-moisture conditions costs nothing and prevents the accumulation that makes late application ineffective. Make it a habit: power reduction → carb heat on.
Expect roughness to worsen briefly when carb heat is applied — hold it in.
When carburetor heat melts accumulated ice, the resulting water ingests through the engine and causes a brief increase in roughness and a further RPM drop of 20–50 RPM. This is normal and expected. The instinct to pull the heat off because 'it made it worse' is the wrong response — that brief worsening is the ice clearing. Hold full carb heat in until the engine smooths out and RPM climbs back. Pulling the heat prematurely re-exposes the carburetor to icing conditions with the ice only partially cleared.
Know your off-field environment before you need it.
The terrain around KSRQ is not uniform. Off Runway 04 (climb-out heading 38°) the environment is marginal — medium development, wooded wetland, low-density development — with limited forced-landing options. Off Runway 22 (heading 218°) the environment includes open water and low-density development: a power loss on approach to Runway 22 may become a ditching. Knowing this before the emergency means you can make better decisions about which runway to target and how much altitude margin you need. Study the NAIP aerial of your home field.
Best glide is 65 KIAS — establish it immediately and know what it buys you.
In any partial or total power loss, the first aerodynamic action is to establish best glide: 65 KIAS in the C172N at gross weight. This maximizes your glide ratio and the distance available to reach a landing surface. At 65 KIAS the C172N glides approximately 1.5 nm per 1,000 ft of altitude (in still air). At 2,200 ft MSL (roughly 2,170 ft AGL) over flat coastal Florida, you have approximately 3.2 nm of glide range — enough to reach KSRQ from 5 miles only if the engine is producing some power. Know the math before you need it.
Built from the real accident record
Scenario built from NTSB CEN24LA362 (2024), CEN14LA276 (2014), and ERA09LA517 (2009) — all Cessna 172N carburetor-ice events. Real accidents occurred at other locations; see outcome_reveal.
NTSB reports: CEN24LA362 · CEN14LA276 · ERA09LA517 · GAA17CA105 · ERA17CA149
ACS tasks: PA.I.C — Weather Information · PA.I.F — Performance and Limitations · PA.IX.C — Emergency Approach and Landing · PA.I.H — Human Factors · PA.II.A — Preflight Inspection
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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