The Rough Engine
Carburetor ice over central Florida — and the decisions that follow
The scenario
Field: Lakeland Linder International Airport (KLAL), Lakeland, FL — elevation 142 ft MSL, Class D airspace, tower in operation. You departed Runway 28 twenty minutes ago on a local proficiency flight and are now returning, planning to land Runway 28.
Aircraft: Cessna 172N, solo, fuel selector on BOTH, within weight and balance limits. Carbureted Lycoming O-320, 160 hp. Steam/vacuum panel. Fixed gear, fixed-pitch prop.
Weather: A classic central Florida summer afternoon — surface temperature 88°F, dewpoint 74°F, scattered cumulus building to the east, light and variable wind. You've been cruising at 2,500 ft MSL at 2,300 RPM. No precipitation, but the air is thick with moisture. You have not applied carburetor heat since run-up.
Pilot: You — a Private pilot with 180 hours, current, comfortable in the local area. KLAL's 8,500-ft Runway 10/28 is your home runway. The flight has been uneventful, and you're already thinking about the post-flight debrief.
The situation: Twelve miles east of KLAL at 2,500 ft MSL, the engine begins to run rough. RPM drops noticeably — maybe 100–150 RPM — and you feel a slight vibration through the airframe. No warning lights. The engine is still producing power, but something is clearly wrong.
- {'label': 'Field', 'value': 'KLAL · Lakeland Linder'}
- {'label': 'Runways', 'value': '5/23 · 10/28'}
- {'label': 'Elevation', 'value': '142 ft'}
- {'label': 'Aircraft', 'value': 'C172N'}
- {'label': 'Dominant phase', 'value': 'Landing / Takeoff'}
The decision
Before you act — which of these are in your head right now? (Pick all that apply; this records your initial mental model.)
What the record shows
What the NTSB files show
NTSB CEN24LA362 (2024) involved a Cessna 172N on a local personal flight that encountered light rain and carburetor ice at 1,800 ft AGL. The engine ran rough and lost power. The probable cause was carburetor ice formation in conditions conducive to serious icing, with insufficient time and altitude for carburetor heat to clear the accumulated ice. That accident did not occur at KLAL — it is used here because the airplane, the engine, and the failure mode are identical to what you just flew.
NTSB CEN14LA276 (2014) involved another Cessna 172N on a cross-country flight that experienced engine roughness and power loss at cruise in carburetor-icing-conducive conditions. The pilot made a forced landing on an island where the aircraft nosed over in soft sand. The lesson: the forced landing environment matters enormously. Soft, uneven, or obstructed surfaces can destroy an otherwise survivable approach.
Central Florida's combination of high surface temperatures and extreme dewpoints — common from May through October — creates conditions that the FAA carburetor-icing chart places squarely in the 'serious icing at glide power' and 'serious icing at cruise power' zones. The Lycoming O-320 in the C172N is carbureted; it has no alternate air, no fuel injection, no boost pump. Carburetor heat is the only tool.
The dominant accident pattern at KLAL itself includes forced landings (17.2% of the local corpus) and loss of control in flight (23.7%). Engine-roughness events that are not addressed promptly become forced-landing events; forced-landing events that are not managed with the right speed and site selection become loss-of-control events on short final.
The correct response to engine roughness in the C172N in humid Florida conditions is immediate, full carburetor heat — before the situation degrades. The transient worsening as ice melts is expected and must be tolerated. Removing carb heat after recovery, in unchanged conditions, guarantees a second event with less altitude and less margin.
Key lesson — In the carbureted C172N, engine roughness in high-humidity Florida conditions means carburetor ice until proven otherwise. Apply full carb heat immediately at the first symptom — not partial, not delayed, not after trying other things. The transient roughness as ice melts is normal; hold it. Remove carb heat only when conditions change. Every minute of delay is altitude and options you will not get back.
Debrief — teaching points
Carburetor ice does not require visible moisture or cold temperatures.
The FAA carburetor-icing probability chart shows that serious icing at cruise power can occur at outside air temperatures between approximately 20°F and 90°F with relative humidity above 50%. Central Florida in summer — OAT in the 80s, dewpoints in the 70s — sits squarely in the serious-icing zone. The Lycoming O-320 in the C172N is carbureted; it is vulnerable every flight in these conditions. Carburetor heat is not an emergency-only tool — it is a routine power-management tool in Florida summer flying.
Full carb heat, immediately, at the first symptom.
Partial carburetor heat is worse than no carb heat in some conditions — it raises the induction air temperature into the most ice-prone range without fully melting accumulated ice. The correct technique is FULL HOT, applied immediately at the first sign of roughness or unexplained RPM loss. Expect the engine to run rougher briefly as ice melts and passes through — this is normal and expected. Hold full carb heat until the engine smooths and RPM recovers. Do not remove it until you are clear of icing conditions.
Best glide is 65 KIAS — establish it immediately if power loss continues.
If carburetor heat does not resolve the roughness within 30–45 seconds, or if power loss continues to worsen, establish 65 KIAS best glide immediately. At gross weight, the C172N glides approximately 1.5 nm per 1,000 ft AGL at 65 KIAS. From 2,500 ft MSL over flat central Florida terrain (~2,358 ft AGL), that gives you roughly 3.5 nm of glide range — enough to reach KLAL from close in, but not from 10+ nm away. Know your numbers before you need them.
The off-field environment around KLAL is not uniform — know which direction gives you options.
East of KLAL (the Runway 05/10 side) the ground cover is low-density development, wooded wetland, and open developed areas — parks and large lots. This is a workable forced-landing environment. West of KLAL (off Runway 28's departure end, 270°) the terrain is rated poor — medium development and evergreen forest. If you are inbound from the east with a sick engine, you have better options behind you than ahead of you. Situational awareness of the off-field environment is part of every flight, not just emergencies.
Full flaps on forced-landing short final minimizes touchdown speed — and impact energy rises with the square of speed.
In a forced off-airport landing, the dominant value of full flaps (30°) is the slowest possible touchdown speed. Vs0 in the C172N is 40 KIAS. Impact energy rises with the square of touchdown speed — touching down at 50 KIAS carries 56% more kinetic energy than touching down at 40 KIAS. Every knot you shed on short final is a direct reduction in injury risk. Configure early, fly a stable approach, and touch down as slowly as the airplane allows. The steeper approach path is a secondary benefit.
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 airports; this scenario is localized to KLAL for training purposes.
NTSB reports: CEN24LA362 · CEN14LA276 · ERA09LA517 · GAA17CA105 · ERA21LA119
ACS tasks: PA.I.F — Weather Information · PA.II.B — Engine Starting / Systems Preflight · PA.IX.C — Emergency Approach and Landing · PA.I.H — Human Factors
Step through the full decision tree, make the calls, and see where each choice leads — then debrief it with your CFI.
Open the interactive scenario →All sample scenarios · More Cessna 172N scenarios · More scenarios at KLAL