The Rough Engine
Carburetor ice over Tampa — a partial power loss and the decisions that follow
The scenario
Field: Tampa Executive Airport (KVDF), Tampa, FL — elevation 22 ft MSL. You departed Runway 05 twenty minutes ago and have been flying a local practice area flight at 3,500 ft MSL. You are now inbound, roughly 8 miles northeast of the field, beginning a gradual descent to pattern altitude.
Aircraft: Cessna 172N, solo, fuel selector on BOTH, within all weight and balance limits. The Lycoming O-320 has been running smoothly — until now.
Weather: A classic Florida summer afternoon. Scattered cumulus at 3,500 ft, tops around 5,000 ft. Visibility 8 miles in haze. Temperature 32°C, dewpoint 26°C — a spread of only 6°C. You flew through a light rain shower about ten minutes ago. The air is saturated.
Pilot: You — a Private pilot with 120 hours, current, comfortable in this airplane. You have been power-back in a gradual descent for the last four minutes, engine at approximately 1,800 RPM.
The situation: The engine begins to run rough. RPM drops noticeably. The tachometer is falling — 1,750 … 1,700 … and still trending down. You are at 2,800 ft MSL, roughly 2,780 ft AGL, 8 miles from KVDF.
- {'label': 'Field', 'value': 'KVDF · Tampa Executive'}
- {'label': 'Runways', 'value': '5/23 · 18/36'}
- {'label': 'Elevation', 'value': '22 ft'}
- {'label': 'Aircraft', 'value': 'C172N'}
- {'label': 'Dominant phase', 'value': 'Landing / Takeoff'}
The decision
Before we branch — which of these are already in your head? (Pick all that apply; this records your mental model going in.)
What the record shows
What the NTSB files show
NTSB CEN24LA362 (2024): A Cessna 172N on a local flight 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 the carburetor heat to clear the accumulated ice once applied. This accident did not occur at KVDF — it is used here because the airplane type, engine, and failure mode are identical.
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. Again — a different location, same airplane, same failure chain.
The pattern across these events is consistent: the Lycoming O-320 in the C172N is highly susceptible to carburetor ice at reduced power settings in humid conditions. The FAA carburetor icing probability chart places a temperature of 32°C and a dewpoint spread of 6°C squarely in the 'serious icing at glide power' zone. Pilots who apply carb heat at the first sign of roughness — or proactively in known icing conditions — survive. Pilots who wait, misdiagnose, or apply it too late face a forced landing.
The off-field environment around KVDF is unforgiving in places. The Runway 36 departure corridor (heading 360°) has open water and medium development to the north — a forced landing off that end is a ditching scenario. The Runway 05 corridor (heading 42°) offers wooded wetland and some pasture. The Runway 23 corridor (heading 222°) offers the best options: pasture/hay and open land. Knowing your field's off-field environment before you need it is not academic — it is the difference between a survivable forced landing and a fatal one.
Key lesson — Carburetor ice in the C172N's Lycoming O-320 is not a rare event — it is a predictable, preventable failure that kills when pilots wait too long to apply carb heat. In high-humidity, visible-moisture conditions at reduced power, carb heat is not optional. Apply it at the first sign of roughness; expect a brief rough patch as ice melts; leave it on. If the engine quits, 65 KIAS buys you the most distance — and knowing what is off each runway end at your home field is the difference between a field landing and a ditching.
Debrief — teaching points
The O-320 ices in conditions you will see every Florida afternoon.
The FAA carburetor icing probability chart is not theoretical. A temperature of 30–35°C with a dewpoint spread of 5–8°C places the Lycoming O-320 at 'serious icing at glide power' — exactly the conditions of a Tampa summer descent. Reduced power settings (descent, approach, pattern) lower the carburetor venturi temperature further, accelerating ice formation. In Florida, this is a year-round threat, not a winter one.
Carb heat is the first response to engine roughness in these conditions — not the last.
When the engine runs rough in high-humidity conditions, the differential diagnosis starts with carburetor ice. Apply carb heat FULL ON immediately. Expect the engine to run rougher briefly — that is partially melted ice passing through the carburetor, and it means the heat is working. If the engine smooths within 30–60 seconds, the diagnosis was correct. If it does not improve or worsens without the expected brief rough patch, you have a different problem. Do not wait, do not cycle, do not lean first.
Leave carb heat on in icing conditions — do not cycle it off.
Once you have cleared ice and confirmed the engine is running smoothly with carb heat on, leave it on for the remainder of the flight if conditions have not changed. Pulling carb heat off to 'check' it in saturated air simply allows ice to re-accumulate. The power reduction from carb heat (typically 100–150 RPM) is acceptable. An iced carburetor is not.
Best glide is 65 KIAS — establish it before you do anything else after engine failure.
If the engine quits, the first action is to establish 65 KIAS best glide. At gross weight, the C172N achieves roughly a 9:1 glide ratio at 65 KIAS. Every knot above or below that speed costs you distance. At 2,300 ft AGL, 65 KIAS gives you approximately 3.5 nm of glide range — enough to reach a field, not enough to reach an airport 7 miles away. Know the number before you need it.
Know your field's off-field environment before you depart.
At KVDF, the off-field options vary dramatically by runway end. Runway 23 (heading 222°) offers the best forced-landing environment: pasture/hay and open land to the southwest. Runway 05 (heading 42°) offers wooded wetland and some pasture to the northeast. Runway 36 (heading 360°) is the most dangerous: open water and medium development to the north means a forced landing off that end is a ditching. Runway 18 (heading 180°) offers low-density development and wooded wetland — marginal. Brief these options before every flight from KVDF, not after the engine quits.
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; this scenario is localized to Tampa Executive Airport (KVDF) for training purposes only.
NTSB reports: CEN24LA362 · CEN14LA276 · ERA09LA517 · GAA17CA105 · ERA17CA149
ACS tasks: PA.I.F — Weather Information · PA.II.A — Preflight Inspection · PA.IX.C — Emergency Approach and Landing · PA.I.H — Human Factors · PA.II.B — Engine Starting / Systems Knowledge
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 KVDF