Engine Roughness Over Tsala Apopka
Carburetor ice in a marginal-climb C172M, high density altitude, and a low-altitude decision window — the 150-hp Lycoming does not forgive delay
The scenario
Departing Brooksville–Tampa Bay Regional Airport (KBKV), Brooksville, FL — Runway 09, climbing out on a 090° heading over open pasture and low-density development. Elevation 76 ft MSL; the field sits on the western shore of the Tsala Apopka lake chain.
It is a warm, humid Florida afternoon in late May: OAT 32°C (90°F), dew point 21°C (70°F), altimeter 29.88. Scattered clouds at 3,000 ft, light rain showers visible to the northeast. Visibility 7 SM in haze. Density altitude is approximately 2,800 ft — well above field elevation. The FAA icing probability chart marks these conditions as 'serious icing at glide power, moderate icing at cruise power.'
You are 500 ft AGL, climbing through 78 KIAS (Vy, best rate of climb), heading 090°, when the engine begins to run rough. Power is noticeably down — the tachometer is dropping. The climb has slowed. KBKV's tower is part-time (0700–2200 local) and is open; you are in Class D airspace.
Aircraft: Cessna 172M, solo, full fuel, within limits. Carbureted Lycoming O-320-E2D, 150 hp, fixed-pitch prop, steam panel, fuel selector on BOTH. Nothing was written up; the airplane was airworthy at departure. The 172M is the lower-powered variant — climb performance is marginal, especially at high density altitude.
Pilot: you — a Private pilot, current, roughly 180 hours total. You did not apply carburetor heat during the run-up because the engine ran smoothly. You did not apply it after takeoff because you were focused on the climb and did not anticipate icing in warm air.
- {'label': 'Field', 'value': 'KBKV · Brooksville–Tampa Bay'}
- {'label': 'Runways', 'value': '3/21 · 9/27'}
- {'label': 'Elevation', 'value': '76 ft'}
- {'label': 'Aircraft', 'value': 'C172M'}
- {'label': 'Dominant phase', 'value': 'Landing / Cruise'}
The decision
Before we get into the decision tree — what do you already know about carburetor ice in the C172M? (Pick all that apply; this records your baseline.)
What the record shows
What the NTSB files show
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. Ambient conditions were 75°F OAT and 55°F dew point — well above freezing but squarely in the FAA icing probability chart's 'serious icing at glide power' zone. The pilot made a forced landing in a field. The probable cause was carburetor icing at glide power.
NTSB DFW05CA237 (2005): A Cessna 172M lost engine power during initial climb due to carburetor icing and made a forced landing in a field. The pilot stalled while maneuvering to avoid a fence — trying to stretch the glide to the runway instead of committing to the field ahead. The accident resulted from engine power loss due to carburetor icing in serious icing conditions, with contributing factors including high density altitude. The pilot did not survive.
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.
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 accident resulted from the pilot's failure to use carburetor heat during the approach and an unsuitable flight profile for the runway configuration. The pilot's failure to use carb heat during the approach meant the ice was already present when the go-around was attempted.
The real accidents cited above occurred at other airports and in other circumstances — NOT at Brooksville–Tampa Bay Regional Airport. KBKV has its own accident history (see field dominant patterns: hard landings, forced landings, runway excursions), but these specific carburetor-icing events happened elsewhere. The scenario is localized to KBKV to make the off-field environment real and consequential for you as a student here.
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 C172M's 150 hp engine is marginal on climb, especially at high density altitude; any power loss is consequential.
Key lesson — In warm, moist Florida air, the C172M's carbureted O-320-E2D can accumulate serious carburetor ice even at cruise power and above-freezing temperatures. Apply full carburetor heat at the first sign of engine roughness or unexplained RPM loss. At low altitude on departure, the decision window is measured in seconds — not minutes. The C172M's marginal climb performance at high density altitude means you cannot afford to lose power twice. Off Runway 09 at KBKV, the off-field environment is open pasture and low-density development — a forced landing is possible if needed, but prevention through early carb heat is the only acceptable strategy.
Debrief — teaching points
Carburetor ice forms in conditions you would not expect.
The FAA icing probability chart shows 'serious icing at glide power' at temperatures between roughly 15°C and 27°C when relative humidity is high — exactly the warm, moist Florida afternoon conditions at KBKV. You do not need visible ice, freezing temperatures, or IMC. Warm, moist air at reduced power is the classic carb-ice environment. The C172M's Lycoming O-320-E2D is carbureted; it has no alternate air system. Carburetor heat is the only tool. The 150 hp engine's marginal climb at high density altitude makes any power loss immediately consequential.
The first symptom is subtle — a dropping tachometer and engine roughness.
In a fixed-pitch airplane like the C172M, carburetor ice first shows as engine roughness and an unexplained RPM decrease. There is no dramatic power cut. Pilots who are not actively monitoring the tachometer miss the early warning. By the time the roughness is obvious, significant ice has accumulated. Scan the tachometer as part of your regular instrument scan, especially in conducive conditions. At 500 ft AGL on departure, you have only 45–60 seconds of useful decision time before altitude becomes marginal.
Apply full carburetor heat — not partial — and expect an initial RPM drop.
When you apply carb heat to an iced carburetor, the RPM will drop further before it rises. This is expected and normal: the heat is melting ice and the resulting water is briefly disrupting combustion. Do not remove carb heat when the RPM drops — that is the heat working. Hold it full on. The RPM will recover as the ice clears, typically within 15–30 seconds depending on ice accumulation. Partial carb heat can worsen the situation by partially melting ice into water ingestion without fully clearing the restriction.
At KBKV Runway 09, an engine failure on departure is a forced landing in open pasture — not a ditching.
The off-field environment off Runway 09's departure end (heading 090°) is mostly open developed land (parks, large lots) and pasture — survivable terrain for a forced landing. This is not a worst-case scenario; it is the geographic reality. Best glide is 65 KIAS. Flaps for slowest possible touchdown speed — impact energy rises with the square of touchdown speed, so the slowest possible speed matters most. Know this before you line up on Runway 09. The C172M's fixed gear and fixed prop mean no extension complications — focus is entirely on speed management and landing distance.
The C172M's 150 hp engine is marginal on climb, especially at high density altitude.
The C172M is the lower-powered variant of the 172 family. At high density altitude (KBKV's density altitude was ~2,800 ft on the day of this scenario), climb performance is marginal — you cannot afford to lose power and regain it twice. Prevention through early carburetor heat application is the only acceptable strategy. If you lose power once and recover it, a precautionary landing is the correct next step, not a continuation of the flight in the same conditions.
Proactive carb heat use in conducive conditions is not optional.
The C172M POH and the FAA Pilot's Handbook of Aeronautical Knowledge both recommend applying carburetor heat when conditions are conducive to icing — before the symptom appears. In a Florida summer departure, with OAT near 30°C and dew point near 20°C, that means applying carb heat during the run-up check (and confirming the expected RPM drop, then recovery) and considering its use during climb in visible moisture or high humidity. Waiting for the roughness to appear at 500 ft AGL is waiting too long. The C172M's marginal climb means the decision window is short.
Built from the real accident record
Scenario built from NTSB ERA09LA379 (2009 C172M carburetor ice / forced landing), DFW05CA237 (2005 C172M carb ice + high density altitude / stall), CEN24LA168 (2024 C172M delayed carb heat / power loss), and CEN22LA181 (2022 C172M carb ice / go-around failure). Anonymized and localized to KBKV.
NTSB reports: ERA09LA379 · DFW05CA237 · CEN24LA168 · CEN22LA181
ACS tasks: PA.I.F — Weather Information · PA.I.G — Cross-Country Flight Planning · PA.IX.C — Emergency Approach and Landing · PA.I.H — Human Factors · PA.II.B — Engine Starting / Systems Preflight
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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