Engine Roughness Over Tampa Bay
Carburetor ice, partial power loss, and a non-towered field surrounded by water — the decision window is measured in seconds
The scenario
Departing Peter O Knight Airport (KTPF), Tampa, FL — Runway 22, climbing out on a 217° heading over open water. Elevation 8 ft MSL; the runway is essentially at sea level. KTPF is non-towered (CTAF 122.8); you are in Class G airspace, but the overlying Tampa Class B (1,200 MSL ceiling) is 4.7 nm away. You are well below that floor on departure.
It is a warm, humid Florida afternoon in late spring: OAT 26°C, dew point 20°C, altimeter 29.94. Scattered clouds at 2,800 ft, light rain shower two miles to the east. Visibility 9 SM. Classic Gulf Coast conditions — warm, moist, and exactly the environment the FAA icing probability chart marks as 'serious icing at glide power, moderate icing at cruise power.'
You are 350 ft AGL, climbing through 78 KIAS (Vy), heading 217°, when the engine begins to run rough. Power is noticeably down — the tachometer is unwinding. The water of Tampa Bay fills the windscreen ahead and below. You are alone in the airplane (solo), full fuel, within limits. The Cessna 172M's 150 hp Lycoming O-320 is carbureted; there is no fuel injection or alternate air system.
Aircraft: Cessna 172M, solo, full fuel, within limits. Carbureted Lycoming O-320-E2D, fixed-pitch prop, steam panel (vacuum-driven attitude and heading), fuel selector on BOTH. Nothing was written up; the airplane was airworthy at departure.
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 the water below.
- {'label': 'Field', 'value': 'KTPF · Peter O Knight'}
- {'label': 'Runways', 'value': '4/22 · 18/36'}
- {'label': 'Elevation', 'value': '8 ft'}
- {'label': 'Aircraft', 'value': 'C172M'}
- {'label': 'Dominant phase', 'value': 'Landing / Approach'}
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. The ambient conditions were 75°F OAT with a 55°F dew point — classic carburetor icing conditions per the FAA icing probability chart (serious icing at glide power). The pilot made a forced landing in a field and survived. 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 and was fatally injured. Contributing factors included high density altitude and the pilot's failure to maintain airspeed. The probable cause was the pilot's failure to maintain airspeed, which resulted in an inadvertent stall; the engine power loss due to carburetor icing was a contributing factor.
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 probable cause was the pilot's failure to use carburetor heat during the approach and his unsuitable flight profile for an upsloping runway.
The real accidents cited above occurred at other airports and in other aircraft types — NOT at Peter O Knight Airport. KTPF has its own accident history (see field dominant patterns: forced landing 19.4%, loss of control 16.7%, ditching 11.1%), but these specific events happened elsewhere. The scenario is localized to KTPF to make the off-field environment real and consequential for you as a student here: Runway 22's departure end is open water — Tampa Bay. An engine failure on the Runway 22 departure at low altitude is a ditching, not a field landing.
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.
Key lesson — In warm, moist Gulf Coast air, the C172M's carbureted O-320 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 over water, the decision window is measured in seconds — not minutes. Off Runway 22 at KTPF, the off-field environment is Tampa Bay: a delayed response means a ditching, not a field landing.
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 20°C and 30°C when relative humidity is high — exactly the Gulf Coast afternoon conditions at KTPF. 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 is carbureted; it has no fuel injection or alternate air system. Carburetor heat is the only tool.
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.
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 KTPF Runway 22, an engine failure on departure is a ditching.
The off-field environment off Runway 22's departure end (heading 217°) is open water — Tampa Bay. There is no alternate landing surface. If the engine quits on the Runway 22 departure and altitude is insufficient to return to the airport, the outcome is a ditching. This is not a worst-case scenario; it is the geographic reality. Best glide is 65 KIAS. Doors unlatched before water contact. Master off just before impact. 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 22.
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 Gulf Coast summer departure, with OAT near 26°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 350 ft AGL over Tampa Bay is waiting too long.
Built from the real accident record
Scenario built from NTSB ERA09LA379 (2009 C172M carburetor ice on base-to-final), DFW05CA237 (2005 C172M carb ice during initial climb, high DA), CEN24LA168 (2024 C172M delayed carb heat use, night IMC), and CEN22LA181 (2022 C172M carb ice during go-around). Anonymized and localized to KTPF.
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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