Rough Air Over the Bay
Carburetor ice in a Cessna 150M at low altitude over Tampa Bay — the decision window is seconds, not minutes
The scenario
Departing Albert Whitted Airport (KSPG), St. Petersburg, FL — Runway 07, climbing out over Tampa Bay on a 062° heading. Elevation 7 ft MSL. You are a Private pilot with 180 hours total time, 12 hours in the Cessna 150M. This is a local VFR flight, solo, full fuel (18 gal usable), within weight and balance.
It is a warm, humid Gulf Coast afternoon in late May: OAT 26°C, dew point 21°C, altimeter 29.92. Scattered clouds at 2,800 ft, light rain shower visible two miles to the northeast. Visibility 8 SM. The FAA carburetor icing probability chart marks these exact conditions — warm, moist air, reduced power — as serious icing risk at glide power, moderate icing risk at cruise power.
You are 350 ft AGL, climbing through 68 KIAS (Vy, best rate of climb for the C150M), heading 062°, when the engine begins to run rough. The tachometer is unwinding — power is noticeably down. The water of Tampa Bay fills the windscreen ahead. KSPG's tower is part-time (0700–2100) and is open; you are in Class D airspace.
Aircraft: Cessna 150M, solo, full fuel, within limits. Continental O-200-A, 100 hp, carbureted, fixed-pitch prop, fixed gear. Nothing was written up; the airplane was airworthy at departure. 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.
Pilot: you — Private pilot, current, 180 hours total, 12 hours in type. You are familiar with the C150's marginal climb performance, especially at gross weight in heat. You know the off-field environment off Runway 07 is open water — Tampa Bay. You have 30 seconds of useful decision time before altitude becomes critical.
- {'label': 'Field', 'value': 'KSPG · Albert Whitted'}
- {'label': 'Runways', 'value': '7/25 · 18/36'}
- {'label': 'Elevation', 'value': '7 ft'}
- {'label': 'Aircraft', 'value': 'C150'}
- {'label': 'Dominant phase', 'value': 'Landing / Takeoff'}
The decision
Before we get into the decision tree — what do you already know about carburetor ice in the C150M? (Pick all that apply; this records your baseline.)
What the record shows
What the NTSB files show
NTSB ERA25LA028 (2024): A Cessna 150H encountered carburetor ice at cruise altitude in conditions with 100% relative humidity and temperature/dew point spread conducive to serious icing. The pilot delayed applying carburetor heat. Probable cause: partial loss of engine power due to carburetor ice as a result of delayed use of carburetor heat.
NTSB ANC25LA005 (2024): A Cessna 150 on initial climb experienced partial loss of engine power due to carburetor ice in conditions with 70% relative humidity conducive to serious icing at glide power. The pilot's improper use of carburetor heat (likely partial application or cycling) worsened the situation. Probable cause: partial loss of engine power due to carburetor ice as a result of improper use of carburetor heat.
NTSB ERA24LA087 (2024): A Cessna 150M on a solo instructional cross-country flight experienced partial engine power loss when the student pilot failed to apply carburetor heat. The pilot made a diversionary landing but failed to attain a proper touchdown point, resulting in a runway excursion. Probable cause: failure to use carburetor heat in icing conditions, with contributing factor of improper touchdown point during the diversionary landing.
NTSB CEN21LA381 (2021): A Cessna 150 experienced partial engine power loss due to carburetor icing during takeoff near Wadsworth, Ohio. The pilot failed to apply carburetor heat despite conditions in the moderate-to-serious icing range. The aircraft made a forced landing to a corn field where it nosed over. Probable cause: partial loss of engine power due to carburetor icing and failure to apply carburetor heat.
NTSB CEN23FA077 (2023, FATAL): A Cessna 150H on an instructional flight conducted a night visual approach in dark conditions with no cultural lighting. The aircraft descended below safe altitude and impacted a farm field 1.2 miles short of the runway. The accident was attributed to loss of engine power due to carburetor icing and the flight instructor's failure to apply carburetor heat. Probable cause: flight instructor's failure to maintain control after loss of engine power due to carburetor icing while maneuvering for forced landing in dark night VFR conditions.
The local environment at KSPG makes this scenario particularly unforgiving: Runway 07's departure end is open water — Tampa Bay. An engine failure on the Runway 07 departure at low altitude is a ditching, not a field landing. There is no open field, no road, no park. The water is the off-field environment. This is not hypothetical; it is the NLCD ground cover off that runway end.
The real accidents cited above occurred at other airports and in other aircraft — NOT at Albert Whitted Airport. KSPG has its own accident history (see field dominant patterns: LOSS_OF_CONTROL_INFLIGHT 20%, FORCED_LANDING 16.4%, DITCHING 12.7%), but these specific NTSB events happened elsewhere. The scenario is localized to KSPG 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 C150 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 C150M's carbureted Continental O-200-A 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 07 at KSPG, 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 KSPG. You do not need visible ice, freezing temperatures, or IMC. Warm, moist air at reduced power is the classic carb-ice environment. The C150M's Continental O-200-A is carbureted; it has no 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 C150M, 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 KSPG Runway 07, an engine failure on departure is a ditching.
The off-field environment off Runway 07's departure end (heading 062°) is open water — Tampa Bay. There is no alternate landing surface. If the engine quits on the Runway 07 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 60 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 07.
Proactive carb heat use in conducive conditions is not optional.
The C150M 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 21°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 ERA25LA028, ANC25LA005, ERA24LA087 (C150 carburetor ice / delayed carb heat), WPR21LA329, CEN21LA381, ERA21LA284 (C150 carb ice on takeoff/climb), and CEN23FA077 (C150 carb ice / night forced landing). Real accidents occurred at other airports — NOT at KSPG. Localized to Albert Whitted Airport, St. Petersburg, FL.
NTSB reports: ERA25LA028 · ANC25LA005 · ERA24LA087 · WPR21LA329 · CEN21LA381 · ERA21LA284 · CEN23FA077
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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