Rough Running Over Tampa Bay
Carburetor ice at low altitude, a partial power loss, and the decisions that follow
The scenario
Departing St. Petersburg–Clearwater International Airport (KPIE), Pinellas Park, FL — elevation 11 ft MSL — Runway 36 in use (heading 351°). You departed northbound about 12 minutes ago and are now at 1,800 ft MSL on a local VFR sightseeing flight over Tampa Bay, roughly 4 nm north of the field.
Aircraft: Cessna 172N, N-number current, solo, full fuel, within limits. Lycoming O-320, carbureted, fixed-pitch prop, steam panel. Everything was normal at run-up.
Weather: KPIE METAR shows OVC025, temperature 26°C, dew point 22°C — a 4°C spread. Visibility 7 SM in light rain showers. The FAA carburetor icing probability chart puts these conditions squarely in the 'serious icing at glide power' and 'icing at cruise power' zone. You did not pull carb heat at run-up because the engine ran smoothly.
Pilot: you — a Private pilot, 180 hours total, 60 in type. You've flown this airplane a dozen times out of KPIE. You know the bay is out there; you've never thought hard about what you'd do if the engine quit over it.
The engine begins to run rough. The RPM drops about 100 RPM and continues to fall slowly. The airplane is at 1,800 ft MSL, roughly 4 nm north of KPIE on a heading of 180°. Runway 36 (heading 351°) is behind you; Runway 18 (heading 171°) would require a 180° turn back to the field. Below and around you: Tampa Bay — open water.
- {'label': 'Field', 'value': 'KPIE · St. Petersburg Clearwater'}
- {'label': 'Runways', 'value': '4/22 · 18/36'}
- {'label': 'Elevation', 'value': '11 ft'}
- {'label': 'Aircraft', 'value': 'C172N'}
- {'label': 'Dominant phase', 'value': 'Takeoff / Landing'}
The decision
The engine just got rough. Before you act — which of these is actually in your head right now? (Pick all that apply; this records your initial mental state.)
What the record shows
What the NTSB files show
NTSB CEN24LA362 (2024): A Cessna 172N on a local personal flight encountered light rain and carburetor ice at 1,800 ft AGL. The engine ran rough and lost power. The NTSB determined 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. This event did not occur at KPIE — it is used here because the aircraft type, altitude, and conditions are directly applicable.
NTSB CEN14LA276 (2014): A Cessna 172N on a cross-country flight experienced engine roughness and power loss at cruise altitude in icing-conducive conditions. The pilot made a forced landing on an island where the aircraft nosed over in soft sand. The probable cause could not be determined due to premature aircraft release, but the conditions and symptoms are consistent with carburetor ice.
The carburetor icing threat is particularly insidious in Florida's climate: high temperatures and high dew points produce a small temperature/dew point spread year-round. The FAA icing probability chart shows that a 4°C spread at 26°C places the C172N's Lycoming O-320 squarely in 'serious icing at glide power' and 'icing at cruise power' territory — conditions that feel like a perfect VFR day.
The KPIE environment adds a specific hazard: Runway 36 departs northbound over Tampa Bay. The off-field environment off the Runway 36 departure end is predominantly open water. Any engine failure shortly after departure from Runway 36, or during low-altitude flight over the bay, is a ditching scenario — not a field landing. Pilots who have not mentally pre-briefed the ditching procedure before departure are unprepared for the decision when it arrives.
The local precedent from LAX89LA222 (a Grumman AA-1C that stalled short of the runway over water after an unstable approach) reinforces the same geography: the water is unforgiving of slow-speed departures from controlled flight. Maintain airspeed, fly the airplane, and commit to the ditching procedure rather than attempting an unreachable runway.
Key lesson — Carburetor ice forms in conditions that feel like a fine VFR day — warm, humid, light rain. The Lycoming O-320 in the C172N is vulnerable at both cruise and glide power settings. Apply carb heat at the first sign of roughness, expect it to run rougher briefly as ice melts, and hold it in. If conditions are conducive, use carb heat periodically before roughness begins. Over Tampa Bay, a complete power loss means a ditching — brief it before you need it, and know that full flaps at Vs0 (~40 KIAS) gives you the slowest, most survivable touchdown speed.
Debrief — teaching points
Carburetor ice forms in conditions that feel benign.
The Lycoming O-320 in the C172N is a carbureted engine — the venturi effect in the carburetor drops air temperature 20–30°F below ambient. A 26°C day with a 4°C temperature/dew point spread is prime icing territory. The FAA carburetor icing probability chart places these conditions in 'serious icing at glide power' and 'icing at cruise power.' A smooth run-up does not guarantee ice-free conditions in flight — ice accumulates progressively. The habit is periodic carb heat application in any conducive conditions, not just reactive use after roughness appears.
Carb heat ON is the first response to engine roughness — and it will run rougher before it runs smoother.
When carb heat is applied to an iced carburetor, the heated air melts the ice and the water passes through the engine — causing a brief increase in roughness and a further RPM drop. This is expected and normal. Pulling carb heat off because 'it got worse' is the wrong response. Hold it in. The roughness will smooth as the ice clears, typically within 10–30 seconds depending on ice accumulation. If the engine does not recover after a full application of carb heat, the ice load may be too heavy — establish best glide (65 KIAS) and begin emergency procedures.
Time and altitude are your margins — spend them on the right problem first.
In the CEN24LA362 accident, the NTSB found there was 'insufficient time and altitude for carburetor heat to clear the accumulated ice.' Every second spent on the wrong response (mixture, throttle, waiting) is altitude lost and ice accumulated. The C172N at 1,800 ft MSL over Tampa Bay has roughly 2.5–3 minutes of glide time at best glide. Apply carb heat immediately at the first sign of roughness — do not troubleshoot other causes first in conditions clearly conducive to icing.
Know your off-field environment before you depart — Runway 36 at KPIE departs over water.
The off-field environment off the Runway 36 departure end at KPIE is predominantly open water (Tampa Bay). Any engine failure shortly after departure from Runway 36, or during low-altitude flight over the bay, is a ditching scenario. The glide math is unforgiving: at 65 KIAS best glide, the C172N descends approximately 700 ft per nautical mile. From 1,000 ft AGL, you can glide roughly 1.4 nm — not enough to reach the runway from 4 nm out. Pre-brief the ditching before every departure from Runway 36.
In a ditching, full flaps give you the slowest touchdown speed — and that is what saves you.
The dominant value of full flaps (30°) in a forced water landing is the slowest possible touchdown speed. Impact energy rises with the square of touchdown speed: a touchdown at 40 KIAS (Vs0, full flaps) has roughly one-quarter the impact energy of a touchdown at 80 KIAS. The steeper approach path is secondary. Select flaps 30° below 85 KIAS (Vfe), touch down nose-high and into the wind, unlatch the door before impact, and fly the airplane all the way to the water. Attempting to stretch the glide to an unreachable runway at the cost of a high-speed, uncontrolled water impact is the worse outcome.
Built from the real accident record
Scenario built from NTSB CEN24LA362 (2024 C172N carburetor ice / engine roughness at 1,800 ft AGL), CEN14LA276 (2014 C172N power loss / forced landing), and ERA09LA517 (2009 C172N total power loss). Real events occurred at other airports. Anonymized and localized to KPIE for training purposes.
NTSB reports: CEN24LA362 · CEN14LA276 · ERA09LA517 · LAX89LA222 · ERA10CA300
ACS tasks: PA.I.C — Weather Information · PA.I.F — Performance and Limitations · 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.
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