Rough Running
Carburetor ice over Tampa Bay — recognizing it, treating it, and deciding where to go when the engine won't cooperate
The scenario
Field: Tampa International Airport (KTPA), Tampa, FL — elevation 26 ft MSL. You departed Runway 28 twenty minutes ago on a local VFR training flight, climbed to 3,500 ft MSL, and are now inbound to land. KTPA is Class B airspace; you are in contact with Tampa Approach and have a clearance through the Bravo.
Aircraft: Cessna 172N, solo, full fuel, within limits. Carbureted Lycoming O-320, steam-gauge panel, fixed gear, fixed-pitch prop, fuel selector on BOTH.
Weather: Classic Tampa summer afternoon — 88°F surface, dewpoint 74°F, scattered cumulus at 2,500 ft, bases building. Visibility 10 SM. Light rain showers in the area; you skirted one cell on the way out. The ASOS is reporting temperature/dewpoint spread of 14°F at the surface. At altitude, it's cooler and moister.
Pilot: You — a Private pilot, 110 hours total, 60 in type. You've flown this airplane a dozen times. You did not apply carburetor heat during the climb or at cruise; the engine has been running smoothly. You are now in a gradual descent, power reduced to approximately 2,100 RPM, passing through 2,800 ft MSL.
The situation: The engine begins running rough. Not a bang, not a silence — a subtle, progressive roughness. RPM is dropping slowly. You are over the built-up western suburbs of Tampa, roughly 8 nautical miles northwest of KTPA. The nearest pavement is KTPA itself.
- {'label': 'Field', 'value': 'KTPA · Tampa'}
- {'label': 'Runways', 'value': '10/28 · 19L/01R · 19R/01L'}
- {'label': 'Elevation', 'value': '26 ft'}
- {'label': 'Aircraft', 'value': 'C172N'}
- {'label': 'Dominant phase', 'value': 'Landing / Takeoff'}
The decision
Before we run the scenario — which of these are in your head right now? (Pick all that apply; this records your starting mental model.)
What the record shows
What the NTSB files show
NTSB CEN24LA362 (2024) involved a Cessna 172N on a local flight that 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 carburetor heat to clear the accumulated ice. The accident did not occur at KTPA — but the airplane, the conditions, and the mechanism are identical to this scenario.
NTSB CEN14LA276 (2014) involved a Cessna 172N on a cross-country that 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. Again, a different location — same airplane, same failure mode.
The pattern across these events is consistent: the pilot was in conditions favorable for carburetor ice (temperature 20–70°F, high relative humidity, visible moisture), power was reduced (descent or approach), and carburetor heat was not applied proactively. By the time roughness appeared, ice accumulation was significant enough that carb heat alone could not clear it in the available altitude.
Tampa's summer environment — high temperatures, dewpoints routinely in the 70s, scattered showers, and visible moisture — is a textbook carburetor-icing environment. The FAA carburetor-icing probability chart places high-humidity conditions with temperatures between 50°F and 70°F in the 'serious icing at glide power' zone. Descent and approach power settings are the highest-risk phase.
The correct procedure is preventive: apply carburetor heat before reducing power for descent, and leave it on through the approach. If roughness appears, apply full carb heat and hold it — expect the engine to run rougher briefly as ice melts, then smooth out. Pulling carb heat off because 'it got worse' stops the melting and leaves the problem unsolved.
Key lesson — Carburetor ice in the C172N is a preventable event. Apply carb heat before reducing power for descent in any conditions with high humidity or visible moisture. If roughness appears, apply full carb heat and hold it through the rough patch — the temporary worsening is the ice melting, not a sign the treatment is wrong. Declare MAYDAY early; ATC and ARFF are assets, not audiences for your embarrassment.
Debrief — teaching points
Tampa's summer air is a carburetor-ice factory.
The FAA carburetor-icing probability chart defines 'serious icing at glide power' as conditions with high relative humidity and temperatures between roughly 20°F and 70°F. Tampa in summer routinely has surface dewpoints in the 70s and temperatures in the upper 80s — at altitude, temperatures drop into the icing-probability zone while humidity remains high. Any flight in or near visible moisture, with power reduced for descent or approach, is a carb-ice exposure. This is not a rare edge case at KTPA; it is a routine summer condition.
Apply carb heat before the roughness, not after.
The POH and the accident record both point to the same lesson: carburetor heat is a preventive tool, not just a treatment. Apply it before reducing power for descent, before entering visible moisture, and before flying through areas of high humidity. Waiting for roughness means ice has already accumulated — and at low altitude, there may not be enough time and altitude for carb heat to clear it. The C172N POH recommends applying carb heat before reducing power below 2,000 RPM.
When you apply carb heat to an iced carburetor, expect it to get worse first.
Full carburetor heat introduces warm air that melts accumulated ice. As the ice melts and passes through the carburetor, the engine will run rougher and RPM will drop further — for 10–20 seconds. This is normal and expected. The instinct to pull carb heat off because 'it got worse' is exactly wrong: that rough patch is the treatment working. Hold full carb heat until the engine smooths out and RPM recovers. If it does not recover, you have a different or additional problem.
Declare MAYDAY early — ATC and ARFF are assets.
A partial power loss over dense Tampa suburbs is an emergency. Declaring MAYDAY to Tampa Approach costs nothing and gains everything: traffic is cleared, you get direct routing to KTPA, ARFF rolls, and emergency services are pre-positioned. The FAA does not certificate-action pilots for declaring emergencies in good faith. Delaying the call to avoid embarrassment costs altitude, options, and — in the worst case — survivability. Under 14 CFR §91.3, you are the final authority; use that authority to ask for help early.
The off-field environment around KTPA is not your friend.
The terrain northwest of KTPA — the direction of this scenario — is dense development, medium development, and open developed lots in every direction. There is no clear survivable forced-landing site. KTPA's runways (Runway 28 at 6,999 ft, Runway 19R at 11,002 ft) are the best surface available, and they are only available if you reach them. This is why altitude and early action matter: every foot of altitude you spend troubleshooting the wrong thing, or waiting to declare, is a foot of glide range you cannot get back. Best glide in the C172N is 65 KIAS — establish it immediately if power is lost or severely degraded.
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 airports; none occurred at KTPA.
NTSB reports: CEN24LA362 · CEN14LA276 · ERA09LA517 · WPR24LA167 · GAA19CA534
ACS tasks: PA.I.F — Weather Information · PA.II.B — Engine Starting / Systems Knowledge · PA.IX.C — Emergency Approach and Landing · PA.I.H — Human Factors
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.
Open the interactive scenario →All sample scenarios · More Cessna 172N scenarios · More scenarios at KTPA