Rough Engine on the Go-Around
Carburetor ice, partial power loss during a low approach, and a marginal off-field environment — the decision clock is short
The scenario
Departing Zephyrhills Municipal Airport (KZPH), Zephyrhills, FL — Runway 19, a local VFR training flight. Elevation 90 ft MSL. It is a warm, humid afternoon in late spring: OAT 28°C, dew point 22°C, altimeter 29.92. Scattered clouds at 2,500 ft, light rain shower two miles to the northeast. Visibility 8 SM. Classic Florida 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 on your third approach to Runway 19 after two go-arounds for traffic. You are now on base leg, 600 ft AGL, descending at 63 KIAS (Vref), heading 180°. The runway is in sight. You are planning to land, but the approach feels unstable — you are a bit high, and the descent is shallow. You decide to go around.
As you apply full throttle to climb out, the engine begins to run rough. Power is noticeably down — the tachometer is dropping. You are at 400 ft AGL, heading 180° (south), over open developed land (parks, small lots, low-density development) and evergreen forest. The runway is behind you. KZPH is non-towered (CTAF); you are in Class G 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.
Pilot: you — a Private pilot, current, roughly 250 hours total. You did not apply carburetor heat during the run-up because the engine ran smoothly. You did not apply it during the approach because you were focused on the landing. You did not apply it during the first two go-arounds because the engine was running fine then.
- {'label': 'Field', 'value': 'KZPH · Zephyrhills'}
- {'label': 'Runways', 'value': '19/1 · 5/23'}
- {'label': 'Elevation', 'value': '90 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 — conducive to serious carburetor icing per the FAA icing probability chart. The pilot made a forced landing in a field. The probable cause was the partial loss of engine power for undetermined reasons — but the conditions and symptoms are consistent with carburetor ice.
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 — a secondary failure after the engine loss. The accident resulted from engine power loss due to carburetor icing in serious icing conditions, with contributing factors including high density altitude. The pilot survived the forced landing but was injured in the stall.
NTSB CEN24LA168 (2024): A Cessna 172M on an IFR flight experienced engine power loss due to carburetor icing during descent in night IMC. 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. The pilot touched down on a building roof and impacted a retaining wall and ground — a fatal accident. The lesson: carb heat must be applied early, before ice accumulation becomes severe.
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 an unsuitable flight profile for the runway configuration. The airplane impacted terrain during the attempted go-around.
The real accidents cited above occurred at other airports and in other aircraft types — NOT at Zephyrhills Municipal Airport. KZPH has its own accident history (see field dominant patterns: forced landing 29.2%, loss of control inflight 29.2%), but these specific events happened elsewhere. The scenario is localized to KZPH 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.
Key lesson — In warm, humid Florida 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. On a go-around at 400 ft AGL, the decision window is measured in seconds — not minutes. Off Runway 19 at KZPH, the off-field environment is marginal but workable — open developed land and forest. A delayed response means a forced landing in that marginal terrain, not a return to the airport.
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 Florida afternoon conditions at KZPH. 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 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 KZPH Runway 19, the off-field environment is marginal but workable.
The off-field environment off Runway 19's departure end (heading 180°) is marginal: mostly open developed land (parks, small lots), evergreen forest, and low-density development. There is no open water or major obstacles. If the engine quits on the Runway 19 departure and altitude is insufficient to return to the airport, a forced landing in that marginal terrain is survivable. Best glide is 65 KIAS. Doors unlatched before touchdown. 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 19.
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 approach, with OAT near 28°C and dew point near 22°C, that means applying carb heat during the run-up check (and confirming the expected RPM drop, then recovery) and considering its use during descent and approach in visible moisture or high humidity. Waiting for the roughness to appear at 400 ft AGL on go-around is waiting too long.
Built from the real accident record
Scenario built from NTSB ERA09LA379 (2009 C172M carburetor ice / forced landing), DFW05CA237 (2005 C172M carb ice / stall on approach), CEN24LA168 (2024 C172M delayed carb heat / power loss descent), and CEN22LA181 (2022 C172M carb ice / go-around failure). Anonymized and localized to KZPH.
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.
Open the interactive scenario →All sample scenarios · More Cessna 172M scenarios · More scenarios at KZPH