Power Loss on Base — Clearwater Air Park
Partial engine failure in the traffic pattern over dense development; a forced landing decision with nowhere good to go
The scenario
Departing Clearwater Air Park (KCLW), Clearwater, FL — Runway 16, returning from a local flight. Elevation 71 ft MSL. You are on base-to-final turn in the traffic pattern, 800 ft AGL, heading 335° (reciprocal of Runway 16's 155° magnetic), descending at 65 KIAS (Vref, approach speed). The runway is in sight, 1.5 nm ahead. It is a warm, humid Florida afternoon: OAT 28°C, dew point 22°C, altimeter 29.92. Scattered clouds at 2,500 ft, visibility 8 SM. KCLW is non-towered (CTAF); you are in Class G airspace, though the overlying Tampa Class B (3,000 MSL ceiling) is above you.
Aircraft: Cessna 172M, solo, full fuel, within limits. Carbureted Lycoming O-320-E2D, 150 hp — the lower-powered variant of the 172 family. Fixed-pitch prop, fixed gear, fuel selector on BOTH. Nothing was written up; the airplane was airworthy at departure. You have 180 hours total time, 40 hours in type.
Pilot: you — a Private pilot, current, roughly 180 hours total. This is your third flight to KCLW. You did not apply carburetor heat during the descent because the engine was running smoothly and you were focused on the approach. You did not apply it during the base turn because you were heads-down on the descent checklist.
At 800 ft AGL on base, the engine begins to run rough. Power is noticeably down — the tachometer is dropping. You are still over dense development (low-density and medium residential, some parks). The runway is ahead. You have roughly 60 seconds to diagnose and decide: continue the approach, go around, or land in the development below.
- {'label': 'Field', 'value': 'KCLW · Clearwater Air Park'}
- {'label': 'Runways', 'value': '16/34'}
- {'label': 'Elevation', 'value': '71 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 partial power loss in the traffic pattern? (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 (75°F, 55°F dew point) were conducive to serious carburetor icing per the FAA icing probability chart. The pilot made a forced landing in a field. The probable cause was carburetor icing at glide power, with insufficient time and altitude for carburetor heat to clear the accumulated ice. The pilot had not applied carburetor heat proactively in conditions that clearly warranted it.
NTSB CEN24LA168 (2024): A Cessna 172M on an IFR flight to Bemidji Regional Airport 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 accident resulted from the pilot's failure to use carburetor heat and maintain an unsuitable flight profile for the runway configuration. The pilot did not apply carb heat during the approach, and when power loss occurred on the go-around attempt, the airplane did not have the climb performance to recover.
NTSB CEN22LA309 (2022): A Cessna 172M experienced engine power loss during cruise flight near Friend, Nebraska due to a stuck exhaust valve. The pilot performed a forced landing in a field between corn crops, resulting in substantial fuselage damage. While this accident was mechanical (not carb ice), it illustrates the C172M's vulnerability to partial power loss — the 150 hp Lycoming O-320 has limited margin for climb and go-around performance.
NTSB WPR13LA035 (2012): A Cessna 172M on an aerial photography mission experienced a loss of engine power when the pilot applied full throttle during climb. The accident resulted from failure of the throttle control cable outer jacket, which fragmented and prevented proper throttle control. This accident illustrates a different failure mode — mechanical throttle failure — but the outcome is the same: partial power loss in the pattern or climb, forced landing.
The real accidents cited above occurred at other airports and in other aircraft types — NOT at Clearwater Air Park. KCLW has its own accident history (dominant pattern: forced landing 22.2%, loss of control inflight 18.5%, gear-up landing 18.5%), but these specific NTSB events happened elsewhere. The scenario is localized to KCLW 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 during descent and approach in warm, moist air. 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. Additionally, the C172M's 150 hp engine has limited climb performance at gross weight in sea-level conditions; a go-around attempt with partial power at low altitude is marginal at best.
Key lesson — In warm, moist Gulf Coast air during descent and approach, the C172M's carbureted O-320 can accumulate serious carburetor ice even at approach power and above-freezing temperatures. Apply full carburetor heat at the first sign of engine roughness or unexplained RPM loss. At 800 ft AGL in the traffic pattern, the decision window is measured in seconds — not minutes. Off Runway 16 at KCLW, the off-field environment is dense residential development: a forced landing there is into houses and streets, not open field. The C172M's 150 hp engine is also marginal on climb performance at gross weight — a go-around attempt with partial power at low altitude is risky. When in doubt, land.
Debrief — teaching points
Carburetor ice forms in descent and approach in warm, moist air — not just in cruise.
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 KCLW. Descent and approach are high-risk phases because the engine is at reduced power (glide power or approach power), which produces less heat and allows ice to accumulate. You do not need visible ice, freezing temperatures, or IMC. Warm, moist air at reduced power during descent 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 the pattern.
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 during descent 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 during descent and approach, especially in conducive conditions. If the tachometer is unwinding without a corresponding change in throttle position, suspect carb ice.
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 KCLW Runway 16, the off-field environment is dense residential development — no good landing surface.
The off-field environment off Runway 16's departure end (heading 155°) and the surrounding area is dense residential development, low-density development, medium development, and scattered parks. There is no open field, no road, no large park. If the engine quits in the pattern at KCLW, a forced landing is into houses and streets. This is not a worst-case scenario; it is the geographic reality. Best glide is 65 KIAS. Doors unlatched before impact. 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 16.
The C172M's 150 hp engine is marginal on climb performance — a go-around with partial power is risky.
The C172M climbs at roughly 600 fpm at gross weight in sea-level conditions. With partial power loss, climb performance is even more marginal. A go-around attempt at 800 ft AGL with a rough, losing-power engine is risky — you may not climb, and you will burn altitude diagnosing the problem. When in doubt about engine behavior in the pattern, land. A precautionary landing is always an option; a forced landing in development is not.
Built from the real accident record
Scenario built from NTSB ERA09LA379 (2009 C172M carburetor icing on base turn), CEN24LA168 (2024 C172M delayed carb heat / night IMC power loss), CEN22LA309 (2022 C172M stuck exhaust valve / forced landing), CEN22LA181 (2022 C172M carb heat failure on go-around), and WPR13LA035 (2012 C172M throttle cable failure). Localized to KCLW.
NTSB reports: ERA09LA379 · CEN24LA168 · CEN22LA309 · CEN22LA181 · WPR13LA035
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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