Power Loss on the Base-to-Final Turn
Partial engine failure in the traffic pattern at KSRQ — carburetor ice, density altitude, and a marginal off-field environment force an immediate land-or-ditch decision
The scenario
Departing Sarasota Bradenton International Airport (KSRQ), Sarasota, FL — Runway 14, a warm Friday afternoon in late May. Elevation 30 ft MSL. You are on a local VFR flight, solo, in a Cessna 172M (150 hp Lycoming O-320, carbureted). Full fuel, within limits.
Weather: OAT 28°C, dew point 21°C, altimeter 29.94, light winds from 130°. Scattered clouds at 3,500 ft. Visibility 10 SM. Classic Gulf Coast spring conditions — warm, humid, and exactly the environment the FAA icing probability chart marks as 'serious icing at glide power.' You are not in IMC, but the moisture is there.
You have completed a local flight — a few turns around the pattern, some slow-flight practice, a touch-and-go on Runway 14. You are now on your second approach to Runway 14, descending through 800 ft AGL on a left base leg. The runway is in sight. You are configured for landing: flaps 20°, airspeed 70 KIAS, descent rate 400 fpm. The tower has cleared you to land.
Aircraft: Cessna 172M, solo, full fuel, within limits. Carbureted Lycoming O-320 (150 hp), fixed-pitch prop, steam panel (vacuum-driven attitude and heading indicators), fixed gear, fuel selector on BOTH. Nothing was written up; the airplane was airworthy at departure.
Pilot: you — a Private pilot, current, roughly 180 hours total. You did not apply carburetor heat during the run-up because the engine ran smoothly. You did not apply it on the initial climb or approach because you were focused on the pattern and the landing. You are not thinking about carb ice on a warm afternoon in VFR conditions.
- {'label': 'Field', 'value': 'KSRQ · Sarasota Bradenton'}
- {'label': 'Runways', 'value': '4/22 · 14/32'}
- {'label': 'Elevation', 'value': '30 ft'}
- {'label': 'Aircraft', 'value': 'C172M'}
- {'label': 'Dominant phase', 'value': 'Takeoff / Landing'}
The decision
Before we get into the decision tree — what do you already know about the C172M's engine and carburetor ice? (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. The ambient conditions (75°F OAT, 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 the pilot's failure to apply carburetor heat in conducive conditions.
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 probable cause was the pilot's failure to use carburetor heat during the approach and an unsuitable flight profile for the runway configuration, which resulted in a partial loss of engine power and an impact with terrain during the attempted go-around.
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. The probable cause was a loss of engine power due to a stuck valve — a mechanical failure, not icing.
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 probable cause was failure of the throttle control cable outer jacket, which fragmented and prevented proper throttle control, necessitating a forced landing. This was a mechanical failure — throttle cable failure — not icing.
The real accidents cited above occurred at other airports and in other aircraft types — NOT at KSRQ. KSRQ has its own accident history (see field dominant patterns: LOSS_OF_CONTROL_GROUND 19.2%, FORCED_LANDING 15.4%, RUNWAY_EXCURSION 11.5%), but these specific events happened elsewhere. The scenario is localized to KSRQ to make the off-field environment real and consequential for you as a student here.
The consistent thread across the carburetor-ice accidents: the C172M is carbureted and susceptible to ice formation in warm, humid air at reduced power. The first symptom is engine roughness and a dropping tachometer — not a dramatic power cut. 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.
Off Runway 14's climb-out (heading 134°), the off-field environment is dense development — a forced landing there is difficult and dangerous. Off Runway 22's climb-out (heading 218°), the off-field environment is open water — a forced landing there is a ditching. Know your off-field options before you line up on any runway at KSRQ.
Key lesson — In warm, humid Gulf Coast air, 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. On base-to-final at 800 ft AGL, the decision window is measured in seconds — not minutes. Off Runway 14's climb-out, the off-field environment is dense development. Off Runway 22's climb-out, it is open water. Know your escape routes before you depart.
Debrief — teaching points
Carburetor ice forms in conditions you would not expect — and the C172M is particularly vulnerable.
The FAA icing probability chart shows 'serious icing at glide power' at temperatures between roughly 15°C and 25°C when relative humidity is high — exactly the Gulf Coast afternoon conditions at KSRQ. You do not need visible ice, freezing temperatures, or IMC. Warm, moist air at reduced power (like on approach) is the classic carb-ice environment. The C172M's Lycoming O-320 is carbureted with no alternate air system. Carburetor heat is the only tool. The 150 hp O-320 is also lower-powered than the later 172N — climb and acceleration are marginal, especially at gross weight or in heat. A partial power loss in a C172M is more consequential than in a higher-powered airplane.
The first symptom is subtle — a dropping tachometer and engine roughness — and it happens on approach when you are heads-down on the landing.
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. On approach, you are focused on the landing — the runway, the descent rate, the flare — and you are not scanning the engine instruments. Scan the tachometer as part of your regular instrument scan, especially in conducive conditions. If the RPM drops unexpectedly, it is carburetor ice until proven otherwise.
Apply full carburetor heat — not partial — and expect an initial RPM drop. Do not remove it in conducive conditions.
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. If ice reforms after carb heat is removed, the atmospheric conditions that caused the icing are still present — leave carb heat on for the duration of flight in those conditions.
At KSRQ, know your off-field environment for each runway before you depart.
Off Runway 14's climb-out (heading 134°), the off-field environment is dense development — a forced landing there is difficult and dangerous. Off Runway 22's climb-out (heading 218°), the off-field environment is open water — a forced landing there is a ditching. Off Runway 04's climb-out (heading 38°), the off-field environment is marginal — medium development, wooded wetland, low-density development. Off Runway 32's climb-out (heading 314°), the off-field environment is poor — medium development, dense development, marsh. A partial power loss on the Runway 22 departure is a ditching. A partial power loss on the Runway 14 departure is a forced landing in dense development. Know these before you line up.
A go-around at low altitude with a rough engine is marginal — but it may be the right call.
At 800 ft AGL on base with a rough engine, a go-around is tight. The C172M climbs at roughly 300 fpm with full throttle and flaps 20° — that is slow. But a go-around gives you altitude, time to apply carburetor heat, and the option to circle and cool the engine before returning for a fresh approach. A go-around is better than continuing a descent on base with a deteriorating engine. If you initiate a go-around, apply carb heat immediately and maintain 65 KIAS (best glide / go-around speed) to maximize climb rate.
On base-to-final with a partial power loss, a shallow descent and reduced flaps maximize glide distance.
If carb heat restores only partial power and you are committed to landing, establish a shallow descent on final, reduce flaps to 10° (not full 40°), and maintain 65 KIAS best glide. This maximizes glide distance and gives you the longest path to the runway. Full flaps increase descent rate and reduce glide distance — they are appropriate only when you have full power and a normal descent rate. In a partial-power emergency on final, glide distance is your margin.
Built from the real accident record
Scenario built from NTSB ERA09LA379 (2009 C172M carburetor ice on base-to-final), 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 KSRQ.
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 · PA.V.A — Approach and Landing
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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