Engine Failure on the Base Turn
Carburetor ice or mechanical failure in the pattern at a water-surrounded airport — the decision window is seconds
The scenario
Departing Peter O Knight Airport (KTPF), Tampa, FL — Runway 04, a local instructional flight in the Cessna 172M. Elevation 8 ft MSL. The runway is essentially at sea level, surrounded by water and dense development.
It is a warm, humid Florida afternoon in late spring: OAT 26°C, dew point 20°C, altimeter 29.92. Scattered clouds at 2,500 ft, light rain shower two miles to the northeast. Visibility 8 SM. Classic Gulf Coast conditions — warm, moist, and exactly the environment the FAA icing probability chart marks as 'serious icing at glide power, moderate icing at cruise power.' Density altitude is approximately 1,500 ft.
You are on base-to-final turn in the traffic pattern at KTPF, descending through 400 ft AGL at 65 KIAS (Vref, approach speed), heading 217° for Runway 22. The engine begins to run rough. Power is noticeably down — the tachometer is dropping. The runway is ahead; you are in the landing pattern. KTPF is non-towered (CTAF); you are in Class G airspace with overlying Tampa Class B above 1,200 ft MSL.
Aircraft: Cessna 172M, solo, full fuel, weight and balance 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. The 172M is the lower-powered variant — climb and acceleration are marginal, especially in warm, humid conditions and at gross weight.
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 descent and landing.
- {'label': 'Field', 'value': 'KTPF · Peter O Knight'}
- {'label': 'Runways', 'value': '4/22 · 18/36'}
- {'label': 'Elevation', 'value': '8 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 the C172M and engine failures in the 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. The ambient conditions were 75°F OAT, 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 carburetor icing at glide power.
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. The accident resulted from engine power loss due to carburetor icing in serious icing conditions, with contributing factors including high density altitude. The pilot's failure to maintain airspeed resulted in an inadvertent stall.
NTSB CEN22LA309 (2022): A Cessna 172M experienced engine power loss during cruise flight due to a stuck exhaust valve. The pilot performed a forced landing in a field between corn crops, resulting in substantial fuselage damage. The loss of engine power was due to a mechanical failure — a stuck valve — 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 accident resulted from failure of the throttle control cable outer jacket, which fragmented and prevented proper throttle control, necessitating a forced landing. This is a maintenance and preflight-inspection issue: a frayed or damaged throttle cable outer jacket can fragment under load.
NTSB CHI07LA177 (2007): A Cessna 172M departed approximately 243 pounds over gross weight and out of balance. During initial climb, the engine lost power at 100–150 feet AGL; the aircraft stalled and impacted terrain. The probable cause was the pilot's improper aircraft weight and balance, and his failure to maintain sufficient airspeed to avoid a stall during takeoff-initial climb. Propeller damage indicated significant engine power at impact — the engine was not the failure; the weight and balance were.
The real accidents cited above occurred at other airports and in other aircraft types — NOT at Peter O Knight Airport. KTPF has its own accident history (forced landings, loss-of-control events, ditchings), but these specific NTSB cases happened elsewhere. The scenario is localized to KTPF to make the off-field environment real and consequential for you as a student here.
The consistent thread across all these events: the C172M is a marginal-performance airplane, especially at gross weight and high density altitude. Engine failures in the pattern — whether from carburetor icing, mechanical failure, or weight/balance issues — leave little time and altitude for recovery. The decision window is measured in seconds, not minutes. Off Runway 22 at KTPF, the off-field environment is open water and dense development: a forced landing there is a ditching or a crash into buildings. Off Runway 04, the off-field environment is dense development: a forced landing there is into obstacles. Early recognition of engine trouble and immediate action — carb heat, go-around, or commitment to the runway — is the difference between a safe landing and a forced landing.
Key lesson — In warm, moist Gulf Coast air at approach power, the C172M's carbureted O-320 can accumulate serious carburetor ice even at above-freezing temperatures. Apply full carburetor heat at the first sign of engine roughness or unexplained RPM loss. At 400 ft AGL in the pattern, the decision window is measured in seconds — not minutes. Off Runway 22 at KTPF, the off-field environment is open water: a delayed response means a ditching, not a field landing. The C172M's marginal climb performance (especially at gross weight and high density altitude) means a go-around is risky; commit to the runway early if power is lost in the pattern.
Debrief — teaching points
Carburetor ice forms in conditions you would not expect — especially at approach power.
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 KTPF. Approach power (roughly 1,500 RPM in the pattern) is a particularly vulnerable power setting: the engine is cool and the air is moving slowly through the carburetor. 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 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 miss the early warning. By the time the roughness is obvious, significant ice has accumulated. In the pattern, at 400 ft AGL, you have roughly 20–30 seconds of decision time. Scan the tachometer as part of your regular instrument scan, especially in conducive conditions. If the tachometer drops 100+ RPM unexpectedly, apply carb heat immediately.
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 KTPF, the off-field environment is water and dense development — a forced landing is a ditching or a crash.
Off Runway 22's approach end (heading 217°), the off-field environment is open water and dense development. Off Runway 04's approach end (heading 37°), the off-field environment is dense development. There is no open field, no road, no park. A forced landing off either runway end is either a ditching or a crash into buildings. This is not hypothetical; it is the NLCD ground cover off those runway ends. If the engine fails in the pattern and you cannot make the runway, a controlled ditching is the correct outcome. Best glide is 65 KIAS. Doors unlatched before water contact. 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.
The C172M is a marginal-performance airplane — especially at gross weight and high density altitude.
The C172M (150 hp) climbs at roughly 400–500 fpm in standard conditions at gross weight. In 1,500 ft density altitude on a warm day, climb performance is even worse. A go-around from 400 ft AGL with a rough engine is risky: you may not have the climb performance to return to pattern altitude safely. If the engine is failing in the pattern, commit to the runway early. A firm landing on the runway is better than a stall/spin trying to climb back to pattern altitude or a ditch trying to stretch a glide to the runway.
Weight and balance matter — especially in the C172M.
An overweight or out-of-balance C172M can stall at low altitude during climb or approach if airspeed is not maintained. Before every flight, verify that the aircraft is within weight and balance limits. A 243-pound overweight C172M (CHI07LA177) stalled at 100–150 feet AGL during initial climb. The engine was not the failure; the weight and balance were. Preflight includes a weight-and-balance check, not just a walk-around.
Built from the real accident record
Scenario built from NTSB ERA09LA379 (2009 C172M carburetor ice / forced landing), DFW05CA237 (2005 C172M carb ice + high density altitude), CEN22LA309 (2022 C172M stuck exhaust valve), WPR13LA035 (2012 C172M throttle cable failure), and CHI07LA177 (2007 C172M weight/balance + stall). Localized to KTPF.
NTSB reports: ERA09LA379 · DFW05CA237 · CEN22LA309 · WPR13LA035 · CHI07LA177
ACS tasks: PA.I.F — Weather Information · PA.VIII.C — Approach and Landing · PA.IX.C — Emergency Approach and Landing · PA.I.H — Human Factors · PA.II.B — Engine Starting / Systems Preflight
Relevant FARs: §91.3 · §91.9 · §91.13 · §91.23
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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