Carburetor Ice on the Descent
A Cessna 172 with a Lycoming O-320 has been cruising at 8,500 feet MSL, throttled back to 2,300 RPM, in air at 5 °C with high relative humidity and visible haze below. The pilot begins a 500-fpm descent toward a nontowered field at 900 feet elevation and spends 15 minutes at reduced power. The engine becomes slightly rough and the tachometer settles at 2,180 RPM. The pilot applies full carburetor heat; the RPM drops to 2,050, the roughness briefly worsens, then over the next 30 seconds the tach climbs back to 2,280 and the engine smooths out. The pilot then enters the pattern at 2,500 feet MSL, turns base with 30 degrees of flaps at 45 degrees of bank, and watches the airspeed decay through 58 knots. Book stall speeds at this weight are 48 knots clean and 43 knots with 30 degrees of flaps.
1. Interpret the RPM behavior. What was happening, and what does the initial drop after applying carburetor heat tell you?
Read the worked answer
This is textbook carburetor icing, and every part of the RPM trace fits.
Carburetor ice forms when fuel vaporizes and the venturi pressure drops, cooling the mixture well below ambient - often 15–20 °C of further cooling. With an ambient temperature of 5 °C and high humidity, the throttle was already partway closed, which puts the throttle plate and venturi in the region where ice accumulates fastest. A 15-minute descent at reduced power is the highest-risk phase for a fixed-pitch, carbureted engine.
Reading the indications:
- Roughness plus a 120-RPM loss is the classic first sign; the mixture is being choked by ice, not by a fouled plug. A mag check or leaning would not fix it.
- The initial drop to 2,050 after applying heat is expected and is the confirmation: carburetor heat routes unfiltered, heated air into the carburetor, which richens the mixture (lower RPM) and melts the ice, briefly sending water through the engine (rougher running).
- The climb back to 2,280 means the ice is gone and the engine is making power again.
A pilot who reads the initial drop as "carb heat makes it worse" and removes heat has misread the sequence and will let the ice re-form. The correct response is full heat, held, until the engine is smooth and the RPM stabilizes.
2. Compute the load factor and the accelerated stall speed for the 45-degree banked turn with 30 degrees of flaps, and evaluate the 58-knot airspeed.
Read the worked answer
Load factor in a level, coordinated turn is n = 1/cos(bank angle):
- 45 degrees: n = 1/cos 45° = 1/0.707 = 1.41 g
- 60 degrees: n = 1/cos 60° = 1/0.5 = 2.0 g
Accelerated stall speed scales with the square root of load factor: Vs(new) = Vs × √n.
- With 30 degrees of flaps, Vs = 43 knots. At 45 degrees of bank: 43 × √1.41 = 43 × 1.19 ≈ 51 knots.
- At 60 degrees of bank: 43 × √2.0 = 43 × 1.41 ≈ 61 knots.
Evaluation: at 58 knots in a 45-degree bank the airplane is at only about 1.14 times the accelerated stall speed - below the 1.3 Vso normally flown on approach, with the airspeed still decaying. The bank itself is not the emergency; the combination of a steep bank, flaps, a decaying airspeed, and the reflex to tighten the turn to avoid overshooting final is.
If the pilot increases bank to 60 degrees to make the turn, the airplane stalls at 61 knots - above the current 58-knot airspeed. That is the base-to-final stall: the wing is already beyond its critical angle of attack while the airspeed indicator still shows a number that feels like flying speed.
3. What should the pilot do right now, and what engine-management items belong in the approach brief?
Read the worked answer
Right now: roll out of the bank, reduce the angle of attack by lowering the nose slightly, and add power. The airplane is slow and steep, so the recovery is to unload the wing first and accept the overshoot - an overshoot is recoverable, a stall at pattern altitude is not. Fly a go-around: full power, carburetor heat cold (full heat with full throttle delivers greatly reduced power and unfiltered air, and in icing conditions the heat may be needed again immediately after), flaps to 20 degrees, positive climb, then rejoin the pattern and fly a stabilized approach with no more than 30 degrees of bank below pattern altitude.
Engine-management items for the brief:
- Carburetor heat on any power reduction - descent, approach, or pattern work - and expect the RPM to drop before it rises as ice melts.
- Leave heat on until the engine is smooth, then decide whether conditions require it to stay on.
- Carburetor heat off before a go-around so full power is available, but be ready to reapply in icing conditions.
- Watch for the other ice indications: rough running, RPM loss, and an unexplained descent in an airplane that was trimmed for level flight.
- Plan the approach: airspeed and configuration set before the turn, no more than 30 degrees of bank, and a decision to go around made early rather than as a save.