The Carb Heat Test You Don’t Know

The Cessna 152 was 200 feet above the ground when its engine quit. Inside, the instructor was preparing his student for the Private Pilot checkride. They were practicing a short-field takeoff and the airplane was ‘dirty’ with flaps, a high angle of attack and at a slim margin above stall speed and now, they were without power.

‘I’ve got it,’ shouted the CFI as he took the controls and shoved them forward to reveal a steep dropoff, a busy road, houses and a school. Given the options, the instructor banked left to reach for the airport behind them. Airspeed decayed and the airplane entered a spin. So close to the ground, the airplane impacted trees before the spin could develop. It flipped inverted and fell, nearly vertically, onto a small shed. As fate would have it, the trees and collapsing building cushioned the impact — both aboard survived.

An NTSB investigator found no evidence of mechanical engine failure, but both pilots independently told him that the engine quit before the spin began. Weather conditions at the time of the crash included cool air and low clouds. The official NTSB report suggests carburetor-icing as the probable cause of the accident.

For carb ice to form, conditions must include muggy air and carb temperatures at or below freezing. Carbureted engines (those without fuel injection) mix fuel and air in a carburetor venturi. The venturi is a narrow passageway in the induction system that accelerates airflow and draws fuel into the manifold. As the air accelerates, its temperature drops — sometimes as much as 40 degrees or more Fahrenheit! Result: If the air coming into the engine is moist and the temperature inside the carburetor is at or below freezing, ice can form. The ice blocks airflow to the engine and, without air, combustion stops and the engine fails.

You should always suspect carb ice when:

  • Relative Humidity is greater than 70%,
  • Temperature/Dewpoint spread is less than five degrees Fahrenheit, and
  • Outside Air Temperature is less than 70 degrees Fahrenheit.

THE TEST — Part 1 (The part you know.)
During engine run-up, apply full carb heat, and watch for a drop in engine rpm. Certified airplane engines often protect against carb ice by ducting hot exhaust air into the carburetor. Hot air is less dense than cold, so when the temperature in the carburetor increases, the mixture gets richer (more fuel per unit of air). Airplane engines are normally set to run ‘super-rich’ unless you do something to change it, so when you pull ‘carb heat on,’ you add fuel to an already rich combustion mixture and the engine runs less efficiently.
The resultant drop in RMP shows you that the system works — but it does *not* show you whether carb icing conditions actually exists.

THE TEST — Part 2 (The important part.)
During your run-up, apply carb heat, but leave it on for a half-minute or so. If the rpm drops with carb heat application and stays at there until you turn it off, then there is no ice. If the rpm drops and then increases, it’s likely you already have carb ice. Why: The rpm drops for the ‘normal’ reason at first, but then rises as the ice melts out, allowing more air into the engine. If you suspect carburetor-icing conditions exist while aloft, run this test. Inside Information: Most carb heat induction air bypasses the engine’s filters so ground-test carb heat away from dusty or rocky areas.

If you discover carb ice while on the ground, you have two choices: abort the flight, or fly with the carb heat on — consult your airplane’s Pilot’s Operating handbook to see if it recommends taking off with carb heat applied. If you discover carb ice in cruising flight, fly with the carb heat on ‘full’ and consider an early landing. In either case, expect decreased engine performance for all operations. In most cases, detecting carb-icing conditions on the ground is reason enough to call it quits and fly another day.