Airworthiness Manual Advisory (AMA) No. 549.13/2

AMA: 549.13/2
Date: 15 April 1987

Subject: Prevention Of Carburettor Icing In Amateur-Built Aircraft

1. Purpose

This advisory material provides guidance for acceptable means, but not the only means, of showing compliance with the requirements of preventing ice build-up in carburettors of amateur-built aircraft.

2. Reference Airworthiness Standards

Chapter 549, Amateur-Built Aircraft Section 549.13 paragraph (a)(3).

3. Background And Discussion. 

Carburettor icing is a constant source of concern in aircraft operation since it may result in partial or complete loss of power.  Factors relevant to carburettor icing are volatility of the fuel, engine design, weather and operating conditions.  Induction icing is possible over a wide range of climatic conditions as a result of the tem­pera­ture drop through the carburettor.  Air passing through the venturi and over the throttle valve expands and local temperature falls.  Heat is lost from the air due to the latent heat of vaporization of fuel, this takes place in the vicinity of the throttle valve, resulting in a local tempera­ture drop up to 33°C (60°F.)  The cumulative effect of both expansion cool­ing and fuel vaporization reduces the temperature to the point where ice may form on the throttle and in the adjacent passages of the carburettor, even though the am­bient temperature is well above freezing.

4. Acceptable Means Of Compliance.

  1. Air Pre-Heater  Carburettor air heating is the most common approach to the icing problem.  This is generally accomplished by using an exhaust gas/intake air heat exchanger upstream of the carburettor.  The pre-heater should be capable, in air free of visible moisture at a  temperature of -1°C (30°F), of increasing the intake air temperature by 50°C (90°F) with the engine at 75% of maximum continuous power. A test to confirm the heat rise capability should normally be required, except where the heater has been previously incorpo­rated in an approved installation.  For the test an ambient tem­perature close to that specified (+2°C) may be accepted but the temperature rise should not be less than the recommended 50°C.
  2. Carburettor Design  Induction icing may be reduced (and possibly eliminated) by using carburettors which through design resist icing, as exemplified by those having a shallow venturi, a coated throttle plate, etc.  The presence of oil in the fuel mixture of 2 cycle engines may reduce the tendency for icing, but is not considered adequate as a sole means to minimize the possibil­ity of icing.  Operational limi­tations based upon the  amount of heat provided, considering the other factors tending to reduce icing may be considered appro­priate and may be applied when  substantiated by temperature measurements.
  3. Other Means  A pre-heater need not be provided if the intake air is continu­ously heated with air taken from the rear of the cylinder(s) or an equivalent temperature rise is provided.  Intake air heat may be provided by directing cylinder cooling air into a carburettor plenum intake.  A substantial heat rise is available at the inlet to the carburettor by this means.  Actual temperature measurements showing the equivalence may be required to ensure that carburettor throat temperature remains high enough to pre­clude icing. Means should be available to supply fresh unheated air to the carburettor inlet for normal operation if the performance loss due to increased charge temperature is significant.  The pos­sibility of fuel vapour lock as a result of the excess carburettor heat must also be considered.

5. Operational Information.

Carburettor heat should be used in all sus­pected icing conditions.  As a rule, total power loss will not occur below 65% relative humidity and/or -1°C (30°F) unless moisture is visible.  Ice formation is most likely to occur at low power settings, when the throttle is almost closed.  When the engine has been operated above idle for several minutes, there is generally sufficient heat to prevent ice formation, if the icing protection procedures have been satisfied.

6. Reference Material. 

The following reference material is provided as an example for the builder's convenience.

  1. AC20-113 Reciprocating Engine Induction System and Fuel System Icing Problems.  (October 22, 1981)
  2. NACA, TR 982, Icing Protection Requirements (1950)
  3. "The Fair-Weather Fiends" FAA General Aviation News, March-April (1982)
  4. "Ice Can Clog Carburettor..." W.P. Dugan, H.A. Toulmin; SAE Journal (March 1955)

M. Khouzam
Chief, Airworthiness Standards
Airworthiness Branch