Aircraft
Certification Service Washington, DC U.S. Department of Transportation Federal Aviation Administration
CE-09-35
June 30, 2009
This is information only. Recommendations
aren’t mandatory.
Introduction
This Special Airworthiness Information Bulletin is written to inform
pilots of the potential hazards associated with carburetor icing.
At this time, this airworthiness concern is not considered an unsafe
condition that would warrant an airworthiness directive (AD) action
under Title 14 of the Code of Federal Regulations (14 CFR part 39).
Background
There were 212 accidents attributed to carburetor icing between 1998
and 2007. Of these accidents, 13 resulted in fatalities. The
certification requirements for carbureted airplanes require that a
heated source of air be provided as mitigation for carburetor icing.
The FAA and the Aircraft Owners and Pilots Association (AOPA) have
addressed the subject of carburetor icing several times in various
forms. Despite the certification requirements, and the information
provided by the FAA and AOPA, the accident trend has remained fairly
steady throughout the years.
Pilots should be aware that carburetor icing doesn’t just occur in
freezing conditions, it can occur at temperatures well above freezing
temperatures when there is visible moisture or high humidity. Icing can
occur in the carburetor at temperatures above freezing because
vaporization of fuel, combined with the expansion of air as it flows
through the carburetor, (Venturi Effect) causes sudden cooling,
sometimes by a significant amount within a fraction of a second.
Carburetor ice can be detected by a drop in rpm in fixed pitch
propeller airplanes and a drop in manifold pressure in constant speed
propeller airplanes. In both types, usually there will be a roughness
in engine operation. The graph below shows the probability of
carburetor icing for various temperature and relative humidity
conditions:
There are some steps a pilot can take to prevent, recognize, and respond to carburetor icing.
To prevent carburetor icing, the pilot should:
Assure the proper functionality of the carburetor heat during the ground (Before Takeoff) check.
Use carburetor heat on
approach and descent when operating at low power settings, or in
conditions where carburetor icing is probable.
To recognize carburetor icing, the warning signs are:
A drop in rpm in fixed pitch propeller airplanes.
A drop in manifold pressure in constant speed propeller airplanes.
In both types, usually there will be a roughness in engine operation.
The pilot should respond to
carburetor icing by applying full carburetor heat immediately. The
engine may run rough initially for short time while ice melts.
The above recommendations are general suggestions. The pilot should
consult the AFM or the pilot's operating handbook for the proper use of
carburetor heat.
Included below are some references, and their associated links, for more information:
AC 20-113, Pilot Precautions
and Procedures to be taken in Preventing Aircraft Reciprocating Engine
Induction System and Fuel System Icing Problems
http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/F5BD7904E845409D862569AE00783347?OpenDocument&Highlight=carburetor%20icing
AC 91-51A, Effect of Icing
on Aircraft Control and Airplane Deice and Anti-Ice Systems
http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/451296DBDF
212C81862569E70077C8F9?OpenDocument&Highlight=carburetor%20icing
