FAA SPECIAL AIRWORTHINESS
Aircraft Certification Service INFORMATION BULLETIN
SUBJ: Carburetor Icing Prevention
This is information only. Recommendations arent mandatory.
SAIB:
Date:
CE-09-35
06/30/2009
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 doesnt 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:
Recommendations
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/F5BD7904E8
45409D862569AE00783347?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
AOPA Safety Advisor Aircraft Icing
http://www.aopa.org/asf/publications/sa11.pdf
AOPA Safety Advisor Aircraft Deicing and Anti-icing Equipment
http://www.aopa.org/asf/publications/sa22.pdf
For Further Information Contact
Peter L. Rouse, Aerospace Engineer, Small Airplane Directorate; phone: (816)
329-4135; fax: (816)
329-4090; e-mail: peter.rouse@faa.gov.