Flight Training - March 2019 - 20

PREFLIGHT ASK AOPA

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BY FERDI MACK

CARBURETOR HEAT

TOP OFF YOUR PLANE

DOES IT NEED TO BE COLD OUTSIDE?

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Q: I'm training in a carbureted Cessna Skyhawk. My instructor insists that carburetor
heat should be applied as per the checklist, regardless of how hot it is outside. Should
I only worry about carburetor ice when it is cold out?

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A: Your instructor is correct. You should use carburetor heat as the checklist instructs,
with limited exception such as when engine performance suffers on days when
carb ice potential is negligible. Also, many pilots forget to remove carb heat during a
go-around, so don't forget to follow the checklist when it says to turn it off as well.
The FAA Pilot's Handbook of Aeronautical Knowledge covers this in section 7 on
aircraft systems. High carb ice potential exists when the outside air temperature
(OAT) is between 20 degrees Fahrenheit and 70 degrees F, and with relative humidity of 80 percent or greater. Some carb icing is possible at an OAT of as high as 100
degrees F and with relative humidity as low as 50 percent.
High OAT doesn't preclude carb icing. Lower air pressure in the carburetor throat
results in colder induction air. Carb ice can occur at outside air temps well above
freezing.
I have experienced carb icing in some rather surprising scenarios. The first was
crossing the mountains west of Rock Springs, Wyoming, in August, and the second
over Maryland's Chesapeake Bay in July. In both cases, the surface temperatures were
typical for summertime heat. The OAT at altitude was lower than the surface but still
well above freezing.
The first indication of a problem in the Cessna 182RG was that the indicated airspeed fell off by 10 knots, then 20, without any pilot-induced power reduction. I was
perplexed, since the common training tip is to look for a drop in rpm as an indication
of carb ice. The constant-speed propeller masked the power loss by allowing the
propeller to move toward coarse pitch in an effort to maintain rpm, so no rpm drop
occurred. The carb ice was only indicated by the airspeed loss.
For more information, contact AOPA's Pilot Information Center at 800-872-2672
or pilotassist@aopa.org.
FERDI MACK is senior manager of the AOPA Pilot Information Center.

PILOT SPEAK

Fix

A point in space. There's no easier
way to think of a fix. The FAA has
a wordier definition, but it comes
down to a geographical point that is
determined visually or with a navaid.
Take the crossing point of any two
random radials or the distance from a
VOR and you have a fix. Sounds like a
waypoint, right? Close, but...

20 FLIGHT TRAINING MARCH 2019

Waypoint

A waypoint is a predetermined geographical position used to define
routes, approaches, visual points, and
so on. In other words, anything you
can find named in a book, on a chart,
and so on. So, a waypoint could be a
fix and a fix could be a waypoint, but
a fix isn't always a waypoint, while a
waypoint is always a fix. Easy peasy.

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Flight Training - March 2019

Table of Contents for the Digital Edition of Flight Training - March 2019