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Pilot's Handbook of Aeronautical Knowledge
Aircraft Systems

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




Float-type carburetor.
Figure 6-10. Float-type carburetor.

A pressure-type carburetor discharges fuel into the airstream
at a pressure well above atmospheric. This results in better
vaporization and permits the discharge of fuel into the
airstream on the engine side of the throttle valve. With the
discharge nozzle located at this point, the drop in temperature
due to fuel vaporization takes place after the air has passed the
throttle valve and at a point where engine heat tends to offset
it. Thus, the danger of fuel vaporization icing is practically
eliminated. The effects of rapid maneuvers and rough air on
the pressure-type carburetors are negligible since their fuel
chambers remain filled under all operating conditions.

Mixture Control
Carburetors are normally calibrated at sea-level pressure,
where the correct fuel-to-air mixture ratio is established
with the mixture control set in the FULL RICH position.
However, as altitude increases, the density of air entering the
carburetor decreases, while the density of the fuel remains
the same. This creates a progressively richer mixture, which
can result in engine roughness and an appreciable loss of
power. The roughness normally is due to spark plug fouling
from excessive carbon buildup on the plugs. Carbon buildup
occurs because the rich mixture lowers the temperature
inside the cylinder, inhibiting complete combustion of the
fuel. This condition may occur during the pretakeoff runup
at high-elevation airports and during climbs or cruise flight
at high altitudes. To maintain the correct fuel/air mixture, the
mixture must be leaned using the mixture control. Leaning
the mixture decreases fuel flow, which compensates for the
decreased air density at high altitude.

During a descent from high altitude, the mixture must
be enriched, or it may become too lean. An overly lean
mixture causes detonation, which may result in rough
engine operation, overheating, and a loss of power. The
best way to maintain the proper mixture is to monitor the
engine temperature and enrich the mixture as needed. Proper
mixture control and better fuel economy for fuel-injected
engines can be achieved by use of an exhaust gas temperature
(EGT) gauge. Since the process of adjusting the mixture can
vary from one aircraft to another, it is important to refer to
the airplane flight manual (AFM) or the pilot's operating
handbook (POH) to determine the specific procedures for
a given aircraft.