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(IFR) requires a gyroscopic rate-of-turn indicator, slip-skid
indicator, sensitive altimeter adjustable for barometric
pressure, clock displaying hours, minutes, and seconds with
a sweep-second pointer or digital presentation, gyroscopic
pitch-and-bank indicator (artificial horizon), and gyroscopic
direction indicator (directional gyro or equivalent).
Flight Instruments
Chapter 3
3-2
Figure 3-1. A Typical Electrically Heated Pitot-Static Head.
Aircraft that are flown in instrument meteorological conditions
(IMC) are equipped with instruments that provide attitude
and direction reference, as well as navigation instruments that
allow precision flight from takeoff to landing with limited or
no outside visual reference.
The instruments discussed in this chapter are those required
by Title 14 of the Code of Federal Regulations (14 CFR)
part 91, and are organized into three groups: pitot-static
instruments, compass systems, and gyroscopic instruments.
The chapter concludes with a discussion of how to preflight
these systems for IFR flight. This chapter addresses additional
avionics systems such as Electronic Flight Information
Systems (EFIS), Ground Proximity Warning System
(GPWS), Terrain Awareness and Warning System (TAWS),
Traffic Alert and Collision Avoidance System (TCAS),
Head Up Display (HUD), etc., that are increasingly being
incorporated into general aviation aircraft.
Pitot/Static Systems
Pitot pressure, or impact air pressure, is sensed through an
open-end tube pointed directly into the relative wind flowing
around the aircraft. The pitot tube connects to pressure
operated flight instruments such as the ASI.
Static Pressure
Other instruments depend upon accurate sampling of the
ambient still air atmospheric pressure to determine the
height and speed of movement of the aircraft through the
air, both horizontally and vertically. This pressure, called
static pressure, is sampled at one or more locations outside
the aircraft. The pressure of the static air is sensed at a flush
port where the air is not disturbed. On some aircraft, air is
sampled by static ports on the side of the electrically heated
pitot-static head. [Figure 3-1] Other aircraft pick up the static
pressure through flush ports on the side of the fuselage or
the vertical fin. These ports are in locations proven by flight
tests to be in undisturbed air, and they are normally paired,
one on either side of the aircraft. This dual location prevents
lateral movement of the aircraft from giving erroneous static
pressure indications. The areas around the static ports may be
heated with electric heater elements to prevent ice forming
over the port and blocking the entry of the static air.
Three basic pressure-operated instruments are found in most
aircraft instrument panels. These are the sensitive altimeter,
ASI, and vertical speed indicator (VSI). All three receive
pressures sensed by the aircraft pitot-static system. The static
ports supply pressure to the ASI, altimeter, and VSI.
Blockage Considerations
The pitot tube is particularly sensitive to blockage especially
by icing. Even light icing can block the entry hole of the pitot
tube where ram air enters the system. This affects the ASI
and is the reason most airplanes are equipped with a pitot
heating system.
3-3
Figure 3-2. A Typical Pitot-Static System.
Indications of Pitot Tube Blockage
If the pitot tube becomes blocked, the ASI displays inaccurate
speeds. At the altitude where the pitot tube becomes blocked,
the ASI remains at the existing airspeed and doesn’t reflect
actual changes in speed.
• At altitudes above where the pitot tube became
blocked, the ASI displays a higher-than-actual
airspeed increasing steadily as altitude increases.
• At lower altitudes, the ASI displays a lower-than-actual
airspeed decreasing steadily as altitude decreases.
Indications from Static Port Blockage
Many aircraft also have a heating system to protect the
static ports to ensure the entire pitot-static system is clear
of ice. If the static ports become blocked, the ASI would
still function but could produce inaccurate indications. At
the altitude where the blockage occurs, airspeed indications
would be normal.
• At altitudes above which the static ports became
blocked, the ASI displays a lower-than-actual airspeed
continually decreasing as altitude is increased.
• At lower altitudes, the ASI displays a higher-than-actual
airspeed increasing steadily as altitude decreases.
The trapped pressure in the static system causes the altimeter
　
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