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Pilot's Handbook of Aeronautical Knowledge
Flight Instruments
Pitot-Static Flight Instruments

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Pilot's Handbook of Aeronautical Knowledge

Preface

Acknowledgements

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

Appendix

Glossary

Index

Pitot-static system and instruments.
Figure 7-1. Pitot-static system and instruments.

Pitot-Static Flight Instruments

The pitot-static system is a combined system that utilizes the
static air pressure, and the dynamic pressure due to the motion
of the aircraft through the air. These combined pressures are
utilized for the operation of the airspeed indicator (ASI),
altimeter, and vertical speed indicator (VSI). [Figure 7-1]

Impact Pressure Chamber and Lines
The pitot tube is utilized to measure the total combined
pressures that are present when an aircraft moves through
the air. Static pressure, also known as ambient pressure, is
always present whether an aircraft is moving or at rest. It is
simply the barometric pressure in the local area. Dynamic
pressure is present only when an aircraft is in motion;
therefore, it can be thought of as a pressure due to motion.
Wind also generates dynamic pressure. It does not matter if
the aircraft is moving through still air at 70 knots or if the
aircraft is facing a wind with a speed of 70 knots, the same
dynamic pressure is generated.

When the wind blows from an angle less than 90° off the
nose of the aircraft, dynamic pressure can be depicted on the
ASI. The wind moving across the airfoil at 20 knots is the
same as the aircraft moving through calm air at 20 knots.
The pitot tube captures the dynamic pressure, as well as the
static pressure that is always present.

The pitot tube has a small opening at the front which allows
the total pressure to enter the pressure chamber. The total
pressure is made up of dynamic pressure plus static pressure.
In addition to the larger hole in the front of the pitot tube,
there is a small hole in the back of the chamber which
allows moisture to drain from the system should the aircraft
enter precipitation. Both openings in the pitot tube need to
be checked prior to flight to ensure that neither is blocked.
Many aircraft have pitot tube covers installed when they sit
for extended periods of time. This helps to keep bugs and
other objects from becoming lodged in the opening of the
pitot tube.

The one instrument that utilizes the pitot tube is the ASI. The
total pressure is transmitted to the ASI from the pitot tube's
pressure chamber via a small tube. The static pressure is
also delivered to the opposite side of the ASI which serves
to cancel out the two static pressures, thereby leaving the
dynamic pressure to be indicated on the instrument. When the
dynamic pressure changes, the ASI shows either increase or
decrease, corresponding to the direction of change. The two
remaining instruments (altimeter and VSI) utilize only the
static pressure which is derived from the static port.

Static Pressure Chamber and Lines
The static chamber is vented through small holes to the
free undisturbed air on the side(s) of the aircraft. As the
atmospheric pressure changes, the pressure is able to move
freely in and out of the instruments through the small lines
which connect the instruments into the static system. An
alternate static source is provided in some aircraft to provide
static pressure should the primary static source become
blocked. The alternate static source is normally found inside
of the flight deck. Due to the venturi effect of the air flowing
around the fuselage, the air pressure inside the flight deck is
lower than the exterior pressure.

When the alternate static source pressure is used, the
following instrument indications are observed:
1. The altimeter indicates a slightly higher altitude than
actual.
2. The ASI indicates an airspeed greater than the actual
airspeed.
3. The VSI shows a momentary climb and then stabilizes
if the altitude is held constant.

 

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