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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

Blocked pitot system with clear static system.
Figure 7-10. Blocked pitot system with clear static system.

For example, take an aircraft and slow it down to zero knots
at given altitude. If the static port (providing static pressure)
and the pitot tube (providing dynamic pressure) are both
unobstructed, the following claims can be made:
1. The ASI would be zero.
2. There must be a relationship between both dynamic
and static pressure. At zero speed, dynamic pressure
and static pressure are the same: static air pressure.
3. Because both dynamic and static air pressure are equal
at zero speed with increased speed, dynamic pressure
must include two components: static pressure and
dynamic pressure.

It can be inferred that airspeed indication must be based upon
a relationship between these two pressures, and indeed it is.
An ASI uses the static pressure as a reference pressure and
as a result, the ASI's case is kept at this pressure behind the
diaphragm. On the other hand, the dynamic pressure through
the pitot tube is connected to a highly sensitive diaphragm
within the ASI case. Because an aircraft in zero motion
(regardless of altitude) results in a zero airspeed, the pitot
tube always provides static pressure in addition to dynamic
pressure.

Therefore, the airspeed indication is the result of two
pressures: the pitot tube static and dynamic pressure within
the diaphragm as measured against the static pressure in case.
What does this mean if the pitot tube is obstructed?

If the aircraft were to descend, the pressure in the pitot
system including the diaphragm would remain constant. It is
clogged and the diaphragm is at a single pressure. But as the
descent is made, the static pressure would increase against
the diaphragm causing it to compress thereby resulting in an
indication of decreased airspeed. Conversely, if the aircraft
were to climb, the static pressure would decrease allowing
the diaphragm to expand, thereby showing an indication of
greater airspeed. [Figure 7-10]

The pitot tube may become blocked during flight due to
visible moisture. Some aircraft may be equipped with pitot
heat for flight in visible moisture. Consult the AFM/POH for
specific procedures regarding the use of pitot heat.

Blocked Static System
If the static system becomes blocked but the pitot tube remains
clear, the ASI continues to operate; however, it is inaccurate.
The airspeed indicates lower than the actual airspeed when
the aircraft is operated above the altitude where the static
ports became blocked, because the trapped static pressure is
higher than normal for that altitude. When operating at a lower
altitude, a faster than actual airspeed is displayed due to the
relatively low static pressure trapped in the system.

Revisiting the ratios that were used to explain a blocked pitot
tube, the same principle applies for a blocked static port. If
the aircraft descends, the static pressure increases on the pitot
side showing an increase on the ASI. This assumes that the
aircraft does not actually increase its speed. The increase in
static pressure on the pitot side is equivalent to an increase
in dynamic pressure since the pressure can not change on
the static side.

If an aircraft begins to climb after a static port becomes
blocked, the airspeed begins to show a decrease as the aircraft
continues to climb. This is due to the decrease in static pressure
on the pitot side, while the pressure on the static side is held
constant.

A blockage of the static system also affects the altimeter and
VSI. Trapped static pressure causes the altimeter to freeze
at the altitude where the blockage occurred. In the case of
the VSI, a blocked static system produces a continuous zero
indication. [Figure 7-11]

 

7-11