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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

Other Airspeed Limitations
Some important airspeed limitations are not marked on the
face of the ASI, but are found on placards and in the AFM/
POH. These airspeeds include:

• Design maneuvering speed (VA)—the maximum
speed at which the structural design's limit load can
be imposed (either by gusts or full deflection of the
control surfaces) without causing structural damage.
It is important to consider weight when referencing
this speed. For example, VA may be 100 knots when
an airplane is heavily loaded, but only 90 knots when
the load is light.

• Landing gear operating speed (VLO)—the maximum
speed for extending or retracting the landing gear if
flying an aircraft with retractable landing gear.

• Landing gear extended speed (VLE)—the maximum
speed at which an aircraft can be safely .own with
the landing gear extended.

• Best angle-of-climb speed (VX)—the airspeed at
which an aircraft gains the greatest amount of altitude
in a given distance. It is used during a short-field
takeoff to clear an obstacle.

• Best rate-of-climb speed (VY)—the airspeed that
provides the most altitude gain in a given period of
time.

• Single-engine best rate-of-climb (VYSE)—the best
rate-of-climb or minimum rate-of-sink in a light
twin-engine aircraft with one engine inoperative. It is
marked on the ASI with a blue line. VYSE is commonly
referred to as "Blue Line."

• Minimum control speed (VMC)—the minimum flight
speed at which a light, twin-engine aircraft can be
satisfactorily controlled when an engine suddenly
becomes inoperative and the remaining engine is at
takeoff power.

A blocked pitot tube, but clear drain hole.
Figure 7-9. A blocked pitot tube, but clear drain hole.

Instrument Check
Prior to takeoff, the ASI should read zero. However, if there
is a strong wind blowing directly into the pitot tube, the ASI
may read higher than zero. When beginning the takeoff, make
sure the airspeed is increasing at an appropriate rate.

Blockage of the Pitot-Static System
Errors almost always indicate blockage of the pitot tube, the
static port(s), or both. Blockage may be caused by moisture
(including ice), dirt, or even insects. During preflight, make
sure the pitot tube cover is removed. Then, check the pitot and
static port openings. A blocked pitot tube affects the accuracy
of the ASI, but, a blockage of the static port not only affects
the ASI, but also causes errors in the altimeter and VSI.

Blocked Pitot System
The pitot system can become blocked completely or only
partially if the pitot tube drain hole remains open. If the pitot
tube becomes blocked and its associated drain hole remains
clear, ram air no longer is able to enter the pitot system. Air
already in the system vents through the drain hole, and the
remaining pressure drops to ambient (outside) air pressure.
Under these circumstances, the ASI reading decreases to
zero, because the ASI senses no difference between ram and
static air pressure. The ASI no longer operates since dynamic
pressure can not enter the pitot tube opening. Static pressure
is able to equalize on both sides since the pitot drain hole
is still open. The apparent loss of airspeed is not usually
instantaneous but happens very quickly. [Figure 7-9]

If both the pitot tube opening and the drain hole should
become clogged simultaneously, then the pressure in the pitot
tube is trapped. No change is noted on the airspeed indication
should the airspeed increase or decrease. If the static port
is unblocked and the aircraft should change altitude, then a
change is noted on the ASI. The change is not related to a
change in airspeed but a change in static pressure. The total
pressure in the pitot tube does not change due to the blockage;
however, the static pressure will change.

Because airspeed indications rely upon both static and
dynamic pressure together, the blockage of either of these
systems affects the ASI reading. Remember that the ASI has
a diaphragm in which dynamic air pressure is entered. Behind
this diaphragm is a reference pressure called static pressure
that comes from the static ports. The diaphragm pressurizes
against this static pressure and as a result changes the airspeed
indication via levers and indicators. [Figure 7-10]

 

7-10