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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

Effects of nonstandard temperature on an altimeter.
Figure 7-3. Effects of nonstandard temperature on an altimeter.

This indicated altitude is correct, however, only when the sea
level barometric pressure is standard (29.92 "Hg), the sea level
free air temperature is standard (+15 degrees Celsius (°C) or
59 degrees Fahrenheit (°F)), and the pressure and temperature
decrease at a standard rate with an increase in altitude.

Adjustments for nonstandard pressures are accomplished by
setting the corrected pressure into a barometric scale located
on the face of the altimeter. The barometric pressure window is
sometimes referred to as the Kollsman window; only after the
altimeter is set does it indicate the correct altitude. The word
"correct" will need to be better explained when referring to
types of altitudes, but is commonly used in this case to denote
the approximate altitude above sea level. In other words, the
indicated altitude refers to the altitude read off of the altitude
which is uncorrected, after the barometric pressure setting
is dialed into the Kollsman window. The additional types of
altitudes are further explained later.

Effect of Nonstandard Pressure and Temperature
It is easy to maintain a consistent height above ground if the
barometric pressure and temperature remain constant, but
this is rarely the case. The pressure temperature can change
between takeoff and landing even on a local flight. If these
changes are not taken into consideration, flight becomes
dangerous.

If altimeters could not be adjusted for nonstandard pressure, a
hazardous situation could occur. For example, if an aircraft is
flown from a high pressure area to a low pressure area without
adjusting the altimeter, a constant altitude will be displayed,
but the actual height of the aircraft above the ground would
be lower then the indicated altitude. There is an old aviation
axiom: "GOING FROM A HIGH TO A LOW, LOOK OUT
BELOW." Conversely, if an aircraft is flown from a low
pressure area to a high pressure area without an adjustment
of the altimeter, the actual altitude of the aircraft is higher
than the indicated altitude. Once in flight, it is important to
frequently obtain current altimeter settings en route to ensure
terrain and obstruction clearance.

Many altimeters do not have an accurate means of being
adjusted for barometric pressures in excess of 31.00 inches
of mercury ("Hg). When the altimeter cannot be set to the
higher pressure setting, the aircraft actual altitude will be
higher than the altimeter indicates. When low barometric
pressure conditions occur (below 28.00), flight operations
by aircraft unable to set the actual altimeter setting are not
recommended.

Adjustments to compensate for nonstandard pressure do not
compensate for nonstandard temperature. Since cold air is
denser than warm air, when operating in temperatures that are
colder than standard, the altitude is lower than the altimeter
indication. [Figure 7-3] It is the magnitude of this "difference"
that determines the magnitude of the error. It is the difference
due to colder temperatures that concerns the pilot. When flying
into a cooler air mass while maintaining a constant indicated
altitude, true altitude is lower. If terrain or obstacle clearance
is a factor in selecting a cruising altitude, particularly in
mountainous terrain, remember to anticipate that a colder than-
standard temperature places the aircraft lower than the
altimeter indicates. Therefore, a higher indicated altitude may
be required to provide adequate terrain clearance. A variation
of the memory aid used for pressure can be employed:
"FROM HOT TO COLD, LOOK OUT BELOW." When the
air is warmer than standard, the aircraft is higher than the
altimeter indicates. Altitude corrections for temperature can
be computed on the navigation computer.

Extremely cold temperatures will also affect altimeter
indications. Figure 7-4, which was derived from ICAO
formulas, indicates how much error can exist when the
temperature is extremely cold.

 

7-4