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 |
Figure 7-4. Look at the chart using a temperature of –10 °C and
the aircraft altitude is 1,000 feet above the airport elevation. The
chart shows that the reported current altimeter setting may place
the aircraft as much as 100 feet below the altitude indicated by
the altimeter.
Setting the Altimeter
Most altimeters are equipped with a barometric pressure
setting window (or Kollsman window) providing a means to
adjust the altimeter. A knob is located at the bottom of the
instrument for this adjustment.
To adjust the altimeter for variation in atmospheric pressure,
the pressure scale in the altimeter setting window, calibrated
in inches of mercury ("Hg) and/or millibars (mb), is adjusted
to match the given altimeter setting. Altimeter setting is
defined as station pressure reduced to sea level, but, an
altimeter setting is accurate only in the vicinity of the
reporting station. Therefore, the altimeter must be adjusted as
the flight progresses from one station to the next. Air traffic
control (ATC) will advise when updated altimeter settings
are available. If a pilot is not utilizing ATC assistance,
local altimeter settings can be obtained by monitoring local
automated weather observing system/automated surface
observation system (AWOS/ASOS) or automatic terminal
information service (ATIS) broadcasts.
Many pilots confidently expect the current altimeter setting
will compensate for irregularities in atmospheric pressure at
all altitudes, but this is not always true. The altimeter setting
broadcast by ground stations is the station pressure corrected
to mean sea level. It does not account for the irregularities at
higher levels, particularly the effect of nonstandard temperature. |
If each pilot in a given area is using the same altimeter setting,
each altimeter should be equally affected by temperature and
pressure variation errors, making it possible to maintain the
desired vertical separation between aircraft. This does not
guarantee vertical separation though. It is still imperative to
maintain a regimented visual scan for intruding air traffic.
When flying over high, mountainous terrain, certain atmospheric
conditions cause the altimeter to indicate an altitude of 1,000
feet or more higher than the actual altitude. For this reason, a
generous margin of altitude should be allowed—not only for
possible altimeter error, but also for possible downdrafts that
might be associated with high winds.
To illustrate the use of the altimeter setting system, follow a
flight from Dallas Love Field, Texas, to Abilene Municipal
Airport, Texas, via Mineral Wells. Before taking off from
Love Field, the pilot receives a current altimeter setting of
29.85 "Hg from the control tower or ATIS, and sets this value
in the altimeter setting window. The altimeter indication
should then be compared with the known airport elevation of
487 feet. Since most altimeters are not perfectly calibrated,
an error may exist.
When over Mineral Wells, assume the pilot receives a current
altimeter setting of 29.94 "Hg and sets this in the altimeter
window. Before entering the traffic pattern at Abilene
Municipal Airport, a new altimeter setting of 29.69 "Hg
is received from the Abilene Control Tower, and set in the
altimeter setting window. If the pilot desires to .y the traffic
pattern at approximately 800 feet above the terrain, and the
field elevation of Abilene is 1,791 feet, an indicated altitude of
2,600 feet should be maintained (1,791 feet + 800 feet = 2,591
feet, rounded to 2,600 feet).
The importance of properly setting the altimeter cannot be
overemphasized. Assume the pilot did not adjust the altimeter
at Abilene to the current setting and continued using the
Mineral Wells setting of 29.94 "Hg. When entering the Abilene
traffic pattern at an indicated altitude of 2,600 feet, the aircraft
would be approximately 250 feet below the proper traffic
pattern altitude. Upon landing, the altimeter would indicate
approximately 250 feet higher than the field elevation.
Mineral Wells altimeter setting 29.94
Abilene altimeter setting 29.69
Difference 0.25
(Since 1 inch of pressure is equal to approximately 1,000 feet
of altitude, 0.25 x 1,000 feet = 250 feet.) |
|