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It should be pointed out, however, that if each pilot in a
given area were to use the same altimeter setting, each
altimeter will be equally affected by temperature and
pressure variation errors, making it possible to maintain
the desired separation between aircraft.
When flying over high mountainous terrain, certain
atmospheric conditions can 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
of a cruising altitude, particularly at higher altitudes,
remember to anticipate that COLDER-THAN-STANDARD
TEMPERATURE will place the glider LOWER
than the altimeter indicates. Therefore, a higher altitude
should be used to provide adequate terrain clearance. A
memory aid in applying the above is "from a high to a
low or hot to cold, look out below." [Figure 4-5]
SETTING THE ALTIMETER
To adjust the altimeter for variation in atmospheric
pressure, the pressure scale in the altimeter setting window,
calibrated in inches of mercury (in. Hg.), is
adjusted to correspond with the given altimeter setting.
Altimeter settings can be defined as station pressure
reduced to sea level, expressed in inches of mercury.
The station reporting the altimeter setting takes an
hourly measurement of the station’s atmospheric pressure
and corrects this value to sea level pressure. These
altimeter settings reflect height above sea level only in
the vicinity of the reporting station. Therefore, it is necessary
to adjust the altimeter setting as the flight progresses
from one station to the next.
Title 14 of the Code of Federal Regulations (14 CFR) part
91 provides the following concerning altimeter settings.
The cruising altitude of an aircraft below 18,000 feet
mean sea level (MSL) shall be maintained by reference to
an altimeter that is set to the current reported altimeter set-
4-5
Figure 4-5. Nonstandard pressure and temperature.
4-6
error, but also for possible downdrafts that are particularly
prevalent if high winds are encountered.
To illustrate the use of the altimeter setting system, follow
a cross-country flight from TSA Gliderport,
Midlothian, Texas, to Winston Airport, Snyder, Texas,
via Stephens County Airport, Breckenridge, Texas.
Before takeoff from TSA Gliderport, the pilot receives
a current local altimeter setting of 29.85 from the Fort
Worth AFSS. This value is set in the altimeter setting
window of the altimeter. The altimeter indication
should then be compared with the known airport elevation
of 660 feet. Since most altimeters are not perfectly
calibrated, an error may exist. If an altimeter indication
varies from the field elevation more than 75 feet, the
accuracy of the instrument is questionable and it should
be referred to an instrument repair station.
When over Stephens County Airport, assume the pilot
receives a current area altimeter setting of 29.94 and
applies this setting to the altimeter. Before entering the
traffic pattern at Winston Airport, a new altimeter setting
of 29.69 is received from the Automated Weather
Observing System (AWOS), and applied to the altimeter.
If the pilot desires to enter the traffic pattern at
approximately 1,000 feet above terrain, and the field
elevation of Winston Airport is 2,430 feet, an indicated
altitude of 3,400 feet should be used (2,430 feet + 1000
feet = 3,420 feet, rounded to 3,400 feet).
The importance of properly setting and reading the altimeter
cannot be overemphasized. Let us assume that the pilot
neglected to adjust the altimeter at Winston Airport to the
current setting, and uses the Stephens CO area setting of
29.94. If this occurred, the glider, when entering the
Winston Airport traffic pattern, would be approximately
250 feet below the proper traffic pattern altitude of 3,200
feet, and the altimeter would indicate approximately 250
feet more than the field elevation (2,430 feet) upon landing.
Actual altimeter setting 29.94
Correct altimeter setting 29.69
Difference .25
(1 inch of pressure is equal to approximately 1,000 feet
of altitude—.25 x 1,000 feet = 250 feet)
The previous calculation may be confusing, particularly
in determining whether to add or subtract the
amount of altimeter error. The following additional
explanation is offered and can be helpful in finding the
solution to this type of problem.
There are two means by which the altimeter pointers can
be moved. One utilizes changes in air pressure while the
　
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