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The two parts of the system are
(1) the pitot tube and pressure line
(2) the static pressure system and lines.
The pitot tube is normally mounted on the leading edge of a wing. The pitot tube on an aircraft used
only for flight under Visual Flight Rules (VFR) may not be heated to prevent icing. Aircraft to be used
under Instrument Flight Rules (IFR) are heated electrically, to prevent icing when operating in visible
moisture and cold temperatures. A switch in the cockpit controls Pitot heat.
The static pressure port is normally found on the side of the fuselage. On later model aircraft, an
alternate static source is provided inside the cockpit. The pilot can select the internal static source if the
outside source becomes clogged with ice.. When the pilot selects the alternative source, the instruments
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relying on the static pressure may operate slightly differently.
1. The altimeter (ALT) may indicate a higher-than-actual altitude.
2. The vertical speed indicator (VSI) will momentarily indicate a climb, then will settle back the initial
indication.
3. The Airspeed Iindicator (ASI) will indicate greater-than-normal airspeed. Altimeter (ALT)
THE ALTIMETER
The Altimeter (ALT) allows the pilot to determine the height above Mean Sea Level (MSL). Correct
altitude indication is very important for several reasons.
a) The pilot must be sure the aircraft is being flown high enough to clear terrain and other obstacles.
b) The pilot must maintain altitude according to certain air traffic rules and instructions to minimize the
possibility of mid-air collision.
c) The pilot can often select more favorable winds at certain altitudes.
d) True Airspeed calculation requires that the altitude be known.
The Altimeter measures the pressure of the outside air. A small bellows inside the altimeter which
contains a constant pressure inside expands when the aircraft climbs, and contracts when the aircraft
decends. This bellows is connected to a gear arrangement which causes the hands to turn as the bellows
expands or contracts. The altimeter is essentially a barometer which is measuring the outside air pressure,
but the indications on the dial indicate hundreds and thousands of feet.
Most altimeters have either 2 or 3 pointers. If 2 pointers, the longer one indicates hundreds of feet, and
the shorter pointer indicates thousands of feet. A third very short pointer which indicates ten's of
thousands altitude exist on some altimeters. The indication in the diagram shown below is 1,430 feet.
Altimeter Setting.
The altimeter is an aneroid barometer. It is correct only when in a known atmosphere. An International
Standard Atmosphere has been defined as a barometric Sea Level pressure of 29.92 inches of Mercury
(Hg), and a temperature of 15 degrees Celsius.
Effects of Atmospheric Pressure Changes
Whenever the altimeter is in a non-standard temperature and pressure the altimeter reads incorrectly, and
an adjustment means must be provided to compensate for the non-standard conditions. Atmospheric
pressure change has the greatest effect on the instrument.
Pitot and Static System
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On modern altimeters, an adjusting knob and scale is provided to allow adjustment for non-standard
pressure. In the diagram of the altimeter face above, a window on the right of the instrument shows a
graduated barometric scale, called the altimeter setting. The pilot can adjust the altimeter setting with the
knob on the lower left. The setting shown is 30.00.
If the altimeter is not periodically readjusted to the local barometric pressure, the plane will be too high if
the local sea level pressure pressure is higher than 29.92 and will be too low if the local sea level
pressure is less than 29.92 in. Hg.
As one flies cross country, the altimeter should be adjusted every 100 miles or so. If flying from a low to
a higher pressure area, the aircraft will be higher than indicated if appropriate altimeter adjustment is not
made periodically. For example, if the altimeter indicates 5000 feet, it will actually be above 5000 feet
when in the higher pressure area.
Likewise, when flying from a high pressure area to a lower pressure area, the aircraft will be falsely low
if no adjustment is made. If the altimeter is indicating 5000 feet, the aircraft will be below 5000 feet
when in the lower pressure area..
The altimeter setting can be obtained in flight from any Air Traffic Control facility or from any FAA
Flight Service Station. If taking off from an airport where no contact can be made with such a facility, set
the altimeter to the altitude of the airport prior to takeoff.
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