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When flying at or above 18,000 feet (Flight Level 180) the altimeter must be set to 29.92. These altitudes
are primarially used by fast jet aircraft. Since there is no possibility of ground collision, all aircraft
operate with the same altimeter setting.
Effects of Temperature
Temperature affects the indicated altitude. The effect is not as drastic as pressure changes. Altimeters in
small aircraft have no simple means to compensate for non-standard temperature. The effect is similar to
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high and low pressure changes. When going from low temperature to higher temperature, the aircraft will
be higher than the indicated altitude. When going from high temperature to low, the aircraft will be lower
than indicated on the altimeter. The pilot should keep this in mind if terrain clearance is a factor in the
flight.
MEMORY AID: From high to low (pressure or temperature) LOOK OUT BELOW.
When flying over mountainous terrain, atmospheric conditions can cause the altimeter to indicate
erroneous altitude by as much as 1,000 feet. Therefore, a generous margin of safety should be planned
when flying over mountainous terrain..
Vertical Speed Indicator (VSI)
The mechanism of the VSI is similar to the altimeter except the bellows contains a small calibrated hole
that allows the pressure inside the bellows to slowly adjust to the same pressure as in the case. Therefore
the pressure inside the bellows is similar to what it was a few seconds ago.
If the change in pressure is slow, the up or down reading will be small. If the up or down altitude change
is large over a short time, the rate of climb or decent will be large. If the pressure both inside and outside
the bellows stays the same, the VSI will indicate zero.
The single pointer indicates level flight (indicating 0), climb in feet per minute ( pointer deflected
upward), and decent in feet per minute (pointer deflected downward).
In small piston engine powered aircraft, the rate of climb will usually be less that 1000 feet per minute.
Usual rate of decent enroute or approach to landing will be in the 500 feet per minute range. When flying
straight and level, the instrument should indicate zero. Also when sitting stable on the ground the
instrument should indicate zero. Most instruments are equipped with a small adjusting screw to calibrate
the zero position when the aircraft is at rest on the ground.
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Airspeed Indicator (ASI)
The Airspeed Indicator (ASI) measures the speed of the aircraft through the air. This should not be
confused with groundspeed. Winds can affect how fast the aircraft tracks over the ground. Groundspeed
is seldom the same as airspeed.
Principle of Operation
"Impact" air hiting the opening of the pitot tube which is pointing in the direction of travel creates a
pressure in the pitot tube line. This pressure is connected by a small tube to the inside of a bellows in the
ASI instrument. The outside atmospheric static pressure enters the case from the static line. The
mechanism inside the ASI therefore measures the difference between the pitot pressure and the static
pressure. Since the impact pressure of the pitot tube is proportional to the speed through the air, the speed
through the air is indicated by the instrument.
Indicated Airspeed (IAS)
The color coding of the airspeed indicator has meaning. The color arcs are as follows.
White Arc - Stall Speeds and Flap operating Range
----- -- Lower end of arc is the Power Off Stall speed with flaps and landing gear in the landing position.
----- -- Upper end of arc is the maximum flaps extend speed.
Green Arc - Normal operating airspeed range
------- Lower end of arc is the power off stall speed clean (flaps and gear up)
------- Upper end of arc is maximum structural cruise speed. (Max normal operating speed).
Yellow Arc - Caution range. Avoid this area unless in smooth air.
Red Line - Never exceed speed.
-------This is the maximum speed at which the aircraft can operate safely. ------- It should never be
intentionally exceeded.
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Calibrated Airspeed (CAS)
Although aircraft designers attempt to keep airspeed errors to a minimum, it is not possible to achieve
complete accuracy throughout the complete range of the instrument. Two types of errors can be
introduced.
a. Installation error caused by the static ports sensing erroneous pressure. This is due to the
unpredictability of the effects of the slipstream around the aircraft at various speeds and attitudes.
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