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b. The pitot tube does not always present the same frontal appearance to the atmosphere at varying
attitudes.
The pilot should consult the Pilot Operating Handbook (POH) for the table applicable to the aircraft
being flown.
True Airspeed (TAS)
As altitude increases , air density decreases. The impact pressure at the port of the pitot tube is less at
higher altitudes. The airplane is actually traveling through the air faster than indicated on the ASI.
Consequently as altitude increase, Indicated Airspeed decreases.
A mathematical correction factor must be applied to Indicated Airspeed (or Calibrated Airspeed) to
arrive at a correct True Airspeed (TAS). This calculation can be made with he E6B Flight computer, or
an approximate correction can be made by adding 2 percent per 1,000 feet of altitude to the IAS.
EXAMPLE: Given IAS is 140kt and ALT is 6,000 feet. Find TAS.
2% x 6 = 12% (.12)
140 x 0.12 = 16.8
140 + 16.8 = 156.8 kt. (TAS)
Some airspeed indicators have built-in adjustment scales that allows the pilot to adjust the instrument for
temperature and pressure. Both the IAS and TAS can be read from such an airspeed indicator.
V Speeds
The Pilot Operating Handbook normally lists various airspeeds for differing situations and conditions.
The definition of the usual V speeds is shown below. is an abbreviation for Velocity.
VA. ..... design maneuvering speed
VFE.... Maximum flap extend speed
VLE.... maximum landing gear extend speed
Pitot and Static System
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VLO....maximum landing gear operating speed
VNE....never exceed speed
VNO...maximum structural cruising speed
VR......rotation speed
VS0.... the power-off stalling speed or minimum flight speed in landing configuration
VS1.... the power-off stalling speed (clean) with flaps and landing gear retracted.
VX...... best angle of climb speed
VY......best rate of climb speed
Best Glide Speed
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Pitot and Static System
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GYROSCOPIC INSTRUMENTS
Gyroscopic Instruments
Gyroscopic instruments may be driven either electrically or by vacuum. In most light aircraft the Turn
Coordinator (TC) is electrically driven. Usually the Heading Indicator (HI) and Attitude Indicator (AI)
are vacuum driven.
Gyroscopic Principles
Any spinning object possesses gyroscopic characteristics. The central mechanism of the gyroscope is a
wheel similar to a bycycle wheel. It's outer rim has a heavy mass. It rotates at high speed on very low
friction bearings. When it is rotating normally, it resists changes in direction.
The gyroscope exhibits two predominant characteristics.
Rigity in Space
Precession
Rigidity in Space.
The gyroscope resists turning. When it is "gimbaled" ( free to move in a given direction) such that it is
free to move either in 1, 2 or 3 dimensions, any surface such as an instrument dial attached to the gyro
assembly will remain rigid in space even though the case of the gyro turns. The Attitude Indicator (AI)
and the Heading Indicator (HI ) use this property of rigidity in space for their operation. The HI
responds only to change of heading. The AI responds to both changes in Pitch and in Roll.
Precession
Precession is the deflection of a spinning wheel 90 ° to the plane of rotation when a deflective force is
applied at the rim. If a force is applied the top of the rim (the plane of rotation), the precession (turn) will
be 90° in the horizontal plane to the left. The Turn Coordinator (TC) uses this precession property. For
example, then taxiing on the ground, the Turn Coordinator will move, with the small airplane in the
instrument showing a bank, even though the aircraft is level. The banking of the small aircraft
presentation indicates only that the aircraft is turning.
The Vacuum System
The Attitude Indicator (AI) and the Heading Indicator (HI) n light aircraft are usually driven by a
vacuum system. The principal components are shown below. Not shown are auxillary devices such as
valves, filters etc. A pump provides the vacuum to the AI and HI through a system of vacuum lines. A
Vacuum Gauge is attached to the lines which gives the pilot an indication that adequate vacuum is being
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generated.
Heading Indicator (HI)
The Heading Indicator (HI) uses the principle of Rigidity In Space for it’s operation. The Gyro is
mounted such that it registers changes around the vertical axis only; i.e. direction changes. The compass
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