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Instrument Flying Handbook
Flight Instruments
Dynamic Pressure Type Instrument

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Instrument Flying


Table of Contents

Chapter 1. Human Factors
Chapter 2. Aerodynamic Factors
Chapter 3. Flight Instruments
Chapter 4. Section I
Airplane Attitude Instrument
Using Analog Instrumentation
Chapter 4. Section II
Airplane Attitude Instrument
Using an Electronic Flight

Chapter 5. Section I
Airplane Basic
Flight Maneuvers
Using Analog Instrumentation
Chapter 5. Section II
Airplane Basic
Flight Maneuvers
Using an Electronic Flight

Chapter 6. Helicopter
Attitude Instrument Flying

Chapter 7. Navigation Systems
Chapter 8. The National
Airspace System

Chapter 9. The Air Traffic
Control System

Chapter 10. IFR Flight
Chapter 11. Emergency

That aircraft must be equipped with an altitude alert system
that signals an alert when the altitude displayed to the flight
crew deviates from the selected altitude by more than (in most
cases) 200 feet. For each condition in the full RVSM flight
envelope, the largest combined absolute value for residual
static source error plus the avionics error may not exceed 200
feet. Aircraft with TCAS must have compatibility with RVSM
Operations. Figure 3-9 illustrates the increase in aircraft
permitted between FL 180 and FL 410. Most noteworthy,
however, is the economization that aircraft can take advantage
of by the higher FL being available to more aircraft.

Increase in Aircraft Permitted Between FL 180 and FL 410.
Figure 3-9. Increase in Aircraft Permitted Between FL 180 and FL 410.

Vertical Speed Indicator (VSI)
The VSI in Figure 3-10 is also called a vertical velocity
indicator (VVI), and was formerly known as a rate-of-
climb indicator. It is a rate-of-pressure change instrument
that gives an indication of any deviation from a constant
pressure level.

Inside the instrument case is an aneroid very much like the
one in an ASL Both the inside of this aneroid and the inside
of the instrument case are vented to the static system, but
the case is vented through a calibrated orifice that causes
the pressure inside the case to change more slowly than
the pressure inside the aneroid. As the aircraft ascends, the
static pressure becomes lower. The pressure inside the case
compresses the aneroid, moving the pointer upward, showing
a climb and indicating the rate of ascent in number of feet
per minute (1pm).

When the aircraft levels off, the pressure no longer changes.
The pressure inside the case becomes equal to that inside
the aneroid, and the pointer returns to its horizontal, or
zero, position. When the aircraft descends, the static
pressure increases. The aneroid expands, moving the pointer
downward, indicating a descent.

The pointer indication in a VSI lags a few seconds behind
the actual change in pressure. However, it is more sensitive
than an altimeter and is useful in alerting the pilot of an
upward or downward trend, thereby helping maintain a
constant altitude.

Some of the more complex VSIs, called instantaneous vertical
speed indicators (IVSJ), have two accelerometer-actuated air
pumps that sense an upward or downward pitch of the aircraft
and instantaneously create a pressure differential. By the time
the pressure caused by the pitch acceleration dissipates, the
altitude pressure change is effective.

Dynamic Pressure Type Instrument

Airspeed Indicator (ASI)
An ASI is a differential pressure gauge that measures the
dynamic pressure of the air through which the aircraft is
flying. Dynamic pressure is the difference in the ambient
static air pressure and the total, or ram, pressure caused by
the motion of the aircraft through the air. These two pressures
are taken from the pitot-static system.

Rate of Climb or Descent.
Figure 3-10. Rate of Climb or Descent in Thousands of Feet Per