| Home | Privacy | Contact |

Pilot's Handbook of Aeronautical Knowledge
Flight Instruments
Pitot-Static Flight Instruments

| First | Previous | Next | Last |

Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




An IVSI incorporates accelerometers to help the instrument immediately indicate changes in vertical speed.
Figure 7-6. An IVSI incorporates accelerometers to help the
instrument immediately indicate changes in vertical speed.

When the pressure differential stabilizes at a definite ratio,
the needle indicates the rate of altitude change.

The VSI displays two different types of information:
• Trend information shows an immediate indication of
an increase or decrease in the aircraft's rate of climb
or descent.
• Rate information shows a stabilized rate of change in

The trend information is the direction of movement of the
VSI needle. For example, if an aircraft is maintaining level
flight and the pilot pulls back on the control yoke causing the
nose of the aircraft to pitch up, the VSI needle moves upward
to indicate a climb. If the pitch attitude is held constant,
the needle stabilizes after a short period (6–9 seconds) and
indicates the rate of climb in hundreds of fpm. The time
period from the initial change in the rate of climb, until the
VSI displays an accurate indication of the new rate, is called
the lag. Rough control technique and turbulence can extend
the lag period and cause erratic and unstable rate indications.
Some aircraft are equipped with an instantaneous vertical
speed indicator (IVSI), which incorporates accelerometers to
compensate for the lag in the typical VSI. [Figure 7-6]

Instrument Check
As part of a preflight check, proper operation of the VSI must
be established. Make sure the VSI indicates near zero prior
to leaving the ramp area and again just before takeoff. If the
VSI indicates anything other than zero, that indication can
be referenced as the zero mark. Normally, if the needle is
not exactly zero, it is only slightly above or below the zero
line. After takeoff, the VSI should trend upward to indicate
a positive rate of climb and then, once a stabilized climb is
established, a rate of climb can be referenced.

Airspeed indicator (ASI).
Figure 7-7. Airspeed indicator (ASI).

Airspeed Indicator (ASI)
The ASI is a sensitive, differential pressure gauge which
measures and promptly indicates the difference between pitot
(impact/dynamic pressure) and static pressure. These two
pressures are equal when the aircraft is parked on the ground
in calm air. When the aircraft moves through the air, the
pressure on the pitot line becomes greater than the pressure
in the static lines. This difference in pressure is registered by
the airspeed pointer on the face of the instrument, which is
calibrated in miles per hour, knots (nautical miles per hour),
or both. [Figure 7-7]

The ASI is the one instrument that utilizes both the pitot,
as well as the static system. The ASI introduces the static
pressure into the airspeed case while the pitot pressure
(dynamic) is introduced into the diaphragm. The dynamic
pressure expands or contracts one side of the diaphragm,
which is attached to an indicating system. The system drives
the mechanical linkage and the airspeed needle.