|Vertical Speed Indicator
The vertical speed indicator (VSI) or vertical velocity indicator
indicates whether the aircraft is climbing, descending, or in level flight.
The rate of climb or descent is indicated in feet per minute. If properly
calibrated, this indicator will register zero in level flight. [Figure
Although the vertical speed indicator operates solely from static
pressure, it is a differential pressure instrument. The case of the instrument
is airtight except for a small connection through a restricted passage
to the static line of the pitot-static system.
A diaphragm with connecting linkage and gearing to the indicator pointer is located inside the sealed case. Both the diaphragm and the case receive air from the static line at existing atmospheric pressure. When the aircraft is on the ground or in level flight, the pressures inside the diaphragm and the instrument case remain the same and the pointer is at the zero indication. When the aircraft climbs or descends, the pressure inside the diaphragm changes immediately; but due to the metering action of the restricted passage, the case pressure will remain higher or lower for a short time causing the diaphragm to contract or expand. This causes a differential pressure which is indicated on the instrument needle as a climb or descent.
The airspeed indicator is a sensitive, differential pressure gauge which measures and shows promptly the difference between (1) pitot, or impact pressure, and (2) static pressure, the undisturbed atmospheric pressure at level flight. These two pressures will be 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 (MPH), knots, or both. [Figure 3-4]
Kinds of Airspeed
There are three kinds of airspeed that the pilot should understand:
Indicated airspeed (IAS) is the direct instrument reading obtained from the airspeed indicator, uncorrected for variations in atmospheric density, installation error, or instrument error.
Calibrated airspeed (CAS) is indicated airspeed corrected for installation error and instrument error. Although manufacturers attempt to keep airspeed errors to a minimum, it is not possible to eliminate all errors throughout the airspeed operating range. At certain airspeeds and with certain flap settings, the installation and instrument error may be several miles per hour. This error is generally greatest at low airspeeds. In the cruising and higher airspeed ranges, indicated airspeed and calibrated airspeed are approximately the same.
It may be important to refer to the airspeed calibration chart to correct for possible airspeed errors because airspeed limitations such as those found on the color-coded face of the airspeed indicator, on placards in the cockpit, or in the Airplane Flight Manual or Owner’s Handbook are usually calibrated airspeeds. Some manufacturers use indicated rather than calibrated airspeed to denote the airspeed limitations mentioned.
The airspeed indicator should be calibrated periodically because leaks may develop or moisture may collect in the tubing. Dirt, dust, ice, or snow collecting at the mouth of the tube may obstruct air passage and prevent correct indications, and also vibrations may destroy the sensitivity of the diaphragm.
The true airspeed indicator (TAS) is calibrated to indicate true airspeed under standard sea level conditions—that is, 29.92 in. Hg. and 15° C. Because air density decreases with an increase in altitude, the airplane has to be flown faster at higher altitudes to cause the same pressure difference between pitot impact pressure and static pressure. Therefore, for a given true airspeed, indicated airspeed decreases as altitude increases or for a given indicated airspeed, true airspeed increases with an increase in altitude.
A pilot can find true airspeed by two methods. The first method, which is more accurate, involves using a computer. In this method, the calibrated airspeed is corrected for temperature and pressure variation by using the airspeed correction scale on the computer.
A second method, which is a “rule of thumb,” can be used to compute the approximate true airspeed. This is done by adding to the indicated airspeed 2 percent of the indicated airspeed for each 1,000 feet of altitude.
Airspeed Indicator Markings
Airplanes weighing 12,500 pounds or less, manufactured after 1945 and certificated by the FAA, are required to have airspeed indicators that conform in a standard color-coded marking system. This system of color-coded markings enables the pilot to determine at a glance certain airspeed limitations which are important to the safe operation of the aircraft. For example, if during the execution of a maneuver, the pilot notes that the airspeed needle is in the yellow arc and is rapidly approaching the red line, immediate corrective action to reduce the airspeed should be taken. It is essential at high airspeed that the pilot use smooth control pressures to avoid severe stresses upon the aircraft structure. [Figure 3-4]
The following is a description of the standard color-code markings on airspeed indicators used on single-engine light airplanes:
• FLAP OPERATING RANGE (the white arc).
• POWER-OFF STALLING SPEED WITH THE WING FLAPS AND LANDING GEAR IN THE LANDING POSITION (the lower limit of the white arc).
• MAXIMUM FLAPS EXTENDED SPEED (the upper limit of the white arc). This is the highest airspeed at which the pilot should extend full flaps. If flaps are operated at higher airspeeds, severe strain or structural failure could result.
• NORMAL OPERATING RANGE (the green arc).
• POWER-OFF STALLING SPEED WITH THE WING FLAPS AND LANDING GEAR RETRACTED (the lower limit of the green arc).
• MAXIMUM STRUCTURAL CRUISING SPEED (the upper limit of the green arc). This is the maximum speed for normal operation.
• CAUTION RANGE (the yellow arc). The pilot should avoid this area unless in smooth air.
• NEVER-EXCEED SPEED (the red line). This is the maximum speed at which
the airplane can be operated in smooth air.
This speed should never be exceeded intentionally.
Other Airspeed Limitations
There are other important airspeed limitations not marked on the face of the airspeed indicator. These speeds are generally found on placards in view of the pilot and in the Airplane Flight Manual or Pilot’s Operating Handbook.
One example is the MANEUVERING SPEED. This is the “rough air” speed and the maximum speed for abrupt maneuvers. If during flight, rough air or severe turbulence is encountered, the airspeed should be reduced to maneuvering speed or less to minimize the stress on the airplane structure.
Other important airspeeds include LANDING GEAR OPERATING SPEED, the maximum speed for extending or retracting the landing gear if using aircraft equipped with retractable landing gear; the BEST ANGLE-OF-CLIMB SPEED, important when a short-field takeoff to clear an obstacle is required; and the BEST RATE-OF-CLIMB SPEED, the airspeed that will give the pilot the most altitude in a given period of time. The pilot who flies the increasingly popular light twin-engine aircraft must know the aircraft’s MINIMUM CONTROL SPEED, the minimum flight speed at which the aircraft is satisfactorily controllable when an engine is suddenly made inoperative with the remaining engine at takeoff power. The last two airspeeds are now marked either on the face of the airspeed indicator or on the instrument panel of recently manufactured airplanes.
Descriptions of these airspeed limitations are, through choice, limited to layman’s language.
The following are abbreviations for performance speeds:
VA—design maneuvering speed.
VC—design cruising speed.
VF—design flap speed.
VFE—maximum flap extended speed.
VLE—maximum landing gear extended speed.
VLO—maximum landing gear operating speed.
VS—the stalling speed or the minimum steady flight speed at which the airplane is controllable.
VS0—the stalling speed or the minimum steady flight speed in the landing configuration.
VS1—the stalling speed or the minimum steady flight speed obtained in a specified configuration.
VX—speed for best angle of climb.
VY—speed for best rate of climb.