Live chat with a certified ground instructor
start 1 2 3 4 5 6 7 8 9 quiz prev next

The Pitot-Static Instrument Family

The pitot-static instrument family is made up of all the aircraft instruments that obtain their information from either a pitot or static source of air. Pitot tube air is sometimes called ram air because it is air forced through a tube. Static air is air measured from a relatively non-moving or ‘static’ source located on or in the aircraft. The three pitot-static instruments are the airspeed indicator, the altimeter, and the vertical speed indicator.

The airspeed indicator is used to determine the speed of air flowing over the aircraft. The airspeed indicator uses a pitot tube that is positioned to give a good reference of the actual airspeed flowing over the wings. This pitot tube utilizes a small ram air tube that runs the air from the pitot to the airspeed indicator. A static air source, an air opening located in a relatively undisturbed place on the side of the fuselage or behind the pitot tube opening, is then sent to the airspeed indicator and the two are compared to determine airspeed. This is usually accomplished by a small barometer, which measures air pressure differential inside the instrument. The barometer looks like a miniature accordion and expands and collapses due to the pressure differentials moving a needle that is seen on the airspeed indicator.

The airspeed indicator is critical in certain flight regimes or phases. During takeoff and landing, it is crucial that the aircraft be flown at the correct airspeed for the flap setting and pitch attitude which you are flying in. If a pilot allows the airspeed to get too slow, he risks the onset of a stall. If he allows it to get too fast, he may not climb or on the landing he may float for a considerable distance.

The airspeed indicator commonly used on light piston single-engine aircraft is color coded and expresses its speed in knots or miles per hour. A white arc indicates the arc in which it is safe to use flaps. The green arc is the normal operating range of the aircraft. The yellow arc is the caution range for the airplane. It is fine to fly the airplane in the yellow range, unless the aircraft is doing maneuvers such as steep banks or dramatic pitch changes, or unless turbulence is encountered. The red line is the highest speed at which the aircraft should be flown. If you fly above this airspeed, it does not necessarily mean the wings will come off, but the aircraft was not designed to be flown this fast, and structural damage could occur in certain conditions.

The altimeter measures the vertical height of the airplane above a reference point, usually sea level. The actual height of an object above sea level is called true altitude. To measure this altitude correctly, the altimeter is equipped with a barometer, which measures air pressure. Standard air pressure at sea level is measured at 29.92" of mercury (hg). The pressure decreases one inch of mercury for every thousand feet of increase in altitude.

As you will learn in the weather chapter, air pressure also varies according to changing weather and atmospheric conditions. The altimeter in your aircraft is equipped with a window to allow you to change your altimeter setting to reflect current atmospheric conditions and thus reflect true altitude. As you fly on cross-country flights, you will occasionally need to reset your altimeter to the closest available airports along your route.

When you are flying above 18,000 feet in higher performance aircraft, however, you will use pressure altitude rather than true altitude for your altimeter. You will do this by setting the altimeter window to 29.92"—the standard altimeter setting. This is done for a couple of reasons. At heights above 18,000' airplanes travel with such speed that it is difficult to reset the altimeter every few minutes to local altimeter conditions. A setting of 29.92" allows aircraft to fly on the same pressure setting for increased safety, even in changing atmospheric conditions. As the atmospheric pressure changes, the altimeter will show the change, and the pilot will make the adjustments necessary to maintain consistent altitudes.

Another reason for using a pressure altitude is that by 18,000' the atmosphere is different than that found at the reporting local airport due to temperature and pressure differences.

Altimeters utilize the static system of the aircraft to obtain air pressure information. This pressure then moves an internal barometer to indicate changes in altitude.

The altimeter can also be affected by changes in temperature. Air molecules become more dense as the temperature becomes lower. A temperature change can adversely affect the altimeter’s measurements, but they are much less a factor than changes in elevation. A common memory aid used to consider the effects of high and low pressure changes due to weather and accompanying temperature changes is, “When flying from high to low or hot to cold, look out below!”

fig3-3.gif (2267 bytes)

The Vertical Speed Indicator or VSI is used to display the rate of climb or descent in feet per minute. The instrument is a secondary instrument because it depends on a barometer for information and tends to lag behind the actual pitch movements of the aircraft. The VSI uses static system information to observe trend changes in pressure, which are then displayed on the instrument. The VSI is often used as a backup instrument on private pilot maneuvers such as steep turns.