Flight Instruments

Figure 8-6. An IVSI incorporates accelerometers to help the instrument immediately indicate changes in vertical speed. 8-8 .50.5 IU P D OI W N 2 2 3 3 4 The VSI displays two different types of information: Airspeed Indicator (ASI) The ASI is a sensitive, differential pressure gauge that • Trend information shows an immediate indication of measures and promptly indicates the difference between pitot an increase or decrease in the aircraft’s rate of climb (impact/dynamic pressure) and static pressure. These two or descent. pressures are equal when the aircraft is parked on the ground • Rate information shows a stabilized rate of change in in calm air. When the aircraft moves through the air, the altitude. pressure on the pitot line becomes greater than the pressure in the static lines. This difference in pressure is registered by The trend information is the direction of movement of the the airspeed pointer on the face of the instrument, which is VSI needle. For example, if an aircraft is maintaining level calibrated in miles per hour, knots (nautical miles per hour), flight and the pilot pulls back on the control yoke causing the or both. [Figure 8-7] nose of the aircraft to pitch up, the VSI needle moves upward to indicate a climb. If the pitch attitude is held constant, The ASI is the one instrument that utilizes both the pitot, the needle stabilizes after a short period (6–9 seconds) and as well as the static system. The ASI introduces the static indicates the rate of climb in hundreds of fpm. The time pressure into the airspeed case while the pitot pressure period from the initial change in the rate of climb, until the (dynamic) is introduced into the diaphragm. The dynamic VSI displays an accurate indication of the new rate, is called pressure expands or contracts one side of the diaphragm, the lag. Rough control technique and turbulence can extend which is attached to an indicating system. The system drives the lag period and cause erratic and unstable rate indications. the mechanical linkage and the airspeed needle. Some aircraft are equipped with an instantaneous vertical speed indicator (IVSI), which incorporates accelerometers Just as in altitudes, there are multiple types of airspeeds. to compensate for the lag in the typical VSI. [Figure 8-6] Pilots need to be very familiar with each type. • Indicated airspeed (IAS)—the direct instrument Instrument Check reading obtained from the ASI, uncorrected for As part of a preflight check, proper operation of the VSI variations in atmospheric density, installation error, must be established. Make sure the VSI indicates a near zero or instrument error. Manufacturers use this airspeed reading prior to leaving the ramp area and again just before as the basis for determining aircraft performance. takeoff. If the VSI indicates anything other than zero, that Takeoff, landing, and stall speeds listed in the AFM/ indication can be referenced as the zero mark. Normally, if the POH are IAS and do not normally vary with altitude needle is not exactly zero, it is only slightly above or below or temperature. 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. Diaphragm Long lever Sector Accelerometer Pitot connection 50 Pitot tube 100 150 200 Inlet from static port Ram air Static air line Handstaff pinion Calibrated leak Figure 8-7. Airspeed indicator (ASI).