IFH 5-9

100 200 50 150 Ram air Static air line Pitot tube Long lever Sector Handstaff pinion Diaphragm Pitot connection Figure 3-11. Mechanism of an airspeed indicator. Figure 5-11. Mechanism of an ASI. Figure 5-12. A true ASI allows the pilot to correct IAS for nonstandard temperature and pressure. 120 130 140 150 2 4 6 MPH TEMP 30 300+ - AIRSPEED TRUE SPEED KNOTS I80 40 60 I60 80 I00 I20 I40 40 I40 I20 60 80 100 Figure 3-12. A true airspeed indicator allows the pilot to correct indicated airspeed for nonstandard temperature and pressure. Some aircraft are equipped with true ASIs that have a temperature-compensated aneroid bellows inside the instrument case. This bellows modifies the movement of the rocking shaft inside the instrument case so the pointer shows the actual TAS. The TAS indicator provides both true and IAS. These instruments have the conventional airspeed mechanism, with an added subdial visible through cutouts in the regular dial. A knob on the instrument allows the pilot to rotate the subdial and align an indication of the outside air temperature with the pressure altitude being flown. This alignment causes the instrument pointer to indicate the TAS on the subdial. [Figure 5-12] Types of Airspeed Just as there are several types of altitude, there are multiple types of airspeed: indicated airspeed (IAS), calibrated airspeed (CAS), equivalent airspeed (EAS), and true airspeed (TAS). Indicated Airspeed (IAS) IAS is shown on the dial of the instrument, uncorrected for instrument or system errors. Calibrated Airspeed (CAS) CAS is the speed at which the aircraft is moving through the air, which is found by correcting IAS for instrument and position errors. The POH/AFM has a chart or graph to correct IAS for these errors and provide the correct CAS for the various flap and landing gear configurations. Equivalent Airspeed (EAS) EAS is CAS corrected for compression of the air inside the pitot tube. EAS is the same as CAS in standard atmosphere at sea level. As the airspeed and pressure altitude increase, the CAS becomes higher than it should be, and a correction for compression must be subtracted from the CAS. True Airspeed (TAS) TAS is CAS corrected for nonstandard pressure and temperature. TAS and CAS are the same in standard atmosphere at sea level. Under nonstandard conditions, TAS is found by applying a correction for pressure altitude and temperature to the CAS.