pitot opening is blocked, but the drain hole is open, the system pressure will drop to ambient pressure, and the ASI pointer will drop to a zero reading. If both the opening and drain holes are blocked, the ASI will not indicate any change in airspeed. However, the ASI pointer will show altitude changes, as the associated static pressure changes. If both the pitot tube and the static system are blocked, the ASI pointer will read zero. If the static ports are blocked but the pitot tube remains open, the ASI will operate, but inaccurately. Types of airspeeds There are four types of airspeed that can be remembered with the acronym ICE-T. Indicated airspeed (IAS), is read directly off the ASI. It has no correction for air density variations, installation or instrument errors. Calibrated airspeed (CAS) is corrected for installation and instrument errors. Equivalent airspeed (EAS) is calibrated airspeed (CAS) corrected for the compressibility of air at a non-trivial Mach number. True airspeed (TAS) is CAS corrected for altitude and nonstandard temperature. TAS is used for flight planning. TAS increases as altitude increases, as air density decreases. TAS may be determined via a flight computer, such as the E6B. Some ASIs have a TAS ring. Alternatively, a rule of thumb is to add 2 percent to the CAS for every of altitude gained. Jet aircraft Jet aircraft do not have VNO and VNE like piston-engined aircraft, but instead have a maximum operating speed expressed in knots, VMO and Mach number, MMO. Thus, a pilot of a jet airplane needs both an airspeed indicator and a Machmeter, with appropriate red lines. An ASI will include a red-and-white striped pointer, or "barber's pole", that automatically moves to indicate the applicable speed limit at any given time. Angle of attack and Lift Reserve Indicators An airplane can stall at any speed, so monitoring the ASI alone will not prevent a stall. The critical angle of attack (AOA) determines when an aircraft will stall. For a particular configuration, it is a constant independent of weight, bank angle, temperature, density altitude, and the center of gravity of an aircraft. An AOA indicator provides stall situational awareness as a means for monitoring the onset of the critical AOA. The AOA indicator will show the current AOA and its proximity to the critical AOA. Similarly, the Lift Reserve Indicator (LRI) provides a measure of the amount of