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• Design maneuvering speed (V$_{A}$)—the maximum speed at which the structural design’s limit load can be imposed (either by gusts or full deflection of the control surfaces) without causing structural damage. It is important to consider weight when referencing this speed. For example, V$_{A}$ may be 100 knots when an airplane is heavily loaded, but only 90 knots when the load is light. 

• Landing gear operating speed (V$_{LO}$)—the maximum speed for extending or retracting the landing gear if flying an aircraft with retractable landing gear. 

• Landing gear extended speed (V$_{LE}$)—the maximum speed at which an aircraft can be safely flown with the landing gear extended. 

• Best angle-of-climb speed (V$_{X}$)—the airspeed at which an aircraft gains the greatest amount of altitude in a given distance. It is used during a short-field takeoff to clear an obstacle. 

• Best rate-of-climb speed (V$_{Y}$)—the airspeed that provides the most altitude gain in a given period of time. 

• Single-engine best rate-of-climb (V$_{YSE}$)—the best rate-of-climb or minimum rate-of-sink in a light twin-engine aircraft with one engine inoperative. It is marked on the ASI with a blue line. V$_{YSE}$ is commonly referred to as “Blue Line.” 

• Minimum control speed (V$_{MC}$)—the minimum flight speed at which a light, twin-engine aircraft can be satisfactorily controlled when an engine suddenly becomes inoperative and the remaining engine is at takeoff power. 

## Instrument Check 

Prior to takeoff, the ASI should read zero. However, if there is a strong wind blowing directly into the pitot tube, the ASI may read higher than zero. When beginning the takeoff, make sure the airspeed is increasing at an appropriate rate. 

## Blockage of the Pitot-Static System 

Errors almost always indicate blockage of the pitot tube, the static port(s), or both. Blockage may be caused by moisture (including ice), dirt, or even insects. During preflight, make sure the pitot tube cover is removed. Then, check the pitot and static port openings. A blocked pitot tube affects the accuracy of the ASI, but a blockage of the static port not only affects the ASI, but also causes errors in the altimeter and VSI. 

## Blocked Pitot System 

The pitot system can become blocked completely or only partially if the pitot tube drain hole remains open. If the pitot tube becomes blocked and its associated drain hole remains clear, ram air is no longer able to enter the pitot system. Air 

already in the system vents through the drain hole, and the remaining pressure drops to ambient (outside) air pressure. Under these circumstances, the ASI reading decreases to zero because the ASI senses no difference between ram and static air pressure. The ASI no longer operates since dynamic pressure cannot enter the pitot tube opening. Static pressure is able to equalize on both sides since the pitot drain hole is still open. The apparent loss of airspeed is not usually instantaneous but happens very quickly. [Figure 8-9] 

If both the pitot tube opening and the drain hole should become clogged simultaneously, then the pressure in the pitot tube is trapped. No change is noted on the airspeed indication should the airspeed increase or decrease. If the static port is unblocked and the aircraft should change altitude, then a change is noted on the ASI. The change is not related to a change in airspeed but a change in static pressure. The total pressure in the pitot tube does not change due to the blockage; however, the static pressure will change. 

Because airspeed indications rely upon both static and dynamic pressure together, the blockage of either of these systems affects the ASI reading. Remember that the ASI has a diaphragm in which dynamic air pressure is entered. Behind this diaphragm is a reference pressure called static pressure that comes from the static ports. The diaphragm pressurizes against this static pressure and as a result changes the airspeed indication via levers and indicators. [Figure 8-10] 

For example, take an aircraft and slow it down to zero knots at a given altitude. If the st atic port (providing static pressure) and the pitot tube (providing dynamic pressure) are both unobstructed, the following claims can be made: 

1. The ASI would be zero. 

2. Dynamic pressure and static pressure are equal. 

3. Because both dynamic and static air pressure are equal at zero speed with increased speed, dynamic pressure 

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