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indicated airspeed slowly drops to zero. If the pitot tube ram pressure hole and drain hole become obstructed, the ASI operates like an altimeter as the aircraft climbs and descends. Refer to the Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25A) for more in depth information on blocked pitot systems along with different scenarios and how they effect the ASI. 

## Blocked Static System 

When a static system becomes blocked but the pitot tube remains clear the ASI continues to operate but is inaccurate. When the aircraft is operated above the altitude where the static ports became blocked the airspeed indicates lower than the actual airspeed because the trapped static pressure is higher than normal for that altitude. The opposite holds true for operations at lower altitudes; a faster than actual airspeed is displayed due to the relatively low static pressure trapped in the system. 

A blockage of the static system can also affect the altimeter and VSI. Trapped static pressure causes the altimeter to freeze at the altitude where the blockage occurred. In the case of the VSI, a blocked static system produces a continuous zero indication. 

An alternate static source is provided in some aircraft to provide static pressure should the primary static source become blocked. The alternate static source is normally found inside of the flight deck. Due to the venturi effect of the air flowing around the fuselage, the air pressure inside the flight deck is lower than the exterior pressure. 

When the alternate static source pressure is used, the following instrument indications are observed: 

1. The altimeter indicates a slightly higher altitude than actual. 

2. The ASI indicates an airspeed greater than the actual airspeed. 

3. The VSI shows a momentary climb and then stabilizes if the altitude is held constant. 

For more information on static system blockages and how to best react to such situations, refer to the Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25A). 

## Effects of Flight Conditions 

The static ports are located in a position where the air at their surface is as undisturbed as possible. But under some flight conditions, particularly at a high angle of attack with the landing gear and flaps down, the air around the static port may be disturbed to the extent that it can cause an error in the indication of the altimeter and ASI. Because of the importance of accuracy in these instruments, part of the certification tests for an aircraft is a check of position error in the static system. 

The Pilot’s Operating Handbook (POH)/Aircraft Flight Manual (AFM) contains any corrections that must be applied to the airspeed for the various configurations of flaps and landing gear. 

## Pitot/Static Instruments 

## Sensitive Altimeter 

A sensitive altimeter is an aneroid barometer that measures the absolute pressure of the ambient air and displays it in terms of feet or meters above a selected pressure level. 

## Principle of Operation 

The sensitive element in a sensitive altimeter is a stack of evacuated, corrugated bronze aneroid capsules. [Figure 5-3] The air pressure acting on these aneroids tries to compress them against their natural springiness, which tries to expand them. The result is that their thickness changes as the air pressure changes. Stacking several aneroids increases the dimension change as the pressure varies over the usable range of the instrument. 

Below 10,000 feet, a striped segment is visible. Above this altitude, a mask begins to cover it, and above 15,000 feet, all of the stripes are covered. [Figure 5-4] 

Another configuration of the altimeter is the drum-type. [Figure 5-5]  These instruments have only one pointer that 

