30.0  29.9  29.8  Figure 5-2.  A typical pitot-static system. 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. 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. Another configuration of the altimeter is the drum-type.  [Figure 5-5]  These instruments have only one pointer that  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] 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. 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. 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. 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. 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).  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. Effects of Flight Conditions Blocked Static System Pitot/Static Instruments Sensitive Altimeter Principle of Operation 5-3