The pitot-static pressure system provides the source of air pressure for the

• ALTIMETER (ALT)
• VERTICAL SPEED INDICATOR (VSI)
• AIRSPEED INDICATOR (ASI)

The pitot tube is normally mounted on the leading edge of a wing. The pitot tube on an aircraft used only for flight under Visual Flight Rules (VFR) may not be heated to prevent icing. Aircraft to be used under Instrument Flight Rules (IFR) are heated electrically, to prevent icing when operating in visible moisture and cold temperatures. A switch in the cockpit controls Pitot heat.

The static pressure port is normally found on the side of the fuselage. On later model aircraft, an alternate static source is provided inside the cockpit. The pilot can select the internal static source if the outside source becomes clogged with ice.. When the pilot selects the alternative source, the instruments relying on the static pressure may operate slightly differently.

• The altimeter (ALT) may indicate a higher-than-actual altitude.
• The vertical speed indicator (VSI) will momentarily indicate a climb, then will settle back the initial indication.
• The Airspeed Indicator (ASI) will indicate greater-than-normal airspeed. Altimeter (ALT)

The two parts of the system are
(1) the pitot tube and pressure line
(2) the static pressure system and lines.

Airspeed errors can occur if either the Pitot tube or the Static vent become blocked.

Pitot tube can become blocked on the ground, either by bugs, or ice. In this event the ASI will read zero. The airspeed will not increase as it should during the takeoff. In Flight Blockage of Pitot - When this first happens, if the aircraft is in steady flight the pilot will not realize there is a problem because the ice simply seals the pressure inside the Expandable Capsule When the aircraft descends for landing however the pressure inside the capsule will remain constant, whether the aircraft accelerates, or decelerates. As the aircraft descends the static pressure will increase. This will squeeze the expandable capsule. Thus, the indicated airspeed will decrease as the aircraft descends regardless of the actual airspeed.

The opposite would happen if the aircraft climbed.

In Flight Blockage of Static Vent - As long as the aircraft remains at the same altitude the Airspeed indicator works normally since the correct static pressure is sealed in the case. When the aircraft descends the static pressure in the case will be lower than it should be. Thus, the airspeed will begin to read progressively higher as the aircraft descends. If the aircraft climbs the airspeed will begin to read progressively lower.

what a pilot should do

• DO consider pitot heat and alternate static sources as mandatory instruments for flight in actual instrument conditions.

• DO conduct a careful preflight of all instrument systems.

• DO keep your scan moving and identify any instrument(s) that give you conflicting information.

• DO identify the instrument or system that is in error by determining what makes sense and what doesn't.

• DO eliminate the offending instrument or system from your attention. Carrying instrument covers (usually used by diabolical CFIIs for partial-panel training) to cover the inoperative instruments can keep them from becoming a constant source of distraction. Remember, your scan will tend to pick up the abnormal.

• DO remember that pitch plus power equals performance. As long as you have vacuum instruments available, you can keep the airplane level. Set the pitch and power for the performance you want and trust that the airplane will deliver.

• DO consider a backup altimeter for serious instrument flying. Even the altimeter watches are good enough to keep you out of the terrain. Sky diving altimeters are also useful as a backup in an emergency.

• DON'T allow your attention to become fixated on any one instrument or system. If you can't figure out why an instrument is giving you the reading it is, there's a good chance there's a problem. Integrate the readings from all other instruments and determine which instrument is lying to you. Eliminate it.