Flying Smart : What it Looks Like

Aircraft pitot-static systems have something in common with animals, including humans: They work properly only if they are allowed to breathe. And, as with people, if the breathing passage is blocked, bad things can happen.

To review some basics, the pitot-static system is plumbed to an airplane's pressure-sensing instruments - including the airspeed indicator, altimeter, and vertical speed indicator. The pitot tube, typically located under the left outboard wing, provides ram air or dynamic pressure to the airspeed indicator. The static port, which on most light aircraft is located flush on the fuselage (many Piper low-wing aircraft have a combination pitot-static head located on the underside of the left wing), provides static pressure to the airspeed indicator, altimeter, and vertical speed indicator.

The altimeter and VSI read ambient pressure. The airspeed indicator measures the difference between dynamic and static pressures and translates that difference as airspeed.

For each of those instruments to function properly, the pitot-static system must breathe - be clear of all contaminants that could hinder or block the free flow of air. Pitot tube covers protect the pitot opening from curious insects and other potential blocking agents while the aircraft is parked. No such protection is provided for the much smaller static port, and that can pose a problem. Some people apply a small piece of tape over the static port before washing an airplane, and certainly before painting the fuselage. The tape can easily be forgotten after such work.

That's why it is important to carefully inspect the static port before flying. A thorough preflight inspection should reveal a blockage in the static port, either from tape that someone forgot to remove, a small insect, or water or ice blocking the port.

You'll know that the static port is blocked if, when in a climb or descent, the airspeed indicator does not respond correctly and the altimeter and VSI needles do not move. If this occurs, try opening the alternate static air source (if the airplane is so equipped), typically by turning a small T-handle valve located under the instrument panel.

Failing that, if you are in instrument meteorological conditions you can break the glass on the VSI to get cabin static pressure. Cabin pressure is slightly lower than ambient pressure, which means the pressure instrument readings will not be exactly accurate, but in that situation it's a lot better than no indications at all.