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Aero-Tips 04.18.06

Although even in Light Sport airplanes we’re entering the glass-cockpit era, it’ll be a long time before the pitot static-driven airspeed indicator (ASI) goes away. As long as we’re dependent on sensed air pressure to determine airspeed, there’s the chance a system blockage will render the indication inaccurate.

How it Works

Very briefly, the airspeed indicator consists of a bellows contained within a sealed instrument case. Ram air pressure (from the pitot tube) enters the bellows; static air pressure (from, where else, the static port[s]) is routed to the case surrounding it. As ram air pressure increases the bellows will expand against the resistance of static air pressure, and vice versa. The bellows expanding or contracting mechanically drives a needle that moves against the marked face of the ASI that you see in the cockpit.

A closed system?

If there was no way for air to escape from the bellows, it would pack full of air as you built up speed. Slow down later (but don’t stop), and ram air could continue to force existing air into the bellows. The result: airspeed would indicate the highest for a flight, but not accurately reduce to match slower airspeeds after that. The answer: a drain system to exchange pitot pressure air. Commonly, the drain is on the back side of the pitot mast; partial vacuum in the lee of the mast draws air out of the pitot lines, creating an exchange of pressure that permits accurate airspeed indications throughout a flight.

Aero-tip of the day: Knowing the basics of how the airspeed indicator works helps you troubleshoot problems that may occur in flight. Tomorrow: failure modes.

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