Aero-Tips!

A good pilot is always learning -- how many times have you heard this old standard throughout your flying career? There is no truer statement in all of flying (well, with the possible exception of "there are no old, bold pilots.") It's part of what makes aviation so exciting for all of us... just when you think you've seen it all, along comes a scenario you've never imagined.

Aero-News has called upon the expertise of Thomas P. Turner, master CFI and all-around-good-guy, to bring our readers -- and us -- daily tips to improve our skills as aviators, and as representatives of the flying community. Some of them, you may have heard before... but for each of us, there will also be something we might never have considered before, or something that didn't "stick" the way it should have the first time we memorized it for the practical test.

It is our unabashed goal that "Aero-Tips" will help our readers become better, safer pilots -- as well as introducing our ground-bound readers to the concepts and principles that keep those strange aluminum-and-composite contraptions in the air... and allow them to soar magnificently through it.

Look for our daily Aero-Tips segments, coming each day to you through the Aero-News Network. Suggestions for future Aero-Tips are always welcome, as are additions or discussion of each day's tips. Remember... when it comes to being better pilots, we're all in this together.

Aero-Tips 04.19.06

Yesterday we looked briefly at how an airspeed indicator (ASI) works.  This sets us up to understand and diagnose potential failure modes. 

Scenario 1: Blocked inlet

If the inlet hole of the pitot tube is obstructed (examples: from dirt, bugs or ice), ram air pressure to the ASI will be reduced.  The bellows inside the ASI will contract as “normal” static pressure pushes against it;  indicated airspeed drops below “actual” and may eventually bleed down to indicate near zero if the blockage is complete.

Note: Forgetting to remove the pitot tube cover during preflight results in zero indicated airspeed on the takeoff roll—and should prompt you to stop and ask what else you may have forgotten before you continue the flight!

Scenario 2: Blocked inlet and drain

If the pitot inlet and drain are both blocked (most likely cause: ice), whatever air pressure is in the bellows when the obstruction occurs remains trapped.  Go up in altitude (static pressure drops), the bellows expands and the indicated airspeed increases.  If you descend, static pressure increases, the bellows is compressed, and indicated airspeed decreases.  This is what the Private Pilot study guides mean when they say “the airspeed indicator acts like an altimeter”—if you go up, indicated airspeed goes up; if you go down, indicated airspeed goes down.

Scenario 3:  No needle movement

If the airspeed indicator is frozen in place, it means either (a) the indicator itself has jammed or otherwise failed, or (b) the pitot inlet, drain and static port(s) are blocked, so the balance between (trapped) ram air and (trapped) static air pressures remains constant.

Any of these scenarios can be confusing, especially as you slow down and go down in preparation to land.  If your airspeed indication appears in error, fly known power settings, pitch attitudes and airplane configurations (flap and gear positions), adjusted as necessary for any ice accumulation that may have caused the obstruction(s).  The airplane doesn’t know that you can’t read airspeed accurately and will behave regardless of what the ASI reads.

Note: I always required pre-solo students to land the airplane with the ASI covered before I’d sign them off for solo, after having two students in the pattern simultaneously with pitot blockages due to a swarm of bugs. 

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

FMI: Aero-Tips