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.
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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.