Mon, Aug 14, 2006
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.")
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. 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.
Look for our daily Aero-Tips segments, coming each day to you
through the Aero-News Network.
Aero-Tips 08.14.06
"My plane's faster than your plane."
Who hasn't at least thought it when comparing an owned, borrowed,
rented or fantasized aircraft to another? But sometimes it's
obvious that even experienced pilots don't always understand the
different kinds of speed.
Aircraft speed is usually expressed in either knots or miles per
hour. One knot is one nautical mile (6000 feet) per hour, or about
1.15 miles per hour. Vocabulary tip: Saying "knots per hour" is
redundant. "Knots" will do.
Here are the kinds of speed, and what they describe:
- True airspeed (TAS). This is the speed the
aircraft is "truly" traveling through the air. TAS corrects for all
instrument error and environmental factors. If there was no wind at
all, TAS would be the speed you cover the ground. TAS is the
correct expression of an airplane's capability.
- Indicated airspeed (IAS). As the name implies,
this is the speed read from your airspeed indicator (ASI) without
any corrections applied. Since the ASI measures impact air
pressure, if the airplane maintains a constant TAS its IAS will
decrease as altitude (and air pressure) decreases. Similarly, if
IAS remains constant, the higher you go the higher your TAS... you
have to fly through space more rapidly in thin air to get the same
impact air pressure to show a constant IAS.
- Calibrated airspeed (CAS). CAS is IAS
corrected for installation error and indicator error. Flight in
some configurations and/or some angles of attack may result in IAS
error. CAS is an engineer's way of compensating for this... from
the pilot's seat, those errors are built into manufacturer's
recommendations for reference IAS. CAS more correctly describes the
amount of air flowing over the airplane's wings.
-
- Example: Generally liftoff in a light
multiengine airplane is done at VMCA, the minimum controllable
airspeed with one engine inoperative, plus five knots. This gives
the pilot a margin above VMCA should the critical engine fail right
at liftoff. In the Beech 58TC Baron I flew for several years, VMCA
is 81 knots. Because of instrument error in the 58TC 86 knots IAS
(VMCA +5) is 81 knots indicated. Lifting off right at the "red
radial" VMCA speed IAS, as recommended in the Pilot's Operating
Handbook, provided the five-knot safety margin in CAS.
- Ground speed (GS). GS is TAS corrected for
wind. For a given TAS, GS increases with a tailwind, and decreases
with a headwind. GS is usually what you read from a GPS. Since GS
is entirely dependent on non-aircraft factors (the wind), it is not
an honest value for comparing one aircraft's performance to
another's. Brag about seeing 200 knots on your Skylane's GPS with a
tailwind if you like, but it's still slower through the air than
the Columbia 400 flying into the wind in the other direction.
Aero-tip of the day: Understand the different
"kinds" of airspeed, and apply them correctly.
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