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 good pilots, we're all in this
together.
Aero-Tips 03.27.06
It’s always good when readers raise questions -- either I
clarify something so readers learn more, or they give me new
information that makes me learn, and provide better instruction. My
recent Aero-Tips article Non-Precision Descent: Dive or
Glide is a good example.
Two readers commented on what they saw as a criticism of what I
call the “glide” procedure -- flying a constant-rate
descent from the Final Approach Fix (FAF) to the Missed Approach
Point (MAP), instead of descending rapidly to the Minimum Descent
Altitude (MDA) and then flying level to the MAP... the “dive
and drive” method.
One reader wrote:
The goal of a stabilized approach (the glide approach) must
be to reach the MDA in a position to land on the runway, not at the
MDA. The Instrument Procedures Handbook states “a descent
rate should be used that will ensure that the airplane reaches the
MDA at a distance from the threshold that will allow landing in the
touchdown zone.” (p. 5-29) Admittedly, this calculation can
be difficult to make... .. The Vertical Descent Angle (VDA)
published on many non-precision approach charts is the angle from
the FAF to the touchdown zone, not the missed approach point. (AIM
5-4-5h) Flying this descent angle will allow you to reach the MDA
in a position for a normal landing. However, the pilot must figure
ground speed to determine the required rate of descent.
My earlier article illustrated the two methods, now modified
with a red line that depicts a VDA profile:
If obstacle clearance permits, this is indeed a superior way to
fly a non-precision descent, because it exposes pilot and
passengers to low altitude for less time in poor conditions, and it
permits a stabilized descent from the FAF all the way to the
MAP.
Everything’s a tradeoff. The VDP descent can result in
higher minima, or (I’ll explain in a moment) higher
effective minima. If an obstacle permits a step-down
fix on the dive-and-drive approach, clearance requirements may
require the glide descent’s MAP be placed at a higher
altitude. More importantly, the minimum altitude in a glide
approach will be higher than MDA for the dive-and-drive. This is
because, unlike a precision approach, the non-precision
provides no clearance below MDA. Whereas in an ILS you can go to
Decision Height (DH) and then apply power for the
miss, inertia carrying you into (cleared) space below DH as descent
turns to climb, in a non-precision approach you’ll have to
add power above MDA so you do not descend below MDA
in the process of missing the approach -- making minima
effectively higher.
Hence, you’ll need to begin the missed approach 50 to 100
feet above MDA if you fly the “glide”-style approach.
Adding power to begin the missed, then catching sight of
the runway just as you hit MDA is a recipe for disaster if
you then chop the throttle and try to land as the nose comes up and
you re-enter the clouds. You need to add power about 100 feet high
to arrest descent using the dive-and-drive technique also, but you
end up at MDA long enough to see the runway if the clouds
are right at minimums.
Conclusion: You get a stabilized approach using the glide
method, but you get lower minima flying dive-and-drive.
Aero-tip of the day: I’ll repeat the tip
from my earlier article: Recognize the advantages and disadvantages
of opposing philosophies toward non-precision approach descents --
and choose the method that best fits conditions.