April 5, 2001 Desired Track (DTK) #2:
They Call Me Mister Headwind! |
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April 5, 2001
I have a 100-knot airplane. Oh, sure; the airspeed indicator usually
reads much higher than that. But when it comes down to what really counts —
rate of movement over the ground — my shiny, expensive, 160-knot airplane is
frequently relegated to speeds closer to an 18-wheeler on the Interstate
highway below me than those approximating its "book" top speed. The
reason? Headwinds. Eastbound, westbound — any direction — it's not a matter
of whether I'll have a headwind, but how strong it will be. It's almost enough
to make me crazy enough to want to work as an aviation Web site editor for a
living.
Uh-oh ...
Like comedian Rodney Dangerfield, I get no respect from the winds. If I
plan a trip for Tuesday, on Monday the route I chose will afford me a nice
little nudge. On the day of the flight, the fickle fates will deal me a
howling 40 knots on the nose. After an unplanned fuel stop, I'll drag myself
into my destination about two hours late, landing only after being forced to
shoot an ILS to near-minimums and well after the FBO has closed. The only food
available will be a warm Pepsi and a package of cheese crackers. On Wednesday,
of course, that route will once again have a nice little tailwind.
 Even
the briefers at the local flight service station have noticed. It used to be
that the briefer invariably collapsed (at least I think they collapsed — it's
hard to see them through the phone...) in a fit of hysterical laughter when
they read the winds to me. Now, they recognize my N-number and are more
prepared for the "headwind punchline," as I call it. I can hear them
gritting their teeth and taking deep breaths just to keep from breaking down
and get through the briefing. When I call up an unfamiliar AFSS, my pre-flight
briefings are usually pretty normal until we get to the winds-aloft portion.
Then I hear the annoying laughter again and I lose the connection. That could
probably explain why I seem to get a lot of full-route clearances, too.
I've compared notes with a few fellow pilots and they seem to get their
share of headwinds. Not nearly to the same extent as me, of course. Somewhere
along the line, a black cat must have crossed my path. Or, maybe I walked
under a ladder.
Of course, there are good, logical reasons that pilots — other than me, of
course — get headwinds. Here's a few of them.
In all seriousness, the likelihood of seeing a head- or tailwind on any
given day is pure chance, right? Wrong. There is such a thing as a prevailing
wind direction. It is even a defined term:
| prevailing wind
In a particular locality, the wind
direction that is most frequent over time. For most areas the prevailing
wind varies, sometimes quite markedly, according to season. It may also
change, or have changed, when the climate changes (as in ice ages). |
In North America (between 30 and 60 degrees north latitude), this means the
nominal wind direction is from the southwest. In fact, these winds are called
"prevailing westerlies," so sayeth NOAA. In other areas of the
globe, the prevailing wind will blow from other directions, of course. In
other areas of the globe, of course, they will still be headwinds for me.
Living on the east coast of the U.S. as I do, there's not much future in
heading due east in a single. So, most of my trips start out heading either
south or west. Right into a headwind.
 Based
on the foregoing, you might conclude that the easiest way to ensure a tailwind
would be to always fly a northeasterly heading, right? Not so fast, partner.
There are such things as local weather patterns, for instance. Two trips I've
flown between the east and west coasts in the past year bear this out. Put
bluntly, I had a lot of time to contemplate the reasons.
One trip, in March 2000, involved flying with AVweb Marketing
Manager Ann Devers in her highly-modified Cessna 172 from Manassas, Va. (HEF)
to the North Las Vegas (Nev.) Airport (VGT), her plane's new home. We made a
few stops along the way during the trip, but one morning after departing
Little Rock, Ark. (LIT), we were able to stay at a relatively low
altitude and pick up a 25-knot tailwind on a leg taking us almost due west
from LIT to Amarillo, Texas (AMA). Even though it didn't last all day, we
thoroughly enjoyed a 150-knot groundspeed for a few hours and, if other
complications had not arisen, would have been able to make it all the way to
VGT that day without undue wear and tear on the crew. Not bad for a 172.
Instead, we called it quits in Albuquerque, N.M. Conversely, had we climbed up
to a "normal," more-efficient altitude for this leg, we would have
had a handful of knots of wind on the nose and been forced to deal with
"only" 100-110 knots over the ground. But, since the name of the
game is groundspeed, the low-level run with this tailwind made much more
sense. Obviously, there was a high-pressure system somewhere to our north and
its clockwise circulation was giving us the push we so richly deserved.
 The
other trip occurred last month as I made my way back east from a week at AVweb
Editor-in-Chief Mike Busch's house in California (since us AVwebbers
work in a virtual office, we have to get together every now and then.) After
dealing with 40- and 50-knot headwinds most of the way from Virginia to
California, I was positively salivating at the idea of those same winds
helping me along on the way home. Alas, I haven't gotten over that black cat
or the infamous ladder incident because it was not to be, at least at first.
After I hung up from my pre-flight briefing and the weather guesser's
hysterical laughter faded away, it dawned on me that the forecast winds (from
150 degrees at 20-30 knots) at the 11,000-foot altitude I needed to be above
most of the MEAs (minimum en route altitudes) along my route would continue my
losing streak. This was because of yet another high-pressure system hovering
out to the east, this time over central Colorado. So, I slogged my way out to
Winslow, Ariz. (INW) for fuel before launching for an overnight at Dave
Higdon's house in Wichita.
The next day was the exception that proves the rule: I made the 940
nautical miles from Wichita's Mid-Continent International Airport (ICT) to my
home plate of Manassas, Va. (HEF), in an easy-to-handle 4:45, thanks to a
blistering tailwind that boosted my 100-knot airplane into one boasting some
220 knots over the ground for most of the trip. After I slowly made my way
past that Colorado high the day before, a low-pressure system over the Great
Lakes took charge and provided the push home. Like winning the lottery or
having dinner with Julia Roberts, stuff like that just doesn't happen to me
every day. The next day, the surface winds howled and locations to the north
saw some snow.
Of course, there's a lot more to local weather patterns than just the
position of highs and lows. Topography — both natural mountains and man-made
obstructions — play a role, as do other weather phenomena like fronts,
troughs and the like.
Even putting aside the local phenomena and the position of highs, lows and
fronts, we will have relative headwinds more then we will have relative
tailwinds. It's a simple fact of life in aviation, so get used to it. (Don't
shoot the messenger, though.) To explain why this is so, one needs to
understand the wind triangle. From there, we can get a better understanding of
the headwind's nasty little secrets.
Every ground-school graduate knows how to compute a magnetic heading using
true airspeed, magnetic variation, true wind velocity and the leg's true
course. Plotting these values on the back of the trusty E6-B computer results
in a diagram called a "wind triangle," because the result is a
three-sided shape the dimensions of which are dictated by the numbers used to
create it. The wind triangle, as its name implies, has three basic components
in its simplest form:
-
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A typical wind triangle depicting a
headwind situation.
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A line (really, a vector) representing heading and airspeed. In a headwind
situation (i.e., when I'm doing the wind triangle), this is the hypotenuse
of the triangle.
- Another line representing the wind direction and velocity. This line
(okay, vector) can be as long as the one representing heading and
airspeed. If it's longer, you're going backwards.
- A final vector which is the course and groundspeed. In my situations,
this is usually the shortest of the three.
The image at the right illustrates the general shape and proportion of a
typical wind triangle.
It's pretty obvious that this arrangement of the vectors will result in a
headwind, since the vector representing course and groundspeed is shorter than
the one representing heading and airspeed. Conversely, if the vector
representing the wind were angled down and to the right instead of down and to
the left, the vector for course and groundspeed would be longer and, since the
vectors have to connect to each other, it would connect with the wind vector
further to the right.
The idea of a headwind when our desired course and the wind direction
directly oppose each other is a relatively easy one to grasp: We don't need a
wind triangle to understand that if the air in which we are flying is moving
at 20 knots in a direction exactly opposite our heading, our groundspeed will
be reduced by 20 knots. But what happens most often is that the wind is not
directly on the nose (unless I'm flying, of course). The result is that we
must turn slightly into the wind to correct the tendency to drift downwind if
we are to arrive at our destination. This is called a "crab," and is
just like a popular crosswind technique, except that we're not close enough to
a runway to land.
Look at the image again. See how the little airplane symbol on the
course/groundspeed vector is angled into the wind? While the angle may not be
correct, the fact that the symbol is angled into the wind is accurate. If we
didn't crab, we would drift downwind and could very quickly be hopelessly
off-course. This difference between the magnetic course and the magnetic
heading is called the wind correction angle. More important, though, is the
question of what happens when we crab our little (or large) airplane away from
our desired heading. Regardless of the wind, the answer is that our
groundspeed between the point of departure and the planned destination
decreases. The fact that a crab reduces our groundspeed is one of the
headwind's nasty little secrets.
Another nasty little secret headwinds present is also related to crabbing.
Again, refer to the wind triangle image and, again, move the wind vector down
and to the right to represent a tailwind instead of down and to the left. Even
in this situation, we would be forced to crab to maintain a direct course to
our destination. In other words, even with a tailwind, because we are
forced to turn into the wind and away from our desired course, our potential
groundspeed is reduced. Put another way, even a tailwind can have a
headwind component.
The final nasty little secret is something I call the double whammy of a
headwind. As we've learned, the headwind slows our groundspeed because the
airmass in which we are flying is moving in a direction opposite our intended
route of flight and because of the crab we often have to employ. As a result,
our groundspeed is reduced. Because our groundspeed is reduced, it takes
longer to get where we're going. The double whammy? Because it takes us longer
to get where we're going, the headwind has more time to do its deed.
Eventually, we get where we're going, but it's not as much fun and often
involves a extra stop for fuel along the way.
Please note that this discussion doesn't include the concept of
pressure-pattern navigation, a method of long-range flying that attempts to
predict local weather patterns to avoid — or at least minimize — headwinds
and take advantage of any tailwind that exists. That's a subject for another
column.
Even though I understand the trigonometry of headwinds, understanding it
doesn't make my lot in life easier. Despite it all, I'll continue to make long
flights. Along the way, I'll pray for a tailwind, expect a headwind and hope
that the FBO at my impromptu fuel stop is still open when I land.
I'll see you there — I'll be the one holding the phone away from my ear as
the AFSS briefer breaks down into fits of hysterical laughter.
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