EAA Sport Aviation

DAVE MATHENY
aircraft transitions from whatever speed
it was making while still attached to the
ground to being part of the flow.)

If that parcel of air is flowing from south to north at 12 mph, then the aircraft will soon be moving north at 12 mph over the ground, along with whatever airspeed it has. In the case of a helicopter that is hovering with no forward airspeed, it would be making 12 mph to the north over the ground while remaining stationary in its parcel of northward-flowing air. If an airplane takes off in any other direction, it, too, will soon be making 12 mph to the north over the ground, while maintaining whatever forward airspeed it happened to be flying at. A jet fighter making Mach 1. 6 to the east would also be moving north at 12 mph.

In the case of an ultralight that just took off to the east, as I had just done, and making 35 mph through the air while pointed east, it will continue to make 35 mph through the air, along with that 12 mph over the ground to the north, so its track over the ground will be well north of east.

If it then turns north, the airspeed will
combine with the groundspeed and it will
make 47 mph over the ground—while still
making 35 mph through the air.
The aircraft, once it is part of the flow,
does not know that it is travelling
upwind, downwind, or in any other direc-
tion. It is just going with the flow.
Turning downwind shortly after takeoff can
be just as perilous, if there’s rising terrain that
way, because you won’t be able to climb at as
steep an angle—relative to the ground—as you
would otherwise.

THE QUESTION OF TERRAIN

But what about my apprehension when I turned downwind on that early flight? Was I wrong? No, as a matter of fact, I really was headed toward those houses a lot faster than I had intended. And that brings us to one of the potential dangers in turning downwind whenever we are close to the ground: terrain. Hard stuff at the bottom of the sky: rocks, bricks, trees, satellite dishes, dogs, piles of industrial waste, angry men with baseball bats. Terrain is bad. Flying toward rising terrain is not good. Flying faster than you intended toward rising terrain is even worse.

The other danger is one that is a special branch of the downwind takeoff (not the turn but the takeoff ), which is not a mythical hazard but rather a very real one. Taking off downwind reduces the actual, real-world deck angle at which you can climb. Turning downwind shortly after takeoff can be just as perilous, if there’s rising terrain that way, because you won’t be able to climb at as steep an angle—relative to the ground—as you would otherwise.

And then there are the effects of a
gusty wind. If, as you are flying down-
wind, a gust comes along, it would reduce
your airspeed by the amount of the gust.

If, for example, you turn downwind
while flying in a parcel of air that’s flow-
ing along at 12 mph, and a gust to 25
mph comes along, your airspeed would
be reduced by 13 mph for the
length of time that the gust
blows. You could conceiv-
ably stall, if your airspeed
were low enough to begin
with. But that’s really
only a problem if you
are flying close to

stall speed.

(While we’re at it, though, let’s note that whenever a gust blows itself out, an aircraft flying downwind will theoretically gain back whatever airspeed was lost. You can’t count on it, of course; whenever flying in rowdy air in a very light aircraft, you will notice occasional slight surges and slowdowns, along with being shoved sideways and up and down. The only thing to be done about it is to maintain a safe airspeed.)

AND BACK TO MYTH

When you put all three of these factors together—unintended speed toward rising ground, flatter-than-usual climb angle, and a chance of momentary reductions in airspeed in gusts—you come up with a strong inadvisability of turning downwind at very low altitudes and low airspeeds, especially in a minimally powered aircraft.

All of which would seem to amount to there being some truth to the legend of the downwind turn. And in fact there is some truth to it, so long as what is being feared is not the aerodynamics of the turn, but its possible consequences in certain circumstances. So we might be tempted to say that if an irrational fear of a downwind turn helps keep a pilot from going the wrong way at low altitudes and low airspeeds, maybe that’s a good thing.

Still, it’s wrong, and I think we’re better off understanding why we should avoid what we avoid. Fear of the downwind turn is irrational, and I got over it a long time ago, mostly.

Dave Matheny, EAA 184186, is a private pilot and an FAA ground instructor. He has been flying light aircraft, including ultralights, for 30 years. He accepts commissions for his art and can be reached at DaveMatheny3000@yahoo.com.