From your planes perspective what happens to the wind when the planes speed matches wind speed?

From : Don Stackhouse

Assuming the wind speed is constant, the plane doesn't know, and doesn't care.

Imagine the plane is flying around inside a giant box of air. It knows its airspeed relative to the air inside the box. If the box happens to be sitting still on the ground, or moving across the ground, the airplane doesn't know and doesn't care. All it knows is its own speed relative to the air inside the box.

Where the trouble comes in is when the pilot is standing on the ground (literally "outside the box"). If the plane is flying in exactly the opposite direction from the motion of the box (the wind speed) at the exact same speed, then its groundspeed is zero. The plane only knows its airspeed inside the box, not its groundspeed, and so it doesn't know and doesn't care that the groundspeed is zero.

However, a pilot standing on the ground will see the groundspeed and not the airspeed. This isn't too much of a problem. The trouble starts when the pilot turns the plane away from the wind onto a downwind heading. The groundspeed in that case is the plane's airspeed plus the wind speed. The plane looks like it's moving really fast, even though its speed through the air is the same as before. The pilot instinctively pulls the nose up to slow the plane down, and if taken to extremes the pilot can slow the plane's airspeed enough to cause it to stall. This is a pilot-induced stall caused by the optical illusion of the plane's high groundspeed, but the pilot assumes that it was the effects of the wind that caused the stall. The real cause was poor piloting in reaction to faulty interpretation of misleading visual cues, and erroneous understanding of the basic physics of flight. In other words, "pilot error".

The same thing can happen to a full-scale airplane in flight close to the ground, such as during landing or takeoff. The pilot gets strong visual cues from the nearby ground, and mistakenly slows the plane down in reaction to the high apparent groundspeed. The plane stalls, crashes, and the myth of the downwind turn gets another boost. At higher altitudes the groundspeed is much less visually apparent, so we don't see this happening in high altitude flight, even though the wind speeds at higher altitudes tend to be much higher than they are close to the ground.

A lot of folks will ask "...but what about the acceleration required when the airplane turns form into the wind to an away from the wind ("downwind") direction? This is also a myth. There is no net acceleration due to the wind in that situation.

Assume we have a plane flying north with a 30 knot airspeed, into a 10 knot headwind. Another way to describe that is to say the plane has a 10 knot "drift" to the south due to the effects of the wind. It's groundspeed is its airspeed, vector summed with this "drift" due to the wind speed. In this case, the drift and airspeed are opposite in direction, so the groundspeed is 20 knots.

Now let's start a 360 degree turn to the left. As the plane reaches a westerly heading, it's airspeed is still 30 knots, but now to the west instead of to the north. However, its drift is still 10 knots to the south, and therefore there has been no acceleration due to the wind. The drift is at right angles to the plane's path, causing its flight path to drift sideways over the ground. Its speed through the air ("airspeed") is 30 knots to the west (i.e.: a compass direction of 270 degrees), but its groundspeed is the vector sum of that plus the 10 knot southward drift, for a groundspeed of about 32 knots to the west-southwest.

As the plane turns to a southerly heading (course 180 degrees), the drift due to the wind is still that same 10 knots to the south (and therefore there has been no acceleration due to the wind), and the groundspeed is now 10+30, or 40 knots to the south. The airspeed is still 30 knots, same as before, only its direction has changed.

As the plane turns to the east (heading 90 degrees), the airspeed is still 30 knots (no change, except it's now to the east), and the wind drift is still 10 knots to the south (same as it has been, so no acceleration there), so the groundspeed is now about 32 knots to the east-southeast.

As the plane turns back to a northerly heading (course 360 degrees) to complete the turn, the wind drift is still 10 knots to the south, the airspeed is still 30 knots (but now to the north due to the turn), so the groundspeed is once again 20 knots to the north.

Note that throughout the turn, the wind drift was always a CONSTANT 10 knots to the south, and therefore imparted no acceleration to the airplane. The drift that subtracted from the plane's groundspeed when heading upwind, and added to it when headed downwind, existed as a sideways drift when the plane was headed crosswind. The drift due to the wind was always constant in magnitude and direction, and so it never caused any acceleration of the aircraft, and therefore no change in airspeed.

All of this assumes that the wind speed is constant. However, if you have a case of wind shear, where the speed of the wind changes as you climb or descend from one altitude to another, or encounter a horizontal gust, the plane will indeed see that as a change in airspeed. It's as if it suddenly flew out of one box of air and into a different box of air that is moving in a different direction or at a different speed. The plane has mass, and it takes a little time to accelerate to match the change in the motion of this new parcel of air that it has just entered.

Because of their higher mass, this can be an even bigger problem for large airliners than for small airplanes, although wind shear has been responsible for the crashes of many airplanes of all sizes. One of the worst forms of wind shear is something called a "microburst", a relatively small, short-lived but very intense downdraft that sometimes occurs under strong thunderstorms. The downdraft forms fairly high inside the storm. The air falls downward till it reaches the ground, then spreads outwards horizontally at high speed in all directions. An airplane entering a microburst at low altitude (such as on final approach for landing) will see a sudden rise in airspeed as it enters the outflow around the base of the microburst, and a tendency to climb due to its natural pitch stability and the increased airspeed. The natural reaction of the pilot is to reduce power to reduce the climb, and pull the nose up to reduce the increased airspeed. Just about the time these two actions are really starting to take effect, the plane flies into the intense downdraft in the center of the microburst. The plane starts to lose altitude rapidly (in other words it starts to drop like a rock!). The pilot firewalls the throttles, but can't shove the nose down because the plane is already too low due to the intense downdraft. This results in further airspeed loss. About that time the plane flies out of the core of the microburst and into the outward flow on the other side, which now acts like a sudden, very intense tailwind. The airspeed was already too low in response to the early portion of this encounter, and the sudden 50 to 100 knot tailwind may be just enough to put the plane's airspeed below its stall speed. This is precisely what happened to a Delta Airliines Lockheed L-1011 Tristar landing at Dallas-Ft Worth some years ago. The airplane was literally swatted out of the sky like a mosquito.

Since then, there has been a lot of study of this phenomenon, along with new tools such as Doppler radar and arrays of wind speed sensors (if the wind speed and/or direction on one side of the airport is substantially different from the value measured on some other part of the airport, wind shear is present, and the tower will warn the pilots of any approaching or departing airplanes) have dramatically improved our ability to avoid these situations. Better training, new flying procedures, and better knowledge of what conditions make wind shear likely have also dramatically reduced the wind shear related accident rate.

Oddly enough, the safety margin for dealing with wind shear is generally better for a twin engined airplane than for one with three or four engines. The required power for each engine of a multi-engine airplane is determined to a great extent by the power needed to sustain flight if one engine is shut down. A twin loses 50% of its available power when one engine is shut down in flight, so its other engine needs to be relatively large. A three engined airplane only loses 33% in an engine-out situation, and a four motor plane loses only 25%. Therefore they do not need as much reserve power from the remaining engines, and therefore they also do not have as much reserve power when all the engines are still running. If the plane encounters a microburst while on landing approach with all engines still running, the twin-engined airplane's proportionately greater reserve power will make it easier for it to power itself safely out of that situation.

In any case, wind shear (and the turbulence that often accompanies it) can be a serious problem. Pilots of all airplanes, large and small, model and full-scale, need to be aware of the situations that cause it, avoid those situations as much as possible, and be ready to react properly if they do encounter it.

Don Stackhouse
DJ Aerotech