Re: [Phys-l] [tap-l] Wind Shear

[John Denker <jsd@av8n.com>]

On 06/12/2009 11:34 AM, Bernard Cleyet wrote:

> I suspect runways are made somewhat longer than necessary for zero  
> wind speeds.  The problem comes when the wind direction changes  
> unexpectedly, or as described below, so the craft must increse its  
> (ground) speed to prevent stalling.  This then requires a longer runway.

1) In many situations, the runway length requirement is dominated
by _takeoff_ considerations (not landing).  See below.

2) A gust that completely changes the _direction_ of the wind is
very unusual.  Typically the gust is small compared to the underlying
steady wind component, for example wind from 270 at 15 gusting to 25.
In this example, if you are landing on runway 27, the 15 knots of
steady headwind _reduces_ the demand for runway length.

The exceptional case, where the wind randomly gusts from 270 at 10
to 090 at 10 (or worse) is very rare but very scary;  see LLWS below.

In practice the *opposite* type of windshear is more common.  Since 
everybody chooses the runway that has a headwind at ground level, 
the windshear that you are most likely to encounter is a tailwind on 
long final that shears to a headwind on short final.  This situation 
reproducibly occurs when there is a warm front moving through the area.

Before the runway was lost to a real-estate fraud, I used to instruct 
out of a fairly small airport:  Only one runway.  Narrow.  2100 feet 
minus 400 foot displaced threshold = 1700 feet available for landing.  
(By way of comparison, runway 13R at JFK is 8.5 times that long.)  
Many a time I had solo students who tried a few times to land there, 
wisely gave up, and landed on a longer and wider runway a few miles 
away.  I would drive over, leave my car, and fly plane and student 
back home.  The landing was usually easy, because by then the front
had moved on, but even if it hadn't, I can land in conditions where 
students don't stand a chance.  It makes a good lesson if handled 
properly.  

  Of course the most important lessons were much earlier, when the
  students learned multiple redundant ways of detecting that an
  approach was not going well, learned to go around sooner rather
  than later, and learned that I would be very happy to pick them
  up if they decided to land somewhere else.

In the reference frame comoving with the wind (which is one of the 
frames that matter) the wind shear causes a change of frame, and the 
change of frame gives you a ton of additional unwanted kinetic energy.  
There is no chance that a student pilot will be able to unload all 
that energy in the time available.  A go-around is the only tenable 
option.  The good part is that all that extra energy makes the go-
around extra easy.
 
> Small (non-commercial) craft use dinky runways?  So in their case, I  
> suspect, the solution is to use full power and not land.  Not a  
> viable option at an international airport w/ commercial craft?

  Tangential remarks:   Small aircraft are more properly called light 
  aircraft.  And they are not necessarily non-commercial.  There are 
  actually *three* grades of pilot certificate:  private, commercial, 
  and airline transport pilot.  There are lots of commercial operations 
  (crop dusting, flight instructing, executive charter, etc. etc. etc.) 
  that you can do even if you aren't an airline.  About half of non-airline
  flying is actually commerce of some kind.

Going around is always an option (except for gliders, which are a
special case).  I've been in several airliner go-arounds.  Obviously
a go-around costs time and money, but it's better than crashing.
The rule is:  If you're not prepared for the go-around, you're not
prepared for the approach.  A modern autopilot / flight director has
a single button marked "go around" to make this a simple as possible.

Jets have a problem here, because if you advance the throttle from
idle to full open, it might take five or six seconds for the engine
to spool up and start producing full power.  This is in contrast to
a piston engine, which responds in a fraction of a second.

> > > Attached is a pdf file showing how a plane descending through  
> > > slowing air can end up with an air speed less than it's stall speed  
> > > and ends up crashing to the ground. This is a good into mechanics  
> > > problem.

That's the nightmare scenario.  That's called LLWS (low-level wind
shear) and is usually associated with a _downburst_ i.e. a narrow
column of air that descends out of the sky at high speed and splatters
against the ground.  Visualize the water flowing out of a garden hose
and impinging vertically against the pavement, so that it flows outward
horizontally in all directions.

If your flightpath takes you through such a thing, you will observe
a headwind that shears to a downdraft and then shears to a tailwind.
This can rob you of quite a bit of energy.  If you even halfway suspect
such a thing is happening, it is time to go around: apply full power 
and get out of Dodge.  Go around first;  figure out what happened later.  
Several airliners have been lost to LLWS.

Airports that are prone to this have robots that detect LLWS so that
dire warnings can be given to approaching aircraft.  The detection 
principle is simple:  They measure horizontal wind velocity Vh (i.e.
the projection of the wind onto the horizontal plane) at several points 
around the airport, and look for a _divergence_ i.e. ∇•Vh.

Downbursts are associated with the late stages of a thunderstorm cell,
so you can avoid this problem by not flying in areas where there is
thunderstorm activity.


> p.s.  is this ever a problem on take off near the ground?   Not as,  
> as ground speed acceleration increases on lift off from absence of  
> rolling friction?  Under adverse conditions, do pilots force their  
> planes to stay grounded longer than normal so lift off speeds are  
> greater?  (again demanding longer runways)

Again: in the usual case, wind from 270 at 15 gusting to 25, you can
get away with *less* runway than in the no-wind case.  The gust term
places extra demands on pilot technique, but that 15 knots of steady
headwind lessens the demands on runway length.

In the exceptional case, wind from 270 at 10 gusting to 090 at 10, you
might want to stay home.  Or you could take off from runway 18 if that's 
available, accepting a 10 knot variable _crosswind_, which should be no 
problem.

Rather than holding the aircraft on the ground, it is better to lift
off and then fly horizontally about 6 inches above the runway.  The
airplane accelerates like crazy without any of the wear and tear that
would be associated with ground contact.  Acceleration is more rapid
than it would be on the runway, because there is no contact friction,
and also more rapid than it would be in ordinary level flight, because
there is no induced drag, because of ground effect.  It's a win/win/win
technique.  The worst-case gust shortly after liftoff would cause the 
airplane to stall and drop onto the runway from a height of 6 inches, 
which is not a problem.

For multi-engine aircraft, minimum runway length is based on "accelerate
stop distance".  That means you can accelerate to a point where you can 
lose an engine and then either (a) take off with N-1 engines using the 
remaining runway, or (b) stop straight ahead on the remaining runway 
(without relying on thrust reversers).

As a corollary, in a normal landing or normal takeoff, there is tons of
extra runway available.  To say the same thing in more unpleasant terms,
you can get into a lot of places you can't safely get out of, so do the
math before deciding to drop in at some arbitrary cute-looking airport.