Wind Shear - Safety Corner 24 jun 21
Glider pilots fly through wind shear all the time, most noticeably on approach.
But, if you're a pre-solo student, maybe you haven't yet experienced that
sinking feeling (pun intended). As you turn final, that 15kt headwind slows your
pace over the ground. On a nice day, the headwind slowly diminishes as you
descend to the runway. That change in wind velocity—direction and/or speed—is
called wind shear. On a blustery day, though, wind shear can have a nasty side.
A 15kt headwind can drop instantly by 5kts or 10kts or even the entire 15kts. In
fact, the headwind can turn into an instant tailwind. All of a sudden, your
comfortable 55kt approach speed, the speed of the lift-generating air over your
wings that keeps you aloft, is now lower; maybe a whole lot lower. What happens?
Your decent rate gets higher and your aim point starts climbing up in your scan.
If you've lost all the headwind, you may even be close to your glider's stall
speed (e.g., 55kts–15kts=40kts), on your way to a hard landing.
What can you do about it? You can give yourself a good margin. Fly faster. You
know the forecast surface winds and gusts at, say, Worcester, you've listened to
the automated weather at Fitchburg (125.1), you've paid attention to your ground
speed and drift in the pattern, and you've accounted for the bumps. One rule of
thumb—discuss this with your instructor—is normal approach speed + half the
steady wind speed + all of the gust velocity ("10 gusting to 20" means a gust
velocity of 10kts). So, if you've flown the approach at 65kts on a bouncy day,
your speed margin might leave you flying at 50kts, a safe speed, despite wind
shear that robbed you of 15kts. Wind shear, however, can be unpredictable. There
are days when the forecast doesn't give you the whole story, and the shear
surprises you. Then, what do you do? Back to margin; this time, altitude. If
you've flown a steep approach, you likely have some altitude under you, even on
short final. If you begin to lose airspeed, lower the nose NOW! That's the only
way you can regain airspeed you are losing to wind shear. Low, shallow
approaches give away your margin. There's little altitude that you can trade for
speed. Yet another reason not to fly flat approaches.
Shear happens. But, if you keep substantial margins—speed and altitude—wind
shear won't bite. The June issue of Soaring contains a piece called "The
Mathematics of Wind Shear" with plenty of nuggets for the less geeky. The
article concludes with the advice to "...simply add some airspeed as a margin to
get yourself safely through the shear layer while allowing your airspeed to
decrease to a pre-selected, safe final glide speed. If you do this, you will not
land short."
* * *
Our Chief Pilot, Roy Bourgeois, wrote an excellent piece on situational
awareness in the current Wings and Wheels newsletter.
It brings into stark relief some critical dos and don'ts to keep us safe;
experienced and student pilots alike. The newsletter is free. If you haven't
subscribed to it, you should.