- "Why would they lose lift while landing?"
What makes microbursts so dangerous? Many things, not the least of which is that they have the potential to swat airplanes out of the sky.
Piston-powered airplanes seem to have a slight advantage since they do not experience a power-lag or spin-up time on the engines as do turbine-powered machines, but they usually also have less power and less inertia to start with than does turbine equipment.
One of the sneaky things about microbursts is that
- if you're a little short on situational awareness, the apparent performance increase that can be the first sign of a microburst in progress
is sometimes ignored and the flight presses on to further danger.
Take a look at the graphic to the right.
This is the classic microburst diagram we've all seen.
Notice the airplane at position A.
It's just beginning to feel the effects of the outflow from the microburst.
This sudden increase in headwind will cause an apparent increase in performance.
The airspeed will increase, the rate of descent will decrease and the almost overwhelming urge of a pilot at this point is to pull the throttles back and push the nose over.
Bad mistake.
This is the time to get out of Dodge.
Microbursts are like everything else associated with thunderstorms —
- there's no way to judge how bad they are from looking at them
and by the time you find out just how bad it is, it's too late.
If you press on, you will find yourself at position B in the graphic.
You will fly out of the headwind and into the tailwind.
The suddenly decreasing headwind and increasing tailwind will cause degradation in performance equal in magnitude to the increase experienced at point A.
You can see that if the headwind portion of the microburst caused a 30-knot jump in airspeed you're now going to lose 30 knots.
If you pulled the throttles back in response to the headwind increase, you'll be in real trouble now.
This is quickly becoming an unrecoverable situation.
When all the talk about wind shear started in the late 70s, I had a real problem understanding how this happens.
After all, we've all had it drilled into our heads since that first hour of dual that
- airplanes only "feel" wind on the ground
and that once you're airborne the airplane moves in concert with the air mass it's in.
The thought that wind could have an effect on airspeed was a hard one to swallow — and still is for many.