CRJ 200 / Wind shear warning - why follow F/D not "Eye brows"?

[D-IITC]

I`m an aviation enthusiast - and I have a question regarding "Wind shear alerts" and the corresponding G/A after such a warning.

Our company OM-A says we shall follow the F/D guidance.

However, if the G/A button is hit - the "eye brows" on the PFD appear.

Why wouldn`t I target the "eye brows" to get out of the situation asap - with max AoA and TOGA thrust I will get a bigger "safety cushion" underneath way faster than by following the F/D pitch...

Can anyone explain? Did I catch something wrong?

Thks!

[FCS Explorer]

don't know if crj's eyebrows are special... but afaik the eyebrows indicate "PULL (over) HERE AND EAT IT"
so in a W/S situation with rapidly changing winds and airspeeds you might find yourself one step beyond the edge...

[safetypee]

Perhaps the aircraft has windshear recovery guidance in the FD system- (TOGA Windshear). If so then the FD should always be followed.
Most recovery control laws will consider the aircraft’s altitude and speed (total energy), and then compute a compromise FD command between not loosing altitude and minimising the time in the location. However, in a severe micoburst, the system’s optimisation would (should) direct a climb at the ‘eyebrow’ attitude thus minimise the likelihood of ground contact.

[Mad (Flt) Scientist]

Originally Posted by safetypee

Perhaps the aircraft has windshear recovery guidance in the FD system- (TOGA Windshear).

Indeed it does, on all built AFAIK

Originally Posted by safetypee

If so then the FD should always be followed.

Just like the book says.

Originally Posted by safetypee

Most recovery control laws will consider the aircraft’s altitude and speed (total energy), and then compute a compromise FD command between not loosing altitude and minimising the time in the location. However, in a severe micoburst, the system’s optimisation would (should) direct a climb at the ‘eyebrow’ attitude thus minimise the likelihood of ground contact.

Correct as discussed below

Originally Posted by D-IITC

Why wouldn`t I target the "eye brows" to get out of the situation asap - with max AoA and TOGA thrust I will get a bigger "safety cushion" underneath way faster than by following the F/D pitch...

No you won't, in fact you'll be using up valuable energy. The windshear escape guidance is designed to get you out of the WS while maintaining as much as possible of your energy, given that you may be losing it at an alarming rate. Flying at shaker-onset (which is what the "eyebrows" represent) is flight at less than 10% above stall speed. That's well into the "back side of the power curve" - in other words you're below minimum drag speed. You're bleeding more energy through induced drag than the optimum, so you're kept away from there.

As you get closer to the ground (assuming you're in a performance limiting WS and cant escape easily) the guidance will gradually eat into your margin to shaker, eventually commanding the FD coincident with the eyebrows at ground contact (thus fulfilling a basic certification requirement for WS guidance, that no "excess energy" be left unused at ground contact). But until you get to that point the margin above shaker represents both a "last ditch" energy reserve and also keeps your energy gain/loss rate as favourable as possible.

Incidentally, although I heartily do not recommend this, the best escape manoeuvre for the certification windshear cases can often be a very close encounter with the ground - the downdraft inevitably tends to zero at the ground plane. of course, for cert purposes it IS a plane, and such inconveniences as houses, power lines and trees are generally absent....