Angle of Attack in Downdraft

[timpara]

In trying to understand wing behaviour in vertical windshear for a (minor) incident analysis, I thought I had it all sussed. However, I came across this in an Airbus Document 'Windshear Awareness'
(http://www.airbus.com/fileadmin/media_gallery/files/safety_library_items/AirbusSafetyLib_-FLT_OPS-ADV_WX-SEQ02.pdf)
Quote:

"downdraft affects both the aircraft Angle-Of-Attack (AOA),
that increases, and the aircraft path since it makes the aircraft sink "


Surely when entering a downdraft, the relative wind vector will be rotated to point more downwards (and hence the AOA will be reduced, not increased)?
The only explanation of this statement I can think of is that
1) The sink triggers a pitch-up response of magnitude greater than the change of relative wind.
2) The AOA increase is after the transient effects of entering the downdraft have occurred.
I'd be grateful for any insight from the professionals.

[Zaphod Beblebrox]

If the citation from Airbus is to be believed then your analysis is not correct. Airbus says the angle of attack increases. I always envision relative wind as a vector that points towards the aircraft and a sudden shift as the result of a strong down draft would increase the angle or move the relative wind vector lower or in a counter clockwise direction, increasing the angle of attack.

Angle of attack is the difference between the wing chordline and the relative wind. If you move the relative wind downward as it strikes the wing the angle increases. There is some reaction by the aircraft because as the aircraft sinks that has the effect of reducing the relative wind angle. I am not sure in a wind shear situation if that makes up for the updraft or not. This type of question is sort of outside the normal aerodynamic text book explanation.

[timpara]

Zaphod Thanks for your reply, but I think part of your statement is wrong.
"If you move the relative wind downward as it strikes the wing the angle increases"
If you add the downward pointing vector of the downdraft to the horizontal vector component of the relative wind, the resultant vector points more downward, hence the AOA reduces. Is that not true?

[PEI_3721]

It may easier to consider the airmass point of view. In a down-burst the airmass is descending, the aircraft has inertial flight path stability and basic aerodynamic stability; these together with any control input to maintain the desired flight path will be seen as a loss of airspeed and increasing AoA, but not instantaneously.

The incident reported below shows the magnitude of the airmass movement in a severe encounter and may provide a better understanding of an actual event.

Windshear Incident (https://ja.scribd.com/mobile/document/35984283/Windshear-Incident)

[scifi]

Hi you must not forget the role that the Tailplane has in this scenario. It will lose more lift than the Mainplane, and because of its moment-arm, will make the rear of the aircraft drop... This will cause a higher nose-up attitude, and is what creates the aircraft's pitch stability.
.

[Basil]

Recollect, as captain in a well respected airline (no, not BA), continually crashing in a sim severe windshear exercise because I refused to pre-empt the windshear.
If, at the first indication of reducing IAS, one had applied full power then one would have survived but that is NOT how a little bit of windshear is handled in real life.
I initially refused to play the game but eventually stuck on full poke and climbed away. To this day I have no idea what they were attempting to teach us.

[timpara]

Scifi wrote "...It will lose more lift than the Mainplane, and because of its moment-arm, will make the rear of the aircraft drop... This will cause a higher nose-up attitude, and is what creates the aircraft's pitch stability."
What you've described is pitch instability, not stability. If it was pitch-stable, the aircraft attitude would remain the same. What the reaction you describe will do though is to maintain a more stable AOA - protecting from stall during a disturbance.
I hope at least that we've established that a wing does experience a decrease in AOA when entering a downdraft. Otherwise we would experience lift in this situation (assuming we're not approaching stall)- which for me defies all experience and logic.

[Goldenrivett]

Hi timpara,

You are correct.
I think someone has mistranslated the Franglaise. It probably should read:
"A downdraft that increases, affects both the aircraft Angle-Of-Attack (AOA) and the aircraft path since it makes the aircraft sink (see Figure 1, item 2)."

[Intruder]

Entering and flying through a downdraft is NOT a single event with a single effect and control response. It is a dynamic situation.

Since the event sequence will START with a stable airplane entering the downdraft, the AOA will temporarily decrease due to the wind vector. However, an almost-immediate control response will be made, pitching up to regain/maintain altitude, INCREASING the AOA. Until the engines compensate, the airplane will then be slower than in the stable state. While the downdraft persists, the pitch attitude will be higher, since the airplane will be effectively climbing.