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Old 24th Feb 2013, 03:29
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AlphaFloor27
 
Join Date: Apr 2010
Location: Q
Age: 34
Posts: 23
SRS mode during windshear - A320

Hello everybody, today I was thinking about what the A320 would do if you have a windshear encounter, assuming managed lateral and vertical modes and AP on, and I'm struggling to really understand what would happen given this situation:

Let's assume the following conditions: takeoff is done with TOGA thrust, flaps 1, and THR RED/ACC ALT is defined by airline policy at 400' AFE. So, everything normal during rotation, AP is engaged, and while below THR RED/ACC ALT (both set on the mcdu at 400' AFE), you start experiencing windhsear conditions. The FCOM procedure states:

"Windshear during initial climb:

THR LEVERS at TOGA: SET OR CONFIRM
AP (if engaged): KEEP
SRS ORDERS: FOLLOW
DO NOT CHANGE CONFIGURATION (SLATS/FLAPS UNTIL OUT OF SHEAR)"

Given those takeoff conditions, at the moment of the windshear encounter all the pilot would have to do is monitor the AP's performance in following SRS orders (TOGA and AP already set, SRS mode engaged, flaps at 1). But, from what the FCOM says, SRS mode would disengage at 400' AFE, and CLB mode would engage, thus losing the SRS guidance and protections, making the whole windshear procedure invalid as you can't "follow SRS orders".

If we take a look at the SRS disengament conditions, it says:

"Disengagement Conditions:

The SRS mode disengages:

- Automatically, at the acceleration altitude (ACC ALT), or if ALT* or ALT CST* mode engages (above 400 ft RA).
- If the flight crew engages another vertical mode
- If the flight crew selects a speed while in SRS mode: SRS reverts to OP CLB mode, and a triple-click aural warning is heard."

And for the CLB mode, it says:

"Engagement conditions:

CLB mode automatically engages when the aircraft reaches ACC ALT, or sequences a waypoint with an altitude constraint while the CLB mode is armed".

From what I remember from my simulator sessions, SRS mode stays engaged during a windshear encounter, but I don't know if that's because it was a FLEX takeoff, and that would force the pilot to engage TOGA thrust, thus engaging SRS again; or because there's another condition I am not able to find in the FCOM that makes SRS mode to stay engaged during a windhsear encounter.

I hope I am making any sense, and would really appreciate some insight into this.

Regards!
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