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-   -   SRS mode during windshear - A320 (https://www.pprune.org/tech-log/508730-srs-mode-during-windshear-a320.html)

AlphaFloor27 24th Feb 2013 03:29

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!

WhyByFlier 24th Feb 2013 11:20

PWS only valid up to 1500 Feet. Reactive WS up to 1300 Feet.

Acceleration altitude (2 engines) is typically 1500 Feet (1000 Feet in easyJet).

SRS changes to CLB at acceleration altitude. Acceleration altitude is limited to single engine acceleration altitude so 400 feet is extremely unlikely.

What's your problem?

AlphaFloor27 24th Feb 2013 12:09

My airline defines both THR RED and ACC ALT at 400' AFE. So the scenario I described is absolutely possible.

What I want to know is why SRS stays engaged during a windshear encounter and the application of the correct procedures (not sure if that's what really happens), given the fact that it should change from SRS to CLB mode due to the reasons I wrote on my previous post.

vilas 24th Feb 2013 12:42

There is no reason for SRS to remain engaged beyond acceleration altitude.What would happen beyond acc.alt is as you correctly said SRS changes to CLB and that means acceleration. In windshear 400 ft is too low for that so pull speed and fly selected speed till out of windshear then go managed. Do not change configuration in windshear.

WhyByFlier 24th Feb 2013 12:51

Some of the worst advice I've ever seen given on pprune there by vilas. Someone doesn't know their memory items. Without SRS you'd take AP off, pitch to 17.5 degrees with TOGA and use full back stick if needed.

Out of interest, which company are you with?

AVApilot 24th Feb 2013 14:37

F/D not available
 
My latest QRH revision states:

Note:
1.If engaged, the autopilot disengages when α is greater than α prot.
2.If the FD bars are not available, use an initial pitch attitude up to 17.5 ° with full backstick, if necessary. If needed, to minimize the loss of height, increase this pitch attitude.

So, while the F/D is still available it is not in the desired mode, so I would have to agree with WhyByFlier, disengage AP and fly a max attitude of 17.5deg Nose up, I'd also keep a good eye on the VSI to keep a positive ROC which is what you need and if you get to a full aft stick deflection then so be it.

I think this applies unless there is a logic in the Flight Envelope part of the FAC where the Windshear Warning would override the CLB engagement at Acceleration Altitude.

If such logic exists I don't know of it, but it would not surpise me if it is there.

AlphaFloor27 24th Feb 2013 15:38

"I think this applies unless there is a logic in the Flight Envelope part of the FAC where the Windshear Warning would override the CLB engagement at Acceleration Altitude."

That would be the optimal behaviour of the FMS, and it would explain why I have the notion in my mind that the SRS keeps engaged during windshear procedures (I may be wrong hehe).

PT6A 24th Feb 2013 16:00

WhyByFlier,

Vilas claims to be ex Air India and an A320 SFI.. sounds about right for those AI boys!:E

WhyByFlier 24th Feb 2013 16:28

He or she might be a great pilot and more qualified than me but that procedure that was offered is non standard, incorrect and less than optimum. In easy it'd be a repeat in the sim for it and on the line it'd be a meeting and new underwear for the captain.

AlphaFloor27 24th Feb 2013 20:41

Found the following:

"Auto Flight, Flight Augmentation, Windshear Detection Function, Guidance:

In windshear conditions, the flight guidance acts on specially adapted FD pitch orders received from the speed reference system (SRS). The pilot must set go around thrust immediately (wich also triggers the FD SRS mode), and follow the pitch order to execute the optimum escape maneuver".

So, from that text and the memory item that states that TOGA thrust must be set or confirmed, and then follow the SRS orders, I assume that the FMGS "knows" you entered a windshear condition and thus keeps or re-engages SRS in the scenario I first described.

That may be the logic that AVApilot thought might exist.

Any other thoughts?

PT6A 25th Feb 2013 02:16

WINDSHEAR (FCOM-PRO-ABN-80 P80, QRH-ABN-80.15 OM B 2.3.90.10)

A red flag "WINDSHEAR" is displayed on each PFD associated with an aural synthetic voice "WINDSHEAR" repeated
three times. If windshear is detected either by the system or by pilot observation, apply the following recovery technique:
¦ At take-off:
* If before V1:
The takeoff should be rejected only if significant airspeed variations occur below indicated V1 and the pilot decides that there is sufficient runway remaining to stop the airplane.
* If after V1:
Announce: "WINDSHEAR TOGA"
THR LEVERS TOGA
REACHING Vr ROTATE
SRS ORDERS FOLLOW


Airborne, initial climb or landing:

Announce: "WINDSHEAR TOGA"
THR LEVERS AT TOGA SET OR CONFIRM
AP (if engaged) KEEP
SRS ORDERS FOLLOW (This includes the use of full back stick, if demanded.)

Note:
1. If engaged, the autopilot disengages when is greater than prot.
2. If the FD bars are not available, use an initial pitch attitude up to 17.5° with full backstick, if necessary. If needed, to minimize the loss of height, increase this pitch attitude.
DO NOT CHANGE CONFIGURATION (SLATS/FLAPS, GEAR) UNTIL OUT OF SHEAR.
CLOSELY MONITOR FLIGHT PATH AND SPEED.
RECOVER SMOOTHLY TO NORMAL CLIMB OUT OF SHEAR.

WINDSHEAR RECOVERY
The best defence against a windshear encounter is to avoid it in the first place. A pilot should
use his knowledge and experience combined with weather reports to assess the possibility of
encountering windshear and plan accordingly. The weather radar may also be a useful tool as
windshear is often associated with thunderstorms and microburst. The use of managed speed
and the groundspeed mini function will give added protection against windshear in strong winds.
The A320 is fitted with two types of Windshear Protection;
 
 
 
REACTIVE WINDSHEAR:
The FACs generate the windshear warning whenever the predicted energy level for the aircraft
falls below a predetermined threshold.
The protection envelope is from just after lift-off until 1300ft on take-off and from 1300ft to 50ft
for landing. For both cases, at least CONF 1 must be selected.
A red "WINDSHEAR" flag is displayed on each PFD (for a minimum of 15 seconds) associated
with an aural warning "WINDSHEAR" repeated 3 times. If windshear is detected either by the
system or by pilot observation, the pilot must take immediate action:
• Apply TOGA
• Call "WINDSHEAR TOGA"
• Follow SRS orders closely (keep A/P if engaged)
• DO NOT change the aircraft configuration
If the flight directors are not available, then the pilot should pitch up to 17.5° nose up. Should thpilot require it, the use of full back sidestick is available. This may trigger Alpha Floor protection
Both pilots should monitor the Pitch, IAS, V/S and the actual W/V during the encounter, as theyare a good indication of the aircraft’s energy state.
In the unlikely event of encountering windshear during the take-off roll, the takeoff should only brejected if significant airspeed variations occur below V1 and the pilot decides that there is
sufficient runway remaining to stop the airplane. Due to changes in the acceleration of the
airplane during a windshear incident, an overrun may occur if a rejected take-off is actioned.

PREDICTIVE WINDSHEAR:
The weather radar has a Predictive Windshear System (PWS) that scans the airspace ahead of
the aircraft up to a range of 5 miles ahead for windshears. The PWS system detects any
‘Doppler shift’ in the pattern of rain droplets and so will not detect windshears in dry atmospheric
conditions.
The protection envelope available for alerts is from the start of the take-off roll up to 100kts, and
then from 50ft to 1500ft and from 1500ft to 50ft for landing.
When the system detects a windshear, a Warning, Caution or Advisory is triggered.
 
 
Warning:
On the PFD: ‘W/S AHEAD’ in red.
On the ND: Windshear icon.
Aural warning: on take-off: "WINDSHEAR AHEAD" twice.
on landing: "GO-AROUND WINDSHEAR AHEAD".
Pilot action: on take-off: Reject take-off.
when airborne: Thrust levers TOGA and turn to avoid the windshear icon.
Follow SRS and normal clean-up provided windshear is not entered.

(On landing, if a positive verification is made that no hazard exists,
the warning may considered cautionary – otherwise go-around.)
Caution:
On the PFD: ‘W/S AHEAD‘ in amber.
On the ND: Windshear icon.
Aural warning: "MONITOR RADAR DISPLAY"
Pilot action: Apply precautionary measures as indicated in ‘FCOM -Supplementary
techniques’
 
 
Advisory:
On the PFD: Nil.
On the ND: Windshear icon.
Aural warning: Nil.
Pilot Action: As Caution.

INDICATIONS ON THE ND:
 

The predicted windshear area is indicated by a red and black icon and two yellow radial lines.
Windshear information is available in ARC and ROSE ND modes.
When the ND range is set above 10 NM, a W/S SET RNG 10 NM message appears, requesting
the crew to adjust the ND range. It is displayed even if the weather radar is switched off,
provided the WINDSHEAR switch on the weather radar panel is set to AUTO.

 
 
 

AlphaFloor27 25th Feb 2013 02:27


I also also wondered why the SRS remained engaged during windshear and did not automatically disengage at the acceleration altitude as it is supposed to (normally, as per FCOM)
So do you agree that the answer to my question is what I posted before about the Windshear Detection Function? From what I now understand, it provides a special kind of SRS, and the FG part of the FMGC knows it is in the middle of a windshear encounter, thus modifying the SRS orders, and keeping it engaged.

I'm still not 100% sure, just the best answer I have right now.

Cheers.

WhyByFlier 25th Feb 2013 03:58

I may not have answered the question but my answer gives the correct memory items in the event the aircraft/ FDs/ AP do not respond the way you'd expect.

I fly for easyJet.

cav-not-ok 25th Feb 2013 08:42

i'm sorry, but can anyone remember if there is a flashing LVR CLB during the windshear maneuver, and if theres none, when does it appear.

thanks

Stuck_in_an_ATR 25th Feb 2013 13:24

IIRC, 'LVR CLB' is flashing on the FMA above the acceleration alt and the FD maintains SRS till the TL's are retarded back to the CLB detent. I can be mistaken, though...

vilas 25th Feb 2013 15:15

Dear WhyByFlier

Memory items are not rocket science everyone includingKowalsky knows them. TOGA was power used for takeoff itself so what’s the pointrepeating it. He has asked for solution in a specific situation, that is theacceleration altitude is 400 feet so SRS has changed to CLB. All Airbusdocuments and DVD’s assume that the acceleration altitude as 3000 feet (noise abetment).Therein lies the problem. No Airbus document gives a procedure for this. It tellsyou to keep the AP on, it tells you to fly the FD but in SRS. Myfriend PT6A who went out his way to find my pedigree (I am honored really) hasgiven a full page reference from FCOM which is completely irrelevant to thepresent case since it keeps on repeating follow SRS, and if AP is on keep iton, because in our case the SRS has changed to CLB and if you do nothing withAP on the AC pitch will reduce to accelerate and may be fatal. Your aggressive suggestionof flying 17.5 has already been answered by a Pilot albeit in your very ownstyle. Since there is no official procedure we have to guess. My suggestionstops the acceleration, allows you keep the AP on and if doesn’t meet therequirement, Airbus golden rule “take over” go above the FD’s, full back stickif required. PT6A can you honestly say you answered the question? Lastly myinterest in pprune is purely professional. Many questions like the present onemake you think out of the box. I don,t flash my experience or background nor Itry to show off if that’s your style good luck to you. In aviation far too manypilots have laughed far too soon and had to eat their hats. So cheer up.

Out of interest Are you a captain yet?

WhyByFlier 25th Feb 2013 15:46

I'm not bickering and I'm not having a go but the question was about no SRS IN A WINDSHEAR caused by a very low thr red/acc alt.

If you think selecting speed with AP engaged in a windshear because there's no SRS is a good idea then I think you need to approach either your training department or Airbus.

I'm not a captain yet no.

Airmann 25th Feb 2013 16:21


IIRC, 'LVR CLB' is flashing on the FMA above the acceleration alt and the FD maintains SRS till the TL's are retarded back to the CLB detent. I can be mistaken, though...
Yes this was my initial thought but we mustn't mix up ACCEL ALT with THR RED. The Situation you mention above is THR RED, not ACCEL ALT. At THR RED you will get flashing LVR CLB message but after selecting CLB you will still be in SRS mode. Once at ACCEL ALT aircraft switches to CLB mode and accelerates. (ref. DSC-22_30-80-20 p1/4 SRS Disengagement Conditions)

What villas is saying is not unreasonable, the key in the windshear situation is to use all the aircraft energy possible for a climb. Pulling speed at ACCEL ALT will maintain v2+10 or whatever SRS had commanded and will get the FG to try and give you the best climb possible with TOGA power. In windshear that's all you can hope for, if TOGA plus max nose up is not going to give you a positive climb then there's not much else left to do but start praying. In my humble opinion this will do the job. I also don't see how TOGA and 17.5degrees is wrong either, both should work, no? I think that selecting OP CLB is a better option than CLB.

The only thing left to analyse now is the difference in behavior between CLB vs. SRS, and are there some inherent differences that would render the CLB mode logic incapable of dealing with windshear (vs. SRS logic, which is obviously programmed to deal with it). SRS limits nose up to 22.5 degrees with windshear, not sure about

PT6A 25th Feb 2013 16:36

Vilas,

I actually think the information I posted does answer the question. As it is tailored company specific information. In our company the standard acceleration altitude is 1000' yet the guidance from our 3 documents remains the same (FCOM, OM-B and Airbus Training Guide) to follow the SRS.

From this I deduce that in the event of wind shear being detected.. The aircraft would remain in SRS.

As for the OP, why is your company using an acceleration altitude of 400'?

(400 being the minimum regulatory acceleration altitude - but far from necessary or desirable in the A-320)

WhyByFlier 25th Feb 2013 16:59

We're talking about a windshear in the Airbus not forgetting to put an NADP 1 in the box. Selecting speed with AP in a windshear is not an appropriate response.

From the QRH:

A red flag “WINDSHEAR” is displayed on each PFD associated with an aural synthetic voice “WINDSHEAR” repeated three times. If windshear is detected by pilot observation, apply the following recovery technique: ■ At takeoff

■ If before V1

The takeoff should be rejected only if significant airspeed variations occur below indicated V1 and the pilot decides that there is sufficient runway remaining to stop the airplane.

■ If after V1 THR LEVERS...................................................... ...................................TOGA REACHING VR.......................................................... .........................ROTATE SRS ORDERS...................................................... .............................FOLLOW If necessary, the flight crew may pull the sidestick fully back.

Note: 1. Autopilot disengages if the angle of attack value goes above α prot. 2. If the FD bars are not displayed, move toward an initial pitch attitude of 17.5°. Then, if necessary, to prevent a loss in altitude, increase the pitch attitude.

■ Airborne, initial climb or landing THR LEVERS AT TOGA........................................................ SETORCONFIRM AP (if engaged).................................................... .........................................KEEP SRS ORDERS...................................................... ..................................FOLLOW If necessary, the flight crew may pull the sidestick fully back.

Note: 1. Autopilot disengages if the angle of attack value goes above α prot. 2. If the FD bars are not displayed, move toward an initial pitch attitude of 17.5°. Then, if necessary, to prevent a loss in altitude, increase the pitch attitude.

DO NOT CHANGE CONFIGURATION (SLATS/FLAPS, GEAR) UNTIL OUT OF SHEAR. CLOSELY MONITOR FLIGHT PATH AND SPEED. RECOVER SMOOTHLY TO NORMAL CLIMB OUT OF SHEAR.


Where does selecting speed come into this?


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