Response of FBW aircraft in updraft
 

Curious as to how automated a/c respond to a severe convective updraft. If this stalls the wing killing the lift, would the a/c respond with ND and descent? Assumed trimmed for cruise and 'uniform' updraft over all the airframe.

Yes, because in your scenario I'm assuming the airspeed would also drop and the short version is that the A/P disconnects (@ alpha protection +1 degree for airbus). If speed decreases fast, and below alpha protection, and beyond the ALPHA FLOOR threshold, the A/THR commands TOGA. In this scenario the autopilot could of course also disconnect earlier due to the turbulence caused by the updrafts but that would ultimately not change how the FBW would react to the loss of airspeed.

There are lots of variables affecting the various speeds, thresholds etc etc, and it could be discussed and debated forever, but that's the simple version.

As always with the FBW threads you can count on this one to also turn in to pages and pages of Boeing vs Airbus and at least 200 versions of what would happen and how the aircraft would react in case of, and what if etc etc.

If you mean airbus FBW, AP OFF, side-stick left at neutral....
The FBW will try to maintain 1.0g in pitch. It has no pitch, speed or altitude targets to aim for, only load factor.
If Alpha exceeds Alpha Prot it will try to maintain Alpha Prot.

If the ATHR protection (Alpha Floor) engages as well, then the a/c will end up maintaining Alpha Prot with TOGA thrust.

Thanks. Wasn't intending to challenge the automatics, real interest is on altitude, with neutral manual control inputs would the initial motion be to climb, descend or hold, or is this a hopeless question because of other factors?

In Normal law the aircraft will not provide a progressive pitch down. This is speed stabilities which are active in alternate law with protections only. It occurs between 5-10kts above the stall. In this case at alpha prot plus 1 the AP disconnects. At alpha floor toga is applied and as said before the aircraft will with a neutral side stick maintain alpha prot. To exit alpha prot the auto thrust must be disconnected and the angle of attack reduced.

That's the theory anyway and it works. However every aircraft and system has its limits and if the downdraft was strong enough insufficient lift would be the result in all cases.

OK, thanks to all.

In theory it could be any of the above as the target will be to maintain Alpha Prot.

I’d have thought it would have been the opposite.

When you encounter an strong updraft, the *immediate* effect is for the IAS to increase along with the effective AoA.

This can have several results. If the resultant AoA is high enough, the wing section will stall (but still be generating significant lift). If it doesn’t stall, then there will be a marked increase in lift which may overstress the structure, depending on the airframe limiting load factors described by the flight envelope.

Correct. But I would assume that if caught in a prolonged updraft (recent AirAsia crash?) the airspeed will start to decrease rapidly as the relative speed of the shear to the aircraft is decreasing.....?

Just been reading other threads about alpha prot and alpha floor: certainly a fertile area of discussion! It says in an AB doc that alpha floor is purely a thrust mode but then goes on to say its activation not only gives TOGA but also a pitch up. Rather glad I don't have to do this for a living..

For an airframe with intrinsic speed stability, the normal response to a step change in relative airflow from a strong updraft would be an increase in g followed by an eventual return to steady state. That is with no constraints, such as trying to maintain an altitude.

Throw in FBW and pilot inputs and what happens? How does a FBW Airbus deal with rapidly increasing airspeed and AoA at the same time?

The FBW protections have a prioritisation logic, the foremost of which is the high alpha protection.
I have not seen the reference to a pitch up with alpha floor in the FCOM. The aircraft is still in Normal Law and it resists pitch changes with large changes of thrust.
The Airbus is not immune to the forces of nature because it has protections. It will try and resist them but if there was a sufficient atmospheric disturbance you will stall it. A normal stall recovery would then be required. A lot of pilots don't seem to appreciate this.

Surely the remedy to most Airbus crashes (Paris Airshow, XL888t, AF447...) is for the Pilot to use the Pitch Trim Control manually.


However this is the last control the plane wants the pilot to touch, it is imbedded under several layers of FBW technology. It is even painted Black and White, and will take your fingers off, like a circular saw, if you get too close to it.


There are only a few aircraft with Anhedral wings that actually need FBW to fly, all others have Dihedral wings, C of G, Centre of Pressure, and inbuilt stability in their airframes.

The response of the Autpilot, Autothrust and FBW Flight Control System of an Airbus A320/A330/A340 to an updraft in cruise will depend on the updraft velocity and rate-of-buildup. If the updraft is strong enough to trigger the high-angle-of-attack protection (alphaprot) of the FBW system, the autopilot disengages. In the absence of pilot control inputs on the sidestick, the FBW system will tend to maintain the angle of attack at a fixed value equal to alphaprot. The airplane will then enter a steep climb at the expense of airspeed up to a certain altitude, and will then start to descend. The speed at the apogee can be well below stall speed, but the airplane will not stall because alphaprot is less than the stall angle of attack.

There have been at least two incidents of this nature to A330/A340 airplanes, one of which is reported here:
Incident Report Airbus A340-314 F-GLZU 22 July 2011 in cruise at FL350.

Alphafloor is an autothrust function that is triggered at an angle of attack greater than alphaprot. If triggered, it will increase thrust to maximum takeoff thrust. However, since takeoff thrust at cruise altitude is not much greater than normal cruise thrust, the effect of alphafloor activation would be relatively insignificant.

Comparing that to the limited data we have on the Air Asia, there are some similarities. The low GS at altitude (close to an unaccelerated stall when IAS is back-calculated) especially...

Fullwings, when I was talking about "prolonged" updraft I was referring to the type of event(-s) Gybreght is referring to in his post.

I was perhaps not that clear (laying by the pool as I'm writing :} ) but there will eventually decrease in airspeed as you gain altitude with the updraft and with increased altitude, close to normal/max cruise altitudes, the stall speed will also increase.

As mentioned already by tubby universal laws of aerodynamics apply also to the Airbus and it WILL stall if the meteological forces at work are strong enough. I have been shot down here before for trying to tell people that the Airbus can and will stall given the "right", wrong, circumstances.

Towards the end of Gysbrecht's A340 F-GLZU article, it mentions that the aircraft had also encountered an increase of headwind of +25knts, which would have contributed to the overspeed and high positive rate of climb.

The protections protect the aircraft from violent pilot inputs, not mother nature if she chooses to be violent

With certain failures of system components the FBW system cannot function normally. It then reverts to Abnormal Law without full flight envelope protections and then it can be stalled.

In NORMAL law that is extremely unlikely, and can only occur transiently while the airplane is already on its way towards an unstalled condition.

There have been more than a few incidents where the Airbus stalled in normal law with no failures. I provided links here before but it was still not "accepted" as enough evidence so I will leave this part of the discussion for now.