AF 447 Thread No. 11

[Chris Scott]

A340 AoA (protection) Law - disengagement conditions

The new (to this thread) BEA Report on the AF A340 Serious Incident of 2011 describes the conditions that allow AoA Law to disengage back into Normal Law (which it refers to correctly - in the context of high altitude - as "load-factor control"). The OR-gate conditions that it lists include a third condition that we seem to have been unaware of so far in our discussion of the A340 AIRPROX event of October 2000.

I raised some controversy recently when I questioned the disengagement logic of AoA Law, having been led to believe that, in the A340 AIRPROX case, once AoA Law was triggered by a short-term, phase-advanced (gust-induced) identification of alpha-prot, it remained engaged until the PF finally made a substantial push on the sidestick. This seemed to be despite the fact that - after the trigger-gust subsided - the AoA and phase-advanced AoA appear to have been below the trigger threshold for a couple of seconds until the EFCS (FBW) in AoA Law selected up-elevator to increase the AoA to alpha-prot.

Accordingly, I wrote:
"...Phase-advanced alpha-protection puts the FBW into AoA Protection Law. Meanwhile, the AoA has returned (fallen) to what it was before the gust. However, FBW uses up-elevator to increase the AoA to alpha-prot. The a/c climbs suddenly until I push the stick forward more than half-travel, OR use less-than-half forward stick for more than a second."

Well, in this new report into the 2011 incident, the BEA lists the disengagement conditions as follows (newbees, note that a sidestick is spring-loaded to neutral):

sidestick forward more than half-travel;
OR
sidestick forward less than half-travel for 1 s while AoA < Alpha MAX;
OR
sidestick neutral or forward for 0.5 s while AoA < Alpha Prot.

It seems now that, either my understanding of the disengagement conditions for the A340 in 2000 was wrong, or they were subsequently modified?

PS by EDIT

Re the report appendix, am looking forward to interpretation by those who are more experienced and better than I am in interpreting FDR traces. (Mentioning no names!) In the interim, I notice that AoA Law disengages (to Normal Law) and re-engages on 5 occasions prior to its final disengagement. So there are 6 periods of AoA Law.

The first period ended in disengagement after only 8 seconds, apparently because the sidestick was released to neutral and an up-gust ceased (or a down-gust was encountered), reducing the AoA below Alpha-Prot (see the third condition listed above) for > 0.5 s. Although this restoration of Normal Law lasted less than 3 seconds, the report suggests it would have been maintained, but for the turbulence and speedbrakes, with the sidestick neutral. During this short period of Normal Law, the pitch increased rapidly from about +5 to about +8.5. The increase in pitch rate and AoA rate was so great that, when AoA Law re-engaged, it was unable to stop the AoA briefly exceeding Alpha-MAX.

Notwithstanding my doubts, expressed a few days ago, about the logic of engagement/disengagement of AoA Law (see above), the DFDR traces in this BEA Report seem to show that the periods of its disengagement may have resulted in greater divergences from normal flight parameters than when it was engaged.

One assumes that, if AoA Law was maintained, the flight profile would approximate a phugoid? That view may be supported by the last (and longest) period of AoA-Law engagement, which started just after the FL382 apogee. During the descent, the VS increases gradually to about 4000 ft/min; then decreases gradually to zero (at FL365). It is at that point that the PF applies just under half forward-stick for more than a second, disengaging AoA law.