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On page 30 of the ATSB report into the QF72 accident (page 50 of the PDF file of the report), in a table describing the actions required of the flightcrew in response to some of the ECAM messages displayed during the flight, there is the following entry:

ECAM message [...] Required actions
[...] [...] [...]
F/CTL ALTN LAW (PROT LOST) [...] Do not use speed brake [...] Maximum speed limited to 330 kts / Mach 0.82

[ATSB Transport Safety Report AO-2008-070, page 30 (PDF page 50), Table 6. Table headings bolded in original; all other bolding is my own.]

Why aren’t the A330/40’s speedbrakes (the term used for the aircraft’s flight spoilers, when deployed symmetrically to increase the aircraft’s drag and reduce its lift) supposed to be used when in alternate law?

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The table in the report omits a condition. Here is the ECAM message in full:

enter image description here

The limitation applies when the aircraft is in emergency electrical configuration (run by the ram air turbine), which is one way to have the flight laws degraded.

The manual does not give a direct reason (not unheard of), but in fact there's more than one reason.

enter image description here

► As a quick summary, you lose 2/3 of the spoilers and the outer ailerons, and if more unlucky, you lose 5/6 of the spoilers.

If you happen to use the now-not-very-helpful spoilers, and you lose more hydraulic pressure, they'll get stuck in the up position (adding drag and degrading roll). So, DO NOT USE makes sense.

The how:

  • In emer elec config, the plane loses 3 of the 5 flight control computers; what remains are P1 and S1 (refer to diagram above).

  • The outer ailerons go into an unpowered state to conserve hydraulic fluid/pressure – the right aileron upfloats* like on the A320 (no P3 and S2).

  • If the planes loses G or Y hydraulic pressure, the plane is left with one panel on each wing, which is likely to be needed for roll control at slower speeds (the outer ailerons are already out).

The references (click to view):

enter image description here enter image description here

All images are from the A340 FCOM, and color highlights are mine. Do not use for flight.

A330 vs A340

Thanks to @JZYL, note that there are some differences between the A330 and A340, while the A340 would announce MOST SPOILERS being INOP, the A330 would announce HALF SPOILERS (an extra computer remains powered). The same rationale given should apply though.

* This is also confirmed from this FCTM (smartcockpit.com) of an airline with both the A330 and A340 in its fleet (see PDF page 207):

(...) The AP will disengage when the failure occurs but AP 1 can be reengaged on the A333 if the EMER GEN is powered by an EDP. As there is no AP available in this configuration in the A340, the Captain must take control and fly the aircraft manually. The A340 aircraft is out of trim in roll due to right outboard aileron upfloat, caused by no power being available to Prim 3 or Sec 2, and neither pitch trim or rudder trim is available. (...)

Note: I haven't found a reason for the different architecture, but ETOPS is a possibility.

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  • $\begingroup$ On the A340, why not remove hydraulic pressure from the left outboard ailerons as well, so they float up too (eliminating the roll mistrim caused by the right-outboard-aileron upfloat)? $\endgroup$
    – Vikki
    Apr 11, 2020 at 21:24
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    $\begingroup$ @Sean: More roll capability is better than less roll capability I think. $\endgroup$
    – user14897
    Apr 11, 2020 at 21:29
  • $\begingroup$ Except that, to judge from the diagram and explanation given, the left outboard ailerons are already inoperative in the emergency electrical configuration, so keeping them under hydraulic pressure doesn't give any additional roll control. $\endgroup$
    – Vikki
    Apr 22, 2020 at 17:37
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    $\begingroup$ @Sean: ahh ok, it's not like they said let's upfloat one, that's simply the result of that specific redundant architecture in a super rare citation not needed scenario, and if it ain't broke $\endgroup$
    – user14897
    Apr 22, 2020 at 18:09
  • $\begingroup$ Also, if hydraulic pressure to the spoilers is lost while they're extended, wouldn't they blow down to the retracted position rather than sticking in the extended position? $\endgroup$
    – Vikki
    Sep 12, 2020 at 23:13

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