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According to the A320 FCOM, capter 22 auto flight, flight guidance, the "automatic speed mode protection" of the auto flight system of the A320 disengages the flight directors upon reaching current VMAX+4kt which will cause A/THR to revert to speed mode. But this protection is only active in climb modes: CLB, OP CLB and EXP CLB.

Does anybody know why the "automatic speed mode protection" is not available in SRS vertical mode where A/THR is also in THR CLB mode?

Also does anybody know if this protection uses the VFE of the selected flap config or of the actual config? E.g. if you are in climb and inadvertently select flaps from UP to CONF ONE then the aircraft won't let you do that but does this (cruise lock) but as far as I know the speed tape on the PFD already shows the maximum airspeed for the selected flap 1 config. So does this already disengage the flight directors? Or do they use the actual flap position or overspeed warning from the flight warning computers?

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2 Answers 2

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High speed protection is still available (in normal law) in SRS (check FCOM DSC 27 20-10-20).

What you will not have is the reversion mode of the autothrust to Speed. You will get the protection (THS freeze, spiral stability to 0deg, max 40 deg bank, nose up order) but thrust will remain in THR CLB.

I suspect the reasoning was that SRS mode implies you are pretty low on altitude, and you do not desire an automatic (and uncommanded) reduction of thrust on, say, a windshear.

Of course, in this particular case the high speed prot will activate at VMO/MMO so if you manage to get that fast while still in SRS you have bigger problems than the thrust

For your second question: mode reversion to Speed will occur at VMAX (which is lower of VMO, VLE or VFE), but for SRS there is no flap overspeed protection so only VMO will trigger it

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  • $\begingroup$ Doesn't the SRS have a "speed protection limiting the target speed to V2+15 kt" according to 22-30-80? $\endgroup$
    – user14897
    May 29, 2019 at 17:14
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    $\begingroup$ yes, but protection might be an unfortunate term here. if you ignore the FDs there is nothing to keep the V2+15.It is just the managed(target) speed that is maxed out to V2+15 $\endgroup$
    – Radu094
    May 30, 2019 at 8:31
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SRS mode is a TAKE OFF or GO AROUND mode, the thrust is fixed and the pitch will maintain the min speed to V2 +10 with 2 engines and V2 in case of an engine failure; this mode requires slats being extended and it stops at acceleration altitude. Speed protection is provided by pitch attitude no need for other speed protection.

With respect to the second part of the question:

It is the SFCC active S/F position not the selected one that is used by APFD

In case of protection, I.e. divergence between the selection and the actual surfaces position, the autopilot flight director uses the actual surfaces position. The speeds prot indications that show on the PFD are computed by the FAC (flight augmentation computer) they do not affect the APFD neither any reversion directly.

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  • $\begingroup$ The thrust is also fixed in OP CLB or CLB and the pilots could chose not to fly V2 + 10kts with SRS engaged, e.g. if they don't pitch up quick enough, so the question is what happens if you fly too fast during takeoff or go-around, e.g. with flaps 3 set and bust the speed limit. Sure nobody does that but the scenario is the same: pilot not pitching up and thrust very high. $\endgroup$
    – Jan
    May 29, 2019 at 14:05
  • $\begingroup$ You can’t fly too fast with flaps at position 3 unless you fly level, nevertheless no changes to the modes will occur before acceleration altitude, however note flap 3 is not a TAKE OFF valid position, so this scenario will trigger the take off config alarm, if by leveling before the acceleration altitude you keep increasing the speed, you will get the over speed alarm but no mode REVERSION $\endgroup$
    – user40476
    May 29, 2019 at 18:00
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    $\begingroup$ Conf 3 is a valid takeoff position $\endgroup$
    – Radu094
    May 30, 2019 at 8:40
  • $\begingroup$ You are right, I was thinking of v2500 $\endgroup$
    – user40476
    May 30, 2019 at 9:38

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