I really can't understand the reason as to why Airbus has put a speed restriction of 72 knots for the activation of autobrakes and deployment of ground spoilers. Could someone explain why this has been imposed?


Not a definite answer: I imagine someone backtracking on a long 4000 m runway might actually taxi at around 71kts?

If armed, MAX braking will occur once the system detects that ground spoilers are deployed. From the FCOM, ground spoilers:

If the ground spoilers are armed and the speed exceeds 72 kt, the ground spoilers will automatically extend as soon as both thrust levers are reset to idle.

Because of Airbus SOPs by the time you enter the active rwy, your autobrakes are on MAX and spoilers armed. Once armed&active, they will deploy the second the ADIRS will sense any deceleration. With thr lvl retarded to idle, the plane now thinks you are actually rejecting a take off, not knowing you are slowing down at the end of the runway for the 180deg turn to lineup. All hell will break loose as your plane insta-handbrakes on the spot with the activation of spoilers+max brakes.

Incidentally, my company used to have a 30kts speed limit for backtrack which has been recently removed.

  • $\begingroup$ Are they armed in takeoff config? Is that in case of rejected t/o? $\endgroup$ – TomMcW Jul 26 '17 at 18:36
  • $\begingroup$ exactly, they are part of T/O ecam memo (done during taxi) and thus active by the time you reach the runway. Any deceleration after reaching 72kts and the plane thinks you are rejecting the takeoff $\endgroup$ – Radu094 Jul 26 '17 at 20:02
  • $\begingroup$ uhm yes.. (IIUC) question was why autobrake max and auto spoiler extension is not active if plane never reaches 72kts. My (attempted) answer was: to prevent a wrongfull deployment after taxiing at high speed. I need to stop posting late at night... $\endgroup$ – Radu094 Jul 27 '17 at 7:20

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