A320 Spoiler/autobrake logic
 

Does anyone know where the 72 knots came from?
Obviously this has some significance as it's such a random number. Does anyone have any ideas why they came up with the 72knots?

72kt=120kmh, maybe from there?

Sounds like as good a guess as any.

72k = 133 km/h

133 seems as random as 72?

Maybe so but 133 is a Blum Integer.

So some sort of mathematical/computing requirement?

I don't know the answer... but in engineering often these sorts of limits come from optimization calculations reflecting tradeoffs from various opposing factors.

E.g., you want the minimum wheelspin speed to be fast enough to ensure that the aircraft is "planted on the ground" (not bouncing) but not so fast that the logic prevents operation during a critical moment (e.g., while hydroplaning in the wet). So there's a tradeoff between the two.

The engineers would collect all these tradeoffs, make a min/max "cost" function, put in some constraints, plug them into a solver, and voila... the computer spits out 37 m/s or 72 kts as the optimum tradeoff.

I'm simplifying of course. I think the Boeing equivalent logic requires higher speeds, 85 or 90 kts, probably based on a different set of tradeoffs.