Airliner brake performance
Some of FlightGear's airliners brake unrealistically well. To support aircraft modelers, this page collects typical maximum decelerations of real-world airliners, achived in real-world RTO tests or otherwise documented.
From all data I could collect so far, airliners' brakes typically generate a mean deceleration in the 3-4 m/s2 range.
RTO and brake test videos on YouTube
A380 brake test
Mean deceleration is given there as 3.62 m/s2
MD-80 Baseline Rejected Takeoff Test
Accelerates slightly above 180 kt, starts braking at 1:40 into the vid, you hear "50" at 1:57, yields 130 kt in 17 seconds, hence mean deceleration 3.9 m/s2
Reporter mentions initial speed of "Above 200 miles or 320 km/h", brakes applied at 1:03, aircraft halts at 1:33. Mean deceleration would be 3 m/s2
The crew applies brakes when reaching V1 at 0:48, the plane stops at 1:17, so that's 29 seconds from V1 to stop. Now what's the value of V1? I estimated it using the length of the centerline markers. When announcing "100", the gear passes by one such dash in 19 frames. When announcing "V1", just before braking, it's one dash in 10 frames, which translates to V1 = 190 kt. So, from 190 kt to 0 in 29 seconds yields 3.4 m/s2.
747 Flight Crew Ops Manual
For max manual braking, no thrust reversers:
Landing dist = ref dist + weight correction + no thrusters
Braking dist = landing dist - 1000 ft (==305 m)
At 200 t
Vref30 = 127 kt
Landing dist = 1010 m - (15 m / 5 t) * (250 t - 200 t) + 75 m = 935 m
Braking dist = 935 m - 305 m = 630 m
Mean deceleration a = Vref302 / (2 * braking dist) = 3.39 m/s2
At 400 t
Vref30 = 184 kt
Landing dist = 1010 m + (20 m / 5 t) * (400 - 250 t) + 75 m = 1685 m
Braking dist = 1685 m - 305 m = 1380 m
Mean deceleration a = 3.25 m/s2
How to perform RTO test in FG
I did some tests with the 787-8 by Omega95. Static friction is set to 0.9 in Aircraft/787-8/Nasal/gear_system.nas. Decelerating from 185 kt to 50 kt (KEDW Rwy 17 is a nice place for that) using full brakes only (no spoilers etc) then takes only 12 seconds, yielding 5.6 m/s2.
Setting static friction to 0.5, it takes 21 seconds to go from 185 kt to 50 kt (3.18 m/s2), and another 7 s to halt (3.4 m/s2). So, values around 0.5 seem to be far more reasonable.