What is the point in specifying a safety margin?
Safety margins are applied to performance data because the calculations involve many assumptions; the margin also considers some variability in the aircraft (engines, brakes, tyres), runway surface (how slippery is ‘wet’), crew reaction, etc, etc. Certification and operational requirements are place markers on which everyday operations have to be built; where possible they cover a reasonable range of the variables and to a lesser degree, account for opportunities for error. The resultant provides an acceptable margin of safety (for public operation) in all routine operations, judged as not taking unacceptable risks.
Take off distance is not necessarily the same as the distance needed to accelerate and then stop from a critical speed.
A rejected takeoff scenario just before V1 is a critical area where the margins are minimal. The identification of the components of a situation on which the decision to stop is perhaps the weakest area.
A fire warning or engine run down is relatively clear cut, but determining the source and effect of a ‘bang’ is complicated. A takeoff ‘surge’ in large engines is normally associated with a big bang (but not always), and the noise could be a tyre, load shift, or a galley door.
Confirming an engine surge – leading to a sustained loss of power, is very complicated. The indications are not always evident on instruments, or if present not always seen; all of this takes time which is a premium.
Why wasn’t reverse used? We must wait and see what the crew have to say, but humans react in unusual ways in stressful situations. Perhaps more so where the decision to stop is made on a particular belief (engine failure) and then coupled with an erroneous belief (asymmetric reverse not authorised, or reverse not required) then the safety margins rapidly disappear.
Attempting to stop beyond the critical point (V1) is fraught with danger; more so if reverse is not used.