Man the FedEx crew who gave the "Southwest Abort" call on the radio earned their wings that day. Just amazing presence of mind to do exactly the right thing in the moment.
It was a good thing to try, but the Southwest plane was above V1 (edit: may not have been, but acted as if they were) and couldn't abort, and didn't. Both planes climbed away from the runway with minimal clearance between them. It was simply luck that they didn't crash anyway.
The fast reaction by the Fedex pilots may have contributed to increased distance between the planes that saved them, but their callout to the Southwest flight didn't fundamentally change anything. Maybe we'll find out that the Southwest flight climbed slower, suspecting the Fedex plane was directly over them. In that case maybe the callout in addition to the Fedex crew being attentive saved the day, but I haven't seen anyone claim that yet.
If the Southwest crew had realized how close the Fedex plane was, maybe they would have aborted even if it meant running off the end of the runway, but they didn't do that.
I'm not sure anymore, given the above replay, that the Southwest plane was at V1. The Fedex flight was at least directly over (according to some other transponder-based replay videos), and might have passed the Southwest plane by about a plane length (as shown in the above video), before the Southwest plane had accelerated enough to close the velocity gap. Maybe Fedex waited, but if they radioed the request for Southwest to abort before the planes were overlapping in the horizontal plane, Southwest should have easily been able to stop.
It's up to investigators to find out. Pilots definitely can (and have the right to) abort after V1, if they consider it safer than taking off. Before V1, you can be sure that abort is safe, after V1, you have to weigh risks of high-speed abort against risks of continuing flying, but staying on the ground is still possible.
Yes, and nitpicking - decision speed is V1, not V0.
No, it's not (always) more dangerous, please read the operation manuals if you don't believe me.
Let me put it another way. V1 is not the speed after which you cannot abort, V1 is the speed after which you can attempt to takeoff. Or, quoting the Boeing flight manual,
> V1 is the maximum speed at which the flight crew must take the first action to reject a takeoff.
If something happens _before_ V1, pilots must abort takeoff, period. However, if something happens after V1, pilots can still make a decision to stay on the ground.
Furthermore, V1 is calculated so that the plane can stop before the end of the runway even if one of the braking systems is not working properly (e. g. one of the engines is out and reverse thrust cannot be used). On a fully working plane, the actual distance necessary to stop will be lower. Then, there is always a safety zone right after the runway, which is not taken into account for V1 calculation.
Anyway, the investigation will thoroughly consider all the factors, let's wait for the results.
