And the safety margin was wide enough to absorb that screw up.
This is _why_ we have wide safety margins. So that one screw up doesn't absorb all of it. People will always screw up, and policy accounts for that fact.
For example, the approach controller may have noticed that a plane they recently handed off to Tower was not yet lined up. The controller would then call up to Tower to make Tower aware of the issue.
Or, Tower may have a screen showing them a radar view of the approach, similar to Approach's screen. They may have noticed it there.
But, the nature of aviation radio is such that you use one frequency to transmit and receive. That means you can't speak over anyone else; you have to wait for them to stop talking before you can start. And if you mess up, there's no easy way for you to know (except when someone else on frequency says "blocked").
* Conditions were clear. The pilot landing would have an increasing probability of noticing their mistake from lights on the runway and taxiway as they got closer to landing.
* After being alerted that the pilot landing could see lights on what they thought was the runway, the ATC would likely have paid closer attention to the various tools available to them to see the location and heading of the aircraft.
And finally, the one factor that happened to be the first of these to happen:
* Multiple pilots looking directly toward the oncoming aircraft, one of whom alerted ATC.
Just because this happened first doesn't mean the other things failed or wouldn't have happened.
> * Multiple pilots looking directly toward the oncoming aircraft, one of whom alerted ATC.
I don't get this.
You're an engineer/pilot/whomever filing for {the current system/pilot/plane/whatever-needs-approval} to get approved. The power-that-be (e.g. FAA) asks you what your safety margins are. You cite "other pilots looking directly at my aircraft in the 30-second span of my mistake" as one of your "safety margins"? ...what?
Incidentally, Aviation Herald reports[1] that "AC-759 had already overflown taxiway C by about 0.25nm when ATC instructed the aircraft to go around" and further that "it is estimated that AC-759 overflew the first two aircraft by 100 feet".
100 feet isn't much, and I imagine this was utterly terrifying for the pilots lined up on the taxiway, but 0.25nm past the beginning of taxiway C is basically touchdown point on the runway (had they been lined up with it), and they were still 100 feet above the ground. That indicates pretty strongly that the pilot of AC-759 had noticed their mistake and was taking evasive action already, before ATC instructed them to go around.
Barely.