What’s an ERO? Engine running ______? Been in the industry 16+ years but have never heard that acronym before.
If you’re implying that they did some sort of engine running quick turn, I can assure you that was definitely not the case. The airplane was not designed to do those sort of operations. It’s all fly-by-wire and FADEC controlled. It needs the engines to be shut down and for things to be reset to work properly. I suspect that this particular plane had an APU that was deferred, requiring a start cart and engine start at the gate before pushing back. Not a totally unusual occurrence, but definitely not routine. In that scenario, the Capt would need to start the Right engine first (due to its ducting being closest and shortest to the hook up for the start cart, minimizing pressure loss and possible abnormal engine start). Normally, the left engine would’ve been started first during pushback and used for single engine taxi if the Capt elected to do so. So now you have an engine running at the gate (dangerous), ground crew having to disconnect hoses and cables after that engine is started (dangerous), chocks must be removed by ground before pushing back with an engine running (dangerous), and the engine that is running, isn’t the one that’s usually running (doubly dangerous).