Hi! That's indeed a bit tricky!
The truth is that, formally, McKinsey and BCG strongly discourage (if not prohibit) working on flights, due to confidentiality. But the truth is also that the majority of consultants actually work on flights, just to cope with the workload.
So different aspects need to balanced against each other here. I for example have frequently worked on very sensitive topics (stock market relevant), so I did not work on flights and I also gave directive to my teams on such projects that working on the plane is an absolute no-go. When working on less sensitive stuff, it was more about applying good judgement.
Hope this helps!