Two grammar questions

Question

In the sentence,"Selon les témoins que j'ai interviewés, et aussi incroyable que cela paraisse, un hélicoptère se serait posé dans la cour de promenade de la prison et La Fuite serait monté à bord avant qu'il ne redécolle, tout ça en quelques minutes à peine !", I don't understand the use of the present tense of redécoller when we are discussing something that happened in the past in particular when it is preceded by two past perfect verbs, se poser and monter.  Also, why is ne inserted before redécoller?

Accepted Answer

After "avant que" , it's the tricky "ne expletive" and the subjunctive mood (just looks like present tense of redécoller). There is a C1 grammar lesson linked to this exercise :  "Avant que + ne explétif + subjunctive" which you can find by clicking on "all related grammar and vocab" just above the transcription of the exercise.
Avant que + ne explétif + subjunctive (Le Subjonctif) = Before I do in French

Answer

And to complement Paul's reply: the use of the subjunctive present is the default. You only use the past subjunctive if you specifically want to emphasize the temporal succession of one event after the other.