"My" rules for forming Imperative, taken from here and there, are rather simple and up to now without an exception:
1) Stem in й (читай-, делай-): nothing is added -> читай, делай
2) The rest of cases
2.1) 1st. p. s. acute (писа-, пишý): и is added. пис(а)и -> пиши
and the previous vocal is suppresed
2.2) id NOT acute (ответи- отвéчу): ь is added. ответ(и)ь ->ответь
and the previous vocal is suppresed
unless accumulation of consonnts is present:
пoмни- пóмню where и is added. пoмн(и)и -> пóмни.

But I found now вылететь II, вылечу whose imperative should be according to what is previously said: вылеть, but I see it everywhere вылети. No mention to an irregularity is found nowhere. I suppose that simply it is like this and can be applied the Spanish proverb: "There are lentils, if you want, eat them; otherwise, leave them" ("Son lentejas, si quieres las comes y si no las dejas"). Is there any additional, and unknown by me, rule that makes this behaviour of вылететь normal or unless not unique? Thank you.