Some people say that if it comes down to an airline having two similar candidates, one with a levels, one without - they'll choose the one with. That probably never happens, and even if it did - they'd choose personality over high school education. Person a might have 2 A grades in Maths and Physics but he's a right pillock - his qualifications wont get him the job.
Except the reality is the airline doesn't have two similar candidates, it has
hundreds. The grades will be used as one method of whittling the lists down to a managable level for interview. Unless you get as far as the interview, your personality is going to have little chance to shine. However imprecise a science it may be, A-levels at good grade and in the right subjects, show the selectors that a candidate made good choices in relevant subjects and appeared to apply the necessary levels of study and aptitude to achieve the grades that they did. For a job that will continue to require the incumbent to apply the necessary levels of study and achieve a high standard (and grade,) that is significant.
In the current and forseeable market, good grades will (in the absence of significant relevant experience) be a likely prerequisite for interview, simply because so many candidates will have them. Personality is a factor at interview, but in itself will not compensate for a lack of accreditable application and achievement. If achievement and personality are required and Candidate "A" has the former but not the latter, and candidate "B" has the latter but not the former, then "A" is likely to get the interview then fail to progress, "B" is likley to get a "Thank you for your interest" letter, and the rest of the alphabet and scores beside will be queueing up to satisfy the limited number of vacancies actually on offer.
Best advice is apply well armed.