The 'Oxford comma' is incorrect with only 2 items conjoined.
To avoid ambiguity, if the desired sentence needs to indicate that it is only engineers who can 'do big sums', although both aircrew and engineers are senior, I would recommend 'Senior engineers, who can do big sums, and senior aircrew'. In this form, the commas are used in parenthesis, so the comma appearing before 'and' is probably acceptable.
PS - I never did idiot scribblers' school! Nor the 'C' exam, for that matter!