The Free On-line Dictionary of Computing (30 December 2018):

aeroplane rule
airplane rule

    "Complexity increases the possibility of failure;
   a twin-engine aeroplane has twice as many engine problems as a
   single-engine aeroplane."

   By analogy, in both software and electronics, the implication
   is that simplicity increases robustness and that the right way
   to build reliable systems is to put all your eggs in one
   basket, after making sure that you've built a really *good*
   basket.

   While simplicity is a useful design goal, and twin-engine
   aeroplanes do have twice as many engine problems, the analogy
   is almost entirely bogus.  Commercial passenger aircraft are
   required to have at least two engines (on different wings or
   nacelles) so that the aeroplane can land safely if one engine
   fails.  As Albert Einstein said, "Everything should be made as
   simple as possible, but not simpler".

   See also KISS Principle.

   (1999-03-22)