▁extra ▁design ▁measures ▁to ▁ensure ▁ade qu ate ▁control ▁response , ▁any ▁attempt ▁to ▁make ▁a ▁last - min ute ▁correction ▁to ▁the ▁flight ▁path ▁is ▁likely ▁to ▁prove ▁too ▁little ▁too ▁late , ▁making ▁a ▁last - min ute ▁landing ▁abort ▁and ▁fly - around ▁difficult ▁and ▁dangerous . ▁ ▁Eng ines ▁The ▁number ▁of ▁engines ▁on ▁an ▁aircraft ▁affect s ▁its ▁reli ability ▁and ▁safety . ▁The ▁more ▁engines ▁there ▁are , ▁the ▁sa fer ▁it ▁is ▁if ▁one ▁engine ▁fails . ▁But ▁on ▁the ▁other ▁hand , ▁the ▁more ▁engines ▁there ▁are , ▁the ▁more ▁likely ▁there ▁is ▁to ▁be ▁a ▁failure ▁of ▁one ▁or ▁more ▁and ▁the ▁greater ▁the ▁work load ▁on ▁the ▁flight ▁engineer . ▁Now ad ays , ▁two ▁engines ▁are ▁preferred ▁in ▁practice , ▁with ▁even ▁quite ▁large ▁wide - body ▁aircraft ▁having ▁only ▁two ▁engines . ▁Four ▁is ▁generally ▁accepted ▁as ▁the ▁limit , ▁for ▁both ▁safety ▁and ▁cost ▁reasons . ▁ ▁Bar ring ▁a ▁few ▁military ▁types , ▁no ▁practical ▁large ▁aircraft ▁has ▁ever ▁had ▁more ▁than ▁four ▁engines . ▁As ▁aircraft ▁get ▁bigger , ▁it ▁therefore ▁becomes ▁necessary ▁to ▁design ▁bigger ▁engines . ▁ ▁The ▁air speed ▁of ▁a ▁fan ▁bla de ▁must ▁be ▁kept ▁below ▁the ▁speed ▁of ▁sound ▁in ▁order ▁to ▁avoid ▁dam aging ▁and ▁no isy ▁shock ▁waves . ▁This ▁maximum ▁speed ▁of ▁the ▁tip ▁sets ▁a ▁limit ▁on ▁the ▁rate ▁of ▁rotation . ▁For ▁a ▁given ▁rate ▁of ▁rotation , ▁the ▁tip ▁of ▁a ▁larger ▁fan ▁will ▁travel ▁faster