CAB orders investation of American Airlines

The Civil Aeronautics Board has ordered an investigation into allegations American Airlines is using corporate ''dirty tricks'' in an attempt to force Braniff International Airlines into bankruptcy.

Among the allegations under investigation are claims that American triggered a critical Braniff cash-flow crisis, American pilots have used runway tactics that delayed Braniff flights, and American executives secretly urged Braniff creditors to ''pull the plug.'' The two airlines compete on numerous lucrative routes.