We don't have synonyms for flat spin yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are flat spot, flat sawn and flat spins.
(n) :
(aviation) An especially dangerous type of spin (abnormal stalled flight mode) with a very high angle of attack (typically between 65° and 90°), such that the aircraft's nose is pointed near the horizon despite an extreme negative flightpath angle (gamma); typically unresponsive to flight control inputs and unrecoverable in most aircraft.
(n) :
(aviation) An especially dangerous type of spin (abnormal stalled flight mode) with a very high angle of attack (typically between 65° and 90°), such that the aircraft's nose is pointed near the horizon despite an extreme negative flightpath angle (gamma); typically unresponsive to flight control inputs and unrecoverable in most aircraft.