The aircraft systems and equipment, considered separately and in relation to each other, must be designed such that any catastrophic failure condition does not result from a single failure not shown to be extremely improbable and an inverse relationship must exist between the probability of a failure condition and the severity of its effect on the aircraft and its occupants.
Córais agus trealamh an aerárthaigh, arna meas astu féin agus ó thaobh an ghaoil atá eatarthu, ní mór iad a dhearadh ionas nach dtarlóidh cliseadh tromchúiseach de thoradh ar chliseadh aonair nár léiríodh gurbh fhíorbheag an seans go dtarlódh sé, agus ní mór coibhneas inbhéartach a bheith ann idir a dhóchúla atá sé go dtarlódh dála cliste agus déine na héifeachta a d'fhéadfadh a bheith ar an aerárthach agus ar na taistealaithe dá bharr.