14 CFR §25.4

1. (a)For the purposes of this part, the following general definitions apply:
   1. (1)Certification maintenance requirement means a required scheduled maintenance task established during the design certification of the airplane systems as an airworthiness limitation of the type certificate or supplemental type certificate.
   2. (2)Significant latent failure is a latent failure that, in combination with one or more specific failures or events, would result in a hazardous or catastrophic failure condition.
2. (b)For purposes of this part, the following failure conditions, in order of increasing severity, apply:
   1. (1)Major failure condition means a failure condition that would reduce the capability of the airplane or the ability of the flightcrew to cope with adverse operating conditions, to the extent that there would be—
      1. (i)A significant reduction in safety margins or functional capabilities,
      2. (ii)A physical discomfort or a significant increase in flightcrew workload or in conditions impairing the efficiency of the flightcrew,
      3. (iii)Physical distress to passengers or cabin crew, possibly including injuries, or
      4. (iv)An effect of similar severity.
   2. (2)Hazardous failure condition means a failure condition that would reduce the capability of the airplane or the ability of the flightcrew to cope with adverse operating conditions, to the extent that there would be—
      1. (i)A large reduction in safety margins or functional capabilities,
      2. (ii)Physical distress or excessive workload such that the flightcrew cannot be relied upon to perform their tasks accurately or completely, or
      3. (iii)Serious or fatal injuries to a relatively small number of persons other than the flightcrew.
   3. (3)Catastrophic failure condition means a failure condition that would result in multiple fatalities, usually with the loss of the airplane.
3. (c)For purposes of this part, the following failure conditions in order of decreasing probability apply:
   1. (1)Probable failure condition means a failure condition that is anticipated to occur one or more times during the entire operational life of each airplane of a given type.
   2. (2)Remote failure condition means a failure condition that is not anticipated to occur to each airplane of a given type during its entire operational life, but which may occur several times during the total operational life of a number of airplanes of a given type.
   3. (3)Extremely remote failure condition means a failure condition that is not anticipated to occur to each airplane of a given type during its entire operational life, but which may occur a few times during the total operational life of all airplanes of a given type.
   4. (4)Extremely improbable failure condition means a failure condition that is not anticipated to occur during the total operational life of all airplanes of a given type.