14 CFR §171.319

1. (a)The monitor system must act to ensure that any of the following conditions do not persist for longer than the periods specified when:
   1. (1)There is a change in the ground component contribution to the mean glidepath error component such that the path following error on any glidepath exceeds the limits specified in § 171.317(d) for a period of more than one second.
   2. (2)There is a reduction in the radiated power to a level not less than that specified in § 171.317(a)(4) for a period of more than one second.
   3. (3)There is an error in the preamble DPSK transmission which occurs more than once in any one second period.
   4. (4)There is an error in the time division multiplex synchronization of a particular elevation function such that the requirement specified in § 171.311(e) is not satisfied and this condition persists for more than one second.
   5. (5)A failure of the monitor is detected.
2. (b)The period during which erroneous guidance information is radiated must not exceed the periods specified in § 171.319(a). If the fault is not cleared within the time allowed, radiation shall cease. After shutdown, no attempt must be made to restore service until a period of 20 seconds has elapsed.