14 CFR §27.1316

1. (a)Each electrical and electronic system that performs a function, for which failure would prevent the continued safe flight and landing of the rotorcraft, must be designed and installed so that—
   1. (1)The function is not adversely affected during and after the time the rotorcraft is exposed to lightning; and
   2. (2)The system automatically recovers normal operation of that function in a timely manner after the rotorcraft is exposed to lightning.
2. (b)For rotorcraft approved for instrument flight rules operation, each electrical and electronic system that performs a function, for which failure would reduce the capability of the rotorcraft or the ability of the flightcrew to respond to an adverse operating condition, must be designed and installed so that the function recovers normal operation in a timely manner after the rotorcraft is exposed to lightning.