5.3. Air-to-ground signals
The following signals by aircraft mean that the ground signals
have been understood and acknowledged receipt of then:
a) Rocking the aircraft wings during the hours of dayling.
b) Flashing on and off twice the aircraft landing lights or, if
not so equipped, by switching its navigation lights on and
off twice during the hours of darkness.
Lack of the above signals indicates that the ground signal is
not understood.