3. Arrival or Through Flight Entry
Requirements. Two−way radio communication
must be established with the ATC facility providing
ATC services prior to entry and thereafter maintain
those communications while in the Class D airspace.
Pilots of arriving aircraft should contact the control
tower on the publicized frequency and give their
position, altitude, destination, and any request(s).
Radio contact should be initiated far enough from the
Class D airspace boundary to preclude entering the
Class D airspace before two−way radio communications
are established.
NOTE−
1. If the controller responds to a radio call with, “[aircraft
callsign] standby,” radio communications have been
established and the pilot can enter the Class D airspace.
2. If workload or traffic conditions prevent immediate
entry into Class D airspace, the controller will inform the
pilot to remain outside the Class D airspace until
conditions permit entry.
EXAMPLE−
1. “[Aircraft callsign] remain outside the Class Delta
airspace and standby.”
It is important to understand that if the controller responds
to the initial radio call without using the aircraft callsign,
radio communications have not been established and the
pilot may not enter the Class D airspace.
2. “Aircraft calling Manassas tower standby.”
At those airports where the control tower does not operate
24 hours a day, the operating hours of the tower will be
listed on the appropriate charts and in the A/FD. During
the hours the tower is not in operation, the Class E surface
area rules or a combination of Class E rules to 700 feet
above ground level and Class G rules to the surface will
become applicable. Check the A/FD for specifics.
.
They will give the plane the light gun and close the field, roll out the rescue equipment, so basically the tower has declared an emergency for that pilot. 
