(2)
They were unable to contact Air Traffic Control on a particular frequency.

NIGHT (P/CG) - The time between the end of evening civil twilight and the beginning of morning civil twilight, as published in the American Air Almanac, converted to local time.
NIGHT (ICAO) - The hours between the end of evening civil twilight and the beginning of morning civil twilight or such other period between sunset and sunrise as may be specified by the appropriate authority.
NOTE - Civil twilight ends in the evening when the center of the sun's disk is 6 degrees below the horizon and begins in the morning when the center of the sun's disk is 6 degrees below the horizon.
NO GYRO APPROACH/VECTOR - A radar approach/vector provided in case of a malfunctioning gyrocompass or directional gyro. Instead of providing the pilot with headings to be flown, the controller observes the radar track and issues control instructions "turn right/left" or "stop turn" as appropriate.
NON-APPROACH CONTROL TOWER - Authorizes aircraft to land or take-off at the airport controlled by the tower, or to transit the airport traffic area. The primary function of a non-approach control tower is the sequencing of aircraft in the traffic pattern and on the landing area. Non-approach control towers also separate aircraft operating under Instrument Flight Rules clearances from approach controls and centers. They provide ground control services to aircraft, vehicles, personnel, and equipment on the airport movement area.
NON-COMPOSITE SEPARATION - Separation in accordance with minima other than the composite separation minimum specified for the area concerned.
NON-DIRECTIONAL BEACON/RADIO BEACON/NDB - A Low/Medium Frequency or Ultra High Frequency radio beacon transmitting non-directional signals whereby the pilot of an aircraft equipped with Direction Finding equipment can determine his bearing to or from the radio beacon and "home" on or track to or from the station. When the radio beacon is installed in conjunction with the Instrument Landing System marker, it is normally called a Compass Locator. (See Compass Locator, Automatic Direction Finder)
NON-FREE FLYING AREA (DIA) - An area normally delineated by an international boundary or the territorial water adjacent thereto wherein penetration by United States military aircraft is likely to result in their being fired upon without warning.
NON-PRECISION APPROACH PROCEDURE/NONPRECISION APPROACH - A standard instrument approach procedure in which no electronic glide slope is provided; e.g., VOR, TACAN, NDB, LOC, ASR, LDA, or SDF approach.