INSTRUMENT FLIGHT RULES WORKBOOK
SLANT RANGE DISTANCES
Figure 1-15 DME Distances
1. Bearing/distance unlock. TACAN bearing and distance signals are subject to line-of-sight
restrictions because of their utilization of UHF frequencies. Because of the
transmission/reception principles, unlock (indicated by rotating of bearing pointer and/or range
indicator) will occur if these signals are obstructed. Temporary obstruction of TACAN signals
can occur in flight when aircraft fuselage, wing, gear; external stores, or wingmen get between
the ground and the aircraft antenna. Aircraft receiver memory circuits prevent unlock for short
periods (approximately 10 seconds for DME and 2 seconds for azimuth). But beyond this,
unlock occurs and will persist until the obstruction is removed and search cycles are completed.
Unlock may occur during maneuvers, such as procedure turns, which cause the aircraft antenna
to be obstructed for longer than 2 to 10 seconds.
INTRODUCTION TO AIRBORNE NAVIGATION AND COMMUNICATIONS
EQUIPMENT AND PRINCIPLES OF OPERATION 1-19