Navigation

in seconds and a 10° bearing change is made, the time from Rate of Intercept the station, in minutes, is determined by counting off one Rate of intercept, seen by the aviator as bearing pointer or decimal point. Thus, if 75 seconds are required to fly a 10° HSI movement, is a result of the following factors: bearing change, the aircraft is 7.5 minutes from the station. • The angle at which the aircraft is flown toward a When the RMI bearing pointer is moving rapidly or when desired course (angle of intercept) several corrections are required to place the pointer on the wingtip position, the aircraft is at station passage. • True airspeed and wind (GS) • Distance from the station The distance from the station is computed by multiplying TAS or GS (in miles per minute) by the previously determined time Angle of Intercept in minutes. For example, if the aircraft is 7.5 minutes from The angle of intercept is the angle between the heading station, flying at a TAS of 120 knots or 2 NM per minute, of the aircraft (intercept heading) and the desired course. the distance from station is 15 NM (7.5 × 2 = 15). Controlling this angle by selection/adjustment of the intercept heading is the easiest and most effective way to control The accuracy of time and distance checks is governed by course interceptions. Angle of intercept must be greater than existing wind, degree of bearing change, and accuracy of the degrees from course, but should not exceed 90°. Within timing. The number of variables involved causes the result this limit, make adjustments as needed, to achieve the most to be only an approximation. However, by flying an accurate desirable rate of intercept. heading and checking the time and bearing closely, the pilot can make a reasonable estimate of time and distance from When selecting an intercept heading, the key factor is the the station. relationship between distance from the station and degrees from the course. Each degree, or radial, is 1 NM wide at Time and Distance Check From a Station Using a a distance of 60 NM from the station. Width increases or CDI decreases in proportion to the 60 NM distance. For example, To compute time and distance from a station using a CDI, 1 degree is 2 NM wide at 120 NM—and ½ NM wide at 30 first tune and identify the VOR station and determine the NM. For a given GS and angle of intercept, the resultant rate radial on which you are located. Then turn inbound and of intercept varies according to the distance from the station. re-center the needle if necessary. Turn 90° left or right, of When selecting an intercept heading to form an angle of the inbound course, rotating the OBS to the nearest 10° intercept, consider the following factors: increment opposite the direction of turn. Maintain heading and when the CDI centers, note the time. Maintaining the • Degrees from course same heading, rotate the OBS 10° in the same direction as • Distance from the station was done previously and note the elapsed time when the • True airspeed and wind (GS) CDI again centers. Time and distance from the station is determined from the formula shown in Figure 16-34. Distance Measuring Equipment (DME) Distance measuring equipment (DME) consists of an ultra Course Intercept high frequency (UHF) navigational aid with VOR/DMEs and Course interceptions are performed in most phases of VORTACs. It measures, in NM, the slant range distance of instrument navigation. The equipment used varies, but an an aircraft from a VOR/DME or VORTAC (both hereafter intercept heading must be flown that results in an angle or referred to as a VORTAC). Although DME equipment is rate of intercept sufficient for solving a particular problem. very popular, not all aircraft are DME equipped. Time-Distance Check Formula To utilize DME, the pilot should select, tune, and identify a VORTAC, as previously described. The DME receiver, A Time to = 60 x minutes flown between bearing change utilizing what is called a “paired frequency” concept, station degrees of bearing change automatically selects and tunes the UHF DME frequency associated with the VHF VORTAC frequency selected by the pilot. This process is entirely transparent to the pilot. B Distance to station = TAS x minutes flown After a brief pause, the DME display shows the slant range degrees of bearing change distance to or from the VORTAC. Slant range distance is the direct distance between the aircraft and the VORTAC and is therefore affected by aircraft altitude. (Station passage Figure 16-34. Time-distance check formula using a CDI. directly over a VORTAC from an altitude of 6,076 feet AGL 16-27