Navigation

difficult to measure to that level of accuracy, and in practical of 200 to 415 kHz. The frequencies are readily available on application, it is rarely necessary. A number of flight planning aeronautical charts or in the Chart Supplement U.S. publications publish airport coordinates and waypoints with this precision and the unit accepts those figures. There is a All radio beacons, except compass locators, transmit subtle but important difference in CDI operation and display a continuous three-letter identification in code, except in the RNAV modes. during voice transmissions. A compass locator, which is associated with an instrument landing system, transmits a In the RNAV modes, course deviation is displayed in terms two-letter identification. of linear deviation. In the RNAV en route mode, maximum deflection of the CDI typically represents 5 NM on either Standard broadcast stations can also be used in conjunction side of the selected course without regard to distance from the with ADF. Positive identification of all radio stations is waypoint. In the RNAV approach mode, maximum deflection extremely important and this is particularly true when using of the CDI typically represents 1¼ NM on either side of the standard broadcast stations for navigation. selected course. There is no increase in CDI sensitivity as the aircraft approaches a waypoint in RNAV mode. NDBs have one advantage over the VOR in that low or medium frequencies are not affected by line-of-sight. The The RNAV approach mode is used for instrument approaches. signals follow the curvature of the Earth; therefore, if the Its narrow scale width (¼ of the en route mode) permits very aircraft is within the range of the station, the signals can be precise tracking to or from the selected waypoint. In VFR received regardless of altitude. cross-country navigation, tracking a course in the approach mode is not desirable because it requires a great deal of The following table gives the class of NDB stations, their attention and soon becomes tedious. power, and their usable range: A fourth, lesser-used mode on some units is the VOR NONDIRECTIONAL RADIO BEACON (NDB) Parallel mode. This permits the CDI to display linear (Usable radius distances for all altitudes) (not angular) deviation as the aircraft tracks to and from VORTACs. It derives its name from permitting the pilot Power Distance to offset (or parallel) a selected course or airway at a fixed Class (Watts) (Miles) distance of the pilot’s choosing, if desired. The VOR parallel Compass Locator Under 25 15 mode has the same effect as placing a waypoint directly MH Under 50 25 over an existing VORTAC. Some pilots select the VOR H 50–1999 *50 parallel mode when utilizing the navigation (NAV) tracking HH 2000 or more 75 function of their autopilot for smoother course following near the VORTAC. *Service range of individual facilities may be less than 50 miles. Navigating an aircraft with VOR/DME-based RNAV can be confusing, and it is essential that the pilot become familiar One of the disadvantages that should be considered when with the equipment installed. It is not unknown for pilots to using low frequency (LF) for navigation is that LF signals are operate inadvertently in one of the RNAV modes when the very susceptible to electrical disturbances, such as lightning. operation was not intended, by overlooking switch positions These disturbances create excessive static, needle deviations, or annunciators. The reverse has also occurred with a pilot and signal fades. There may be interference from distant neglecting to place the unit into one of the RNAV modes by stations. Pilots should know the conditions under which these overlooking switch positions or annunciators. As always, the disturbances can occur so they can be more alert to possible prudent pilot is not only familiar with the equipment used, interference when using the ADF. but never places complete reliance in just one method of navigation when others are available for cross-check. Basically, the ADF aircraft equipment consists of a tuner, which is used to set the desired station frequency, and the Automatic Direction Finder (ADF) navigational display. Many general aviation-type aircraft are equipped with ADF radio receiving equipment. To navigate using the ADF, The navigational display consists of a dial upon which the the pilot tunes the receiving equipment to a ground station azimuth is printed and a needle which rotates around the known as a nondirectional radio beacon (NDB). The NDB dial and points to the station to which the receiver is tuned. stations normally operate in a low or medium frequency band 16-29