The station identifiers and frequencies of the selected radio navigation aids are displayed on the (prior to U7) PROGRESS and (U4, U5) POS SHIFT pages or (U7 and on) NAV STATUS page 1/2.
With the AUTO/MANUAL switch in AUTO, DME radios are automatically tuned by the FMC. The stations to be tuned are selected based upon the best available signals (in terms of geometry and strength) for updating the FMC position. Radio position is determined by the intersection of two DME arcs.
The dual frequency–scanning DME radios are always automatically tuned by the FMC.
Flight Management, Navigation -Flight Management System Operation 737 Operations Manual
AUTO tuning uses any of several radio modes. The primary mode tunes the radios to separate DME–capable stations. If required, a single radio can cycle between two DME–capable stations. This process is referred to as “agility tuning.” The lowest priority radio mode is the use of both VOR and DME from a single station. The FMC does not use VOR information beyond a range of 25 NM.
Note: With both navigation radios in AUTO tuning, if a U3 series FMC determines that a navigation radio is unreliable for position updating, both VHF nav control panels will display dashes in the automatic frequency indicators. This creates the appearance that both radios are agility tuning. However, the FMC is not using both radios for position updating. It deselects the unreliable radio and uses the good radio for agility tuning. U4 and later FMCs operate in the same manner, but 108.0 is displayed in the automatic frequency indicator.
With the radios selected to MAN, the FMC continues to update its position using manually tuned DME or VOR/DME stations if the tuned stations meet the FMC position updating requirements. ILS frequencies can only be tuned manually, although the FMC will also tune a DME associated with an ILS frequency.
FMC position fixing is more accurate when using DME information only. Best accuracy is with AUTO DME–DME tuning, because the FMC selects stations based on best geometry. Position fixing based on VOR/DME updating is less accurate due to inherent VOR bearing errors, especially when tuning manually. If desired, the crew may inhibit the use of VOR bearing or DME information from a specific VOR/DME station.
If the DME radios fail, or if suitable DME stations are not available, FMC navigation is based on IRS position information only. The two VHF Nav radios are used by the FMC for localizer updating during an ILS approach and by the crew for navigation monitoring.
Note: The FMC is designed to automatically reject unreliable navaid data during FMC position updating. However, in certain conditions, navaids which are in error may satisfy the reasonableness criteria and provide the FMC with an inaccurate radio position. One of the most vulnerable times is when a radio position update occurs just after takeoff. This is usually manifested in an abrupt heading correction after engaging LNAV. The position shift can be seen on the map, which will shift the desired track and runway symbol to a position significantly different from that displayed during ground roll. If the flight crew observes either of these indications, and an extended period of IRS NAV ONLY flight follows, the FMC should be carefully monitored.
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11.31.6 December 1, 2000
Flight Management, Navigation -Flight Management System Operation
737 Operations Manual
When adequate radio updating is not available, the EHSI map may display a shift error. This error results in the displayed position of the airplane, route, waypoints, and navigation aids being shifted from their actual positions.
An across track, undetected map shift may result in the airplane flying a ground track that is offset from the desired track. An along track, undetected map shift may result in the flight crew initiating altitude changes earlier or later than desired. In either case, an undetected map shift may compromise terrain or traffic separation.
Map shift errors can be detected by comparing the position of the airplane on the EHSI map with data from the ILS, VOR, DME, and ADF systems.
Navigation Performance (U7 and on)
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