The best navigation aids are broadly defined to be the 10 navigation aids closest to the aircraft's present position. In order to qualify as candidates for the best navigations aid list, these facilities must be within a range which is suitable for its class of operation and the present altitude of the aeroplane.
From the best navigation list the FMCS determines the best navigation aid pair as a function of the line-of-position (LOP) crossing angle. The crossing angle of LOPs must lie between 30° and 150°. The best navigation pair is defined as the combination which procuces a crossing angle closest to the ideal of 90°. Reselection of the best navaid pair will be performend every two minutes.
If one of the DME interrogators has been switched to MAN mode, the FMCS will attempt to use the manually tuned DME station and to find a complementary DME station that provides an acceptable LOP crossing angle. When this is not possible, the DME interrogator in the AUTO mode may be operated by the FMCS in a frequency-agile submode (if agility DME is installed) in order to preserve the primary dual-DME navigation solution. When this occurs, the single AUTO mode DME interrogator will be tuned alternately to each of two DME stations, selected by the FMCS as the best pair, at intervals of 5 seconds; otherwise, the FMCS may operate in a VOR/DME mode.
Boeing Flight Management Computer System Guide.