ALTERNATES AND ENABLE ALTN: STORED AIRPORT VERSUS USABLE DIVERSION
EGKK is the teaching alternate for EGLL. Entering that code establishes the alternate destination; it does not establish a realistic diversion route, airspace clearance or landing-performance case. The source distinguishes an alternate route actually entered in the plan from a bare airport pair. This difference can materially affect fuel and time predictions, particularly for nearby airports whose practical routing is much longer than the direct distance. [66, PDF pp. 378-388,445-446]
1. Inspect the alternate record. Open the destination's lateral revision and ALTN/ALTERNATES where available. Read the selected airport, any associated route and the basis of the displayed distance/extra-fuel result. Do not interpret a company/database-listed alternate as automatically suitable for current weather, performance and operational requirements. The planning decision remains separate from retrieving a candidate. [66, PDF pp. 378-388,445-446]
2. Build the actual alternate route where required. Enter the intended connecting route and suitable arrival rather than leaving the predictor to assume direct geometry. Inspect the alternate portion beyond the primary destination in F-PLN. A plan that contains only the original destination and alternate separated by a gap is not a completed diversion route simply because both airport names are correct. [66, PDF pp. 378-388,445-446]
3. Select the real diversion point. If the diversion is now required, open LAT REV at the appropriate point and use ENABLE ALTN when that is the intended supported operation. The source constructs different temporary results depending on whether an alternate route exists. Inspect what is retained, removed and connected from the actual flight situation before INSERT. [66, PDF pp. 378-388,445-446]
4. Complete the missing route and data. Without a prepared route, the documented result contains discontinuities requiring a new connection. Establish that route according to clearance, review the ND and reconcile destination weather, radio/approach, minima and landing performance. Read any new cruise reference or phase state rather than accepting a historical default as the desired diversion level. [66, PDF pp. 378-388,445-446]
5. Reassess fuel honestly. Compare the revised predicted arrival fuel and actual remaining fuel with the appropriate plan and reserve requirements. A short great-circle assumption can make EXTRA/HOLD time optimistic. Do not reduce reserves to preserve an attractive figure. Confirm the active destination and first leg, then complete company and ATC coordination as separate tasks. The MCDU's accepted change does not itself notify either party. [66, PDF pp. 378-388,445-446]