Calculation of Forecast Time Period
Due to the daily route change and the variable diversion times it is necessary to calculate the required forecast time window on an individual basis, which should be Dispatchers task or is recently done by the flight planning systems.
Data for this can easily be obtained from the ETP calculation of the Company Flight Plan (CFP). Just the first and the last enroute alternate requires a little bit more time consuming effort, because time slot depends on the ETOPS entry point, which is not pointed out on the CFP.
The weather forecast must indicate the weather to above given ETOPS-minima for a time from one hour before earliest possible time of arrival until one hour after latest possible time of arrival.
Earliest and latest decision point for a specific alternate is considered to be at the ETP before and after that aerodrome. Therefore a continuous chain of computer-calculated ETPs is very helpful.
Calculation is done as follows:
| Beginning of time period | ||
| Time of departure | PLUS | time to the ETP prior the aerodrome |
| PLUS | time from that ETP to the aerodrome | |
| MINUS | one hour | |
| End of time period | ||
| Time of departure | PLUS | time to the ETP beyond the aerodrome |
| PLUS | time from that ETP to the aerodrome | |
| PLUS | one hour | |
| Beginning of time period at first enroute alternate | ||
| Time of departure | PLUS | time to the ETOPS entry point |
| PLUS | time from that point to the aerodrome | |
| MINUS | one hour | |
| End of time period at last enroute alternate | ||
| Time of departure | PLUS | time to the ETOPS exit point |
| PLUS | time from that point to the aerodrome | |
| PLUS | one hour | |
Remark: Time to ETOPS exit / entry points and to the enroute alternates is calculated as follows:
- Take the time to the next waypoint prior or after the exit / entry point out of CFP.
- Measure the distance from that waypoint to the exit / entry point, as well as from exit / entry point to the enroute alternate using NAT Plotting Chart.
- Calculate flight times for these legs considering TAS (out of CFP) +/- wind component.
- Add / subtract these times accordingly.
Examples:
| Enroute alternate A (first enroute alternate) | ||
| Beginning of period | ||
| Time of departure | take ETD | |
| PLUS | time to some waypoint "X" (next to the ETOPS entry point (ENP)) | frome CFP |
| PLUS / MINUS | time from "X" to ENP (plus or minus depending on position of "X") | calculate |
| PLUS | time from ENP to enroute alternate "A" | calculate |
| MINUS | one hour | |
| End of period | ||
| Time of departure | take ETD | |
| PLUS | time to ETP1 | from CFP |
| PLUS | time from ETP1 to enroute alternate "A" | from CFP |
| PLUS | one hour | |
| Enroute alternate B (second enroute alternate) | ||
| Beginning of period | ||
| Time of departure | take ETD | |
| PLUS | time to ETP1 | from CFP |
| PLUS | time from ETP1 to enroute alternate "B" | from CFP |
| MINUS | one hour | |
| End of period | ||
| Time of departure | take ETD | |
| PLUS | time to ETP2 | from CFP |
| PLUS | time from ETP2 to enroute alternate "B" | from CFP |
| PLUS | one hour | |
Read Maximum distance from an adequate aerodrome for two-engined aeroplanes without an ETOPS approval