En-route Performance, Cruise Control and In-flight Replanning
En-route performance, cruise control and in-flight replanning test your ability to fly the cruise efficiently and change the plan when fuel, weather or ATC demands it. The exam covers specific range, cost index, step climb, equal time point (ETP) and point of no return (PNR).
Cruise control
- Specific range (NM/kg) = TAS/fuel flow. Maximum range occurs at the speed for best specific range; long-range cruise trades time for fuel.
- Cost index balances fuel cost and time cost; a higher CI gives a higher speed.
- Step climb as mass decreases to stay near optimum altitude.
- Monitor fuel against the operational flight plan at each waypoint.
Replanning
ETP is the point where time to return equals time to continue. PNR is limited by fuel to return to a safe aerodrome. Re-dispatch allows a flight to continue under a new fuel plan from an en-route decision point.
Worked example
A to B = 1,200 NM; GS outbound 420 kt; GS home 480 kt. ETP from A = 1,200 × 480/(420 + 480) = 640 NM. Time to ETP = 640/420 = 1.52 h, or 1 h 31 min. At the ETP, an engine failure gives equal time to A or B, ignoring descent and approach differences.
Traps
- Using TAS instead of GS in ETP or PNR calculations.
- Forgetting that wind changes with altitude and time.
- Calculating PNR with final reserve included as usable fuel.
- Not updating the plan after a level change or route shortening.