PET and PSR Calculations

Point of Equal Time (PET) / Critical Point (CP)

The point along the track where it takes the same time to continue to destination as to return to departure (or proceed to an alternate).

• Important for Engine Failure or Depressurization scenarios.
• Concept: Since wind affects Ground Speed (GS) differently outbound vs inbound (headwind becomes tailwind), the PET shifts into the wind.
• Formula: $ Distance_{PET} = \frac{D \times GS_{Home}}{GS_{Home} + GS_{Dest}} $
  • $D$: Total Distance.
  • $GS_{Home}$: Ground Speed returning.
  • $GS_{Dest}$: Ground Speed continuing.

Point of Safe Return (PSR) / Point of No Return (PNR)

The furthest point along the track from which the aircraft can return to the departure airport (or alternate) with the required fuel reserves still intact.

• Beyond this point, the aircraft must proceed to the destination (or another en-route alternate).
• Formula: $ Time_{PSR} = \frac{Endurance_{Safe} \times GS_{Home}}{GS_{Home} + GS_{Out}} $
  • $Endurance_{Safe}$: Total usable endurance minus reserves.
  • $GS_{Out}$: Ground speed outbound.