033 Flight Planning and Monitoring topic guide
Point of Safe Return (PSR)
The point of safe return (PSR) is the furthest point along track from which an aircraft can return to its departure aerodrome and land with the required reserves intact. Beyond the PSR you are committed to continuing; before it, turning back remains an option. That operational meaning is what separates it from the point of equal time, which is about time, not fuel.
The PSR is driven by safe endurance: the time you can fly on the useable fuel after setting aside everything that must still be in the tanks on landing. Feed the formula the wrong endurance, for instance one that includes final reserve, and the PSR moves dangerously far out even though every subsequent step is arithmetically perfect.
The formula
With safe endurance E, groundspeed out O, and groundspeed home H, the time to the PSR is T = E multiplied by H, divided by (O plus H). The distance to the PSR is then that time multiplied by the groundspeed out, O.
The structure mirrors the PET formula, which is exactly why the two get confused. The PET divides a distance; the PSR divides an endurance. If the stem gives you hours of fuel, you are in PSR territory. If it gives you a route distance and asks where times are equal, it is a PET question.
A useful check: in still air the PSR time is exactly half the safe endurance, and wind in any direction brings the PSR distance closer than the still-air maximum, because whatever helps you outbound penalises the return leg for longer than it helped.
Where the endurance comes from
Exam stems usually quote safe endurance directly, but harder variants make you build it: take the useable fuel, remove final reserve and any fuel that must remain for an alternate or holding, and convert what is left to time at the planned fuel flow. Only that remainder is spendable on the out-and-back exercise.
Variable fuel flow is the other twist. If the outbound and return fuel flows differ, the simple time formula no longer applies and the question must be balanced in fuel terms instead. At ATPL level the single-flow version is what is examined, but recognising the assumption protects you from misreading a stem.
Worked example
Worked example: PSR with a tailwind outbound
Not quite. The correct answer is B.
Correct answer: B
- A. This multiplies half the endurance by the TAS, ignoring the wind on both legs. Groundspeed, not TAS, covers ground.
- B. Correct: time to PSR = 4.8 x 200 / 480 = 2 hours, and 2 hours at the outbound groundspeed of 280 kt covers 560 NM.
- C. This multiplies the PSR time by the groundspeed home (200 kt) instead of the groundspeed out. The outbound leg is flown at 280 kt.
- D. This multiplies the correct 2 hour PSR time by the 240 kt TAS instead of the 280 kt outbound groundspeed. The last step is flown over the ground, so it takes the groundspeed out.
Step by step
- Convert the endurance: 4 hours 48 minutes is 4.8 hours.
- Groundspeeds: out O = 240 + 40 = 280 kt, home H = 240 - 40 = 200 kt.
- Time to the PSR: T = E x H / (O + H) = 4.8 x 200 / 480 = 2.0 hours.
- Distance: 2.0 hours at 280 kt is 560 NM.
- Check the return: 560 NM at 200 kt takes 2.8 hours, and 2.0 + 2.8 = 4.8 hours, exactly the safe endurance.
Common mistakes
Using total fuel instead of safe endurance
The PSR protects the fuel that must still be aboard on landing. Building E from total useable fuel, final reserve included, pushes the PSR beyond the true commit point.
Multiplying the PSR time by the wrong groundspeed
The distance out is flown at the outbound groundspeed. After correctly computing T, multiplying by H instead of O is the most common final-step slip.
Confusing PSR with PET in the last minute of a question
The formulas look alike. Anchor on the given quantity: endurance means PSR, route distance with an equal-time question means PET.
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Last reviewed July 2026