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Equal time point and point of no return

Two calculations that look alike and answer completely different questions: where the diversion decision flips, and how far you can go and still get back.

Flight planning15 min read

Video scheduled

The lesson video for this topic is in production on The Flight Dispatch Room.

Written lesson

The lesson

These two are almost always taught together and almost always confused. One is about time and distance to two airports. The other is about fuel. Getting the distinction clear first makes both formulas obvious.

Equal time point

The equal time point, sometimes called the critical point, is the position along the route from which it takes the same time to continue to the destination as to return to the departure airport. Before it, turning back is quicker. After it, continuing is quicker. It has nothing to do with how much fuel is on board.

Because the wind is usually not symmetrical, the ETP is rarely at the midpoint. A headwind on the outbound leg means the return is faster, which pushes the ETP beyond halfway.

Point of no return

The point of no return, sometimes called the point of safe return, is the furthest position along the route from which the aircraft can return to the departure airport within its safe endurance. It is entirely a fuel question, and the endurance figure used must be safe endurance, meaning total usable fuel less the reserves that must remain on landing.

The two calculations use the same pair of groundspeeds and share the same denominator, which is why they are easy to mix up. The ETP is built from the route distance and returns a distance. The PNR is built from endurance and returns a time, which is then converted to a distance.

Where these matter operationally

On extended overwater and remote routes these points feed directly into the release. The ETP defines the diversion decision boundary between a pair of airports, so the weather at both must be monitored against alternate minimums at the relevant arrival times. The PNR constrains how long a flight can wait for a destination to improve before committing.

Examination focus

Key exam concepts

  • The ETP is a time problem: equal time to continue or return. It is independent of fuel on board.
  • The PNR is a fuel problem: the furthest point from which the aircraft can return within safe endurance.
  • Both use the outbound groundspeed and the return groundspeed, with the sum of the two as the denominator.
  • The ETP is at the midpoint only in still air or with symmetrical winds.
  • Safe endurance is total usable fuel less required reserves.

Reference

Important formulas

Distance to the equal time point

D(ETP) = (D x GS return) / (GS on + GS return)

D is the total distance between the two airports. Result is the distance from the departure airport.

Time to the point of no return

T(PNR) = (E x GS return) / (GS on + GS return)

E is safe endurance in hours. Result is a time; multiply by the outbound groundspeed for the distance.

Method

Worked examples

Equal time point with an outbound headwind

Given

  • Distance between the departure and destination airports: 1,800 NM.
  • Groundspeed continuing to the destination: 420 knots.
  • Groundspeed returning to the departure airport: 500 knots.

Steps

  1. 1Apply the formula: (1,800 x 500) / (420 + 500).
  2. 2Numerator: 1,800 x 500 = 900,000.
  3. 3Denominator: 420 + 500 = 920.
  4. 4Divide: 900,000 / 920.

Answer

978 NM from the departure airport, which is beyond the 900 NM midpoint. The headwind on the outbound leg makes the return quicker, so the point at which continuing becomes faster lies further along the route.

Point of no return with reserves applied

Given

  • Total usable fuel gives 7.0 hours of endurance.
  • Reserves that must remain on landing amount to 1.0 hour.
  • Groundspeed continuing outbound: 460 knots.
  • Groundspeed returning: 380 knots.

Steps

  1. 1Safe endurance: 7.0 - 1.0 = 6.0 hours. The reserve is not available for the return.
  2. 2Apply the formula: (6.0 x 380) / (460 + 380).
  3. 3Numerator: 6.0 x 380 = 2,280.
  4. 4Denominator: 460 + 380 = 840.
  5. 5Time to PNR: 2,280 / 840 = 2.714 hours, which is 2 hours 43 minutes.
  6. 6Distance: 2.714 x 460.

Answer

The PNR is reached 2 hours 43 minutes after departure, about 1,248 NM out. Using the full 7.0 hours instead of safe endurance would put it about 208 NM further out, which the aircraft could not actually return from with legal reserves.

Watch out

Common exam traps

  • Using total endurance instead of safe endurance for the PNR.
  • Assuming the ETP is always at the midpoint of the route.
  • Multiplying the ETP result by a groundspeed. It is already a distance.
  • Forgetting that the PNR formula returns a time, not a distance.
  • Applying the outbound groundspeed in the numerator. The numerator uses the RETURN groundspeed in both formulas.

Test yourself

Practice questions

Choose an answer to reveal the discussion. Every option is explained, including why the wrong ones are attractive.

  1. Q01

    The distance between departure and destination is 1,200 NM. Groundspeed on is 400 kt and groundspeed return is 500 kt. How far from departure is the equal time point?

  2. Q02

    The point of no return is primarily a function of:

  3. Q03

    With a strong headwind outbound and therefore a tailwind on the return, the equal time point will lie:

Verify

Official FAA references

  • FAA-H-8083-25Pilot's Handbook of Aeronautical Knowledge — flight planning.
  • FAA-G-ADXAircraft Dispatcher Airman Certification Standards.
  • 14 CFR 121.639Fuel supply: Domestic operations.
  • 14 CFR 121.645Fuel supply: Flag and supplemental operations.

Current regulation text is published on eCFR. Always confirm against the current version before relying on any citation.

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