Point of No Return (PNR) vs Point of Equal Time (PET): When to Use Which?

If something goes wrong now… do I turn back, or keep going? 👀

That’s where Point of No Return (PNR) and Point of Equal Time (PET) come in.

Two bits of “ATPL theory” that suddenly become very real when the weather turns, the fuel burns faster than planned, or you start questioning some unplanned events mid-flight.

They sound similar, but they have very different purposes!

So, what’s the difference, and when to use which one? 💡

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⛽️ Point of No Return = Decision Based on Fuel

The point of no return (PNR) is the point during a flight at which an aircraft is no longer capable of returning to the airfield from which it took off due to fuel considerations.

After the PNR, you’re committed to continuing ahead, because turning back wouldn’t leave you with enough fuel reserves.

It’s a fuel-based calculation, and answers the question:

“How far can I go and still make it back safely?”

The answer can be either time or distance, depending on the equation (we’ll show both). So what are the factors?

🔸 Fuel endurance (excluding final reserve)
🔸 Groundspeed outbound
🔸 Groundspeed homebound

We can calculate by using this short equation:

Let’s look at an example based on a 20 kts tailwind while outbound with a 120 kts IAS:

🔸 Fuel endurance: 3 hours
🔸 Groundspeed outbound: 140 kts
🔸 Groundspeed homebound: 100 kts

We can convert this into distance by using the outbound groundspeed above the divide line:

Completing this gives:

Keep in mind that it’s not a solid circle around your departure airport, because of winds, performance (burnrate) etc.

It’s a point that is constantly changing as you continue along your route.

⏱️ Point of Equal Time = Decision Based on Time

The Point of Equal Time (PET) is the point along your route where it will take the same amount of time to continue to your destination as it would to turn back to your departure aerodrome (or an alternate).

It’s sometimes called the Critical Point (CP), or the Equal Time Point (ETP).

Before the PET, it’s quicker to turn back. After the PET, it’s quicker to continue ahead.

It’s a time-based calculation, and answers the question (during emergencies for instance):

“From this point, is it faster to turn back, or continue on?”

The answer will be in nautical miles. So what are the factors?

🔸 Outbound groundspeed
🔸 Homebound groundspeed
🔸 Total distance

We can calculate by using this short equation:

Let’s use this as an example:

🔸 Total distance: 200 NM
🔸 Outbound groundspeed: 140 kts (tailwind)
🔸 Homebound groundspeed: 100 kts (headwind)

Let’s complete the equation:

Keep in mind that a massively different groundspeed (unplanned level changes etc), will completely throw these figures!

💭 Conclusion

So, with the same numbers, we get different figures:

🔸 PNR (175 NM)

🔸 PET (83 NM)

So, at 83 nm we will take an equal amount of time to turn back or continue, but the point where we can no longer return at all is only at 175 nm.

The PNR is all about fuel and less restrictive in this case: it marks the point where you’ve just got enough to get back home with your reserves intact.

Once you pass it, turning back isn’t an option without dipping into your legal reserves.

The PET is all about time: it’s where continuing ahead or turning back would take the same amount of time.

Have you ever had to use a PNR or PET decision for real, offshore, long-haul, or otherwise? We’d love to hear your story!