PNR vs PET Explained: The Key Flight Planning Difference
At first, PNR and PET look like two versions of the same idea.
Both involve a point on the route. Both matter in flight planning. Both can appear in ATPL questions with wind, distance, groundspeed and endurance mixed into the wording. That is exactly where students get caught. Before checking what the examiner is really asking, it is easy to jump to the wrong formula.
The difference is small in wording, but huge in calculation logic. PNR is mainly about fuel or endurance. PET is mainly about time.
In this guide, we will explain Point of No Return and Point of Equal Time, also known as the Critical Point, show how each one is calculated, and highlight the exam traps that make students choose the wrong formula.
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Point of No Return
Let’s start with the Point of No Return, or PNR.
The PNR is the point in a flight after which the aircraft can no longer return to the departure aerodrome with the fuel or endurance available. Beyond this point, the aircraft must continue to another suitable destination. PNR is mainly a fuel planning concept. It answers one practical question:
How far can we fly before returning is no longer possible?
Several factors affect the PNR, including aircraft configuration, fuel burn, wind component, groundspeed, altitude and any abnormal planning assumptions.
Note: You may also see the term Point of Safe Return, or PSR. It is closely related to PNR and is sometimes used to stress that the aircraft can still return while keeping the required fuel reserves or safe endurance. In exam questions, read the wording carefully. If the question mentions fuel available, endurance, reserves or returning safely, it is testing the same basic logic: how far can the aircraft continue before returning is no longer possible within the planned fuel limits?
PNR Formula and Calculation Logic
The basic PNR formula is:
Time to PNR = E × H ÷ (H + O)
Where:
E = safe endurance in hours;
H = groundspeed when returning to the departure aerodrome;
O = groundspeed when proceeding away from the departure aerodrome.
Safe endurance is normally calculated from the usable fuel available at take-off, minus the required minimum overhead fuel, divided by the average fuel burn per hour.
Once you have the time to PNR, you can calculate the distance:
Distance to PNR = time to PNR × O
Example
Safe endurance: 5 hours
Groundspeed outbound: 420 kt
Groundspeed homebound: 330 kt
Time to PNR = 5 × 330 ÷ (330 + 420).
Time to PNR = 1,650 ÷ 750 = 2.2 hours.
So the PNR is reached after about 2 hours 12 minutes.
To find the distance: 2.2 × 420 = 924 NM. So the PNR is about 924 NM from departure.
Notice what happened here. The aircraft has a higher groundspeed outbound than homebound. That means returning would take longer, so the PNR occurs earlier than students might expect.
One Engine Inoperative PNR
In some planning scenarios, the PNR may be calculated using one engine inoperative assumptions.
A common logic is:
Distance to PNR = fuel available ÷ (specific fuel flow out + specific fuel flow back)
Where: Specific fuel flow = fuel flow ÷ groundspeed
This version is more performance-specific because it accounts for different fuel flows and groundspeeds in each direction. The basic idea remains the same: the aircraft must still have enough fuel to return safely if the turnback is made at the PNR.
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Point of Equal Time
Now let’s move to the Point of Equal Time, or PET.
PET is also known as the Critical Point or Equal Time Point. It is the point on the route where the time to continue to the destination equals the time to return to the departure aerodrome.
In practical terms, PET helps answer this question:
If something happens here, will it take the same time to continue or turn back?
This can matter during an emergency, especially on long sectors with a large distance between suitable en-route aerodromes. In real operations, crews may consider PET between suitable alternates, not only between departure and destination.
For ATPL exams, however, the calculation is often presented between the departure and destination aerodromes.
With no wind, the idea is simple. If both groundspeeds are equal, the PET is halfway along the route. If the total flight takes two hours, the equal time point is after one hour.
With wind, it becomes more interesting. A tailwind one way may become a headwind in the other direction. That changes the groundspeed and moves the PET away from the simple halfway point.
A headwind outbound pushes PET closer to departure.
A tailwind outbound pushes PET further along the route.
The key idea: PET is mainly about comparing time to return with time to continue, the moment when your options are equal.
PET Formula and Calculation Logic
The PET formula is based on a simple time principle: Time = distance ÷ groundspeed
At the PET: time to return = time to continue. In EASA-style questions, the formula is often written as:
P = D × H ÷ (O + H)
Where:
P = distance from departure to the PET
D = total route distance
H = groundspeed homeward, when returning to departure
O = groundspeed outbound, when continuing to destination
Example
Total distance: 900 NM
Groundspeed homeward: 300 kt
Groundspeed outbound: 450 kt
P = 900 × 300 ÷ (450 + 300)
P = 270,000 ÷ 750 = 360 NM
So the PET is 360 NM from departure.
At that point:
Time to return = 360 ÷ 300 = 1.2 hours.
Distance to destination = 900 − 360 = 540 NM.
Time to continue = 540 ÷ 450 = 1.2 hours.
The times are equal, so the calculation works.
A simple mnemonic for the formula is: Pretty ducks have orange hats. It is silly, but it helps students remember the order under exam pressure.
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PNR and PET in Pilot Operations
PNR and PET are particularly useful on oceanic sectors, polar routes and flights towards isolated destinations, where diversion options are limited. Crews consider them during planning, using forecast winds, aircraft performance, fuel requirements and suitable alternates.
Before passing PNR, crews review whether continuing remains appropriate. This includes checking destination weather, aircraft serviceability and actual fuel consumption while the planned return option is still available. Passing PNR rules out a return within the calculation’s fuel assumptions, but another suitable diversion aerodrome may remain accessible.
PET helps crews compare diversion options between selected aerodromes, which may be en-route alternates rather than departure and destination. Operational flight plans often call it an equal time point, or ETP.
During flight, crews check actual conditions against the plan and reassess their options when circumstances change. The quickest aerodrome to reach is not automatically the best choice: weather, runway suitability, available facilities and the emergency itself also matter.
Pilot reminder: PET compares flying times; it does not confirm fuel sufficiency. Crews must check that the chosen diversion is both suitable and achievable with the fuel remaining.
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Common ATPL Questions and Traps about PNR and PET
An ATPL question may pack distance, TAS, wind, fuel, endurance and several aerodromes into one paragraph. Before calculating anything, identify what the examiner is asking you to find.
You may see PSR (Point of Safe Return) instead of PNR. Both concern the furthest point from which the aircraft can return within the available fuel or endurance. For PSR calculations, the required reserves must remain protected. Always check the question’s fuel assumptions rather than relying on the name alone.
Use the wording to identify the task:
Furthest point from which you can return with reserves intact: think PNR / PSR.
Point where continuing and returning take the same time: think PET / Critical Point.
Use groundspeed, not TAS. If wind is given, calculate the relevant outbound and return groundspeeds before using the formula.
Protect reservesб but do not subtract them twice. Usable fuel can include reserve fuel; “usable” does not mean freely available for the outbound and return legs. Deduct the required reserve or minimum fuel remaining overhead to the return aerodrome, as specified. If safe endurance is already given, check whether this allowance has already been made.
Example: With 15,000 kg of usable fuel on board and 3,500 kg required on return, 11,500 kg remains for the calculation, assuming no other deductions.
Some PSR questions use specific fuel flow, meaning fuel consumed per nautical mile:
Specific fuel flow = fuel flow ÷ groundspeed
Dividing kg/h by NM/h gives kg/NM. Calculate each leg separately when outbound and return values differ.
A calculated PSR can lie beyond the destination. Under the question’s assumptions, this means sufficient fuel is available to reach the destination and return with the required reserve. Do not automatically limit your answer to the destination distance.
PET is not automatically halfway along the route. In the standard calculation, it lies halfway only when the groundspeeds towards the two aerodromes are equal. When they differ, PET lies closer to the aerodrome that takes longer to reach per nautical mile.
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Quick reference: PNR vs PET
Here is the essential difference in one place, so you can check the logic before reaching for the formula.
Concept | Also called | Main question | Main driver | Key formula logic | Exam trap |
PNR | Point of No Return | Can we still return with the fuel/endurance available? | Fuel, endurance, fuel burn, reserves and wind | Time to PNR = E × H ÷ (H + O) Distance to PNR = time to PNR × O | Treating it as a time comparison instead of a fuel limitation |
PSR | Point of Safe Return | What is the furthest point we can reach and still return with reserves intact? | Safe endurance, usable fuel and required reserve fuel | Often calculated like PNR, but with reserves clearly protected | Forgetting to subtract minimum reserve fuel before calculating |
PET | Point of Equal Time / Critical Point | Will it take the same time to continue or return? | Distance, wind and groundspeed in each direction | P = D × H ÷ (O + H) | Assuming the point is halfway along the route when wind is present |
Keep Practising
Want to practise this with real exam-style logic? Open the Airhead ATPL Question Bank and try more Flight Planning questions on PNR, PSR, PET, fuel, wind and groundspeed.
