On this page
- Tests
- DGCA CPL and ATPL Air Navigation
- Core skill
- Time and fuel balance points, track error correction
- Inputs needed
- Groundspeeds home and out, still-air distance, endurance
- Typical weight
- At least one CP/PNR question plus 1-in-60 questions per paper
Critical Point, Point of No Return and the 1-in-60 rule are grouped together in this guide because they solve three different real-world questions with the same underlying tool: a distance divided proportionally between two groundspeeds. Once you see that shared skeleton, all three stop looking like separate topics to memorise and become one calculation applied to time, then to fuel, then to angles.
What Critical Point is
The Critical Point, also called the equal time point or ETP, is the point along a route where the time remaining to the destination equals the time it would take to turn around and fly back to the departure point. It depends only on the still-air distance between the two points and the groundspeed you would achieve going onward versus going back, since those differ whenever there is a headwind or tailwind component on the route.
If the wind is calm, CP sits at the exact midpoint. Any wind pushes CP towards the aerodrome you would reach slower, because it takes longer to get there and so the equal time point shifts to compensate. A tailwind on the outbound leg (meaning a headwind on the way back) pushes CP further from departure, since returning becomes slower and the balance point moves out to meet it.
What Point of No Return is
PNR is a fuel question, not a pure time question. It is the furthest point from departure from which the aircraft can still turn back and land at the departure aerodrome using no more fuel than its safe endurance allows. Past PNR, there is not enough fuel left to return, so continuing to the destination, or a suitable alternate, becomes the only option. PNR is used heavily in overwater and remote-area flight planning where diversion options are limited, which is exactly the scenario DGCA questions usually frame it in.
Because PNR depends on endurance rather than a fixed destination distance, it is calculated differently from CP even though the setup looks similar. CP asks where two travel times are equal. PNR asks how far you can go before your fuel forces you to turn around.
The 1-in-60 rule
The 1-in-60 rule is a piece of small-angle trigonometry pilots use instead of a calculator in flight. Over short angles, 1 nautical mile of displacement after 60 nautical miles travelled corresponds to almost exactly 1 degree of angular error. It scales linearly: 3 nm off track after 60 nm flown is about 3 degrees of track error, and if you have covered a different distance, you rescale accordingly.
Two numbers come out of a 1-in-60 calculation. Track error angle tells you how far off your intended track you have drifted, given how far you have flown and how far off track a fix shows you to be. Closing angle tells you how many degrees to turn to converge back onto a point ahead, given the distance still to run. Add these together correctly and you get a single heading change that both corrects your track and re-aims you at your destination, which is the standard "double the angle" style question DGCA sets.
Formula sheet
| Quantity | Formula |
|---|---|
| Critical Point distance (from departure) | CP = D × GS(home) / (GS(out) + GS(home)), where D is total still-air distance, GS(out) is groundspeed continuing onward, GS(home) is groundspeed returning to departure |
| Time to Critical Point | Time(CP) = CP / GS(out) |
| Point of No Return distance | PNR = GS(out) × GS(home) × E / (GS(out) + GS(home)), where E is safe endurance in hours |
| Time to PNR | Time(PNR) = PNR / GS(out) |
| 1-in-60 track error angle | Track error (deg) = 60 × distance off track / distance flown |
| 1-in-60 closing angle | Closing angle (deg) = 60 × distance off track / distance remaining to run |
| New heading to regain original track and destination | New heading = original heading + (track error angle + closing angle), signed towards the side needed to converge |
Example 1: Critical Point with different outbound and return groundspeeds
Total still-air distance D = 720 nm Groundspeed continuing onward, GS(out) = 180 kt Groundspeed returning to departure, GS(home) = 220 kt (There is a headwind component on the outbound leg, hence the lower outbound groundspeed.) CP = D x GS(home) / (GS(out) + GS(home)) = 720 x 220 / (180 + 220) = 720 x 220 / 400 = 158,400 / 400 = 396 nm from departure Time to CP = CP / GS(out) = 396 / 180 = 2.2 hours = 2 hr 12 min Check: time to return to departure from CP at GS(home) = 396 / 220 = 1.8 hours = 1 hr 48 min... That does not look equal at first glance, so recheck: time onward from CP to destination remaining distance = 720 - 396 = 324 nm time onward = 324 / 180 = 1.8 hours = 1 hr 48 min This matches the time to return (1 hr 48 min), confirming CP is correctly balanced.
Answer: Critical Point is 396 nm from departure, reached 2 hours 12 minutes after takeoff. From there, continuing onward and turning back both take exactly 1 hour 48 minutes.
Example 2: Point of No Return given safe endurance
Groundspeed outbound, GS(out) = 160 kt
Groundspeed returning, GS(home) = 200 kt
Safe endurance (total fuel available for flight, minus reserves), E = 5.5 hours
PNR = GS(out) x GS(home) x E / (GS(out) + GS(home))
= 160 x 200 x 5.5 / (160 + 200)
= 176,000 / 360
= 488.9 nm from departure
Time to PNR = PNR / GS(out) = 488.9 / 160 = 3.06 hours = 3 hr 3 min
Check: fuel used to reach PNR and then fly all the way back
time out = 3.06 hr
distance to fly back from PNR = 488.9 nm, time back = 488.9 / 200 = 2.44 hr
total time = 3.06 + 2.44 = 5.50 hours, exactly matching safe endurance E
Answer: PNR is about 489 nm from departure, reached 3 hours 3 minutes after takeoff. Beyond that point there is not enough endurance left to return to departure within the safe fuel reserve.
Example 3: 1-in-60 track correction back to the original track
Planned track: 090°T. Total distance to destination: 300 nm.
After flying 100 nm on heading 090°T, a fix shows the aircraft is 8 nm right of the
planned track.
Step 1: Track error angle
Track error = 60 x distance off track / distance flown
= 60 x 8 / 100
= 4.8°, to the right of track (meaning heading needs to correct left)
Step 2: Distance remaining to destination from the fix point
Distance remaining = 300 - 100 = 200 nm
Step 3: Closing angle to regain the original track by destination
Closing angle = 60 x distance off track / distance remaining
= 60 x 8 / 200
= 2.4°
Step 4: New heading
To get back ON TRACK you need to turn left by the track error angle (4.8°).
To then CONVERGE on the original track by the destination, add the closing angle (2.4°)
in the same direction.
New heading = original heading - (track error + closing angle)
= 090 - (4.8 + 2.4)
= 090 - 7.2
= 082.8°T, round to 083°T
Answer: turn left onto heading 083°T. This corrects the 8 nm displacement and re-converges on the original track by the time the aircraft reaches the destination.
Example 4: combined in-flight replanning scenario
Planned track: 300°T, planned distance 400 nm from departure to destination.
After 120 nm flown on heading 300°T, a fix shows the aircraft is 6 nm LEFT of planned track.
Remaining safe endurance from this fix point is 3.2 hours, and groundspeed on the
present heading is 160 kt. The crew wants a single new heading that flies DIRECTLY
to the destination from the fix point, not merely a rejoin of the old track.
Step 1: Track error so far
Track error = 60 x 6 / 120 = 3.0°, to the left (so track flown has drifted 3° left of plan)
Step 2: Closing angle to destination (distance remaining, not to a rejoin point but straight
to the original destination, still 400 - 120 = 280 nm along the original track line)
Closing angle = 60 x 6 / 280 = 1.3°
Step 3: New heading direct to destination
Since the aircraft is left of track, the correction to head back towards the track line
and on to the destination is a turn to the RIGHT.
New heading = original heading + (track error + closing angle)
= 300 + (3.0 + 1.3)
= 300 + 4.3
= 304.3°T, round to 304°T
Step 4: Fuel check using remaining endurance
Remaining direct distance to destination, approximately equal to remaining track
distance for small angles, = 280 nm
Time required = 280 / 160 = 1.75 hours
Compare to remaining safe endurance of 3.2 hours: 1.75 hours is comfortably within
endurance, so no PNR concern exists on this leg from the fix point.
Answer: turn right onto heading 304°T for a direct track to the destination, and fuel is not a constraint since the leg needs only 1 hour 45 minutes against 3 hours 12 minutes of remaining endurance.
FAQ
What is a Critical Point in DGCA Navigation?
The Critical Point, also called the equal time point, is the point on a route from which it would take exactly the same time to continue to the destination as it would to turn back to the departure point, given the groundspeeds in each direction.
What is Point of No Return and why does it matter?
Point of No Return is the furthest point along a route from which the aircraft can still return to the departure aerodrome and land with its required fuel reserves intact. Beyond PNR, continuing to the destination is the only option that keeps reserves intact, which matters for fuel planning on long overwater or remote sectors.
What is the 1-in-60 rule?
The 1-in-60 rule states that an error of 1 nautical mile off track after flying 60 nautical miles corresponds to a track error of approximately 1 degree. It scales linearly, so 2 nm off track after 60 nm is about 2 degrees, and it is used to calculate track error angle and the closing angle needed to regain a planned track or reach a new point.
Do CP and PNR use the same formula?
They are related but not identical. Critical Point only balances time home against time onward using groundspeeds. Point of No Return also brings in safe endurance, because it is fundamentally a fuel question rather than a pure time question.
How many CP/PNR questions typically appear?
DGCA Air Navigation papers typically include at least one CP or PNR calculation alongside several wind triangle and 1-in-60 questions, since all three sit under the same general navigation and flight planning portion of the syllabus.
We build the DGCA question banks these guides link to. Corrections welcome and credited.
Sources & references
- Oxford ATPL Navigation series, flight planning and general navigation chapters
- DGCA CPL and ATPL Air Navigation syllabus
- ICAO Annex 6, Operation of Aircraft, for fuel and endurance planning principles
Formulas and worked examples reviewed for the current DGCA syllabus. Spotted something out of date? Tell us and we will fix it.