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Consider this reroute on a VFR nav trip from Great Malvern to Chipping Norton.

Maintaining 120 TAS.

Track 110 °M Heading 103 °M (taking 3/4 of Max Drift - MD)

In still air this 30nm section would take 15 minutes. However, with a 5-10 headwind the groundspeed would be lower than TAS, around 110-115 kts

Is there a good rule-of-thumb, or quick method in the air to estimate the timings and ETA when considering the effect of headwind or tailwind on the timing of a reroute leg?

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    $\begingroup$ Do we assume, as per your question, that you've already estimated your groundspeed. Or are you asking how to estimate ETA based only on TAS and wind info? Also, 50NM in still air at 120kts TAS would take 25 minutes not 15. $\endgroup$
    – Jamiec
    Sep 10 '20 at 10:08
  • $\begingroup$ its 30 miles not 50 (maybe my 3 isnt curly enough) $\endgroup$ Sep 10 '20 at 11:19
  • $\begingroup$ Ah sorry my bad. I thought it said 50 $\endgroup$
    – Jamiec
    Sep 10 '20 at 12:28
  • $\begingroup$ @ob318, Why do you say that in still air GS would be lower than TAS? $\endgroup$ Sep 10 '20 at 16:49
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    $\begingroup$ That makes sense. @ob318, I get that you are asking about headwind, but without stating the actual winds your question wasn't as clear as it could be. I made a very minor edit, please make sure you agree. $\endgroup$ Sep 10 '20 at 20:52
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If you know or have a reference to Ground Speed, estimating reasonably accurate flight timings is straightforward. Whatever your GS, you will cover 1/10th of the distance in 6 minutes. So at 90kts, 9 miles in 6 mins. At 70kts, 7 miles in 6mins and at 120kts, 12 miles in 6 mins etc. If you mark your pen/pencil with 5 & 10nm distance marks measured in the same scale as your map, it’s easy to interpolate say, 7nm (for say 70kts GS) and mark your map with 6minute marks. If needed dissect these markings in half, for 3 minute markers.

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  • $\begingroup$ Thanks @Rob Wilkinson. 1/10th distance in 6 mins makes sense. I suppose the most challenging part of it is calculating an accurate Groundspeed, w/o GPS or ADC. But even this task isn’t too difficult. $\endgroup$ Sep 11 '20 at 11:00
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The rules of thumb for time estimates are:

  • 60kts = 1NM per minute
  • 90Kts = 1.5NM per minute
  • 120kts = 2NM per minute

In between is either a) easy enough to interpolate or b) somewhat irrelevant and might make the difference of plus/minus a minute or so for a long leg.

Your leg is what I would say is somewhat long at 30NM but if you were to calculate using 120kts it would be 15 minutes. For the purpose of estimating using interpolation that time for 110Kts ground speed I would add 10% and round up to the minute giving me 17 minutes.

For the purpose of a PPL diversion I would think this would work acceptably. Pulling out my trusty wizz wheel on the ground, 30NM at 110Kts gives 16.4 minutes. I still think estimating 17 in the air would be just fine!

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