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It's generally hard for humans to estimate how far away an aircraft really is.

Question: Is there a standard rule for estimating the distance to an aircraft?

For example:

  • The entry/exit hatches are 2.4 metres high and your thumbnail at arms length translates to a 2.4 metre high door at 400 metres

  • Adults with normal 20/20 eyesight can make out an airliner's passenger windows in normal sunlight, so the plane is under 500 metres away, and over 500 metres if you can't make out individual windows.

  • Aircraft's registration code must be XX cm high and is therefore readable at up to 1000 metres.

Of course lighting and elevation differences, paint designs and model of plane, and even heading differences would mess it up a bit, but this is a rule of thumb in lieu of any other information.

For example, the popular fallout character is not giving a thumbs-up. Instead he's comparing the size of a distant mushroom cloud to his thumb to see if he's outside the nuclear fallout zone.

https://pbs.twimg.com/profile_images/421959231776894977/qHC0YoMM.jpeg

Inspired by How close can planes fly to each other over the ocean?

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  • $\begingroup$ Tags are not very good, sorry. Feel free to edit/add. $\endgroup$ – Criggie Jul 31 '17 at 10:51
  • $\begingroup$ the fallout guy thing sounds like an urban legend to me. $\endgroup$ – Federico Jul 31 '17 at 11:08
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    $\begingroup$ @Federico Mostly it is, with a tiny sliver of truth: skeptics.stackexchange.com/questions/28918/… $\endgroup$ – Jamiec Jul 31 '17 at 12:09
  • $\begingroup$ Related $\endgroup$ – Pondlife Jul 31 '17 at 12:46
  • $\begingroup$ @Pondlife thank you - definitely relevant. I figured since doors and windows and registration numbers have a fairly-fixed size/height then such a working test might be possible. $\endgroup$ – Criggie Jul 31 '17 at 19:35
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A rule-of-thumb would imply frequent usage. Such as this list on SKYbrary.aero: Rules of Thumb.

For usage by pilots and/or controllers, a distance-to won't be beneficial even if there was a rule-of-thumb (there isn't).

  • Imagine two planes on approach to a runway, the controller uses his thumb, now the following plane is also higher, so we have different slant angles: it just won't work to judge their separation (and on a foggy day it's of no use of course).

  • Imagine a pilot flying VFR, at night. The pilot spots the navigation lights of a plane, but nothing else to judge the plane's size: again won't work.

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  • $\begingroup$ Yep - I figured most parts and dimensions of the plane depend on its model, so a 737 would look smaller at 1000m than a 787. Wingspan and therefore navigation lights would vary too, But doors and windows are generally fairly uniform in size, and the $\endgroup$ – Criggie Aug 4 '17 at 3:32

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