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When planning for a takeoff alternate, according to 14 CFR 121.617

(1) Aircraft having 2 engines. Not more than one hour from the departure airport at normal cruising speed in still air with one engine inoperative.

What does "still air" mean?

Is it because of the performance tables for Aircraft?

Or just don't take the effect of winds on aircraft range?

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The latter - it means you don't need to take winds into account for that portion of the takeoff alternate selection.

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    $\begingroup$ The FAR is talking about being able to reach the alternate airport in a certain amount of time. And it says "normal cruising speed" not "speed optimized for conditions". Updrafts and downdrafts won't significantly affect groundspeed if a/c is flying at some fixed speed (e.g. "normal cruising speed".) The edits to mention updrafts and downdrafts (including my own) were not really necessary or appropriate. However there are other cases in the FAR's where it might matter. IN GENERAL, "still air" means air with no wind and no vertical air motion. $\endgroup$ – quiet flyer May 8 '19 at 20:01
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    $\begingroup$ As far as I'm aware (and I'm pretty confident) there are no regulations that require vertical air motion to be considered for any flight planning purpose for powered aircraft. Consequently I agree that your edit was unnecessary and inappropriate (and irrelevant) so I have rolled it back :) $\endgroup$ – pericynthion May 9 '19 at 0:51
  • $\begingroup$ @pericynthion: So it's perfectly legal (albeit stupid) to fly into a microburst? $\endgroup$ – Sean May 9 '19 at 3:04
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    $\begingroup$ @Sean I was discussing flight planning, not operations. Penetrating microbursts would be covered under catch-all 91.13 if not something more specific. $\endgroup$ – pericynthion May 9 '19 at 3:15

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