Cruise speeds and how close do jets go to "optimal"
Hi all,
Question on cruise speeds, hope this is an appropriate place to ask.
Take for example on Wikipedia or Janes etc it might say the cruise speed of a 747 is .85M. I guess this is an idealized approximation and it's actually quite rare to cruise @ .8500M, because of temperature, weighting, altitude etc. My question is not about a 747 in particular, but passenger jets in general, how much variance on the "ideal" value would you typically see, first of all during a "typical" flight. So from the start of cruise to end of cruise, through stepped cruise etc, would you be looking at eg .85M +/- .01, .03, .05M? More than that?
Second of all, suppose you follow the same route from time to time over so many months or years. Is there much variance due to occasional / unforeseen issues that might cause a flight to occasionally have to adopt a bit slower or faster cruise due to unexpected events? How much variance from an intended cruise speed might a flight encounter on an occasional basis.
I'm sure there is no special "ideal cruise speed" for a given plane but I'm just wondering how much variance there is due to regular or unexpected operational factors from what might be deemed "ideal" for a given flight on a given day.
Hope it makes sense!