\\411a,
I hardly think cruising at FL290 at M0.86 is fuel efficient especially in a Classic. My comment was made tongue-in-cheek. Maybe they were on a special flight plan to recover delayed time. Certainly should not be the norm!\\
Well, as usual, it depends.
Now I don't fly the B747, but I do operate the Lockheed TriStar, and have done so for a very long time.
The heavy weight models of these (-200. -250) have an FMS which specifies a rather high speed cruise, M.86, even at lower altitudes...FL290/310, for example, at higher enroute weights.
This is the 'min cost' provision, which takes into account fuel cost, among several other variables.
The TriStar was designed as a high speed cruiser, and was the first wide-body jet transport to use a true laminar flow wing.
As the fuel on longer flights is consumed, higher altitudes are requested, but the speed remains fairly constant, until closer to TOD, where a lower speed (M.84) is commanded by the FMS, in the thrust management mode of operation, using min cost.
Any slower, and the deck angle becomes excessive...then the fuel flows really go up...ugh.
The -500 model is the exception. On this model (using an extended span wing, with active ailerons for gust load alleviation), the high speed cruise is M.84, reducing to M.83 at lighter weights...again using min cost.
Darn Airboos aircraft keep getting in the way.
