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Don't try that "a bit over .7" stuff on my aircraft Intruder.
Are you for real? More like " a bit under .8" would be a lot safer.
Can't find an equivalent chart for the -400, but there is a "minimum cruise speed" chart in the Classic PHB. At FL390 and 220 Tonnes GW, it is .728 with a 1.3G buffet margin, and all the way down to .693 at 200 T.
So, "the book" corroborates my answer to the OP's question.
Would I normally fly at that speed? No. However, if I'm running short on fuel over the Pacific and I have a strong tailwind (e.g., the 170 kt I had the other day), that gives me my limiting speed.
Yes, capt Intruder, I know a few chaps who like to fly the 74 slow like a Bus, but I have never cruised in the bird at less than .84 [above FL280], no matter what the weight, the wind or altitude. Besides the fact that my employer is not in the business of flying empty birds, the typical LRC at optimum altitude and max payload averages M.855
At the risk of a bit of thread drift, if you are short of fuel you want Max Range Cruise. This is CI 0 for all intents and purposes. Using 220T @ FL 390, CI 0 is listed in B744 RR book as M.808. We are told to reduce by M.01 per 100Kt tail wind. Now that brings us to not much below M.80. Hence my liking for M.80. Holding speed is, naturally, below this but en route hold is a special case and is not to save fuel en route.
Be very aware of Zero Rate of Climb Speed. Below it and you must descend to regain speed. I know it well, been there, done that and have the scars from a CP biting my head off (almost) to prove it.
Below around M.80 at high altitude is brave, count me out.
At the risk of a bit of thread drift, if you are short of fuel you want Max Range Cruise. This is CI 0 for all intents and purposes. Using 220T @ FL 390, CI 0 is listed in B744 RR book as M.808. We are told to reduce by M.01 per 100Kt tail wind. Now that brings us to not much below M.80. Hence my liking for M.80. Holding speed is, naturally, below this but en route hold is a special case and is not to save fuel en route.
The minimum cruise speed I cited above gives almost 30 KIAS buffet margin (225 KIAS/M0.728 at 220 Tonnes; buffet 194 KIAS at 227 Tonnes). At ISA+10, the limit altitude is 430, so that is not a limiting factor, either.
Also, the OP asked for MINIMUM cruise speed, not what you or I use as normal cruise speed. While M0.85 may be close to LRC at FL390, that is not anywhere near minimum cruise speed.
If you use Max Range Cruise in the 744 and set CI=0, then the FMC computes the speed based on current wind, and you do not need to make any adjustment. For the Classic, again based on the available chart, Max Range Cruise at FL390, 150 Kt tailwind goes down to M0.76 at 180 Tonnes (.81 at 220T).
Holding speed is effectively max endurance speed (i.e., minimum fuel flow) adjusted for bank angle and the racetrack pattern, further defined by Boeing as Vmindrag. In the regime under discussion, that is 228-254 KIAS.
Would I normally cruise at "minimum cruise speed"? Again, no. However, there are certainly times when M0.85 is excessive, and M0.80 is not optimum for the conditions. It's a good idea to know your airplane's limits, even if you don't exercise them.
The minimum cruise speed I cited above gives almost 30 KIAS buffet margin (225 KIAS/M0.728 at 220 Tonnes; buffet 194 KIAS at 227 Tonnes). At ISA+10, the limit altitude is 430, so that is not a limiting factor, either.
That is getting dangerously close to zero RoC speed. Beware.
I am aware that the FMC allows for wind. I was quoting figures out of our CRZ pages.
In my airline your figures are simply unreal, our OEW is above 180T for a start so we rarely fly at lower than 230T.
Linear Mode
