Originally Posted by hawk37
Misd-agin,
any chance you could expand on too low FMC generated speed, or the Boeing emails?
Hawk
For minimum speeds at altitude we use Vref +80 below FL250 and Vref +100 above FL 250. It works. Obviously it's just a swag but Vref +80 gets below 1.3(?) Vs at the higher altitudes. So they came up with a easy to remember number that covers all situations. I'd be B.S. ing you if I tried to explain the problem but I believe it's a KIAS/KEAS problem. Discovered by experience when guys tried to slow to FMC holding speeds at high altitude and reported back that it wasn't a very good idea!
It was obvious if you ever tried it. I remember holding at FL410(?) (10 n.m. box pattern, expanded to 20 n.m. box pattern). FMC hold speed was probably 210 +/- range, plane wasn't happy below the low 220's so we held using high 220's. Based on later policy (Vref +100) the higher speed was correct and appropriate.
Boeing emails? I asked some questions regarding various speeds and their relationships to each other - L/D max, best glide, best rate, min sink, best angle, best holding speeds, etc.
My memory of their answer ("FMC data is correct") is that L/D max = best glide = FMC holding speed = best angle.
Next question I asked was why in the 757 FMC holding speed wasn't the same as the best angle speed(prior email said it was). "It wasn't?" and havn't heard from them again. (nothing sinister just lost touch...)
"we don't use min sink speeds".