B777 VREF Question
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B777 VREF Question
I was just comparing the V REF Speeds in the QRH PERF inflight section for a 200LR and then 300ER.
Question -
200LR tables include Pressure Altitude Versus A/C LDG WT for whatever flap setting you may choose to land (30,25,20)
However, 300ER VREF TABLES don't include PA at all, just FLAPS versus A/C LDG WT
Why is pressure altitude included in calculation for 200 LR and not 300 ER?
apologies if I have missed something obvious.
Question -
200LR tables include Pressure Altitude Versus A/C LDG WT for whatever flap setting you may choose to land (30,25,20)
However, 300ER VREF TABLES don't include PA at all, just FLAPS versus A/C LDG WT
Why is pressure altitude included in calculation for 200 LR and not 300 ER?
apologies if I have missed something obvious.
200LRs and 200Fs have a "Vref Min" feature. Being the short fuselage with the large engines, Boeing feels they need a minimum speed on approach to ensure control authority to handle the asymmetric thrust in the event of a single-engine go-around, as the rudder is acting through a shorter arm than on the 300ER
As thrust decreases with pressure altitude, the minimum approach speed like-wise decreases as PA increases.
The minimum Vref is approximately 136.4Kts IAS at sea level. Below a certain landing weight (about 195T) Vref 25 will be the same as Vref30 at sea-level, so it is good practice to land flap 25 so you are nearer the "actual" vref- increased landing distance due to less drag is minimal and offset by the decreased likelihood of float.
The 300ER doesn't have this feature as it has sufficient rudder authority at the normal Vref at its' a lightest weight, due to having the longer fuselage and thus longer arm.
As thrust decreases with pressure altitude, the minimum approach speed like-wise decreases as PA increases.
The minimum Vref is approximately 136.4Kts IAS at sea level. Below a certain landing weight (about 195T) Vref 25 will be the same as Vref30 at sea-level, so it is good practice to land flap 25 so you are nearer the "actual" vref- increased landing distance due to less drag is minimal and offset by the decreased likelihood of float.
The 300ER doesn't have this feature as it has sufficient rudder authority at the normal Vref at its' a lightest weight, due to having the longer fuselage and thus longer arm.
