vref speeds vs field elevation

[casablanca]

Previously I flew the 777, and remember that altitude affected the vref... for example at a given weight and sea level vapp would be 140 Kias.....then the same airplane landing in Johannesburg or Quito(7-8000 feet) would have approach speeds in the 120s.
Currently I fly the 75/76 and vapp seems to be the same at sea level or an 8000 foot airport. Just trying to understand how or why?

[Alex Whittingham]

Maybe Vmcl was keeping Vref high at sea level? It should be the higher of 1.23 Vsro, Vmcl and a speed that allows an AEO 40 degree bank turn without stall warning.

[rudestuff]

If you're high you'll have a higher TAS and GS, but why would elevation effect IAS?

[Goldenrivett]

Maybe Vmcl was keeping Vref high at sea level? It should be the higher of 1.23 Vsro, Vmcl and a speed that allows an AEO 40 degree bank turn without stall warning.
I agree.
Boeing - The migration to higher thrust and the effects on Control Speeds (https://www.smartcockpit.com/docs/The_Migration_To_Higher_Thrust_Engines_And_The_Effect_On_Con trol_Speeds.pdf)

[ScepticalOptomist]

If you're high you'll have a higher TAS and GS, but why would elevation effect IAS?

I was wondering the same.

[Goldenrivett]

I was wondering the same.
The answer is given above and the Boeing document explains it.

Depending on the maximum thrust available from the engines, the Vref has a lower limit (floor) to ensure control in the event of a full power GA with an engine failed.

[seagull967]

All good, but still wondering about that 777 approach speed bump. Vmca is approximately 136-137 KIAS and so, so depending on flap settings you'll hit the floor at weights something below 460,000 pounds . But something else happens at the lighter weights that I cannot find. I show Vref go from 136 at 340,000 pounds to 143 at 339,000 pounds and below. I have yet to find a satisfactory explanation for this.

[ScepticalOptomist]

The answer is given above and the Boeing document explains it.

Depending on the maximum thrust available from the engines, the Vref has a lower limit (floor) to ensure control in the event of a full power GA with an engine failed.

Thank you. Of course! I missed it on first read.

[FullWings]

All good, but still wondering about that 777 approach speed bump. Vmca is approximately 136-137 KIAS and so, so depending on flap settings you'll hit the floor at weights something below 460,000 pounds . But something else happens at the lighter weights that I cannot find. I show Vref go from 136 at 340,000 pounds to 143 at 339,000 pounds and below. I have yet to find a satisfactory explanation for this.
Which variant are you looking at? I have four in the QRH and they all have Vref decreasing with weight and pressure altitude. On one of them, at very light weights, Vref for F30, F25 and F20 are the same, indicating a Vmca floor.

[seagull967]

Which variant are you looking at? I have four in the QRH and they all have Vref decreasing with weight and pressure altitude. On one of them, at very light weights, Vref for F30, F25 and F20 are the same, indicating a Vmca floor.

777F. Here are flap 30 values. Notice the below 340k line. Below 340k the speed is the same for all flaps settings. It does decrease with altitude but why the increase below 340K?

Weight
(1000 lb)
Pressure Altitude (ft)
0 2000 4000 6000 8000 10000
780 184 184 184 185 185 185
740. 178 178 178 179 179 179
700 172 172 172 172 172 173
660 164 164 164 164 164 164
620 158 158 158 158 158 159
580 151 151 151 151 151 152
540 145 145 145 145 145 146
500 140 140 140 140 140 140
460 137 134 134 134 134 134
420 137 134 130 128 128 128
380 137 134 130 126 121 121
340 137 134 130 126 121 116
Below 340 143 140 137 134 132 128