Hi Pierre,
The numbers I posted earlier about the B777 are typical for econ cruise at altitude. At our cruise altitudes, everything we do is relative to mach number, not IAS or TAS. That's because we're operating well into the area where compressibility in the flow has huge effects on the aerodynamics on the wing.
At FL390 (from memory here) we see around 260 knots IAS at a mach of about .83 to .84. TAS will be in the vicinity of 460 knots, which is 530 mph (like you said). The redline on the airspeed tape at that point will be about mach .87, and we're running up there at mach .84, which is only about 6-7 knots under Vne at altitude. But the B777 is flying at a very efficient point right there, even though we're just a hare below redline.
Now to the point regarding your question:
If you're showing 260K with a VNE of 360 indicated, your TAS must be over 520 MPH, not near your real redline, which must be near Mach 1, no? Do you have a redline that moves South with increasing density altitude or a manual that shows decreasing redlines with altitude?
Redline is a moving target in these transport category jets that operate at high altitude and fairly fast speeds. We have a redline that respects either a dynamic pressure limit on the airframe, OR a mach limit, which ever is lower. It moves south as mach number becomes the limiting factor on our airspeed. With increasing altitude, the airspeed that will give us mach .87 (Vne) is less and less.
The "cross over" point where 340 indicated equals .87 mach is in the upper 20's if I remember right. So above the middle altitudes, the airspeed indicator's redline will move lower to reflect the mach limit rather than the IAS limit. (Its dynamically updated by the air data system) At high altitude, say around FL390, the Vne is still mach .87 but that equates to only around 270 knots indicated.
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Thanks to y'all who pointed out the Vans article on Vne. Now I see what they're talking about - they're concerned about FLUTTER. Considering that, I understand Ken Krueger's attention being turned to TAS. He's an accomplished engineer, and rightly points out the TAS concerns relative to
flutter onset. Flutter is a dynamic phenomenon and physically is linked to the real flow speed around the aircraft - we call it TAS.
But this is a totally different animal than dynamic pressure limitiations. Flutter is something that Vans Aircraft Inc has as a concern about since they really don't know where these aircraft will flutter. And they will, if pressed hard enough.
Its not realistic for them to predict any kind of flutter onset speed since variations in build quality, control surface skin tension, trim tab slop, etc, would move that speed around in a big way.
BUT - Vne is defined in FAR23 as an indicated airspeed, like Ken points out in the article. They just don't want us Cowboys strapping on lots of HP and taking it up high where we could get into some super-high TAS territory, and then Ken and Van would be losing sleep at night.