Simviation Forums

Hi everyone, when I am flying the 747-400, I set speed to 450knts, however when it gets to 380 to 400, Overspeed warning comes on, why ?
It should be able to sit on 450knts.
Thanks again....Tomcat61

Hi Tomcat,

It shows overspeed at high altitudes at such a small speed because of the INDICATED airspeed. At 50000 feet the indicated airspeed is half of your true airspeed. So, unless you know about this you think you're travelling half the speed that you really are. You think that the air is moving quite slowly over the wing, though it's really quite fast. I think that overspeed comes at a true airspeed rather than an indicated airspeed. I'm not sure, but you're really travelling faster than you think. If you're cruising at 35000 feet at 400 knots of indicated airspeed, to find your true airspeed do this sum:

Take the first 2 digits of it (35) and multiply it by 2 = 70. Then, devide it by 100 and add it to one. This gives 1.7...
With the 1.7 multiply it by the 400 knots and that equals 680 knots - quite a lot faster than you expected eh? Hope it helped,

- Stevie

450 knots is a True Airspeed (TAS) reference, not to be confused with Indicated Airspeed (IAS).

At altitude, in the 30,000's, the 747 can do 450 knots True Airspeed (similar to groundspeed), but the indicated airspeed is closer to 260 knots. That's why you have to reference your speed as Mach when you're that high, because airspeed becomes too difficult to gauge.

Basically, using indicated airspeed hold on jets is useful up to 320 knots. While climbing at 320 knots you can watch the mach reading gradually increase. The air is getting thinner and thinner so the aircraft actually moves faster and faster to hold 320 as it climbs. When the mach get's to .84 you set speed hold to M.84. The autopilot then holds your mach cruising speed, and as you continue to climb the airspeed reading will begin to drop which is a sign that the air is REALLY thin.

When you're flying a heavy at altitude you don't use airspeed - you use mach.

You're using your autopilot aren't you??

Just switch from Speed to Mach - it will cruise all day long at 0.8 - 0.85 mach

Then just pop up your GPS and check out your groundspeed......