My NGX followed FMC high alt. cruising speed at 0.78 Mach, but PFD speed tape stopped by 268 Knots(red dots over 270 Knots). PFD speed was 0.78 Mach, and tape at 268 Knots. My PFD speed tape always shows red dots over 340 Knots. I can't figure out what is wrong with that. I think, it is the matter of setup or preprogramming. Any idea why, please ?

No problems here, 0.78 mach is roughly 270knots at higher altitudes, remember this, 300 knots for example at sea level is slower than 300 knots at FL360. So it is completely normal to see those types of speeds at that altitude.

I was at FL350. Mach 0.78 is over 500 Knots. Would never realize, high alt. can change speed that much. Thanks, ddswh1pk0s.

 

 

Would never realize, high alt. can change speed that much.

 

Yep - that's why you'll notice the plane switches from knots to Mach as it climbs (watch the magenta speed numbers at the top left of the primary flight display: they'll go from about 250 to upper 200s during the climb, and then to Mach, indicated by .##).

Interesting, perhaps:

 

If the company really needs to get the plane back to the the stable, sometimes they'll file us for FL260 or FL270. That's the "crossover altitude." It's where the limits M.82 and 340kts meet. You'll also get the highest TAS there.

Interesting, perhaps:

 

If the company really needs to get the plane back to the the stable, sometimes they'll file us for FL260 or FL270. That's the "crossover altitude." It's where the limits M.82 and 340kts meet. You'll also get the highest TAS there.

 

Off topic but thanks for that information about the highest TAS altitude I will keep it in mind when I need to fly quicker. Thanks again

Thanks, guys.