For example, I set the autopilot to go to an altitude of 32,000 ft and I set the speed at 300 knots, I also set the climb rate at say 1000-1200 fpm. But I still think there's something wrong (with the speed) here. I am going to try and see if I get this warning at higher altitudes, although I remember this warning hardly goes away. It's actually more complex than I've described, but this should give you the general idea. The above figures are all very rough, not figuring actual temps and barometers (altimeter settings) and humidities, but will be in the right ballpark. 85 * 573 = 487 KTAS, with KIAS obviously being much less at about 278. 85 (that is, 85% of the speed of sound), then. So if the limiting mach for your aircraft is. So that 2%/1000 ft (it's actually less than that) would say that the 350 kts IAS when you're at 35,000 feet will true out at about 595 knots TAS, which is actually somewhat beyond the (roughly) 573 knot (660 mph) speed of sound at 35,000. The set in IAS are basically pressure related and the one in TAS is largely relative to the effects from getting too close to the speed of sound (mach 1), which is much lower at higher altitudes. So back to the OP question, the drastically reduced pressure at jet altitudes means that the airspeed indicator 'sees' less pressure for a given speed, therefore you're going a lot faster than indicated, losing roughly 2% per thousand feet under standard conditions (the 'standard' temp drops 3.5Âº F per thousand feet), and the barometer at 18,000 feet reads about half the pressure that it does at sea level.Īlso note that (and this is why mach is used at high altitudes) there are two different sets of limiting speeds for jets (all aircraft, actually), one set in IAS and one in TAS. By the same token, increased air pressure (reversal of above conditions) means it will indicate faster than TAS. Less dense air (higher temperature, higher altitude, more humidity, lower barometer) means that the indicator will read less than actual speed. But any variation from those conditions means that IAS no longer matches TAS. That instrument actually does match TAS at sea level under standard conditions, that is, 29.92' Hg, 59Âº F (or 15Â✬), and 0% humidity. The actual reason for the disparity is that IAS (Indicated AS) is pressure dependent, since the indicator is nothing but an instrument that reads the air pressure coming in to the pitot tube, and is calibrated in knots. Groundspeed is strongly affected by winds, while TAS is relative to the surrounding air, and is all the aircraft actually 'knows' about.

The '600' MPH/Knots are NOT KIAS (Knots indicated) but GROUND SPEED.Īctually, it is KTAS, Knots True AirSpeed.

Fsx passenger wing view