Hello everybody Well i owned this forum because i have been dealing with a problem for a while in which the highest the altitud on any of my FSX airplanes the narrowest the speed limit i have. I had even changed the information on the airplanes.cfg documents so that the cruise speed and the limit speed is over 500knts in all of them and i can reach those speeds at low altitud but as the altitud is increased the power of the engines is lower.For example:Flying a 747-400 with max charge and 100,000 pounds of gas i can rech 505 as maximum cruise speed below let's say FL240 but as i start increasing the altitud reechoing the FL340 the power of the engines even with 99.9 on the N1 is just enough to reach about 345knts.Well i hope someone has an answer for my problem. :(

Indicated airspeed will decrease with altitude due to a reduction in air pressure based on altitude. Changing your aircraft.cfg wasn't a good idea as it won't change physics.At higher altitudes mach speed is used not indicated.

As Ed said, at flight level altitudes the indicated airspeed will decrease because of the lower air density and air temperature.There is most likely nothing "wrong" with the aircraft.cfg file to begin with, just a case of not understanding why the indicated airspeed is so "slow." Actually, at 345 KIAS and that altitude, the real airspeed was closer to 500 knots!

Wow guys thank you rely much I am not exactly an expert about the physics haha but i just have another question: which is the cruise MACH speed of an airliner? And when must i change the The indicator to MACH mode?

Just to add to the previous good replies :- The airspeed is measured by the pressure of the impacting air (from the speed of the aircraft) blowing into a little pipe sticking out the side of the aircraft. It is called a pitot tube. In bygone days, this pressure used to inflate a small diaphragm inside the airspeed indicator, and with the help of a few gears and levers, would make the needle move around the dial. The faster you went, the further round the dial the needle moved. Now, it's no secret that there is more air at sea level than at say, 35,000 feet. Therefore, at any given speed through the air, the needle will be blown further around the dial at sea level than it will at 35000 feet. This gives rise to the term "Indicated Airspeed" which is the figure on the airspeed indicator. There are calculators (many and varied) that will take your altitude, temperature and IAS (indicated airspeed) to calculate the real speed you are moving through the air. This is called "True Airspeed" or TAS. Just to add a couple of numbers, at sea level at IAS 320 you would be doing Mach 0.484 and TAS 320. At 25000 feet an IAS of 320 gives Mach 0.761 and TAS 458. At 35000 feet IAS 320 gives Mach 0.925 and TAS 533.So your next question was when do we change from IAS to Mach? Suppose we start our climb at 320 IAS - we know that the Mach number at low level will be quite low and therefore we use IAS. As we continue our 320 knot climb, we will notice that the Mach number is steadily increasing. At very close to 29000 feet you will have a situation where 320 IAS is now the same as M0.82. We now maintain M0.82 for the rest of our climb, and you will now notice that the IAS slowly decreases.