# 747-400 speed

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hi all, i have the 747-400, and it is bugging me. This problem is the same for the 737-600/700 too.When i am climbing to thirthy thousand feet, the red line that tells me that i can't go faster than that comes down to 360 knots as i climb. Then the red line gets closer and closer to 340 knots. Then my speed drops. What is happening? Look at the picture to see...robbie

Im not a real world pilot so im not the most qualified to answer this, but ill give it my best shot. as you climb, air pressure decreases, the air gets thinner. and sound needs a medium to travel through, air, water, ect. so as you go higher there is less air for sound to travel through, and if im not mistaken the speed of sound decreases. and the aircraft has a limit on its speed in mach number. so as you climb the speed of sound decreases, your IAS indicator will decrease your speed limit so you dont go past a certain mach number. so if u are traveling at M.85 at sea level, and M.85 at FL370, your IAS would be faster at sea level since the speed of sound is faster there. so you may think that you are slowing but you are still going very fast as indicated by your TAS and GS. look at both pictures, in the first one you are at M.87 and close to 350 IAS, in second picture you are higher and at M.90 but at only 337 IAS, remember, speed of sound decreases with altitude. looking at the AOM, for FL300 to FL400 the limit is M.900. as for speed dropping, make sure you are not over weight for that altitude, because wings need air to work, and air gets thinner with higher altitudes. once again, i am only a high school senior and this is my first year taking physics, so im not the most qualified to answer this. so someone with more knowledge may correct me.Adam

right, i've tried that. so what your saying is if i set 340 knots into the auto pilot and then wait for the aircraft to reach it, and when it drops because of my altitude, am i still going 340 knots?robbiePS. I'm getting lighter every second, so that will help!

Your speed indicator shows indicated airspeed(IAS) which i believe is not corrected for altitude. Fly at 250 IAS at say, 1500 feet, and take note of your TAS. Then fly at 250 IAS at, FL250 and take note of your TAS. at fl250 your TAS should be higher although your IAS is the same. in the 744 AOM, it shows speed limits at FL300, FL350, and FL400. and the limits are (KIAS/Mach) 340/.900, 309/.900, 269/.900, respectively. so as you see, the mach is the same in the limits, but the IAS is significantly reduced because the speed of sound is less as altitude increases. at higher altitudes the MCP speed indicator should switch over to show mach instead of IAS. so you should be using mach to select speed at higher alts. but say that u are using IAS to set speed in the MCP. So you are cruising at FL300 with an IAS speed of 340, that would put u right on the limit at M.900. Then lets say you did a step climb to FL350. If you where to stay at 340 IAS you would overspeed as the speed of sound will decrease thus putting you over M.900. You would have to reduce speed to 309kts, which would still give you M.900.adam

yes, thank you. I ended up flying at FL320 at 332knots. I belive that is OK. 332 is about Mach number M.900. i do thank you for your help. I will keep my caution on this very confuesing topic.robbie

no problem. if u have any more questions regarding the 744, you can drop me an email at, airbusa340300@gmail.comadam

Robbie,you should not cruise at Mach .90, this burns rediculoulsy too much fuel. Furthermore, one little wind gust and you end up in overspeed...No airline cruises their B744 at more than .86 Mach. If you have set up the FMC with the correct performance figures and chosen a realistic cost index you'll get the speed information on your VNAV pages.Regards,Markus

Markus Burkhard

Robbie,You're trying to fly the plane like an MS default aircraft (ie using V/S, autothrottle SPD mode) etc all the time. This is not at all how the aircraft should be flown.You also seem to have a basic misunderstanding of how indicated airspeed works as you gain altitude - you're attempting to make the plane exceed its maximum airspeed. You need to switch the speed window to Mach once you get up that high.Better yet, download copies of the Type Rating Course 1-3 and start learning how to fly the plane like the pros do!

Ryan Maziarz

For fastest support, please submit a ticket at http://support.precisionmanuals.com

Robert S. Randazzo

PLEASE NOTE THAT PMDG HAS DEPARTED AVSIM

You can find us at:  http://forum.pmdg.com

thanks you guysrobbiea happy, but very confused userps. What on earth shall i put in the FMC cost index? It keeps on saying insuffient fuel on the screen. i'm taking a flight from hanade to singapore, with full tanks, light, so what should i put in the reserves and the cost index?

right, spent a while figuring this out, looked through the manual, and nothing. who knows about these watcha-ma-call it?

Robbie-All of this information was covered in the manual. You probably want to spend some time reading the fuel planning methodology section- and you definitely want to spend some time in the FMC section too....

Robert S. Randazzo

PLEASE NOTE THAT PMDG HAS DEPARTED AVSIM

You can find us at:  http://forum.pmdg.com

Ahh, found the problem.It seemed to me i could fuel the 747 by the default FS9 options, but noo ser-iy! I should of used the 747 fuel loader or something. The fuel weight was 136,000 kilograms, but the fuel should of been 176,000 kilograms. i should of took some time to read the manual afterall.Thanks you guys, especially you, Captain. mushy guy aint I?regards,Robbie

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