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Help with cruise speed


steve1975
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I am doing my take off with VNAV and LNAV. Take off great climbs 250 to 10000 ft then FMC has itnclimb at 280 till my cruising alt of 32000. Then the fmc says it will cruise at .71 mach. Then my speed drops down to .71 Mach on the panel ad a speed of 254 knots . I looked online and it says itnshould he higher . Is the 254 knots really Mach .71 using ground speed and if so on my descent it wants 280 knots using vnav 

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I am doing my take off with VNAV and LNAV. Take off great climbs 250 to 10000 ft then FMC has itnclimb at 280 till my cruising alt of 32000. Then the fmc says it will cruise at .71 mach. Then my speed drops down to .71 Mach on the panel ad a speed of 254 knots . I looked online and it says itnshould he higher . Is the 254 knots really Mach .71 using ground speed and if so on my descent it wants 280 knots using vnav 
Mach number is a quantity that defines how quickly a vehicle travels with respect to the speed of sound. The Mach number (M) is simply the ratio of the vehicle's velocity (V) divided by the speed of sound at that altitude (a). 

Therefore: Mach is the division of Velocity and Altitude. 

So above 25000ft use Mach as said before... 
I usually do that so the velocity precision is better as the air is much thinner... 

If you fly with Knots (IAS) your actual speed would be much faster...

Maybe this helps...
Jet

Gesendet von meinem SM-T810 mit Tapatalk

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1 minute ago, Cebulon said:

Mach number is a quantity that defines how quickly a vehicle travels with respect to the speed of sound. The Mach number (M) is simply the ratio of the vehicle's velocity (V) divided by the speed of sound at that altitude (a). 

Therefore: Mach is the division of Velocity and Altitude. 

So above 25000ft use Mach as said before... 
I usually do that so the velocity precision is better as the air is much thinner... 

If you fly with Knots (IAS) your actual speed would be much faster...

Maybe this helps...
Jet

Gesendet von meinem SM-T810 mit Tapatalk
 

I don’t understand . I know about Mach is safer than Ias. But my fmc has cruise sped at .71 mach. On th climb I’m doing 280 but on my cruise at .71 it says the ias is 254 . But I would think mach .71 would be over 300 on the ias. Then when I go into decent it want to go up in speed into 280 as the fmc says this 

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IAS in the aircraft (your big analogue speedometer) is measured by a pitot tube sticking out the side of the aircraft, the tube simply measures the pressure of the air against a sensor as you move through the air and changes this to an indicated air speed (very basic terms).  Imagine you are in a car, doing 100mph, you don't know what speed your going, your blindfolded and ear muffs on, stick you hand out the window and guess the speed, about 100mph you would say.  Now drive your car at 100mph across a bridge at 36'000ft, do the same test, you would probably guess your speed as 50mph, your doing 100mph, but the air is thin so it feels much less.   This is why your IAS is showing 250kts only, because the air is less dense.

So if you are doing 300kts IAS at 200ft above ground, and there is no wind,  your ground speed is also 300, and your TAS is 300.  If you go the exact same speed at 30'000ft, you ground speed is 300kt, but your IAS will only show 200kts as the air is thin and not pushing so hard against the sensor, so you have this "false" reading so to speak.  On the 737, have a look at the top corner of the ND and you can see the ground speed reading, you will see while cruising at Mach 0.71 and 254kts, that your ground speed is over 400kts, so all is fine. 

Thats why its important not to chase after IAS as you climb and descent as you might overspeed the aircraft.  You will climb at 280kts, then as you get closer to your cruise height, you will see the FMC changes from targeting IAS to target a Mach number.   This is because at at 200ft, 280kts is maybe Mach 0.5, but at 26'000ft, 280kts is Mach 0.71 so it switches to Mach 0.71 at 26'000ft as you continue to climb.  Your Mach then stays 0.71 as you climb, you are not speeding up or slowing down, but your IAS gets lower and lower as the air thins out, but your not slowing down :)      If you could just keep climbing, you would get an IAS that goes all the way down to ZERO, as there is no more air, but you might be going MACH 36 like a rocket.


It's hard to explain clearly, everyone is totally confused by it at some point in the virtual or real flying, but once it clicks, you will understand.  

Edited by Iain
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The mach number is an indicator for the aerodynamic regime you are in. It is expressed as fraction of the speed of sound in the air you are currently surrounded by and for sub sonic flight you'll want to stick below 0.8 or whatever number the manufacturer has set. 

The speed of sound does not change with pressure , so the mach number does not drop as you increase altitude - it does change somewhat with temperature though.

For the 733 it mainly serves as ceiling that you do not want to cross.

 

The true airspeed (TAS) is the speed at which the aircraft is moving relative to the surrounding air and can be calculated from the indicated airspeed IAS and the altitude.

It is however not that important to the actual flying, since the behavior of the plane depends on the forces the air exercises, not its actual speed - as long as you don't change the aerodynamic regime. If you go from subsonic to trans/supersonic the forces change dramatically.

If you combine the true airspeed with the motion of the air relative to the ground you get the ground speed, which is only important during landing/takeoff and for navigation. These days ground speed is usually provided by GPS.

 

The (calibrated) indicated airspeed (cIAS) is the speed that is displayed on the instruments and derived from the pressure difference between the moving air (at the current speed) and the static pressure.

I like to think of the (calibrated) IAS as "aerodynamic speed" because it determines the forces that the flowing air applies to the aircraft, i.e. as the air gets thinner you need to go faster (the true airspeed is higher) for the same amount of force.

When it comes to flying this is the most important speed, because it tells you how the aircraft will behave: if it is low you need a large angle of attack too keep the aircraft level and are in danger of stalling if you go slower. If it is high the aircraft will be very responsive to control input and you might exceed the allowed forces of the air frame if you speed up further.

 

The FMC in the 733 will track the IAS setting as long as it does not exceed the selected mach number.

So at low altitudes you will be at 280 knots IAS and as you climb higher your true airspeed needs to go up to keep the IAS at 280 as the air gets thinner and at some point you will reach a speed that equals mach 0.72.

At this point the FMC will track the mach number instead and basically keep your true airspeed more or less the same (the speed of sound will change with temperature) while the IAS drops as you get higher because the pressure goes down.

On descend this is reversed: you will fly at mach 0.72 until the IAS reaches 280 and then the FMC will reduce the TAS during your descent to keep the IAS at 280.

 

Here's a quick table of the corresponding speeds in a standard atmosphere: 

Altitude IAS TAS Mach   Mach TAS IAS
0 280 280 0.42   0.72 476 476
10000 280 323 0.51   0.72 402 460
20000 280 375 0.61   0.72 442 333
30000 280 437 0.74   0.72 424 271
35000 280 473 0.82   0.72 415 242
40000 280 521 0.91   0.72 413 215

To get the ground speed you simply need to add the current winds to your TAS (as vectors).

Looking at that table during the climb you aircraft will stay at 280 knots up until roughly FL300 - you TAS/GS will have gone up from 280 to ~437 at that point. (More precisely you'll reach mach 0.72 at 28500 where your TAS is 427.)

Climbing further will cause the speed to stay at mach 0.72 and your IAS will drop to 215 at FL400 whereas the TAS/GS will only decrease slightly due to the temperature changes.

 

Edited by mfor
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