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Mach Switch?


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New to the forum... Hi to all...

Just noticed something odd/strange today...

V-Pilot in FS Red Airlines, I was flying a 747-400ER YBBN - YPDN: got to TOC and in level flight @ FL320 and flicked on the Mach switch...

Odd thing happened...

N1 & N2 both reduced, FF (fuel flow) decreased from 6.4 to 2.8

Meant I was doing ca 270kn AIS (= 430kn GS) but using less than half the fuel...

How can this happen..??

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Did you try manually setting the power to the same level without the mach/IAS activated? Presumably you just had too much power set in, and at those speeds a large power increase only makes a small airspeed increase.


Of course those more familiar with the aircraft might have other thoughts...


Larry N.

As Skylab would say:

Remember: Aviation is NOT an exact Science!

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Hmmm. I thought you had to switch to mach once above FL180? Works for me any way.

Bob G.

[sIGPIC][/sIGPIC]Intel i7 7700K 4.5 Ghz, GTX 1080 SC 1TB 7200RPM HDD, Samsung 500Gb SSHD, 750W PSU, P3DV4, Windows 10 64 bit Home Premium.REX,GEX,UTX. UTLive
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I used to switch to mach at FL240. Then I found a tool that, along with a whole host of other things, calculates the change over altitutde when given the IAS and desired mach speed. Surprisingly, depending on the IAS and desired mach speed, a lot of my change over altitudes are between FL280 and FL310.


Changeover altitude is where the IAS and desired mach are the same., for example when 290 kts is also mach 0.82


The tool is adcalc and can be found at http://www.aero.sors.fr/adacalc.html

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  • 4 weeks later...

Just curious-what does pressing the 'B' key do? Does NOT pressing the 'B' key influence the accuracy of the mach speed function?

Chuck B


i7 2600K @ 3.4 Ghz (Turbo-Boost to 3.877 Ghz), Asus P8H67 Pro, Super Talent 8 Gb DDR3/1333 Dual Channel, XFX Radeon R7-360B 2Gb DDR5, Corsair 650 W PSU, Dell 23 in (2048x1152), Windows7 Pro 64 bit, MS Sidewinder Precision 2 Joy, Logitech K-360 wireless KB & Mouse, Targus PAUK10U USB Keypad for Throttle (F1 to F4)/Spoiler/Tailhook/Wing Fold/Pitch Trim/Parking Brake/Snap to 2D Panel/View Change. Installed on 250 Gb (D:). FS9 and FSX Acceleration (locked at 30 FPS).
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