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THRUSTER57

MACH AND KNOTS

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Hello,when i am looking at the FMC in flight some speeds show in MACH and some in KNOTS? is this normalthanksjames.

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Makes sense when you know what Mach means. It is altitude what defines what you'll see there.

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Thanks Omar so at what altitude does the display change from knots to .mach read outs in the FMC? from new boy many thanksjames.

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Thanks Omar so at what altitude does the display change from knots to .mach read outs in the FMC?from new boymany thanksjames.
In Timothy Metzingers Vol 2 tutorial on the PMDG 737, he indicates that the changeover from kts to Mach occurs at 20,000 ft or above. Jim D.

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Hello,when i am looking at the FMC in flight some speeds show in MACH and some in KNOTS? is this normalthanksjames.
According to Boeing !! Your speed on the MCP will change from Knots to Mach at about 26,000 ft. Fred.

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Such altitude is called the cross over altitude. It is defined by the speeds selected for climb (IAS and MACH). If your schedule is to climb at 300/0.8 then the cross over altitude will be that at which both 300KIAS and M0.78 result in the same TAS. The crossover altitude depends only on those speeds because it is a result of them, not a cause. It is the speeds what have been chosen depending on different criteria (mainly cost index). Since i don't feel like drawing this graph in photoshop... here's a photo I just took from my performance book.img6361gw.jpg

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The changeover will depend on what your set climb speed and planned mach speed is for climb

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For me and my usual climb and cruise speeds, the crossover is usually around theFL 310 range plus or minus.. 20,000 feet? No way... In fact, I don't think I've everhad the change be any less than FL290, and for my usual flights, that would bequite low. If you climb at 300 knots and go to a mach speed of say .78, it willbe pretty close to 31,000 plus or minus.Also, I've never seen it change at 26,000.. That's too low unless you are almostredlining the IAS with a high cost index.It should change at the exact crossover point depending on speeds, like Jack_Csays, not at any "set in stone" Boeing spec. Those are just guidelines to go by. Like I say, on almost every flight I run, it will usually end up being about 30,000-31,000 + or -.. And it's been that way on every 737 I've ever flown in the sim world.Course, on the lesser jets like the default, I had to do the change manually.. :/The graph looks accurate.. Note how 300 knots crosses over to mach ,78 atabout 30,31K.. I generally don't climb faster than about 310 IAS, so that maybe one reason why I never see the crossover as low as FL260.

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The faster the IAS on the climb the higher the C/O altitude will be. The higher the IAS is planned on descent, the lower the C/O altitude will be.

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The faster the IAS on the climb the higher the C/O altitude will be. The higher the IAS is planned on descent, the lower the C/O altitude will be.
The complete opposite. The higher the IAS for climb, the lower the crossover altitude. Look at the graph again. 320/M0.8 will result in a lower x-over altitude than 280/M0.8

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