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Guest shadowman76

Two questions about 737 FMC speeds

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Guest shadowman76

Hi!I've got two questions about the calculation of speeds in the 737 NG FMC. I did a flight from Hamburg (EDDH) to Goteborg (ESGG) with the 737-600 (although I have the update for the 800/900 installed). I had 120 PAX and about 4.4 tons of baggage/cargo, resulting in a ZFW of about 50.7 tons. CI was 80 and I choosed FL300 for cruise. With this configuration, the FMC calculated a cruise speed of Mach 0.6 for econ cruise, which for me seeme to be much too low, even for an economically calculated flight. LRC cruise speed has also been calculated to Mach 0.6. Are these low values realistic or did I do something wrong?Now my second question is about the Vref calculation of the FMC. The same configuration as above (it was actually the same flight), the FMC calculated a Vref for an approach with 40 degree flaps of 104 kts, which for me seems to be too low, too. Nevertheless, when entering final approach, I dialled that speed into the MCP speed setting and activated speed hold. When indicated by the PFD, I lowered the flaps to the indicated setting during my approach down the glideslope. The aircraft slowed down and at about 110kts the 737-600 stalled. I was able to recover and continued my approach and made the touchdown at about 120kts. So the Vref calculation of the FMC must have been wrong, otherwise the aircaft would not have flown into a stall. Now again, can this happen or may I have entered something wrong into the FMC. All values I entered were in kg/tons (e.g. 50.7 as ZFW).I hope somebody can help me with this.yours, Thorsten

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Guest mruane

Thorsten, sounds like you could be using Kilograms in the Load Manager and Pounds in FS. Is that a possibility?

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Guest shadowman76

In the load manager, I us kilogramms. And I enter kilogramms into the FMC. I'll check if the FMC awaits values in lbs, thanks for that hint. BTW, if the FMC wants lbs, is there a way to change it to kg?

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Guest mruane

Thorsten, Select the PMDG/737NG/Styles menu option and review the Various setting (from the left of the screen). On the Various page you will find an option to select either LBs or KGs. I am fairly sure that the default is LBs, so that would explain what you are getting if you have not changed that setting. Select KGs and you should be good to go.

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Hi Thorsten,The only thing I can think of that caused the aircraft to go into a stall on the approach is that the airplane was actually flying too slow. For example, you said you had a ZFW of 50.7 tons. The question is in the units of measurment. Was the FMC interpreting this as 50,700 lbs, 50,700 kilograms, or 50.7 tons? I don't think the FMC interprests these units in tons, but I could be wrong. If you're using english units of measurments, then the 50.7 you entered into the FMC would be 50,700 lbs., not 50.7 tons, unless there's a way to change those units of measurments to tons so that the FMC will know to enter the weight in tons. If the actual weight is 50.7 tons and the FMC enterprets the weight in pounds, just convert tons to lbs. Multiply 2000 X 50.7 or 101,400 lbs. This would be entered into the FMC as 101.4. Anyway, the issue is make sure you're using the correct units of measurments and that your FMC will enterpret those units. Ken.

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Guest meyomyx

I'm surprised you populate the ZFW box in the FMC manually. The FMC modelled in PMDG differs in that by clicking the LSK1 button, it will fill in the weight boxes for you (I think this differs from the real thing - as it does with V speeds).So you can populate the weight boxes and always amend them if you want to ..... but why would you want to?

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