# Boeing FMC help

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Hay guys whats going on, I am looking for the meaning and correct format for the ISA Dev, its on the desent forcast page and also the SEL/OAT on the N1 Limit page. I know the OAT is outside air temp but the SEL/ part of it is confusing, I know it has to be a larger number then the OAT but what is the format and how do I get the number on both parts. Thanks in advance Lar

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Hi,=================================A formula on how to calculate ISA Deviation=================================ISA deviation = OAT - ISA (International Standard Atmosphere) temperature for the heightISA = 15 - 1.98 * Hp/1000but you would normally use 2 in lieu of 1.98.e.g. planned cruise at pressure height (Hp) 3000 ft. Forecast OAT is 20C.ISA dev becomes= 20 - (15 - 2 * 3)= 20 - (15 - 6)= 20 - 9= +11e.g. 8000 ft and OAT -10dev is= -10 - (15 - 2 * 8)= -10 - (15 - 16)= -10 - (-1)= -10 + 1= -9If the minuses cause a problem, this is the same as sayingISA = -1Cdev is the number of degrees above or below ISA, so -10C is 9C colder so the deviation is -9C. and so on. This all presumes that you can get the numbers above correct.================================================================orIf you're looking for a "one liner", the following should suit your purposeISA Deviation = SAT - (15-PH/1000*1.9812)x(PH<36089.24)-56.5X-(PH=>36089.24)Where SAT = Static Air Temperature in °C, and PH = Pressure Height in feet.That will take care of business up to 82021 feet Pressure Height, after that adifferent set of parameters apply.================================================================orJust double the altitude minus 15 until FL360. So it's -54 until about Fl450.Mach to TAS: Multiply Mx6.1 - Ram rise.(example: M8x6.1=488-28=460)To convert IAS to Mach: TAS + RAT=488 divided by 6.1=M

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Hi Al,Yeah, I've had a few long post get dumped. It makes me hesitant to post sometimes, wish they would find a way to fix the problem once and for all.

Former Beta Tester - (for a few companies) - As well as provide Regional Voice Set Recordings

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Yes, it puts a strain on being benevolent when your lengthy helpful post that you've just typed out disappears, but I'm sure we all remember how confusing the FMC looked when we first saw it, so I figured I'd be kind and take the trouble to explain it.If I'd have known you were also coming to the rescue, I'd have probably not bothered :( Al

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Guys I want to tell you thank you again fo your Info, it helps very much. I've been looking for this info for weeks, and I'm sure its out there but its hard to find.The reason I was asking is I know the basics but I want to learn alot more about the FMC.Now 1 more question, in the desent forcast page, if you inter the desent forcast (I know you have to start with your cruse alt. and go down) and if you DO NOT have the ISA DEV intered, does the forcast still take effect?P.S. can you guys recommend a book I can buy to help me learn more about the FMC? Thanks Lar

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1 more question, in the desent forcast page, if you inter the desent forcast (I know you have to start with your cruse alt. and go down) and if you DO NOT have the ISA DEV intered, does the forcast still take effect?P.S. can you guys recommend a book I can buy to help me learn more about the FMC? Thanks Lar
On the real B737, if you don't enter specific temperature and pressure details, the FMC will still calculate a T/D, but the calculations will be based on standard ISA settings (i.e. they are unlikely to be especially accurate), so if you wanted a more accurate prediction and a better-controlled descent, it'd help to give the FMC as much accurate data as you can. You can find the required pressure and temperature data while in flight from a number of sources, such as the FS map, Flight Keeper, Active Sky, EFB, Radar Contact etc.Of course some of the accuracy of how that descent calculation occurs in FS will depend on how well the FMC has actually been simulated (and that depends on how good the developer is who made your FS airliner), so you might find it doesn't make a huge deal of difference on less accurately simulated FMCs and quite a big difference on the better ones. Expect it to be very good on the forthcoming PMDG NG for example, since they are apparently spending a lot of time on fine tuning those aspects of the FMC's behaviour.With regard to a good detailed book on FMCs, I would recommend Chris Brady's 'The Boeing 737 Technical Guide', which in addition to all the other systems on a B737, covers typical Boeing FMC features in detail. Chris runs the 737 technical site and is a very experienced 737 pilot. You can find a good deal of info about the various FMCs found on 737 Classics and NGs there, and that information will largely suffice for understanding FMCs on other Boeing aircraft, which are not all the same, but similar in most respects. Be sure to give his website a look:http://www.b737.org.uk/fmc.htmIf you want a less technical book about all that stuff, you might try 'Flying the Boeing 700 Series Simulators' by Mike Ray, which is a good all-round guide on how to fly an airliner properly, and a very entertaining and easy read. Mike was a United Airlines Captain, and like Chris Brady, he knows his subject well. That book covers how to program your FMC amongst other things:http://www.amazon.com/Flying-Boeing-700-Flight-Simulators/dp/0936283106/ref=sr_1_1?ie=UTF8&s=books&qid=1308196661&sr=8-1Al

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Hi,Just to add another option to what Al has said, Bill Bulfer has produced a few Boeing FMC User's Guides. You may find one of these useful.http://www.cockpitcompanion.com/servlet/the-FMC-USER-GUIDES/Categories

Former Beta Tester - (for a few companies) - As well as provide Regional Voice Set Recordings

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Ok just so I understand, by looking at the chart above, if I'm flying at 38000 and the and the TAT is -22 I would want to put +30 in the ISA DEV in the forcast page,am I right or worng, sorry its confusing. Lar

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Hi,I'm running out the door and just spotted your question, just follow the formula.38,000 ft and an OAT -22dev is= -22 - (15 - 2 * 38)= -22 - (15 - 76)= -22 - (-61)= +39Sorry, I was running out the door when I did my calculations. If you take a look at the chart above, you will see it goes up to 36,089 feet. That's because above 36,000 feet the temp will stay at approx. -56 degrees. So you can see there is a difference in the above calculation by 5 degrees (-61 and -56 above 36,000 feet), so the dev with the altitude correction (since we are above 36,000 feet) would be +34 degrees Celsius.Think of it this way, if your above 36,000 feet its the OAT minus -56. From 36,000 feet and below you would use the above formula.

Ok just so I understand, by looking at the chart above, if I'm flying at 38000 and the and the TAT is -22 I would want to put +30 in the ISA DEV in the forcast page,am I right or worng, sorry its confusing.Lar

Former Beta Tester - (for a few companies) - As well as provide Regional Voice Set Recordings

Four-Intel I9/10900K | One-AMD-7950X3D | Three-Asus TUF 4090s | One-3090 | One-1080TI | Five-64GB DDR5 RAM 6000mhz | Five-Cosair 1300 P/S | Five-Pro900 2TB NVME        One-Eugenius ECS2512 / 2.5 GHz Switch | Five-Ice Giant Elite CPU Coolers | Three-75" 4K UHDTVs | One-24" 1080P Monitor | One-19" 1080P Monitor | One-Boeing 737NG Flight Deck

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