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LarryImler

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=Menvironmentalenvelope.gif

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You'll be pleased to know I typed all this out and then the forum bombed on me, so this is attempt number two LOL...ISA is the International Standard Atmosphere, which is the 'standard daytime weather' setting that was agreed upon by various organisations around the world so that things such as the thrust ratings of engines could be agreed upon, i.e. if an engine is claimed to develop 29,000lbs of thrust, it should be able to do that in ISA conditions.ISA is: 1013.25Mb of pressure, or 29.92Hg inches of mercury, at a temperature 15°C when at sea level. Anything other than that when at sea level is known as an ISA deviation.Where FMCs are concerned, ISA is important because if you tell the FMC some info about temperature and pressure, it uses that to calculate various things, for example to calculate how much thrust to set the engines to for a take off when you press the TOGA (Takeoff/Go Around) levers on the throttles (Control+G in FS by default), or where your Top of Descent should be for the ideal descent point on your flight plan. Knowing this, you can fool the FMC into doing some fancy tricks for you, such as using reduced thrust to prevent wear on the engines if you kind of 'lie to it' about what the temperature is; otherwise known as a derated take off.To do a derated take off, there are in fact two places on a Boeing FMC where you can enter the necessary info (depending on what type of FMC it is). On an older FMC, you go to the 'Takeoff Ref' page and enter two values, one in the OAT field (Outside Air Temperature) and one in the SEL TEMP field (Selected Temperature). Your OAT will be the real temperature, your SEL TEMP will be your fake one.On a newer FMC, such as the one in a 737 NG, you enter that info on the 'N1 Limit' page, where you will find both those temperature input fields on the same line, i.e. the SEL/OAT field up at the top. You put the fake temperature in the first bit and the real temperature in the second bit. So the format for that would be something like 32/15 (i.e. 32 degrees is your fake temp, and 15 degrees is the real one).But you don't just take a wild &amp;@(&#036;* guess at that, what you do is consult the take off tables for the aircraft and determine what is the highest temperature at your given weight where the aircraft will still take off. This is then your maximum assumed temperature setting that you can use. You do of course also have to ensure you have enough runway to do this, since the whole point of all this is to use less thrust, which reduces the wear on the engines, but also means you will accelerate down the runway up to your take off speed a bit slower. As your aircraft climbs, it gradually ditches this pseudo temperature data, so that by 15,000 feet it will be back to normal parameters assuming all your genuine temperature data is correct. It is important to get this right on a real airliner incidentally, because if you spend ages accelerating down the runway, you might overheat the tires, and then when you retract them, you could have a wheel well fire or a tire explosion since they will no longer be cooled by the airflow.In case you were curious about why the FMC will reduce thrust if you tell it the air is hotter than it really is, a jet engine can get swamped with warm air and that will reduce its efficiency, so even though the air is theoretically thinner when warmer, the engine will still need to suck it in more slowly to avoid being swamped, so it reduces the N1 speed in order to do that.Once merrily flying on your way, you will notice that your FMC has calculated a T/D (Top of Descent) for you, based on a vertical bearing of 3.5 degrees or FPA (Flight Path Angle) if you are using VNAV. But to make that accurate and useful, you have to enter some info, and you do that on the DES FORECAST page (Descent Forecast). You can again have it do different things based on what you put in the ISA DEV (International Standard Atmosphere Deviation), but you should put in the real temperature at sea level and the real pressure at sea level (i.e. the deviation from ISA). If you do that, it should then start you down with VNAV at the optimum distance for your flight plan.Hope that helps.Al

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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.

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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

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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

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