Greetings;

 

Two questions.   Can someone please confirm the correct procedure.

 

In the FMC PERF INIT page there is T/C OAT that needs inputting.

 

I have always inputted the top of climb temperate, by calculating the value by this formula:  altitude x 2 -15 which gives you the temperature at that altitude (a negative number).  You can also look at the weather and the find the temperature.

 

Another flyer has told me I am incorrect.  He states the value that should be entered here is the outside temperature as observed on the EICAS screen (Total Air Temperature / TAT).

 

Which is correct ?

 

Further, page 2/2 of the PROGRESS Page has  the entries: SAT / ISA DEV.

 

Is this only for information, or should this be read and inputted elsewhere in the FMC?

 

I hope someone can clarify this for me - thank you,   WilloW

You've got it right. As the title of that field in the FMC says, it wants the temperature at the top of climb, not the current temperature at ground level.

 

The formula you're using is to calculate the ISA temperature at a given altitude. If you don't have the actual temperature of the altitude you'll be flying, you don't need to put anything in that field. The computer will use the standard one (ISA) by default.

 

SAT / ISA DEV shows your current static air temperature and how much it is deviating from the standard atmosphere value. While on the ground, it will show outside air temperature. I believe it is provided only for reference.

You've got it right. As the title of that field in the FMC says, it wants the temperature at the top of climb, not the current temperature at ground level.

 

The formula you're using is to calculate the ISA temperature at a given altitude. If you don't have the actual temperature of the altitude you'll be flying, you don't need to put anything in that field. The computer will use the standard one (ISA) by default.

 

SAT / ISA DEV shows your current static air temperature and how much it is deviating from the standard atmosphere value. While on the ground, it will show outside air temperature. I believe it is provided only for reference.

Hi Matheus

 

Thanks for the prompt reply - I appreciate your time.

 

Just a little further clarification.  Lets say you have no idea what the temperature is aloft.  I do the calculation as written above to determine the ISA.  Where do I input the ISA figure?  Do I input this into the T/C OAT space?  Is there something I am missing like a conversion from ISA to OAT?

 

Cheers,   WilloW

There is another field - ISA DEV right above T/C OAT on perf init. You fill only one of this two fields and other value will be calculated automatically.

 

If you don't have got temp aloft forecast, you may not fill this fields. After takeoff FMC will be using value from progress page for correction. Putting temperatures on perf init gives you accurate fuel predictions and T/C point.

There is another field - ISA DEV right above T/C OAT on perf init. You fill only one of this two fields and other value will be calculated automatically.

 

If you don't have got temp aloft forecast, you may not fill this fields. After takeoff FMC will be using value from progress page for correction. Putting temperatures on perf init gives you accurate fuel predictions and T/C point.

Thanks for the reply.  I understand that the ISA DEV and T/C OAT are linked.

 

My confusion is in "what" do I place into either of these two fields.    The value needed changes dependent upon the cruise altitude.  I am led to believe that the below equation is correct...

 

Do I calculate the ISA as per the equation above (altitude x 2 -15 + ISA (negative number) and place this into the ISA DEV line ?   If I do and the figure is quite low (say -45), the FMC will not allow its entry, so this does not appear correct...

 

Without wanting to confuse everyone,  I need to know what number goes where and how do I find this number? 

 

Many thanks,   WilloW

Hi Matheus

 

Thanks for the prompt reply - I appreciate your time.

 

Just a little further clarification. Lets say you have no idea what the temperature is aloft. I do the calculation as written above to determine the ISA. Where do I input the ISA figure? Do I input this into the T/C OAT space? Is there something I am missing like a conversion from ISA to OAT?

 

Cheers, WilloW

The FMC Already knows what the Standard temperature is. You need to find the forecast temperature for that altitude or the deviation.

Thanks you for everyone's comments.   I cannot help but feel as if I am missing something here.....  

 

If the weather can supply aloft temperatures for the cruise altitude, then great.  It's easy to plug this into the T/C OAT line.

 

I'm trying to figure out what to do if this data is not available.  I thought it was the equation mentioned above.

 

I understand that the FMC can calculate the standard temperature.  If I cannot determine temperature at altitude, what do I do ?

 

Obviously something is not clicking to allow me to understand this concept fully,   WilloW

Iain,

 

Your equation calculates the "standard" or "ISA" (International Standard Atmosphere) temperature at your top of climb. It's a simple equation and the FMC assumes this by default. That entry is for when it *isn't* standard.

 

If you know the outside air temperature (OAT) at your top of climb then enter it in T/C OAT.

If your information is given as a deviation from ISA then you can add that deviation to the line above (ISA DEV).

Only enter information into one of these above two.

 

If you don't know the outside air temperature (OAT) at your top of climb then enter nothing and the FMC will assume a "standard" or "ISA" temperature.

 

Hope this helps,

 

Ian

Iain,

 

Your equation calculates the "standard" or "ISA" (International Standard Atmosphere) temperature at your top of climb. It's a simple equation and the FMC assumes this by default. That entry is for when it *isn't* standard.

 

If you know the outside air temperature (OAT) at your top of climb then enter it in T/C OAT.

If your information is given as a deviation from ISA then you can add that deviation to the line above (ISA DEV).

Only enter information into one of these above two.

 

If you don't know the outside air temperature (OAT) at your top of climb then enter nothing and the FMC will assume a "standard" or "ISA" temperature.

 

Hope this helps,

 

Ian

Hi Ian

 

Ahhhhhhh, I see clearer now......    It's so simple I cannot see how I could not understand it....    :rolleyes:

 

If you have the temperature or ISA deviation from the weather provider for the cruise altitude - then enter it..   If you have not a clue what the temperature is, leave the lines unpopulated and the FMC will calculate things based on the standard temperature.

 

Thank You,      WilloW