Assumed temp. calculation B737

[Crossunder]

(Yes, I have used the search function)

I think I understand the basics of assumed temperature - the whys and partly the hows. There seems to be a formula, using Kelvin - not Celsius;

Reduced N1 = Full N1 x Square route of (Ambient temp / Assumed temp)

...this from Chris Brady's web page.

This formula does not take into consideration the aircraft mass and available runway - so there HAS to be something more to it. How does the FMC use the excess runway available, and actual take-off mass, to calculate the N1 for take-off?
Does it do a series of calculations or interpolations? Is it really a single formula? What have I missed?

Anyone care to shed some light on the subject?

[BOAC]

Basic misunderstanding there, Cu. The FMC has no part (on a 737) in calculating assumed temperature, but merely effects the selected temperature in the engine settings. The AT itself is derived from airport analysis tables which do of course allow for those factors.

I don't know anyone who gets involved in that equation in the cockpit.

[mutt]

How does the FMC use the excess runway available

It doesnt.

and actual take-off mass, to calculate the N1 for take-off?

It isnt used.

The "standard" FMS has extremely limited calculation power, it doesnt calculate the takeoff weight so it doesnt need to know the runway length. The N1 is calculated based on pressure altitude and temperature, it doesnt use weight.

Mutt

[Todders]

Ok ill just add my bit to help make this clear cause im not sure it is.

Assumed Temp is something that operationaly in an airline you get from tables produced for your airline. These take into account the runway length, climb out clearance etc. Not only do you get one per runway but also one per intersection you could use to enter the runway. Also you have diff. corrected tables for different flap settings, wet runway, bleeds on/off etc. Into these tables you put the weight of the aircraft and read off the assumed temp (and also V1,Vr,V2 by the way).

The assumed temp is then input to the FMC via the CDU and the FMC then calculates the reduced N1 for the given Assumed temp (ambient is entered also). So as you can see all the other factors have been taken into account by the company designing and producing the Tables.

Note: If your lucky enough and your company has paid for the extra these tables may be replaced by the Boeing Laptop Tool (BLT) or as it's now known OPT. Into this laptop you select rnw in use, enter the weather conditions, aircraft weight, flap setting and even any MEL/CDL issues with the aircraft and it gives you assumed temp,N1(so you can check it),V1,Vr,V2.

[Crossunder]

I see. Yes, we have these tables where we find the Tass. The new EFB is due to arrive next year, and we have the equivalent of BLT (from EAG) available in our crew rooms.
But how then, does one calculate the Tass? Step-by-step?

[Todders]

I'm sorry seems i was preaching to the converted as they say what i see now is you know what your talking about and are asking what is the formular used to derive the tables etc.

To be honest i dont know it off the top of my head but i shall dig out my performance stuff and see if i can find something for you.

[Crossunder]

Sweet - appreciate it if you could find the time!

My initial Q was kinda poorly phrased i guess

[mutt]

does one calculate the Tass?

What is the TASS?

Into these tables you put the weight of the aircraft and read off the assumed temp (and also V1,Vr,V2 by the way).

We dont have V-speeds on the takeoff analysis chart (non Airbus), they are obtained from the FMS.

Mutt

[Crossunder]

Tass = Temperature(assumed).

[framer]

Reduced N1 = Full N1 x Square route of (Ambient temp / Assumed temp)

...this from Chris Brady's web page.

This formula does not take into consideration the aircraft mass and available runway - so there HAS to be something more to it. How does the FMC use the excess runway available, and actual take-off mass, to calculate the N1 for take-off?
Does it do a series of calculations or interpolations? Is it really a single formula? What have I missed?

I think I may see the misunderstanding, and this may expose a misunderstanding of my own so please feel free to correct me.
Assume you enter an assumed temp figure of 54 degrees into the FMC, the FMC thinks to itself "hmmmm, what N1 can my engines safely produce at 54 degrees" and comes up with a figure unrelated to weight, location, runway in use, wet/dry, field elevation etc. So really it can be thought of as a power setting. Now to the runway analysis book, someone ages ago sat in a dark room and looked at the runway slope, the obstacles in the take-off path, the TODA, TORA, etc etc etc and decided on what power setting you need for certain temperatures and weights and combinations thereof.
So the assumed temps of 30 degrees through to 60 degrees could be thought of as power setting 1 through to power setting 30. And some poor sod has worked out which power setting you need for that particular runway with that temp and at that weight (of course there are the QNH and wet runway adjustments as well).

That is how I think of it. I have to admit I have pretty much come to these conclusions on my own so if they need adjusting I am open to wise and constructive input

[mutt]

and decided on what power setting you need for certain temperatures and weights and combinations thereof.

Generally, the weight/temperatures on a runway analysis chart are not limited by the permitted 25%-40% assumed thrust reduction. Therefore to obtain thust reduction limit, one has to enter a thrust chart or have the FMS do it.

Mutt

[FCS Explorer]

how de do it

we punch all the rwy,wx and load sheet data into our sneaky little laptop and we get
-derate (22k,24k,26k)
-T ass
-Flap
-V-speeds
-min flap retr. alt

and if you really want it shows u a very basic numeric presentation of just how close the non-normal case would get (feet-wise)

and then you make a nice take-off with 82%N1 and it takes forever to get airborne.....SCARY

[downwindabeam]

I have never flown B737, but if I'm understanding the question correctly the FMC gets it the same way a human would.


1) You go to the MAX N1 (bleeds on/off table) SETTINGS Table.
2) Put OAT or in your case you use the "Assumed Temp" as your OAT.
3) Find the MAX N1 allowable for that temp. So if it's an Assumed Temp you use that temp.

Now, how do you find what's the maximum assumed temp you can do.

You basically have to reverse walk on a Runway limit and Climb limit graph, or airport analysis table as they like to call them these days.

You find your weight maybe add a couple hundered or thousand pounds to allow for some leeway (depending on airplane size). Then trace backwards on the graph or table to account for runway length, pressure altitude and so forth and find out what's the maximum temprature you could be assured of that your airplane will not be limited at your takeoff weight. You use that temp as your assumed temp.


TIP:

If you can get your hands on Section 5 & 6 of some old 737 classic, or 747-100/200 or some 757-200 books from Boeing. They should contain all the graphs (notice I wrote GRAPHS) of Runway limit, climb limits etc. If you trace your finger to figure out the limits everything becomes clear.

[Rainboe]

FCS, you haven't lived! How about a max weight 747-100 with low powered engines, Nairobi, 5,300' elevation, 10,000'+ runway, +30 deg. You want low acceleration! You got it! Full power, end of runway coming up, 55 seconds to lift off, over the end of the concrete at 50' every time. Then juggle flap retraction or climb, but you can't have both. Sometimes neither happens! Your little 737 at 82% still goes up like a space shuttle compared to that

[FCS Explorer]

if you add multiple-derate + T ass on a wet rwy with clearway you will get to see those rwy designator numbers on the opposite side, too.
not funny.
luckily we still get to decide "how much we save" on our own.