TO Derate charts and assumed temperature

[severniae]

Hi,

Could someone please point me to the charts that allow one to work out whether a de rated takeoff is permissible and the assumed temperature please?

I see references to them all over but can't find them - I tried buying TOPER but it appears I wasted my money as it only references a 22K engine and the 733 modelled by IXEG is the 20K variant.

Thanks in advance,

James

[Litjan]

Hi James,

I don´t think you can find them on the internet... Before we used computers to calculate those values, we had "Runway Weight Charts" in the cockpit - several charts for every single runway, they were thick folders.

The actual TASS depends on the environmental conditions and the runway lenght plus the obstacle situation - and the path the plane would fly after an engine failure. This was calculated by every airline (depending on their engine failure procedures), so there is no standard chart for a runway.

We had "sample airports" in there, that assumed no obstacles and just different runway lenghts at different altitudes - those could be helpful to give you a ballpark setting.

Mostly the TASS on our aircraft ranged between 35 and 50 degrees - it was rare to do a full power takeoff. When we started using the laptops in the cockpit, the TASS increased by ca. 5C on average, I would say. Just due to better calculation (less conservative).

Cheers, Jan

 

[severniae]

Thanks Jan - I'm indeed looking for the runway charts or an example of them. If I can find something to use as a reference I hope to code up a quick and dirty calculator that will perform a basic calculation.

Do you know anywhere I might be able to purchase/acquire the charts?

[Litjan]

I don´t know - my airline stopped using them about 20 years ago, if I remember correctly... I don´t know where I would look for those...

[severniae]

Hi @Litjan

So I found the chart below in the FPPM for the -300 and it appears to have the data that I suspect the runway weight charts you referenced are based on. 

The trouble is making sense of the tables but I suppose I shall get there...! 

I don't suppose you have any advice for interpreting them? There is a similar chart for landing distance for any given weight, which is quite a bit simpler to interpret - I can't quite figure out the relation between the vertical and horizontal lines on the this chart...

[Litjan]

Yes - this chart will give you the TASS for a flaps 15 takeoff - but only taking into account the runway length, not the obstacle situation.

You read it like this (there is an example-dashed line in the diagram):

1.) Enter on left with airport OAT - go up until you hit the airport pressure altitude (elevation + QNH correction of 30`per hPa). Then go horizontal to right, until you hit the REF LINE.

2.) Now start on right with field length available, go up to first REF LINE, then follow it PARALLEL to the "slanted lines" for runway slope, then go up again to next REF LINE then follow it PARALLEL to the slanted lines for "head/tailwind", then go straight up (make a long line).

Now follow from the final point of "1" PARALLEL to the "slanted lines" until you hit the vertical long line from 2. From that intersection go horizontal to the right to read off the maximum brake release weight.

If you want to find out the TASS, you need to start the process in reserse. Start with 2.) to get the vertical line. Then go from the right (current weight) to the vertical line, followe PARALLE to slanted lines to the REF LINE (vertical), then go left until you hit the airport pressure altitude then go straight down to read the "theoretical" OAT (=TASS) that would still allow a takeoff with your current weight.

Cheers, Jan

 

[severniae]

Thanks Jan for taking the time to explain that to me! Hugely appreciated!

I'm going to give it a run a few times until I'm confident in my calculations and then maybe put together an app or spreadsheet that will give read outs in the ball park.