G’day,

While experimenting with the perf calculator to work out a pseudo flex temp at KVPS i got some rather strange results.

So off runway 19 full length at 390T i found the max temp i could get without an exceedence weight message was 44C.

Then by increasing the temp to 45C i get a message saying the max take off weight is 359825Kg.

So 1 degree makes a 30175Kg diff in take off weight.

Another 1C to 46 at the limit is now 356884 so just under 3000Kg.

So from 44 to 45 there is a 30 ton change in limit weight but from 45 to 46 its 3 tonne change.

Just wondering what is going on here to cause such a huge change in limit weight of only 1 degree C when other temp changes are in the order of 3000Kg per degree.

Cheers

Darren

 

Darren,

First a tip: If you want to determine the maximum allowed takeoff weight leave the TakeOff Weight field empty. OPT will calculate the max weight and display it in the results area.

Setting the OAT is not the same as using an assumed temperature. The OAT value does not only affect engine thrust. The actual environment OAT also plays a role.

As for the huge change on the weight limit you observed it is probably caused by reaching some limit. Takeoff calculations are much more complicated than simple linear interpolations and multiple limitations are considered: max certified weight, field length limit, climb limit, tire limit, brake energy limit, etc.

Darren,

I checked the data you provided and there is indeed a bug in the displayed max weight message. If you have requested OPTIMUM flaps it will display the minimum weight of flaps 10 and 20 instead of the maximum as it should. This will be fixed in the update that will be available in a few days. 

In any case, the proper way to determine maximum weight is by doing the calculation with an empty TakeOff Weight field. This is not affected by this bug.

Thanks figured something was wrong.

 

Great work on the 747-8!

Edited September 27, 20187 yr by DEHowie