Originally Posted by
flybywire380
For the 737NG, it appears that using 10 minute take-off thrust time limit compared against extended second segment with 5 minutes thrust gives the same limiting performance weight.
No it does not. It depends mostly on obstacle clearance and only in some cases. The difference is to be able to use the thrust for more than 5 minutes, ever so much a factor when flying in mountainous regions where obstacle clearance is key. Think about a very heavy aircraft taking off towards a range of high terrain, no option to perform an airborne turn thus required to climb above the terrain. By not requiring the aircraft to reduce to climb power much later (compared to same high MFRA) and while maintaining throughout this climb the takeoff weight can be increased for the same profile.
Originally Posted by
flybywire380
why would one pay for the 10 minute AFM entry, when you could use extended second segment instead?
As per above, to facilitate higher takeoff weights for same conditions in predominantly high terrain airports which need to clear these obstacles where the duration of full climb thrust is key to clearing the obstacles.
Originally Posted by
flybywire380
Could it be to do with the requirement to have 1.2% gradient capability above 400ftAAL?
No, more so with the requirement to clear terrain.
BTW: be careful when performing these calculations unless your tools are correct. Being performance files should be used for these calculations are very specific. It is not always (easily) possible to enable calculations by simply changing a setting (e.g. 5-10 minutes) unless the provided file from Being allows you to compare both.