When I try to mimic some of Singapore Airlines' KEWR or KJFK routes, SimBrief displays a "Flight exceeds aircraft range" or exceeds MTOW, etc. message.  Is there a way to manually tweak anything so I can do these flights with realistic passenger/payload counts?

I belive you may need to wait for the A350ULR which I belive is due out Q2 as a free update.

These flight were operated with normal A350, so it should be possible without the ULR:

Singapore Airlines Flights 23 and 24 - Wikipedia

According to my experience Simbrief sometimes is too pesimistic regarding fuel burn. You can also get this warning for longer real world MAX flights. The only way to plan these flights with SimBrief I am aware is unloading paylod to unrealisticly low levels (like 10pax).

Well there you go. I didn’t realise that they have been operated by the standard A359. Cool to know. 

Simbrief is very conservative with fuel planning. In real life, they probably have some form of redispatching. What you can do is manually tweak the contingency and reserve fuel, as well as change the alternate airport. 

Edited March 29, 20251 yr by V1ROTA7E

1 hour ago, V1ROTA7E said:

they probably have some form of redispatching.

This video from FS Expo last year was very informative on the ways  international dispatching occurs. And also some of the limitations of SImbrief.