Last night, having already made 2 successful flights, I decided to casually fly from one airport to another a couple of hundred miles away after closing down the default state, and doing an APU start-up.

Assumed fuel was what the default state had onboard.

But climbing to FL350, the plane in AP,on Heading Track,on manual V/S, all of a sudden,decided to descend at 21000ft.

Any reason why it should do this? Just for future reference as all previous as per CDU prepped flights have so far been perfect except for a black overhead screen when trying night flights

If you finished a flight, the fuel remaining at the end of your previous flight would be all you have for your next flight. Even loading a different 757 III model will not reset the fuel, so you need to redo the fuel after completing a flight.

I'm not sure if, by what you are saying, you think that changing/resetting a panel state will reset the fuel. It doesn't. It only resets the panel state.

If none of therse is the case, I have no idea why it tumbled out of the sky. This has never happened to me. I have stalled after takeoff though, but that was because of pilot (me) error!

Markoz wrote on Jan 15^th, 2019 at 1:18am:

If you finished a flight, the fuel remaining at the end of your previous flight would be all you have for your next flight. Even loading a different 757 III model will not reset the fuel, so you need to redo the fuel after completing a flight. I'm not sure if, by what you are saying, you think that changing/resetting a panel state will reset the fuel. It doesn't. It only resets the panel state. If none of therse is the case, I have no idea why it tumbled out of the sky. This has never happened to me. I have stalled after takeoff though, but that was because of pilot (me) error!

Let me clarify. It was not a flight carrying on from a flight just ended. It was a fresh flight that loads the default 'engines running, ready to fly mode.

All I did was shut the engines down power up by the EP and then run a normal flight except without going through all the CDU set up rigmarole just for fun on a very short flight.

I did not need any LNAV or VNAV because I already had a map to follow whatever Heading I chose.

And everything was running fine, except when climbing to around 20000ft, all of a sudden the engines cut out and the aircraft just plummeted out of the sky.