787 weight question

[Just a spotter]

Hi

the Boeing 787 is touted as allowing a higher cabin pressure/lower cabin altitude during flight. Presumably this then means that once pressurised, the aircraft is hauling around more air inside in order to maintain the higher pressures.

So, my question is this; what's the weight penalty for operating this higher pressure/greater volume of air in the 787 cabin and how adversely does that affect fuel burn? Will airlines choose operate their cabins run at "traditional" lower pressures in order to save weight and fuel or is weight more a fuel burn concern during climb more so than cruise?

JAS

[TopBunk]

I'll have a quick stab at an answer.

I am led to believe that the mass of air in a B747 is about 1 tonne - lets assume that this is the case at a normal cruise altitude.

A B787 is at a guess about 2/3 the volume of a B747, so the mass of air inside would be about 660kg at typical pressurisation levels.

Lets further assume that the air is pressurised to say 11psi rather than 9psi in the B747 (all numbers approximate), then this would equate to about 20% more air, so the 660kg would increase to about 800kg, a difference of about 140kg.

This is a negligible amount, and the increase in fuel burn due to it would likely be in the range of a few kilos (less than10) per hour, imho.

[TSR2]

On the subject of the B787 weight, a leading aviation magazine is quoting that Vietnam Airlines is switching its B787 order from the B787-8 to the B787-9 varient due to 'unexpected performance limitations' on the B787-8 and stating that the -8 model is too heavy to economically operate long haul routes to Europe and North America.

[Spooky 2]

Right? I bet that -9 is a lot lighter at 550,000+:confused::confused::confused: Something must have lost in the translation.