Light rain or heavy rain?

747 press
 

you guys .....i dont beleive that you actually fly them ...its simple you push the throttles forward and the aircraft moves forward and the packs feed air into the cabin the outflow vales move closed untill there is a pressure increase in the cabin and its all auto after that untill something goes wrong ....right fishey i know you will tell me if its not

Weight of air in 747 cabin
 

I was intrigued by this, so I ran some numbers through excel.

At sea level, temperature 27 deg C (300K), density of air is 1.18kg/m3 so with a cabin volume of 885.9m3 the mass is 1042kg. The weight of this air is balanced by the weight of air it displaces (Archimedes) which means that there is no effect on the overall weight of the plane. It is however an extra ton of mass that has to be accelerated to take-off speed, over and above what the aircraft would weigh on a weighbridge.

At 2000m, the air density is 1.006kg/m3 and the mass of a cabinful of the outside air falls to 891kg. The air temperature at this altitude is 2 degrees C, so assuming the cabin is heated to 300K and at the same pressure, the air inside is less dense than that outside. In fact its mass is only 817kg, so the aircraft is actually 74kg lighter at this point.

At 10000m, the density falls to 0.417kg/m3 and the OAT is -50 deg.C. The air displaced weighs only 365kg, the pressurised air inside (2000m, 27 deg.C) still weighs 817kg. Although the weight of air in the cabin is less than at sea level, the air displaced weighs even less. So the wings have to produce an additional 452kg of lift - nearly half a ton - to keep all that air in the air, so to speak.

I hope the beancounters at a certain low-cost airline aren't reading this...

fizz57 well done :) that has to be the clearest answer ever.

THANKS PJ

Not sure when the Classic does it!!!
 

Muduckace The Classic usually takes off with the packs OFF. Packs introduced after airborne by the old bloke who no longer gets a position in the cockpit. Always set the cabin pressure controller to 1000' above cruise level, to reduce the chnces of getting an overpressure, and ideally the cabin would be fully depressurised by touch-down. All manual, no "electronic magic" involved, just the guy whose presence allowed the MEL to be a lot thicker than it is without him being there.

Fizz57' answer.
 

Brilliant. One of the best, clearest and most interesting answers I have ever read. :) Well done sir. So interesting in fact, that have save it on my lappie so I can remember the details - someone, somewhere will ask.:8

Roger.

That must be the best first post ever on PPRuNe. :) Thanks fizz from Malta!

U dont pump air in, you flow it through.
 

H3dxb , sorry to disillusion you, but the air is both "pumped" in and "flowed" through. Pumped in from the engine bleeds and flow controlled by the outflow valves. Outflow is controlled to maintain the required differential pressure such as to have the selected cabin altitude, and at the same time not exceed max permissible differential pressure. Safety valves are fitted to relieve the pressure in the event of a malfunction. Also, the cabin altitude will be determined by the level of pressurisation. Sea level can be maintained up until Max Differential is reached, however it is usual for the cabin altitude to increase at a rate of around 500'/min in the climb. Likewise if the cabin altitude is at, say 6000' in the cruise, it will gradually decrease at a lesser rate than the aircraft descends so that it is at destination airfield altitude for arrival. Ideally, a cabin altitude decrease of around 300'/min is desired to minimize passenger discomfort.

Prompted by Fizz57's excellent analysis, me and the bath have just worked out that I occupy around 0.09 cubic meters. So stick several hundred of me in Fizz57's 747, we'd only displace 30 cubic meters of air or so, affecting Fizz57's calcs by only 3.5%.