Calculation Cabin ALT

[kampfnudel]

Hy all,

can anyone tell me how i can calculate the best Cabin alt on a Flightlevel? I rework our Checklist, and i want to insert a list for the Pilots where the can see (as example) on FL350 the must have a Cabin alt about 7000ft. I can't find anything in the Documentation. Maybe I'm blind but i can't find anything in Google

Regards

Dirk

Edited May 5, 2016 by kampfnudel

[mgeiss]

There's a plackard below the pressurization panel showing the cabin alt corresponding to the flt. level. But IIRC you only need to set this in manual operation. In AUTO mode you would set it to 200ft below destination field elevation according to my handbook:

 

 

 

Edited May 5, 2016 by mgeiss

[archibit]

So if destination field elevation is 0 then it is right to set cab alt -200?

[lanmancz]

I set the cabin altitude according to the little ruler under the panel (which is there probably for this reason ;-)) - ie. for FL300 the CAB ALT will be 5100. You set the destination elevation into the LAND ALT. CAB ALT is the cabin pressure setting during cruise.

btw I think the CAB ALT setting is relevant only in manual mode. I'm not sure about this but I think that in AUTO mode the cabin pressure setting in controlled automatically according to the current altitude.

Edited May 5, 2016 by lanmancz

[sparkie66]

Put it in manual above 15k, it gets funny then.

Hypoxia

[kampfnudel]

35 minutes ago, lanmancz said:

...btw I think the CAB ALT setting is relevant only in manual mode. I'm not sure about this but I think that in AUTO mode the cabin pressure setting in controlled automatically according to the current altitude....

@ This point im not realy shure. A few days ago we have tested it during a flight. Just for fun i have put in the Cabin alt 13000ft. after few minutes a cabin altert as been start.

but now i have understand the Cabin Pressure. Thanks a lot

Regards

Dirk

[Litjan]

lanmancz is partly right:

You need the little black placard for both manual and standby operation - not for auto.

Flying in manual is an absolute emergency, fallback mode. You want to somehow keep the aircraft pressurized - any movement of the thrust levers will change the influx of air, and you would have to change the position of the outflow valve - try it, it is fully modeled!

In standby you will set the CABIN ALT to departure elevation -200, then switch to FLT. During cruise you set the CAB ALT to the suggest value according to your cruising altitude. When descending, you set the CABIN ALT to landing elevation -200.

This pretty much mimics what the AUTO mode is doing, too.

Jan

 

[Alpha Floor]

Just to complement on what Jan said:

The pressurization panel consists of 3 mode sections and a "master" section where you'll find the FLT/GND switch and the pressurization mode selector. With the mode selector knob you select your pressurization mode and then go to the corresponding section. Each section is labeled and separated with the white line. In AUTO operation you only really "need" to input values into the AUTO section of the panel, that is: FLT ALT and LAND ALT. In MANUAL the only thing you control is the outflow valve directly (must be fun, haha).

Setting the proper CAB ALT into the standby section is good airmanship in case you have to revert to standby, have it already in there. But not "essential" for pressurization to work correctly if only using AUTO.

The minus 200 is to make sure that the aircraft de-pressurizes and the doors can be opened. I guess the slight negative pressure helps in opening the doors (they move into the fuselage first, then out)

3 hours ago, Litjan said:

try it, it is fully modeled!

I definitely will be trying this!

[Litjan]

32 minutes ago, Alpha Floor said:

The minus 200 is to make sure that the aircraft de-pressurizes and the doors can be opened. I guess the slight negative pressure helps in opening the doors (they move into the fuselage first, then out)

 

No, at -200 the airplane is slightly pressurized and the doors and windows can not be opened. Remember - low cabin alt = high cabin pressure

The real reason is that the outflow valve in this condition is "almost closed" (to enable the pressure to build), so no air can rush into it during rotation and hurt the passengers ears. The slight pressure also seats the windows into the frames, so they don´t rattle on takeoff (just a nice side-effect).

Jan

 

[Alpha Floor]

Ah yes, thanks for the correction! I messed the "mental experiment" up in my mind

Also: What I said did't make much sense, there's no such thing as "making sure it de-pressurizes". As long as you open the outflow valve without any engines/packs blowing air into the cabin, the pressures in and out will equal each other. It's simple physics.

18 minutes ago, Litjan said:

The slight pressure also seats the windows into the frames, so they don´t rattle on takeoff

Now the next question would be:

Is this also simulated on the IXEG? Should we try taking-off in AUTO and then taking-off in manual with the valve full open?

 

Edited May 5, 2016 by Alpha Floor

[Litjan]

1 hour ago, Alpha Floor said:

Is this also simulated on the IXEG? Should we try taking-off in AUTO and then taking-off in manual with the valve full open?

 

Please don´t .

By the way, the cabin can never have less pressure than the outside - there are big, flat depressurization panels that will simply "flip in" to make sure that never happens. You can push them in during outside-check to test them.

Jan

 

[Alpha Floor]

2 hours ago, Litjan said:

By the way, the cabin can never have less pressure than the outside - there are big, flat depressurization panels that will simply "flip in" to make sure that never happens. You can push them in during outside-check to test them

Yes, it's a negative pressure relief valve! The structure doesn't like negative pressures... I checked the EASA CS-25 (Certification Specification) and the requirement for pressurization is "the structure must be strong enough to withstand the flight loads combined with pressure differential loads from zero up to the maximum relief valve setting" (25.365). This "maximum relief valve" applies in either direction, positive AND negative.

 

And here are two pictures of these valves. The negative (right) will open at -1.0 psi. Or is it -0.1? Source here

The -200 can be "seen" here on this diagram. The FCOM says the valve closes when the switch is set to FLT. I think in other aircrafts this happens when TO thrust is set, could it be?

Edited May 5, 2016 by Alpha Floor

[kampfnudel]

Wow Thanks guys,

Ok then i know it now for the Future that i only need the Cruise Alt and Landing alt in Auto. I am always surprised by how deep the IXEG is Simulates

Regards

Dirk