supplementary B737
 

according the supplementary :
"Automatic Pressurization Control – Landing Airport Elevation
Above 6000 Feet"

why the reason ? and why 6000ft ?
there is a thread talking about it ?:ugh:

tks

nobody knows the answer ? :sad::{

Hello,


I'll take a stab at it. Why 6000 feet? It is a nice round number that is close enough to your land altitude that it will have time to correct during descent. Why is there a Supplementary procedure like this? It is for passenger comfort.

Let's imagine a flight from JFK to MEX. During cruise at lower altitudes (roughly FL340 for example), the automatic controller will maintain a cabin altitude just below the LAND ALT until about 8.35 psid, then climb the cabin above the LAND ALT to maintain 8.35psid. Therefore, while cruising at FL340 and FL360, the passengers in my example will breathe easier (more oxygen density at 6000' than 7300') for about 5 hours by using this supplementary procedure.


I hope that helps.

PCars

tks very much ;)

Pcars, I am afraid I do not get it. And I read it twice.

Allow me to rephrase the second paragraph:


On a flight from anywhere, to any destination where the airport elevation is above 6000 feet; we will set 6000 in the LAND ALT window. At initial descent or approximately 20 minutes prior to landing, we will set the real, actual destination airport elevation in the LAND ALT window. There is a cruise altitude (I can't name it exactly but my estimate is FL360) above which the auto pressurization controller reaches 8.35 psid and it must increase the cabin altitude on schedule which will mean the cabin altitude goes above 6000'. Therefore, until the cruising altitude exceeds FL360 or the LAND ALT window is reset for top-of-descent, the cabin will be 6000' and the passengers will be breathing easier.

Put another way, on a flight from JFK to MEX (MEX field elevation 7300'), with a cruising altitude of FL360, we could have the cabin pressure at 6000' instead of 7300' for 5 hours with no cost to the operation. Those with marginal cardio-vascular health will benefit most, but all will feel better during the flight.



Pcars

I agree with PCars...

The SP says to set 6,000 feet in the LAND ALT window till either TOD or around 20 minutes prior.

That is to keep the cabin altitude to 6,000 for as long as possible before setting the actual landing altitude in the window, and having the cabin go to a possibly uncomfortable altitude for the pax.

Also, (as it says), if the cabin altitude should go above 9,000 feet (due to the setting of the LAND ALT), it introduces other problems for the crew for longer than necessary, like use of oxygen when the cabin is above 10,000 feet.

That's my take on it...

EW73

I am getting your point. Thanks.

if u set the true landing alt at the selector. the airplane will do the math and find out the differencial pressure between FLT ALT and LANDING ALT(the one on the selector) and compare with the pressure here:
below FL280 7.35
280-360 7.8
above 360 8.35
and if the if the actual diff is lower than the one according to the cruzing level.the airplane will set the cabin alt abit lower than the figure on LANDING ALT SELECTOR so she only withstand a lower diff pressure .
the airplane is pretty smart:)
but higher cabin alt means less oxygen to us.so we played the trick and fooled the airplane by set a number lower than the real one .

this can explain why 6000 not the real landing alitude .