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
Last edited by PCars; 18th Sep 2010 at 19:55.