Depressurization critical point

[Cheju]

Hi, I'm just about to start my training on long haul operations and I wasn't 100% sure about the concept of "depressurization critical point" - what's the concept of it? And why do we have that on flight plan?

My "limited"understanding is lets say.. U hav got depress'n crit point between A(company designated ERA) and B(destination) and company added mandatory fuel because if you have depress'n at or after crit point and you divert to that ERA, it require more fuel than continuing destination. I mean..is this the concept of the depress crit point? Ie greater fuel/time requirement beyond crit point etc? Similar to ETP concept in some way?

Second question is, if you require diversion to random(other than company specified: no mand fuel) ERA following depress'n event, how would you estimate fuel required by manipulating FMC? Ie put fl140 or 10000 ft all the way? I heard there is a simple formula can be used for quick estimation: ie use prog page 3,put ERA destn, get the dist, then dist X3 plus 3? Or something like that..can anyone help? Im on -400 and Appreciate any comments. cheers

[RAT 5]

I'm not 100% sure, but we had such a point, identified differently on a 2 engined Boeing. It was the point, where if you had a depressurisation at CRZ FL you could make an emergency descent to 10,000' within the specified pax 02 duration time, considering the MSA. This had an effect on easterly routes in the region of the high mountains of Asia. The routing had to be adjusted to allow this, or increased pax 02 had to be fitted. THis was a planning operation and in the end we did not fly the route. If this is the correct answer, and you are on a 4 engine a/c, I wonder at its relevance.

Regarding fuel after a descent you should be able to insert new cruise FL in the FMC and read calculated arrival fuel at diversion or destination. Either that or good old fashioned time v fuel flow.

[FE Hoppy]

Are we talking ETOPS/EROPS here?

If so it's the most critical ETP with regard to the de-presurisation AEO. It's the point from which your ADD fuel is calculated.