Depressurization Escape Routes

[AC560]

No one is supposed to descend below an MEA and if you read something into this as a approval for that, you are wrong.

Original question "How can a pilot legally descend below the MEA to comply with an airplanes rapid depressurization profile?".

What is wrong in my answer. Nobody is supposed to land a plane in the middle of a freeway either, given the appropriate circumstances though one does what one must. The legal clearance that the poster is looking for is 91.3 (b). Whether that decision is wise or not is an entirely different subject.

 

[SuperFLUF]

....How can a pilot legally descend below the MEA to comply with an airplanes rapid depressurization profile? ......

Captain's emergency authority.

 

[pianoman]

I understand the question, but really who cares about the airplane's pressurization schedule if you aren't sure where the terrain is?

The plane will fly fine unpressurized. Maybe the pax take a nap for a bit.

 

[Beetle007]

I understand the question, but really who cares about the airplane's pressurization schedule if you aren't sure where the terrain is?

The plane will fly fine unpressurized. Maybe the pax take a nap for a bit.

I think this issue matters from a flight planning standpoint. As a pilot you must be capable of following the depressurization profile of the aircraft and therefore must know the location of terrain and how low you can descend.

Or, imagine an FAA line check when they ask you "What would you do if you had a rapid decompression right now?"

Things are always easier from a practical standpoint (i.e., emergency authority) than from a regulatory standpoint.

 

[SuperFLUF]

I understand the question, but really who cares about the airplane's pressurization schedule if you aren't sure where the terrain is?

The plane will fly fine unpressurized. Maybe the pax take a nap for a bit.

How long will your crew O2 last? At what altitude? Was your O2 bottle serviced full before you took off or just within limits? It gets awfully cold up there...etc.

What if an engine fails and the engine inop altitude is below the terrain? Better have a plan on where to go before you're dealing with an engine out. Especially if its accompanied by control issues or a fire.

If you've got the low alt charts on board, in most cases that's all you need to find an airway that will get you down and headed to a diversion airport.

 

[Beetle007]

The Airbus crew oxygen system will "guarantee" 13 minutes of diluted oxygen during an emergency descent. You have LESS time if you accidentally keep the oxygen selector in the preflighted position of 100%. 100% will give you 15 minutes at 8,000 ft, so I don't imagine it will be much at 39,000 ft.

That doesn't seem like much time with papers flying around, moisture condensation on your oxygen mask and trying to look at a chart to figure out the location of terrain and how low you can descend.

 

[Spooky 2]

The Airbus crew oxygen system will "guarantee" 13 minutes of diluted oxygen during an emergency descent. You have LESS time if you accidentally keep the oxygen selector in the preflighted position of 100%. 100% will give you 15 minutes at 8,000 ft, so I don't imagine it will be much at 39,000 ft.

That doesn't seem like much time with papers flying around, moisture condensation on your oxygen mask and trying to look at a chart to figure out the location of terrain and how low you can descend.

What am I missing here? Maybe about 3 hours of oxygen for starters. Are you telling me that the Airbus has less than 20 minutes total of oxygen? What if you had smoke and fumes in the cockpit and you were say 180 minutes from landfall or an airport. You should have enough oxygen onboard for the entire flight deck crew whether it be two, three or four pilots to get you safely to an airport. Forgive me if I'm missing the obvious as I'm one of the over 60 guys!

 

[michael707767]

I've heard airlines create depressurization escape routes over mountainous terrain. How can a pilot legally descend below the MEA to comply with an airplanes rapid depressurization profile?

the whole point of a depressurization route is that you don't go below MEA. Don't confusue the Grid MORA (in laymans terms, the lowest altitude within a grid on a chart) with the MEA. The Grid MORA within a certain area can be 14600, but there can be routes within that grid that have an MEA below 14600.

Look at it this way, within a certain area on a chart there could be a 12,000 foot mountain. So the Grid MORA in that area would have to account for that. But at the other end of that grid there could be an airway that goes nowhere near the 12K mountain and thus has an MEA a lot lower.

The point of the depress routes is to get you on a known route and down to the lowest altitude allowed by that route.

 

[Spooky 2]

the whole point of a depressurization route is that you don't go below MEA. Don't confusue the Grid MORA (in laymans terms, the lowest altitude within a grid on a chart) with the MEA. The Grid MORA within a certain area can be 14600, but there can be routes within that grid that have an MEA below 14600.

Look at it this way, within a certain area on a chart there could be a 12,000 foot mountain. So the Grid MORA in that area would have to account for that. But at the other end of that grid there could be an airway that goes nowhere near the 12K mountain and thus has an MEA a lot lower.

The point of the depress routes is to get you on a known route and down to the lowest altitude allowed by that route.

With 12,000' terrain in a grid the Grid Mora alt would be 14,000. 2,000 above in mountainous terrain, 1,000' in all others is the way I believe it's set up. Of course I know you knew that anyway.

 

[Beetle007]

What am I missing here? Maybe about 3 hours of oxygen for starters. Are you telling me that the Airbus has less than 20 minutes total of oxygen?

Im not sure how much oxygen you will have if you stay at cruising altitude, but the manuals says you only have 15 minutes of 100% oxygen at 8,000 ft. If you begin your descent immediately you will have 2 hours of diluted oxygen. The following is a quote from the aircaft limitations section on the Airbus.

Protection after loss of cabin pressure with mask regulator on NORMAL (diluted oxygen):
- During emergency descent: For all cockpit members for 13 minutes
- During cruise at FL 100: For 2 flight crewmembers for 107 minutes​

or​

Protection in case of smoke, with 100 % oxygen: For all cockpit members for 15 minutes at a cabin altitude of 8000 feet.

 

[Beetle007]

Don't confusue the Grid MORA (in laymans terms, the lowest altitude within a grid on a chart) with the MEA.

THe MEA also guarantees Navaid reception and might be well above terrain. When I lose cabin pressure on a GPS/IRS equipped aircraft I don't really care about navaid reception. I think that is why some airlines publish charts with minimum altitudes below the MEA.

 

[michael707767]

THe MEA also guarantees Navaid reception and might be well above terrain. When I lose cabin pressure on a GPS/IRS equipped aircraft I don't really care about navaid reception. I think that is why some airlines publish charts with minimum altitudes below the MEA.

absolutely correct. In that case, your escape route would likely list the MOCA instead of the MEA.

 

[Spooky 2]

The Airbus crew oxygen system will "guarantee" 13 minutes of diluted oxygen during an emergency descent. You have LESS time if you accidentally keep the oxygen selector in the preflighted position of 100%. 100% will give you 15 minutes at 8,000 ft, so I don't imagine it will be much at 39,000 ft.

That doesn't seem like much time with papers flying around, moisture condensation on your oxygen mask and trying to look at a chart to figure out the location of terrain and how low you can descend.

Beetle, looking at 121.333 it appears that a minimum oxygen requirement would be two hours including the time required to descend down to 10,000. So is that at 100%? I'm still doing the research but in any case it is significantly higher than your example. Lets see what else we find out before closing out this post.

 

[Beetle007]

Beetle, looking at 121.333 it appears that a minimum oxygen requirement would be two hours including the time required to descend down to 10,000. So is that at 100%?

My original post was confusing but I corrected myself. On the Airbus you have 13 minutes of oxygen for the emergency descent then 107 minutes at 10,000 ft. This gives you 2 hours but is only diluted oxygen. 100% will not give you 2 hours. And if you stay at a high altitude you won't have 2 hours either.

Another item of trivia, the Airbus will automatically give you 100% at FL350 or above. So if you stay at cruise (Above FL350) while looking at the charts for terrain, you will have significantly less oxygen. I could be wrong on this but I think it is written somewhere because it is in an oral exam guide.

 

[Spooky 2]

Well I suppose it could be that the "authorities" on this subject feel that having 2 hours at 100% would be a moot point since any pax in the rear would have expired withing the first 15 minutes and the airframe would also be consumed within some similar short period of time.

Obviously depending on what size bottles are in you particular aircraft your time of useful consciousness would be less than two hours.

Interestingly, the GV/550 has a seperate bottle for the crew and the pax, however, the crew can use the pax oxygen. Supplemental oxygen for life pax life support above FL250 is not recomended.