Depressurization Escape route
 

Hi,
When on a depressurization escape route which is planned on an airway can the plan descend flight level/Altitude be below the MOCA of that airway. Example: a planned escape route states to follow "XYZ" airway to an enroute alternate and descend to FL100. However, the MOCA for "XYZ" airway is 11000ft, GRID MORA is 12400ft and MEA is FL150. ? Thanks

I would say it depends the provider of the escape route, the level of analysis they have done and the navigation accuracy they have assumed during the drift down. Sometimes you can offset an airway if the major obstacles are on the other side of it and get different terrain clearances for a particular RNP.

A DNP route is used to avoid terrain so that the aircraft can descend to 10,000 feet as quickly as possible due to limited oxygen supply.

If following an airway whose MOCA is above 10,000 feet prevents descent to 10,000 feet, the crew will instead select a route that allows descent to 10,000 feet as soon as possible while ensuring the oxygen duration is sufficient.

This might involve continuing ahead, turning back, diverting sideways, or even leaving the airway — as long as the aircraft remains clear of terrain and obstacles.

In the example you mentioned, the aircraft would maintain 11000 feet until it is safe to descend to 10000 or lower altitude.However, if a turn back would take the aircraft into an area where the terrain is higher than the MOCA, the crew might instead use the GRID MORA until rejoining the route.(You cannot descend below the terrain clearance altitude — whether it is the MOCA, GRID MORA, or any other applicable minimum if you are going to fly through that area. Unless your company has a more detailed terrain database and has defined the DNP route based on that information.

The calculation method may vary, but the key requirements are:
• The escape path must ensure terrain and obstacle clearance, and
• The available oxygen must be sufficient for the planned descent and routing.(above 10000)
Also, different aircraft types may have different DNP routes depending on their oxygen duration.

For example, some aircraft that have only 12 minutes of oxygen supply cannot operate on certain routes in the Kabul area, as those routes do not allow descent to a safe altitude(10000) within that oxygen time limit.

If it is possible to descend to 10,000 feet immediately, there is no need for a DNP route.

Thanks for the detailed reply. My company procedure for said route provides an escape procedure on an airway(as mentioned) However, the escape procedure dictates descend to FL100 on the airway but as mentioned the MOCA for said airway is F11000 and later increasing to F12000. Hence wanted to know if there is a legal leeway to descend below MOCA or am I missing something?

If that’s the case, I don’t think it’s legal. My company has had several similar incidents. When we asked where the terrain database came from, it turned out that the person who created the DNP made a mistake, so it had to be corrected. You might want to check with your company — it could be the same situation.

Or maybe they have a more up-to-date database indicating that it’s safe to descend as stated — but if not, that would be quite concerning.

Wouldn’t the minimum altitude for a decompression descent be defined by MEA not MOCA?
Especially in these days of satcom capability.
Grinding the coffee a little fine…..just curious.

Depends. If you fly down to Mexico or Chile crossing los Andes ridge, there are instances where MEA is above FL180 and MORA too. But if you divert to the sides, the MORA will go below FL180 and keep descending to lower levels. This opens the way for us to descent FL180 within 5 minutes, maintain FL180 for another 6-7 minutes and then be able to descend lower.

So, finally spoke with my company's performance engineer. And what he told me is that they plan the escape route based on the Route MORA of the airway or escape route/airway. Since route MORA caters to 10nm either side of the airway, and the route MORA for said airway is all below F100, the descent to F100 can occur. The only catch here is you need to perform the escape maneuver strictly following assigned procedure

Complete to concurr with that.
Between Chile and Bolivia FIR, on the UM688 OTPOP-BOKAP, the MORA is above 180 while the MEA is 125. This gives you a good chance of descending for a depressurization case in this particular area. Otherwise the situation gets tight for many operators.