Originally Posted by
Noknoipobin
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?