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Based on this question ("What are the 'parts' of a SID procedure?"), I would like to ask for clarification concerning the role of the Common Route in a SID.

As far as I have understood, the most important part of the SID (the "runway transition" as it is called in the ARINC424 specification manual) is visualized with bold lines on the related charts. During this phase, the aircraft performs an obligatory climb with a minimum climb gradient of 200 ft/NM, unless otherwise specified. The remaning routes (common route and enroute transitions) are visualized with thinner lines and their respective Minimum Enroute Altitudes are also depicted.

What I find confusing is that the Common Route is sometimes visualized with bold lines and without Minimum Enroute Altitudes and other times with thinner lines and with their respective MEAs.

An example can be found if one compares FINZZ2 with EPKEE:

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Is there a reason for having the Common Route sometimes as part of the Runway Transition (without MEAs) and other times as part of the Enroute Transitions (with MEAs)?

EDIT:

Τhe same distinction also applies to STARs, where common route can either be part of enroute or runway transitions. An example can be found here and here (OHSEA TWO Arrival, KSNA). Notice that common route is visualized with bold lines, same as runway transitions. However, most of the arrival procedures, as far as I can tell, use it as part of the enroute phase (containing MEA, drawn with thinner lines etc.).

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    $\begingroup$ Obstacle or terrain clearance probably? $\endgroup$
    – Michael Hall
    Aug 23, 2019 at 17:11
  • $\begingroup$ @MichaelHall Thank you for your input. It seems reasonable but in this case why not just end the Runway Transition at the waypoint which provides the desired obstacle clearance, instead of renaming the remaining path as "common route"? There is already a common path which is part of the runway transition (APUUU TO EPKEE). Why ending runway transition at EPKEE and deciding that the rest will be the Common Route? I know I am probably overthinking this, but I am really curious if there is any reason behind it. Notice that there is no way of coding this distinction with ARINC424. $\endgroup$
    – Vector Zita
    Aug 24, 2019 at 16:05
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    $\begingroup$ Is it possible that what you are observing is simply normal "process variation" where different designers working on different areas to a common design guideline just come up with a slightly different way of doing things? If either way of depicting this is permissible then does it really matter? $\endgroup$
    – Michael Hall
    Aug 29, 2019 at 21:43
  • $\begingroup$ What airports are these SIDs from? Your links are going to break every time the FAA makes a website change. $\endgroup$
    – Max Power
    Oct 2, 2022 at 3:08

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This took some digging. There seems to be some mixing of the terms in the question, using different publications, mixing between STARS and SIDs, and maybe neglecting to read the textual route descriptions. The following is only about SIDs.

SIDs, unlike obstacle departure procedures, are primarily developed to assist ATC with traffic management. They get you up and away from other airport traffic.

There are two types of DPs; those developed to assist pilots in obstruction avoidance (referred to as ODP) and those developed to communicate air traffic control (ATC) clearances (referred to as SID). FAA 8260.46j

According to the FAA chart legend, FAA chart user guide, and FAA charts. The thick lines in the FAA charts are the "departure route" the thin lines are the "transition routes".

There is no "common route" or "runway transition" for a SID these terms are only used for STARs (searching 8260.3E)

Transition routes are not part of the SID, they are engineered much like mainline en-route airways. (V J T Q routes)

The SID ends where the bold lines end, which is also where the detailed textual description ends. Normally this ending fix is also the name of the SID and it is the generic transition fix from which you can file to some other en-route fix in your flight plan such as FINZZ3.FINZZ ALBAS V25 MZB

EPKEE7 (and EPKEE6) seems to be an exception, but I found this in 8260.46j

Design SIDs to terminate at a fix/NAVAID depicted on an IFR en route chart, at an altitude that will allow random IFR flight, or at a position and altitude where ATC radar service is provided.

Which likely fits EPKEE Along with

When ATC requests an altitude restriction for a fix located on a transition route, it must be at or above the specified minimum en route altitude (MEA) for the route (see note in paragraph 2-1-1.e(2)(b)). Do not raise an MEA to support ATC operational requirements; use fix crossing altitudes where operationally needed.

There are also some guidelines on how departures should be worded without excess complexity, to reduce pilot workload. The textual description of EPKEE7 says to maintain fl230 on transitions unless filed lower, so the SID section from EPKEE to POIZN is a way of ensuring at least 15000 between EPKEE and POIZN for ATC purposes. (APUUU to POIZN gives clearance over some V routes)

The OROCA for the EPKEE-POIZN is 9100 with several victor MEAs of 7000, so making this a transition route would end up with a low MEA. Also the transition routes after POIZN go over a corner of an MOA. So it is apparently just a matter of altitude control.

Especially as this is a jet-only RNAV SID and the only practical time those transitions would really be used is with a radio failure.

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