1
$\begingroup$

I am studying ARINC 424 file format, and I have noticed that there is not any reserved space for coding Minimum Enroute Altitude (MEA), as far as Instrument Flight Procedures are concerned.

I find this value to be really important for properly specifying the characteristics of a procedure. It seems strange that in ARINC 424 specification, MEA is omitted. I have also checked FAA's Coded Instrument Flight Procedures file (CIFP), which are based on ARINC 424, and MEAs are nowhere to be found.

Perhaps I am missing something? Could someone point me in the right direction?

$\endgroup$
1
$\begingroup$

It's in the Altitude/Minimum Altitude data field:

5.30 Altitude/Minimum Altitude

Definition/Description: The Altitude/Minimum Altitude field indicates the reference altitude associated with (1) Enroute Airways (MEA, MFA or other minimum altitudes as defined by source), (2) holding pattern path of Holding Pattern record, (3) altitudes at fixes in terminal procedures and terminal procedure path termination defined by the Path Terminator in the Airport or Heliport SID/STAR/Approach Record and (4) lowest altitude of the blocked altitudes for a Preferred Route.
...
On Airport and Heliport SID, STAR and Approach Route records, the first Altitude field will contain an altitude when Altitude Description field contains a plus (+), a minus (-), or one of the following characters: B, G, or V. The second Altitude field will contain an altitude when the Altitude Description field contains one of the following characters: B, C, D, G, or V. In approach procedure coding, some fix Altitudes may be below sea level, in the case of altitudes at runway fixes when the runway threshold elevation is below sea level. In these cases, the Altitude will be expressed in feet with a minus (-) sign in the first character of the five-character field, see examples.

In 4.1.9.1 SID/STAR/Approach Primary Records you will find columns 85-89 and 90-94 are the two altitude fields referenced in 5.30.

The characters referenced in 5.30 are defined in:

5.29 Altitude Description (ALT DESC)

Definition/Description: The Altitude Description field will designate whether a waypoint should be crossed at, at or above, at or below or at or above to at or below specified altitudes. The field is also used to designate recommended altitudes and cases where two distinct altitudes are provided at a single fix.

| improve this answer | |
$\endgroup$
  • $\begingroup$ Thank you Gerry for the provided quotes from the ARINC-424's specification manual. However, isn't the aforementioned field used to describe the altitude constraints imposed on a waypoint (e.g. "CROSS LARRY AT 2500 OR ABOVE"), as far as flight procedures are concerned? Isn't this different from MEA, which applies to the actual segment? $\endgroup$ – Vector Zita Jul 3 '19 at 16:17
  • 1
    $\begingroup$ This supports both concepts. All data is tied to waypoints; e.g., an airway is defined by a list of waypoints. As stated in the first definition in 5.30 "...(1) Enroute Airways (MEA..." You should also note the 5.29 descriptor where it addresses "cases where two distinct altitudes are provided at a single fix." Thus it can define the MEA as the minimum crossing altitude based on the direction crossing the fix. $\endgroup$ – Gerry Jul 3 '19 at 16:53
  • $\begingroup$ Thank you, I really appreciate your thoughtful answers. I understand it more clearly now, however I wonder what will happen if there are both an altitude constraint at a specific waypoint and minimum enroute altitudes assigned to its adjacent legs? Take HAWKZ7, where one can see PTERA "AT/ABOVE FL220" but at the same time MEA "10000" assigned to both adjacent legs (and additionally MOCA values, not even mentioned in ARINC manual...). Altitude data field would be populated by FL220. Which field MEA would populate? $\endgroup$ – Vector Zita Jul 8 '19 at 15:19
  • 1
    $\begingroup$ First, your link it HAWKZ7 is incorrect. Having found it, I can see your question. The issue is that you're looking at the 'altitude field' for PTERA as it is used in the STAR. The 10000 ft MEA is a moot issue for the STAR. You would need to find the Airway record for PTERA. The airway definition, not part of a procedure, should have the actual MEA listed in the altitude field. $\endgroup$ – Gerry Jul 8 '19 at 16:23
  • 1
    $\begingroup$ I don't have an answer to that. I'm a database user, not a developer. I suspect what you're looking for is in the Aeronav 'master' database. As a terminal waypoint, the MEA is a factor for designing the procedures, so it obviously exists. Since the waypoint isn't intended for use outside of the procedure, the MEA doesn't appear as part of the procedure in the published database as the procedure altitude takes precedence. $\endgroup$ – Gerry Jul 8 '19 at 17:13
1
$\begingroup$

As the name suggests, MEA is coded for enroute segments lik ethe airways (ARINC permits this). In so far as Instrument Approaches are concerned, the templates takes into account vertical clearances in Primary and secondary areas and then arrives at safe heights at each segment. Geometry of primary and secondary areas and the clearance therein depends on types of approach. For example, in ILS approach, half full scale deviation is considered safe. Also, it stems from this, obstacle clearance at different segments of the approach would be differen, so, specifying a minimum altitude may not be applicable. Secondly, in emergency, Minimum Sector altitude (MSA) centered 25 nm around airport/nav aid provides requisite obstacle clearance..

| improve this answer | |
$\endgroup$
  • 1
    $\begingroup$ This comment does not answer the question. $\endgroup$ – Juan Jimenez Jun 28 '19 at 7:17
  • 2
    $\begingroup$ Thank you Sandeep for your contribution and welcome to aviation stack exchange! Although you have made some valid points concerning the definition of Minimum Enroute Altitude, still in most SID, STAR or IAP procedures one can clearly see that MEA values are important. Take, for example, this STAR, this SID and this IAP from JFK Airport. Even intermediate segment has a coded MEA (1900). $\endgroup$ – Vector Zita Jun 28 '19 at 8:18

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.