Narrative:

I thought I understood descend via clearances. I have read info in publications put out by company; but the reality is that I did not understand the final altitude cleared to. The descend via at another airport I fly to regularly (sao paulo) has published altitude constraints all the way to the FAF. This is not the case at MMMX; and I was mistaken when I thought 12000 (the MEA) was a valid final target altitude for the descend via clearance. The communication with ATC was: mexico center: abc 123 cleared to descend via the datul 2A; expect ILS 05R. Next frequency (10 minutes later); and after passing slm: mexico approach: abc 123; you have descended into military airspace without authorization. You have descended below 13000. Would you like abc 123 to climb to 13000 feet? No; that is not necessary. Further information: 1) we received no altitude clearance with the descend via clearance; so we selected 12000 on the MCP before vacating our cruise flight level. Twelve thousand is the MEA for the slm-smo leg; but I have just learned from aim 5-4-1 that only published restrictions are applicable to descend via clearances; not MEA's. MEA's are published on the chart; but are not considered published restrictions. Slm is published as between 16000 & 13000; so I should have remained at 13000 until further cleared by approach.2) we passed slm with no further instructions from approach control. We continued below 13;000 to 12000; the altitude I thought we were cleared to with the descend via clearance. This altitude is appropriate to prepare for the ILS 05R. However; after reading the aim on descend via clearances; I should have remained at 13000 until cleared to a lower altitude. Had I been unsure; I would have asked ATC if I was cleared to 12000. 3) there is no altitude depicted for smo; only the 12000 MEA for the slm-smo leg. It is now evident to me that this is not considered a published altitude; but only an informational altitude.4) if there is an altitude restriction after slm; why not put that on the chart? As it is; there are no published restrictions after slm. Why not finish the puzzle by publishing exactly what they want?5) if there is a military airfield near slm with controlled airspace; could they put that on the area chart? Perhaps that may have given me a clue that there is a reason not to descend below 13000 (I'm not sure if that would have gotten my attention; but I'm surprised it is not on the commercial area chart).6) we normally fly the VNAV path; but ATC gave us two airspeed reductions that resulted in the captain using speed brakes. He went below the VNAV path; and I eventually asked him if he meant to do that (we were still well above the minimum altitude for slm). He retracted the speed brake; and shallowed out our descent. Had we been on the VNAV path; we would not have been that low; and I would not be writing this report. However; I also would not have learned that I misunderstood the final altitude cleared to on a descend via STAR when ATC does not give an altitude (maybe they should give an altitude?).aim 5-4-1 a. 2.2. Pilots navigating on STAR/RNAV STAR/fmsp procedures shall maintain last assigned altitude until receiving authorization to descend so as to comply with all published/issued restrictions. This authorization will contain the phraseology descend via. (A) clearance to descend via authorizes pilots to: (1) vertically and laterally navigate on a STAR/RNAV STAR/fmsp. (2) when cleared to a waypoint depicted on a STAR/RNAV STAR/fmsp; to descend from a previously assigned altitude at pilot's discretion to the altitude depicted for that waypoint; and once established on the depicted arrival; to navigate laterally and vertically to meet all published restrictions. Note-1. Air traffic is responsible for obstacle clearance when issuing a descend via instruction to the pilot. The descend via is used in conjunction with stars/RNAV stars/fmsps to reduce phraseology by not requiring the controller to restate the altitude at the next waypoint/fix to which the pilot has been cleared. 2. Air traffic will assign an altitude to cross the waypoint/fix; if no altitude is depicted at the waypoint/fix; for aircraft on a direct routing to a STAR/RNAV STAR/fmsp. 3. Minimum en route altitudes (MEA) are not considered restrictions; however; pilots are expected to remain above meas.preventive measures: I think this was just me. I did not know that I was not fully knowledgeable about descend via clearances. I thought I knew what I was doing; but I was wrong. The controller did not seem too concerned because there was no other traffic in the area. However; I pride myself on airspace and chart procedures; and I missed the rule for final altitudes on descend via clearances. I have been asked to publish an article for my company about this. They have also asked me to review company manuals on descend via clearances to see if it needs to be re-worded.

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Original NASA ASRS Text

Title: A B757 First Officer discovered during the DATUL 2A arrival to MMMX that he was not fully versed in all the nuances of a descend via clearance; along with the flying pilot. This resulted in a descent below the final published altitude.

Narrative: I thought I understood descend via clearances. I have read info in publications put out by company; but the reality is that I did not understand the final altitude cleared to. The descend via at another airport I fly to regularly (Sao Paulo) has published altitude constraints all the way to the FAF. This is not the case at MMMX; and I was mistaken when I thought 12000 (the MEA) was a valid final target altitude for the descend via clearance. The communication with ATC was: Mexico Center: ABC 123 cleared to descend via the DATUL 2A; expect ILS 05R. Next frequency (10 minutes later); and after passing SLM: Mexico Approach: ABC 123; you have descended into military airspace without authorization. You have descended below 13000. Would you like ABC 123 to climb to 13000 feet? No; that is not necessary. Further information: 1) We received no altitude clearance with the descend via clearance; so we selected 12000 on the MCP before vacating our cruise flight level. Twelve thousand is the MEA for the SLM-SMO leg; but I have just learned from AIM 5-4-1 that only published restrictions are applicable to descend via clearances; not MEA's. MEA's are published on the chart; but are not considered published restrictions. SLM is published as between 16000 & 13000; so I should have remained at 13000 until further cleared by approach.2) We passed SLM with no further instructions from Approach Control. We continued below 13;000 to 12000; the altitude I thought we were cleared to with the descend via clearance. This altitude is appropriate to prepare for the ILS 05R. However; after reading the AIM on Descend via clearances; I should have remained at 13000 until cleared to a lower altitude. Had I been unsure; I would have asked ATC if I was cleared to 12000. 3) There is no altitude depicted for SMO; only the 12000 MEA for the SLM-SMO leg. It is now evident to me that this is not considered a published altitude; but only an informational altitude.4) If there is an altitude restriction after SLM; why not put that on the chart? As it is; there are no published restrictions after SLM. Why not finish the puzzle by publishing exactly what they want?5) If there is a military airfield near SLM with controlled airspace; could they put that on the area chart? Perhaps that may have given me a clue that there is a reason not to descend below 13000 (I'm not sure if that would have gotten my attention; but I'm surprised it is not on the Commercial Area Chart).6) We normally fly the VNAV path; but ATC gave us two airspeed reductions that resulted in the Captain using speed brakes. He went below the VNAV Path; and I eventually asked him if he meant to do that (we were still well above the minimum altitude for SLM). He retracted the speed brake; and shallowed out our descent. Had we been on the VNAV path; we would not have been that low; and I would not be writing this report. However; I also would not have learned that I misunderstood the final altitude cleared to on a descend via STAR when ATC does not give an altitude (maybe they should give an altitude?).AIM 5-4-1 a. 2.2. Pilots navigating on STAR/RNAV STAR/FMSP procedures shall maintain last assigned altitude until receiving authorization to descend so as to comply with all published/issued restrictions. This authorization will contain the phraseology DESCEND VIA. (a) Clearance to descend via authorizes pilots to: (1) Vertically and laterally navigate on a STAR/RNAV STAR/FMSP. (2) When cleared to a waypoint depicted on a STAR/RNAV STAR/FMSP; to descend from a previously assigned altitude at pilot's discretion to the altitude depicted for that waypoint; and once established on the depicted arrival; to navigate laterally and vertically to meet all published restrictions. NOTE-1. Air traffic is responsible for obstacle clearance when issuing a descend via instruction to the pilot. The descend via is used in conjunction with STARs/RNAV STARs/FMSPs to reduce phraseology by not requiring the controller to restate the altitude at the next waypoint/fix to which the pilot has been cleared. 2. Air traffic will assign an altitude to cross the waypoint/fix; if no altitude is depicted at the waypoint/fix; for aircraft on a direct routing to a STAR/RNAV STAR/FMSP. 3. Minimum en route altitudes (MEA) are not considered restrictions; however; pilots are expected to remain above MEAs.Preventive Measures: I think this was just me. I did not know that I was not fully knowledgeable about descend via clearances. I thought I knew what I was doing; but I was wrong. The Controller did not seem too concerned because there was no other traffic in the area. However; I pride myself on airspace and chart procedures; and I missed the rule for final altitudes on descend via clearances. I have been asked to publish an article for my company about this. They have also asked me to review company manuals on descend via clearances to see if it needs to be re-worded.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.