Narrative:

Takeoff from iah runway 15L; climbing via MMUGS1 RNAV SID captain checked in with houston departure control; advised climbing to 4;000 ft. The controller then cleared us to climb via MMUGS1; except climb to 16;000 ft. The captain read back cleared to climb via MMUGS1; except climb to 16;000 ft. After passing ttaps [and climbing through] 4;000 ft simultaneously the aircraft kept climbing [so as] to cross botll at or below 5;000 ft [but] I stopped the climb at 4;400 ft. Due to a TCAS TA for an aircraft on a 6;000 ft downwind for runway 27 at iah. Simultaneously the controller asked the captain to verify that we were climbing to 5;000 ft. The captain advised him we were cleared to 16;000 ft. The controller said yes via the MMUGS1. The captain replied in the affirmative; yes via the MMUGS1. We were still at 4;400 ft because of the TCAS TA. I had the autopilot engaged and LNAV and VNAV engaged. Autopilot was disengaged momentarily for the TCAS TA to stop climb at 4;400 ft so as not to get the TCAS RA which would have been a more aggressive maneuver. The aircraft crossed ttaps at 4;000 ft and was climbing for botll at 5;000 ft or below on the MMUGS1 RNAV SID.; based on the aircraft's FMC's navigational system.[our company's] B737-400's navigational system uses a single IRU as the primary navigational positioning platform with auto tuned DME-DME or VOR-DME or VOR-VOR position updating; all of which gives the FMC the capability of being within 1/3 to 1/2 a mile of accuracy. [It is] not GPS accurate! The FMC cannot auto tune and update until after takeoff. Even then it sometimes can't update due to external or internal factors; or finding a viable VOR and/or DME signal to auto tune. I have personally seen the FMC off by 10 NM on 2 hour trips; because it was not updating properly. The FAA does not allow this type of navigational system to fly an RNAV approach to a runway using it for lateral and vertical guidance because it is not accurate enough; but thinks it is accurate enough to fly lateral and vertical guidance for a precise RNAV SID that has vertical criteria to meet; with other RNAV sids off other close airports flying 1;000 ft. Above or below your precise altitude. Precise altitudes at three dimensional waypoints are only as good as the lateral precision your navigational system gives you![we should] not let navigational systems that use only one IRU with ground receiving updating (i.e.: no GPS updating or even 2 or more irus that talk to each other) fly precision three dimensional RNAV sids.

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Original NASA ASRS Text

Title: Confusion and disagreement reigned with respect to appropriate altitudes between ATC and the flight crew of a B737-400 assigned the MMUGS RNAV SID from IAH departing Runway 15L. A single IRU based FMS was cited as a major contributing factor.

Narrative: Takeoff from IAH Runway 15L; climbing via MMUGS1 RNAV SID Captain checked in with Houston Departure Control; advised climbing to 4;000 FT. The Controller then cleared us to climb via MMUGS1; except climb to 16;000 FT. The Captain read back cleared to climb via MMUGS1; except climb to 16;000 FT. After passing TTAPS [and climbing through] 4;000 FT simultaneously the aircraft kept climbing [so as] to cross BOTLL at or below 5;000 FT [but] I stopped the climb at 4;400 FT. due to a TCAS TA for an aircraft on a 6;000 FT downwind for Runway 27 at IAH. Simultaneously the Controller asked the Captain to verify that we were climbing to 5;000 FT. The Captain advised him we were cleared to 16;000 FT. The Controller said yes via the MMUGS1. The Captain replied in the affirmative; yes via the MMUGS1. We were still at 4;400 FT because of the TCAS TA. I had the autopilot engaged and LNAV and VNAV engaged. Autopilot was disengaged momentarily for the TCAS TA to stop climb at 4;400 FT so as not to get the TCAS RA which would have been a more aggressive maneuver. The aircraft crossed TTAPS at 4;000 FT and was climbing for BOTLL at 5;000 FT or below on the MMUGS1 RNAV SID.; based on the aircraft's FMC's navigational system.[Our company's] B737-400's navigational system uses a single IRU as the primary navigational positioning platform with auto tuned DME-DME or VOR-DME or VOR-VOR position updating; all of which gives the FMC the capability of being within 1/3 to 1/2 a mile of accuracy. [It is] not GPS accurate! The FMC cannot auto tune and update until after takeoff. Even then it sometimes can't update due to external or internal factors; or finding a viable VOR and/or DME signal to auto tune. I have personally seen the FMC off by 10 NM on 2 hour trips; because it was not updating properly. The FAA does not allow this type of navigational system to fly an RNAV approach to a runway using it for lateral and vertical guidance because it is not accurate enough; but thinks it is accurate enough to fly lateral and vertical guidance for a precise RNAV SID that has vertical criteria to meet; with other RNAV SIDs off other close airports flying 1;000 ft. above or below your precise altitude. Precise altitudes at three dimensional waypoints are only as good as the lateral precision your navigational system gives you![We should] not let navigational systems that use only one IRU with ground receiving updating (i.e.: no GPS updating or even 2 or more IRUs that talk to each other) fly precision three dimensional RNAV SIDs.

Data retrieved from NASA's ASRS site 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.