Date: 201406
Local time of day: 0601-1200
Location: IAH.Airport, TX
Flight conditions: VMC
Light: Daylight
Aircraft: B737-400
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class B IAH
ATC / advisory: TRACON I90
Aircraft component: INS / IRS / IRU
Component problem: Design
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Human factors: Communication Breakdown; Distraction
Second reporter function: Captain; Pilot Not Flying
Anomaly: ATC Issue All Types; Aircraft Equipment Problem Critical; Conflict Airborne Conflict; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Aircraft TA; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew FLC Overrode Automation
Contributing factors: Human Factors; Chart Or Publication; Aircraft; Procedure
Primary problem: Ambiguous

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.


Second narrative: After departure we checked in with IAH Departure Control and advised that we were climbing to 4;000 FT. He then cleared us to 16;000 VIA MMUGS1. We read back 'Cleared to 16;000 VIA MMUGS1.' After passing TTAPS Intersection at 4;000; we continued our climb to cross BOTTL at or below 5;000 FT. When we landed; we called the Controller; and he advised us that we were above 5;000 FT north of the TTAPS Intersection on the MMUGS1 departure and that the restriction is at or below 4;000 FT. We explained that this could not be possible as we had stopped our climb per the T/A at 4;400 FT. [Note that] there is no DME backup for the TTAPS Intersection; so all we have is the FMS for [track/fix] identification. The [single IRU driven] FMS is not GPS-accurate. We explained this fact to the Controller; but he said he was submitting the situation report and that we may or may not hear anymore about it.This is a new procedure for Houston Departure. To insure that this does not recur; I suggest that the Departure Controller not issue a clearance above 4;000 FT prior to TTAPS Intersection. Only issue a step climb to 5;000 FT to BOTLL.


Synopsis: 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.