Narrative:

I gave a farewell PA and directed flight attendants to secure the cabin while level at 10000 ft between tyssn and suzsi fixes on tyssn STAR. I could not do it earlier because first officer had been off getting gate information from las operations. During my PA, ATC controller said something. I understood only part of transmission. Upon finishing the PA, the first officer said, 'prino at 8000 ft.' I understood that to mean he or ATC controller was directing me to descend now to 8000 ft. I saw 8000 ft in altitude window and only 3 mi from next fix. Our company installs only 1 FMC on first officer side of cockpit and it had been left on a page that shows arrival times/arrival fuel instead of fixes. I started down rapidly in attempt to make prino at 8000 ft. The first officer said, 'wait! You are not at suzsi yet, you are not cleared to descend!' I punched altitude hold and started power back in to climb back up, but I had descended about 300 ft. I regained altitude and nothing was said by controller. Miscom -- first officer was stating a reset altitude, not a cleared altitude. I was unable to hear approach clearance accurately because of company's requirement to secure the cabin at 10000 ft. Lack of proper equipment. In my opinion, the company should not be allowed to use RNAV procedures with only 1 control head to the RNAV data. I was unable to quickly determine which fix was 3 mi away: suzsi or prino, because the fixes/legs page had been replaced by another page. The FAA should require dual head FMC's as boeing sold/certified the plane. Furthermore, the company continues to micro manage PA's and in-range calls instead of adopting the philosophy of allowing the crew to make the 'secure the cabin PA' and 'in range call' when ATC arrival conditions permit. Callback conversation with reporter revealed the following information: reporter states that this air carrier utilizes a variety of different models of B737, utilizing both rudimentary RNAV system with a single CDU and no moving map display and dual FMC with moving maps. Neither system includes VNAV functionality. Flight crews fly either RNAV system at random. There is no SOP protocol regarding the use of the system and flight crews from different backgnds utilize them in different ways at their discretion. The rudimentary system utilizes a single data entry/programming unit that is located at the first officer's position. The programmed data is inputted to a simple flight path deviation device on the instrument panels with a distance to go and path guidance, but there is no means of identing the 'active' fix save by reference to the single data entry unit (CDU). Positional awareness is seriously compromised by the lack of a situation display common to more comprehensive RNAV system. In this case the CDU was engaged in administrative tasks to alert the company as to their expected arrival. The first officer is also out of the loop at this time because the data thus received must be transmitted by radio. Thus, both the data source and the second crew member are not available for air navigation purposes. As a result, the PF guessed incorrectly as to the identify of the active waypoint and initiated a descent based on that assumption.

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Original NASA ASRS Text

Title: FLT CREW OF B737-500 (B737-300 PER FO RPT) ADVISE IMPROPER DSCNT FROM 10000 FT PRIOR TO REACHING SUZSI INTXN ON TYSSN RNAV STAR TO LAS. CITE INADEQUATE ACCESSIBILITY TO RNAV DATA ON AIRBORNE SYS AS A CONTRIBUTING FACTOR.

Narrative: I GAVE A FAREWELL PA AND DIRECTED FLT ATTENDANTS TO SECURE THE CABIN WHILE LEVEL AT 10000 FT BTWN TYSSN AND SUZSI FIXES ON TYSSN STAR. I COULD NOT DO IT EARLIER BECAUSE FO HAD BEEN OFF GETTING GATE INFO FROM LAS OPS. DURING MY PA, ATC CTLR SAID SOMETHING. I UNDERSTOOD ONLY PART OF XMISSION. UPON FINISHING THE PA, THE FO SAID, 'PRINO AT 8000 FT.' I UNDERSTOOD THAT TO MEAN HE OR ATC CTLR WAS DIRECTING ME TO DSND NOW TO 8000 FT. I SAW 8000 FT IN ALT WINDOW AND ONLY 3 MI FROM NEXT FIX. OUR COMPANY INSTALLS ONLY 1 FMC ON FO SIDE OF COCKPIT AND IT HAD BEEN LEFT ON A PAGE THAT SHOWS ARR TIMES/ARR FUEL INSTEAD OF FIXES. I STARTED DOWN RAPIDLY IN ATTEMPT TO MAKE PRINO AT 8000 FT. THE FO SAID, 'WAIT! YOU ARE NOT AT SUZSI YET, YOU ARE NOT CLRED TO DSND!' I PUNCHED ALT HOLD AND STARTED PWR BACK IN TO CLB BACK UP, BUT I HAD DSNDED ABOUT 300 FT. I REGAINED ALT AND NOTHING WAS SAID BY CTLR. MISCOM -- FO WAS STATING A RESET ALT, NOT A CLRED ALT. I WAS UNABLE TO HEAR APCH CLRNC ACCURATELY BECAUSE OF COMPANY'S REQUIREMENT TO SECURE THE CABIN AT 10000 FT. LACK OF PROPER EQUIP. IN MY OPINION, THE COMPANY SHOULD NOT BE ALLOWED TO USE RNAV PROCS WITH ONLY 1 CTL HEAD TO THE RNAV DATA. I WAS UNABLE TO QUICKLY DETERMINE WHICH FIX WAS 3 MI AWAY: SUZSI OR PRINO, BECAUSE THE FIXES/LEGS PAGE HAD BEEN REPLACED BY ANOTHER PAGE. THE FAA SHOULD REQUIRE DUAL HEAD FMC'S AS BOEING SOLD/CERTIFIED THE PLANE. FURTHERMORE, THE COMPANY CONTINUES TO MICRO MANAGE PA'S AND IN-RANGE CALLS INSTEAD OF ADOPTING THE PHILOSOPHY OF ALLOWING THE CREW TO MAKE THE 'SECURE THE CABIN PA' AND 'IN RANGE CALL' WHEN ATC ARR CONDITIONS PERMIT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THAT THIS ACR UTILIZES A VARIETY OF DIFFERENT MODELS OF B737, UTILIZING BOTH RUDIMENTARY RNAV SYS WITH A SINGLE CDU AND NO MOVING MAP DISPLAY AND DUAL FMC WITH MOVING MAPS. NEITHER SYS INCLUDES VNAV FUNCTIONALITY. FLT CREWS FLY EITHER RNAV SYS AT RANDOM. THERE IS NO SOP PROTOCOL REGARDING THE USE OF THE SYS AND FLT CREWS FROM DIFFERENT BACKGNDS UTILIZE THEM IN DIFFERENT WAYS AT THEIR DISCRETION. THE RUDIMENTARY SYS UTILIZES A SINGLE DATA ENTRY/PROGRAMMING UNIT THAT IS LOCATED AT THE FO'S POS. THE PROGRAMMED DATA IS INPUTTED TO A SIMPLE FLT PATH DEV DEVICE ON THE INST PANELS WITH A DISTANCE TO GO AND PATH GUIDANCE, BUT THERE IS NO MEANS OF IDENTING THE 'ACTIVE' FIX SAVE BY REF TO THE SINGLE DATA ENTRY UNIT (CDU). POSITIONAL AWARENESS IS SERIOUSLY COMPROMISED BY THE LACK OF A SIT DISPLAY COMMON TO MORE COMPREHENSIVE RNAV SYS. IN THIS CASE THE CDU WAS ENGAGED IN ADMINISTRATIVE TASKS TO ALERT THE COMPANY AS TO THEIR EXPECTED ARR. THE FO IS ALSO OUT OF THE LOOP AT THIS TIME BECAUSE THE DATA THUS RECEIVED MUST BE XMITTED BY RADIO. THUS, BOTH THE DATA SOURCE AND THE SECOND CREW MEMBER ARE NOT AVAILABLE FOR AIR NAV PURPOSES. AS A RESULT, THE PF GUESSED INCORRECTLY AS TO THE IDENT OF THE ACTIVE WAYPOINT AND INITIATED A DSCNT BASED ON THAT ASSUMPTION.

Data retrieved from NASA's ASRS site as of July 2007 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.