Narrative:

During the captain's originating preflight flow the no 2 stall warning system did not test. A call to dispatch and maintenance control resulted in the determination that the no 2 smyd computer had failed. We discussed the applicability of MEL X (stall warning systems) and determined that the aircraft was dispatchable in this condition. Unfortunately; this is not a simple MEL as it demands not only its own logbook entry and placards but also activates nine other MEL item references; each with their own unique placard requirements. To assure this complex MEL was performed correctly I first asked the dispatcher to issue a new release reflecting MEL and its associated counterparts. This enabled the operations agent to print a hard copy of the MEL document. With 16 pages of MEL text in hand; the maintenance controller talked me through each of the individual MEL items to make sure we accurately identified the exact locations and labeling of the circuit breakers to be pulled and placards to be placed. Finally; the first officer and I repeated the entire process a second time to verify that we both agreed with all the steps that had been performed. A full hour after beginning the paperwork we were finally ready to leave the gate. Following pushback and engine start we performed our after start flows. Upon 'recall' of the system annunciator panel we expected that several lights would remain on and; in fact; we noted the following flight control systems were illuminated: mach trim fail; automatic slat fail; and the yaw damper. The first officer asked if the yd light should be on. I recalled from a previous experience with MEL that the yaw damper had been inoperative during that flight so I was not surprised to see the yd light illuminated. I do not recall if I attempted to engage the yd switch since I believed an inoperative yd to be a logical and correct condition for a failed yaw damper computer. At any rate; I stated that I expected the yd light to be on and that the MEL permitted us to takeoff without a yd. Thus; not having experienced this situation in the B737; the first officer had no basis on which to identify my assumption as being wrong. (I also failed to recall that when the yd is failed we normally file for lower altitudes; not the FL380 that was on my flight plan. Had I remembered this; it would have keyed me into our failed yd not being part of the MEL that had just been performed.) to my embarrassment; later in the flight as we reviewed the complete MEL yet another time; I would learn that my previous experience must have been a failure of the no 1 smyd; not the no 2 smyd. This was apparent because the MEL omits the deactivation of the yaw damper if only the no 2 smyd fails. With this new knowledge I attempted to engage the yaw damper switch but was unable to do so. We forwarded this information to dispatch (via ACARS) who confirmed from maintenance that the yd should be operational. (Dispatch performed a recalculation of fuel requirements thus assuring us we had adequate fuel to continue the flight at a lower altitude and provide a more stable ride for the passengers.) in hindsight; however; the fact remains that by not identifying the illuminated yaw damper light as being a condition separate from this specific application of MEL. I allowed the aircraft to depart with an undocumented inoperative main yaw damper. Curiously; upon arrival at destination; I told a mechanic that I had put the yaw damper write up on a separate page because it wasn't included as a reference MEL for a no 2 smyd failure. He commented that it was quite possible the yd failure was related to the no 2 smyd failure because the aircraft alternates which computer (smyd 1 or 2) it uses to feed inputs to the yaw damper. Therefore; if it was no 2's turn to give the input but it wasn't working then of course the yd wouldn't work either. If this is actually the way the yd works in the -700; I could find no mention of it in the flight reference manual; the MEL; or any other company provided document. Perhaps there is either an error in the mechanic's knowledge or there is a big divide between our aircraft systems resources and reality. In either case; although I'm sure I made errors in the preflight documentation paperwork; I'm no longer sure that the failure of the yd was actually unrelated as I had previously thought. Note: a report would have been filed regardless of any other issues purely because my experience with this MEL has shown there are many different and varying methods to interpret what must be deactivated/placarded/collared/etc; thus making a 'perfect' execution of this MEL virtually impossible! (The last time I encountered this MEL I filed a report to report the contradicting interpretations given by different maintenance control personnel leading to the confusion as to what the end result of a 'proper' application of this MEL would yield. I still have no confidence that a fully uniform procedure yet exists.) the MEL is lengthy and cumbersome. If only the part of the MEL applicable to the specific aircraft could be provided (ie; -700 with blended winglets) while all other text was removed it would eliminate a lot of the frustration of having to search for the correct section of the document. 2) a short summary stating what should be seen at the end of applying the MEL would be helpful. For example; '10 MEL stickers; 4 circuit breakers collared; and upon recall; system failure lights for the automatic slats and speed trim.' as it is; it required an entire reproduction of the implementation process to verify what had been done. Even at that; it is apparent we may have failed to identify an illuminated light that was not related to the problem at hand. 3) although it seems contradictory to my charge to simplify the MEL; adding some information about what is not affected by the deactivation process would help identify any unrelated problems. For example; while a failed no 1 smyd also fails the yd; a failed no 2 smyd does not. Had it been clear that I should anticipate a working yd I would have been sure to apply that separate MEL item prior to takeoff. 4) it would eliminate much confusion if there were a clear and consistent explanation of how reference MEL items should be handled. For example; proviso F of MEL X states 'speed brake load alleviation must also be considered inoperative. Refer to (MEL Y speed brake load alleviation system). Open and collar the speed brake autostow circuit breaker on the P6-2 panel per MEL sp #2 - circuit breaker deactivating/safetying.' it is unclear whether the intent is to simply look at MEL Y; say 'hmmmm' then pull the circuit breaker and proceed to the next item in MEL X or whether there should be a full logbook entry implementing MEL Y. (The procedure we followed under the direction of maintenance control was the former although some pilots I talked to who have had the same problem said they were instructed to make an individual logbook entry for each reference MEL.) 5) lastly; it is important to remember that experience does not always prove beneficial. My expectation of seeing the yaw damper light illuminated after engine start led me to believe this was a proper indication so the failure of the yd to engage before takeoff did not seem out of place. Besides; there were already two stickers in close proximity to the yd light so subconsciously I may have considered all the lights in that area of the overhead panel to be related to the MEL we had just implemented and double checked. However; looking back; the MEL would have identified that the inability to engage the yd switch and extinguish the yd light was not an appropriate condition for a failure of only the no 2 smyd.callback conversation with reporter revealed the following information: the reporter believes that the smyd system seems to be reliable. He stated that the MEL concerning this system is certainly not user-friendly; especially for the pilots involved. The documentation included conditions and requirements for all versions of the B737 in the air carrier's fleet. This deferral references other affected systems and their associated MEL's. With the numerous systems affected; this required sifting through the many pages of material.

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Original NASA ASRS Text

Title: FLT CREW CONFUSION RE THE COMPLEX SMYD MEL PROCS MASKED A SEPARATE YAW DAMPER MALFUNCTION.

Narrative: DURING THE CAPTAIN'S ORIGINATING PREFLIGHT FLOW THE NO 2 STALL WARNING SYSTEM DID NOT TEST. A CALL TO DISPATCH AND MAINTENANCE CONTROL RESULTED IN THE DETERMINATION THAT THE NO 2 SMYD COMPUTER HAD FAILED. WE DISCUSSED THE APPLICABILITY OF MEL X (STALL WARNING SYSTEMS) AND DETERMINED THAT THE AIRCRAFT WAS DISPATCHABLE IN THIS CONDITION. UNFORTUNATELY; THIS IS NOT A SIMPLE MEL AS IT DEMANDS NOT ONLY ITS OWN LOGBOOK ENTRY AND PLACARDS BUT ALSO ACTIVATES NINE OTHER MEL ITEM REFERENCES; EACH WITH THEIR OWN UNIQUE PLACARD REQUIREMENTS. TO ASSURE THIS COMPLEX MEL WAS PERFORMED CORRECTLY I FIRST ASKED THE DISPATCHER TO ISSUE A NEW RELEASE REFLECTING MEL AND ITS ASSOCIATED COUNTERPARTS. THIS ENABLED THE OPS AGENT TO PRINT A HARD COPY OF THE MEL DOCUMENT. WITH 16 PAGES OF MEL TEXT IN HAND; THE MAINTENANCE CONTROLLER TALKED ME THROUGH EACH OF THE INDIVIDUAL MEL ITEMS TO MAKE SURE WE ACCURATELY IDENTIFIED THE EXACT LOCATIONS AND LABELING OF THE CIRCUIT BREAKERS TO BE PULLED AND PLACARDS TO BE PLACED. FINALLY; THE FIRST OFFICER AND I REPEATED THE ENTIRE PROCESS A SECOND TIME TO VERIFY THAT WE BOTH AGREED WITH ALL THE STEPS THAT HAD BEEN PERFORMED. A FULL HOUR AFTER BEGINNING THE PAPERWORK WE WERE FINALLY READY TO LEAVE THE GATE. FOLLOWING PUSHBACK AND ENGINE START WE PERFORMED OUR AFTER START FLOWS. UPON 'RECALL' OF THE SYSTEM ANNUNCIATOR PANEL WE EXPECTED THAT SEVERAL LIGHTS WOULD REMAIN ON AND; IN FACT; WE NOTED THE FOLLOWING FLIGHT CONTROL SYSTEMS WERE ILLUMINATED: MACH TRIM FAIL; AUTO SLAT FAIL; AND THE YAW DAMPER. THE FO ASKED IF THE YD LIGHT SHOULD BE ON. I RECALLED FROM A PREVIOUS EXPERIENCE WITH MEL THAT THE YAW DAMPER HAD BEEN INOP DURING THAT FLIGHT SO I WAS NOT SURPRISED TO SEE THE YD LIGHT ILLUMINATED. I DO NOT RECALL IF I ATTEMPTED TO ENGAGE THE YD SWITCH SINCE I BELIEVED AN INOP YD TO BE A LOGICAL AND CORRECT CONDITION FOR A FAILED YAW DAMPER COMPUTER. AT ANY RATE; I STATED THAT I EXPECTED THE YD LIGHT TO BE ON AND THAT THE MEL PERMITTED US TO TAKEOFF WITHOUT A YD. THUS; NOT HAVING EXPERIENCED THIS SITUATION IN THE B737; THE FO HAD NO BASIS ON WHICH TO IDENTIFY MY ASSUMPTION AS BEING WRONG. (I ALSO FAILED TO RECALL THAT WHEN THE YD IS FAILED WE NORMALLY FILE FOR LOWER ALTITUDES; NOT THE FL380 THAT WAS ON MY FLIGHT PLAN. HAD I REMEMBERED THIS; IT WOULD HAVE KEYED ME INTO OUR FAILED YD NOT BEING PART OF THE MEL THAT HAD JUST BEEN PERFORMED.) TO MY EMBARRASSMENT; LATER IN THE FLIGHT AS WE REVIEWED THE COMPLETE MEL YET ANOTHER TIME; I WOULD LEARN THAT MY PREVIOUS EXPERIENCE MUST HAVE BEEN A FAILURE OF THE NO 1 SMYD; NOT THE NO 2 SMYD. THIS WAS APPARENT BECAUSE THE MEL OMITS THE DEACTIVATION OF THE YAW DAMPER IF ONLY THE NO 2 SMYD FAILS. WITH THIS NEW KNOWLEDGE I ATTEMPTED TO ENGAGE THE YAW DAMPER SWITCH BUT WAS UNABLE TO DO SO. WE FORWARDED THIS INFORMATION TO DISPATCH (VIA ACARS) WHO CONFIRMED FROM MAINTENANCE THAT THE YD SHOULD BE OPERATIONAL. (DISPATCH PERFORMED A RECALCULATION OF FUEL REQUIREMENTS THUS ASSURING US WE HAD ADEQUATE FUEL TO CONTINUE THE FLIGHT AT A LOWER ALTITUDE AND PROVIDE A MORE STABLE RIDE FOR THE PASSENGERS.) IN HINDSIGHT; HOWEVER; THE FACT REMAINS THAT BY NOT IDENTIFYING THE ILLUMINATED YAW DAMPER LIGHT AS BEING A CONDITION SEPARATE FROM THIS SPECIFIC APPLICATION OF MEL. I ALLOWED THE AIRCRAFT TO DEPART WITH AN UNDOCUMENTED INOPERATIVE MAIN YAW DAMPER. CURIOUSLY; UPON ARRIVAL AT DEST; I TOLD A MECHANIC THAT I HAD PUT THE YAW DAMPER WRITE UP ON A SEPARATE PAGE BECAUSE IT WASN'T INCLUDED AS A REFERENCE MEL FOR A NO 2 SMYD FAILURE. HE COMMENTED THAT IT WAS QUITE POSSIBLE THE YD FAILURE WAS RELATED TO THE NO 2 SMYD FAILURE BECAUSE THE AIRCRAFT ALTERNATES WHICH COMPUTER (SMYD 1 OR 2) IT USES TO FEED INPUTS TO THE YAW DAMPER. THEREFORE; IF IT WAS NO 2'S TURN TO GIVE THE INPUT BUT IT WASN'T WORKING THEN OF COURSE THE YD WOULDN'T WORK EITHER. IF THIS IS ACTUALLY THE WAY THE YD WORKS IN THE -700; I COULD FIND NO MENTION OF IT IN THE FLIGHT REFERENCE MANUAL; THE MEL; OR ANY OTHER COMPANY PROVIDED DOCUMENT. PERHAPS THERE IS EITHER AN ERROR IN THE MECHANIC'S KNOWLEDGE OR THERE IS A BIG DIVIDE BETWEEN OUR AIRCRAFT SYSTEMS RESOURCES AND REALITY. IN EITHER CASE; ALTHOUGH I'M SURE I MADE ERRORS IN THE PREFLIGHT DOCUMENTATION PAPERWORK; I'M NO LONGER SURE THAT THE FAILURE OF THE YD WAS ACTUALLY UNRELATED AS I HAD PREVIOUSLY THOUGHT. NOTE: A REPORT WOULD HAVE BEEN FILED REGARDLESS OF ANY OTHER ISSUES PURELY BECAUSE MY EXPERIENCE WITH THIS MEL HAS SHOWN THERE ARE MANY DIFFERENT AND VARYING METHODS TO INTERPRET WHAT MUST BE DEACTIVATED/PLACARDED/COLLARED/ETC; THUS MAKING A 'PERFECT' EXECUTION OF THIS MEL VIRTUALLY IMPOSSIBLE! (THE LAST TIME I ENCOUNTERED THIS MEL I FILED A REPORT TO REPORT THE CONTRADICTING INTERPRETATIONS GIVEN BY DIFFERENT MAINTENANCE CONTROL PERSONNEL LEADING TO THE CONFUSION AS TO WHAT THE END RESULT OF A 'PROPER' APPLICATION OF THIS MEL WOULD YIELD. I STILL HAVE NO CONFIDENCE THAT A FULLY UNIFORM PROCEDURE YET EXISTS.) THE MEL IS LENGTHY AND CUMBERSOME. IF ONLY THE PART OF THE MEL APPLICABLE TO THE SPECIFIC AIRCRAFT COULD BE PROVIDED (IE; -700 WITH BLENDED WINGLETS) WHILE ALL OTHER TEXT WAS REMOVED IT WOULD ELIMINATE A LOT OF THE FRUSTRATION OF HAVING TO SEARCH FOR THE CORRECT SECTION OF THE DOCUMENT. 2) A SHORT SUMMARY STATING WHAT SHOULD BE SEEN AT THE END OF APPLYING THE MEL WOULD BE HELPFUL. FOR EXAMPLE; '10 MEL STICKERS; 4 CIRCUIT BREAKERS COLLARED; AND UPON RECALL; SYSTEM FAILURE LIGHTS FOR THE AUTO SLATS AND SPEED TRIM.' AS IT IS; IT REQUIRED AN ENTIRE REPRODUCTION OF THE IMPLEMENTATION PROCESS TO VERIFY WHAT HAD BEEN DONE. EVEN AT THAT; IT IS APPARENT WE MAY HAVE FAILED TO IDENTIFY AN ILLUMINATED LIGHT THAT WAS NOT RELATED TO THE PROBLEM AT HAND. 3) ALTHOUGH IT SEEMS CONTRADICTORY TO MY CHARGE TO SIMPLIFY THE MEL; ADDING SOME INFORMATION ABOUT WHAT IS NOT AFFECTED BY THE DEACTIVATION PROCESS WOULD HELP IDENTIFY ANY UNRELATED PROBLEMS. FOR EXAMPLE; WHILE A FAILED NO 1 SMYD ALSO FAILS THE YD; A FAILED NO 2 SMYD DOES NOT. HAD IT BEEN CLEAR THAT I SHOULD ANTICIPATE A WORKING YD I WOULD HAVE BEEN SURE TO APPLY THAT SEPARATE MEL ITEM PRIOR TO TAKEOFF. 4) IT WOULD ELIMINATE MUCH CONFUSION IF THERE WERE A CLEAR AND CONSISTENT EXPLANATION OF HOW REFERENCE MEL ITEMS SHOULD BE HANDLED. FOR EXAMPLE; PROVISO F OF MEL X STATES 'SPEED BRAKE LOAD ALLEVIATION MUST ALSO BE CONSIDERED INOPERATIVE. REFER TO (MEL Y SPEED BRAKE LOAD ALLEVIATION SYSTEM). OPEN AND COLLAR THE SPEED BRAKE AUTOSTOW CIRCUIT BREAKER ON THE P6-2 PANEL PER MEL SP #2 - CIRCUIT BREAKER DEACTIVATING/SAFETYING.' IT IS UNCLEAR WHETHER THE INTENT IS TO SIMPLY LOOK AT MEL Y; SAY 'HMMMM' THEN PULL THE CIRCUIT BREAKER AND PROCEED TO THE NEXT ITEM IN MEL X OR WHETHER THERE SHOULD BE A FULL LOGBOOK ENTRY IMPLEMENTING MEL Y. (THE PROCEDURE WE FOLLOWED UNDER THE DIRECTION OF MAINTENANCE CONTROL WAS THE FORMER ALTHOUGH SOME PILOTS I TALKED TO WHO HAVE HAD THE SAME PROBLEM SAID THEY WERE INSTRUCTED TO MAKE AN INDIVIDUAL LOGBOOK ENTRY FOR EACH REFERENCE MEL.) 5) LASTLY; IT IS IMPORTANT TO REMEMBER THAT EXPERIENCE DOES NOT ALWAYS PROVE BENEFICIAL. MY EXPECTATION OF SEEING THE YAW DAMPER LIGHT ILLUMINATED AFTER ENGINE START LED ME TO BELIEVE THIS WAS A PROPER INDICATION SO THE FAILURE OF THE YD TO ENGAGE BEFORE TAKEOFF DID NOT SEEM OUT OF PLACE. BESIDES; THERE WERE ALREADY TWO STICKERS IN CLOSE PROXIMITY TO THE YD LIGHT SO SUBCONSCIOUSLY I MAY HAVE CONSIDERED ALL THE LIGHTS IN THAT AREA OF THE OVERHEAD PANEL TO BE RELATED TO THE MEL WE HAD JUST IMPLEMENTED AND DOUBLE CHECKED. HOWEVER; LOOKING BACK; THE MEL WOULD HAVE IDENTIFIED THAT THE INABILITY TO ENGAGE THE YD SWITCH AND EXTINGUISH THE YD LIGHT WAS NOT AN APPROPRIATE CONDITION FOR A FAILURE OF ONLY THE NO 2 SMYD.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE REPORTER BELIEVES THAT THE SMYD SYSTEM SEEMS TO BE RELIABLE. HE STATED THAT THE MEL CONCERNING THIS SYSTEM IS CERTAINLY NOT USER-FRIENDLY; ESPECIALLY FOR THE PLTS INVOLVED. THE DOCUMENTATION INCLUDED CONDITIONS AND REQUIREMENTS FOR ALL VERSIONS OF THE B737 IN THE AIR CARRIER'S FLEET. THIS DEFERRAL REFERENCES OTHER AFFECTED SYSTEMS AND THEIR ASSOCIATED MEL'S. WITH THE NUMEROUS SYSTEMS AFFECTED; THIS REQUIRED SIFTING THROUGH THE MANY PAGES OF MATERIAL.

Data retrieved from NASA's ASRS site as of May 2009 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.