Narrative:

The flight was the first flight of the day. I was the PF; and the first officer was performing PNF duty. During the pre-departure check; we noticed the flight director flag on the first officer's ADI. We found one yellow capped circuit breaker behind first officer seat had popped. Since it was a yellow circuit breaker; I told first officer to reset it although I was pretty sure I had set that circuit breaker during the initial set up. That reset and cycling FD switches on MCP cleared the flag. All checks were completed; and the flight departed normally. During the climb out; I noticed that the TCAS fail flag appeared on both nds. I asked first officer to check to see if any circuit breaker was popped. We found all cbs were in. I asked first officer to send maintenance message when he got a chance. While curious about this TCAS fail flag; ATC told us that they had lost radar contact. The controller asked us our altitude and position. While first officer made the report; he cycled the xpdr to stby then back to TA/RA. No change; ATC still was not receiving our code. So; I switched the xpdr from left to right. That solved the problem; and ATC had us in contact. However; that was short-lived. Xpdr failed again; and selecting lt or rt had no effect. ATC kept asking us about our position and altitude throughout that time. At this point; I was concerned about separation in rvsm airspace due to ATC not receiving our code; and my pre-departure knowledge of required WX deviation ahead. Thinking about both TCAS fail and xpdr failure; I checked all cbs; and I asked the first officer to double check that all cbs were in place. They were. While we were trying to get our position established with ATC; we leveled off at our initial assigned altitude. Autoplt a was in cmd mode. After seeing altitude acq; I started to talk more about the failed systems with first officer. Shortly after; I noticed the airspeed needle was reaching the barber-pole. I found it strange how the airspeed increased; so I retarded the throttles. However; the airspeed clacker sounded twice then speed was reduced to the command bug speed. During this time; ATC cleared us to climb to FL330. When I commanded VNAV; it seemed to me that autoplt a wasn't responding. This is when I noticed the automatic throttle was disconnected; and wasn't re-engaging. To initiate the climb; and to feel control; I pulled back the yoke which turned cmd to cws mode. And; I advanced the throttles manually to a climb thrust value indicated on my CDU N1 page. We tried to re-engage automatic throttle numerous times; but the outcome was unsuccessful. Autoplt was reengaged; but I was very concerned about the operational integrity of the automatic flight system at this point. And; still we were trying to re-establish our position with ATC after again losing xpdr. Then; while climbing; I noticed another system failure: the GPWS inoperative light on first officer panel was illuminated. I looked around at all other indicators and instruments to see what else we were losing. I believe at this point I asked my first officer to; again; make sure cbs were in correctly? I think this is the time I asked my first officer to get off the radio telephony duty; and turned around to check the entire electrical panel again. Shortly after first officer returned to his radio duty; we heard a rather faint intermittent cabin altitude warning horn. Since I had experienced a false intermittent warning horn in flight last year; I initially thought it was a similar faulty warning. (During that previous case; pressurization was normal and; initiating a descent immediately silenced the horn. Normal flight continued thereafter.) we both looked up at the overhead indicator to check the cabin altitude. It was approaching 10000 ft. Since; my ears were feeling normal; I thought that initiating a descent would solve the problem immediately just as the previous case. So instead of performing qrc; I told first officer to ask ATC for lower altitude; and we were cleared to descend. Contrary to my hope; the descent didn't silence the horn. When I checked the cabin altitude; it was still climbing. So; I ordered first officer to run the qrc; and told him not to rush though it. (Reason behind this was that my first officer during the last proficiency check a few months ago skipped a qrc item because he was rushing; and I didn't want that to happen this time.) we put oxygen masks on and; declared an emergency with ATC. On top of this; during the descent; we were approaching the area of buildups and WX. I was looking straight ahead and saw the need of deviation and turbulent sky conditions. That was the last thing our passenger needed; I thought. I told first officer to ask ATC for a 180 degree turn while we decided where to proceed to. Since I was concerned about the automatic flight system; initially; I flew it manually. Then; the pass oxy light illuminated which confirmed us that the passenger oxygen system had automatically deployed. We both checked that the cabin altitude was above 15000. While we continued to descend; I was concerned about physiological conditions of the passenger; especially the elderly and children. I suspected that they were scared to see all those oxygen masks deployed. So as to not add anymore strange sensation; I elected (due to rather slower loss of pressurization) to continue to descend at around 310 knots pitch attitude instead of steeper vmo. When the throttles were idled; another system had failed. This time; I saw engine master caution. Initially; I suspected a thrust loss but that confirmed that was not the case by checking the thrust integrity by advancing the throttles. We confirmed on the overhead panel that this was due to low idle caution light. I refocused on the emergency descent process. A simple way of describing the cockpit environment was; busy and noisy. It was very difficult to hear my first officer and ATC over the speakers while wearing oxygen masks. First officer was running the checklist; ATC kept asking us questions; and the flight attendant dinging the flight interphone. Of course; the thought process in my own head was creating additional noise also. ATC suggested an airport of diversion; while I was checking blue circled airports on my nd. I wasn't familiar with any of them; and felt that none of them were stations equipped to handle the situation after we landed. So; I asked first officer to tell ATC that until we gathered more information; we requested to be returned to ZZZ. And; I asked first officer to complete the checklist items. He told me that was the case. At least; we knew that the WX and field support capability were best suited for our return. When we leveled off at 10000 ft and slowed down to 250 knots; I checked the cabin altitude. Both first officer and I confirmed that we were at 7000 ft; and the situation was under control; I ordered to take our oxygen masks off. I gave control over to first officer and asked him to start preparing for an arrival and approach into ZZZ while I checked with dispatch and the flight attendants. We checked our landing performance and arrival procedures. Though we were bit annoyed by another ATC (tower controller) asking us of nature of the emergency on the short final; we gave him the information. We suspected that loss of xpdr caused more than usual emergency ATC inquiries due to loss of our radar contact (though; we were informed that the approach controller was receiving our transponder code). Still a bit suspicious of automatic flight system; I made a manual ctrl approach and normal landing.callback conversation with reporter revealed the following information: the reporter stated that the multiple failures resulted in hand flying while conducting an emergency descent in the vicinity of thunderstorms. Dealing with the numerous malfunctions was a challenge while wearing oxygen masks. The source of the numerous anomalies was identified as the air/ground sensing system. Initially; it repeatedly transitioned from the air to the ground mode. When the sensor finally transferred to the ground mode; the aircraft depressurized.

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Original NASA ASRS Text

Title: B737-300 EXPERIENCED MULTIPLE ANOMALIES REPORTEDLY DUE TO AN AIR/GROUND SENSING FAILURE DURING FLT; CULMINATING IN A LOSS OF PRESSURIZATION WITH AN EMER DSCNT AND LNDG.

Narrative: THE FLT WAS THE FIRST FLT OF THE DAY. I WAS THE PF; AND THE FO WAS PERFORMING PNF DUTY. DURING THE PRE-DEP CHK; WE NOTICED THE FLT DIRECTOR FLAG ON THE FO'S ADI. WE FOUND ONE YELLOW CAPPED CB BEHIND FO SEAT HAD POPPED. SINCE IT WAS A YELLOW CB; I TOLD FO TO RESET IT ALTHOUGH I WAS PRETTY SURE I HAD SET THAT CB DURING THE INITIAL SET UP. THAT RESET AND CYCLING FD SWITCHES ON MCP CLEARED THE FLAG. ALL CHKS WERE COMPLETED; AND THE FLT DEPARTED NORMALLY. DURING THE CLBOUT; I NOTICED THAT THE TCAS FAIL FLAG APPEARED ON BOTH NDS. I ASKED FO TO CHK TO SEE IF ANY CB WAS POPPED. WE FOUND ALL CBS WERE IN. I ASKED FO TO SEND MAINT MSG WHEN HE GOT A CHANCE. WHILE CURIOUS ABOUT THIS TCAS FAIL FLAG; ATC TOLD US THAT THEY HAD LOST RADAR CONTACT. THE CTLR ASKED US OUR ALTITUDE AND POSITION. WHILE FO MADE THE RPT; HE CYCLED THE XPDR TO STBY THEN BACK TO TA/RA. NO CHANGE; ATC STILL WAS NOT RECEIVING OUR CODE. SO; I SWITCHED THE XPDR FROM LEFT TO RIGHT. THAT SOLVED THE PROBLEM; AND ATC HAD US IN CONTACT. HOWEVER; THAT WAS SHORT-LIVED. XPDR FAILED AGAIN; AND SELECTING LT OR RT HAD NO EFFECT. ATC KEPT ASKING US ABOUT OUR POSITION AND ALTITUDE THROUGHOUT THAT TIME. AT THIS POINT; I WAS CONCERNED ABOUT SEPARATION IN RVSM AIRSPACE DUE TO ATC NOT RECEIVING OUR CODE; AND MY PRE-DEP KNOWLEDGE OF REQUIRED WX DEV AHEAD. THINKING ABOUT BOTH TCAS FAIL AND XPDR FAILURE; I CHKED ALL CBS; AND I ASKED THE FO TO DOUBLE CHK THAT ALL CBS WERE IN PLACE. THEY WERE. WHILE WE WERE TRYING TO GET OUR POSITION ESTABLISHED WITH ATC; WE LEVELED OFF AT OUR INITIAL ASSIGNED ALTITUDE. AUTOPLT A WAS IN CMD MODE. AFTER SEEING ALT ACQ; I STARTED TO TALK MORE ABOUT THE FAILED SYSTEMS WITH FO. SHORTLY AFTER; I NOTICED THE AIRSPEED NEEDLE WAS REACHING THE BARBER-POLE. I FOUND IT STRANGE HOW THE AIRSPEED INCREASED; SO I RETARDED THE THROTTLES. HOWEVER; THE AIRSPEED CLACKER SOUNDED TWICE THEN SPEED WAS REDUCED TO THE COMMAND BUG SPEED. DURING THIS TIME; ATC CLEARED US TO CLB TO FL330. WHEN I COMMANDED VNAV; IT SEEMED TO ME THAT AUTOPLT A WASN'T RESPONDING. THIS IS WHEN I NOTICED THE AUTO THROTTLE WAS DISCONNECTED; AND WASN'T RE-ENGAGING. TO INITIATE THE CLB; AND TO FEEL CONTROL; I PULLED BACK THE YOKE WHICH TURNED CMD TO CWS MODE. AND; I ADVANCED THE THROTTLES MANUALLY TO A CLB THRUST VALUE INDICATED ON MY CDU N1 PAGE. WE TRIED TO RE-ENGAGE AUTO THROTTLE NUMEROUS TIMES; BUT THE OUTCOME WAS UNSUCCESSFUL. AUTOPLT WAS REENGAGED; BUT I WAS VERY CONCERNED ABOUT THE OPERATIONAL INTEGRITY OF THE AUTO FLT SYSTEM AT THIS POINT. AND; STILL WE WERE TRYING TO RE-ESTABLISH OUR POSITION WITH ATC AFTER AGAIN LOSING XPDR. THEN; WHILE CLBING; I NOTICED ANOTHER SYSTEM FAILURE: THE GPWS INOP LIGHT ON FO PANEL WAS ILLUMINATED. I LOOKED AROUND AT ALL OTHER INDICATORS AND INSTRUMENTS TO SEE WHAT ELSE WE WERE LOSING. I BELIEVE AT THIS POINT I ASKED MY FO TO; AGAIN; MAKE SURE CBS WERE IN CORRECTLY? I THINK THIS IS THE TIME I ASKED MY FO TO GET OFF THE RADIO TELEPHONY DUTY; AND TURNED AROUND TO CHK THE ENTIRE ELECTRICAL PANEL AGAIN. SHORTLY AFTER FO RETURNED TO HIS RADIO DUTY; WE HEARD A RATHER FAINT INTERMITTENT CABIN ALTITUDE WARNING HORN. SINCE I HAD EXPERIENCED A FALSE INTERMITTENT WARNING HORN IN FLT LAST YEAR; I INITIALLY THOUGHT IT WAS A SIMILAR FAULTY WARNING. (DURING THAT PREVIOUS CASE; PRESSURIZATION WAS NORMAL AND; INITIATING A DSCNT IMMEDIATELY SILENCED THE HORN. NORMAL FLT CONTINUED THEREAFTER.) WE BOTH LOOKED UP AT THE OVERHEAD INDICATOR TO CHECK THE CABIN ALTITUDE. IT WAS APCHING 10000 FT. SINCE; MY EARS WERE FEELING NORMAL; I THOUGHT THAT INITIATING A DSCNT WOULD SOLVE THE PROBLEM IMMEDIATELY JUST AS THE PREVIOUS CASE. SO INSTEAD OF PERFORMING QRC; I TOLD FO TO ASK ATC FOR LOWER ALTITUDE; AND WE WERE CLRED TO DSND. CONTRARY TO MY HOPE; THE DSCNT DIDN'T SILENCE THE HORN. WHEN I CHKED THE CABIN ALT; IT WAS STILL CLBING. SO; I ORDERED FO TO RUN THE QRC; AND TOLD HIM NOT TO RUSH THOUGH IT. (REASON BEHIND THIS WAS THAT MY FO DURING THE LAST PROFICIENCY CHECK A FEW MONTHS AGO SKIPPED A QRC ITEM BECAUSE HE WAS RUSHING; AND I DIDN'T WANT THAT TO HAPPEN THIS TIME.) WE PUT OXYGEN MASKS ON AND; DECLARED AN EMER WITH ATC. ON TOP OF THIS; DURING THE DSCNT; WE WERE APCHING THE AREA OF BUILDUPS AND WX. I WAS LOOKING STRAIGHT AHEAD AND SAW THE NEED OF DEV AND TURBULENT SKY CONDITIONS. THAT WAS THE LAST THING OUR PAX NEEDED; I THOUGHT. I TOLD FO TO ASK ATC FOR A 180 DEG TURN WHILE WE DECIDED WHERE TO PROCEED TO. SINCE I WAS CONCERNED ABOUT THE AUTO FLT SYSTEM; INITIALLY; I FLEW IT MANUALLY. THEN; THE PASS OXY LIGHT ILLUMINATED WHICH CONFIRMED US THAT THE PAX OXYGEN SYSTEM HAD AUTOMATICALLY DEPLOYED. WE BOTH CHKED THAT THE CABIN ALT WAS ABOVE 15000. WHILE WE CONTINUED TO DSND; I WAS CONCERNED ABOUT PHYSIOLOGICAL CONDITIONS OF THE PAX; ESPECIALLY THE ELDERLY AND CHILDREN. I SUSPECTED THAT THEY WERE SCARED TO SEE ALL THOSE OXYGEN MASKS DEPLOYED. SO AS TO NOT ADD ANYMORE STRANGE SENSATION; I ELECTED (DUE TO RATHER SLOWER LOSS OF PRESSURIZATION) TO CONTINUE TO DSND AT AROUND 310 KNOTS PITCH ATTITUDE INSTEAD OF STEEPER VMO. WHEN THE THROTTLES WERE IDLED; ANOTHER SYSTEM HAD FAILED. THIS TIME; I SAW ENG MASTER CAUTION. INITIALLY; I SUSPECTED A THRUST LOSS BUT THAT CONFIRMED THAT WAS NOT THE CASE BY CHKING THE THRUST INTEGRITY BY ADVANCING THE THROTTLES. WE CONFIRMED ON THE OVERHEAD PANEL THAT THIS WAS DUE TO LOW IDLE CAUTION LIGHT. I REFOCUSED ON THE EMER DSCNT PROCESS. A SIMPLE WAY OF DESCRIBING THE COCKPIT ENVIRONMENT WAS; BUSY AND NOISY. IT WAS VERY DIFFICULT TO HEAR MY FO AND ATC OVER THE SPEAKERS WHILE WEARING OXYGEN MASKS. FO WAS RUNNING THE CHKLIST; ATC KEPT ASKING US QUESTIONS; AND THE FLT ATTENDANT DINGING THE FLT INTERPHONE. OF COURSE; THE THOUGHT PROCESS IN MY OWN HEAD WAS CREATING ADDITIONAL NOISE ALSO. ATC SUGGESTED AN ARPT OF DIVERSION; WHILE I WAS CHKING BLUE CIRCLED ARPTS ON MY ND. I WASN'T FAMILIAR WITH ANY OF THEM; AND FELT THAT NONE OF THEM WERE STATIONS EQUIPPED TO HANDLE THE SITUATION AFTER WE LANDED. SO; I ASKED FO TO TELL ATC THAT UNTIL WE GATHERED MORE INFORMATION; WE REQUESTED TO BE RETURNED TO ZZZ. AND; I ASKED FO TO COMPLETE THE CHKLIST ITEMS. HE TOLD ME THAT WAS THE CASE. AT LEAST; WE KNEW THAT THE WX AND FIELD SUPPORT CAPABILITY WERE BEST SUITED FOR OUR RETURN. WHEN WE LEVELED OFF AT 10000 FT AND SLOWED DOWN TO 250 KNOTS; I CHKED THE CABIN ALTITUDE. BOTH FO AND I CONFIRMED THAT WE WERE AT 7000 FT; AND THE SITUATION WAS UNDER CONTROL; I ORDERED TO TAKE OUR OXYGEN MASKS OFF. I GAVE CONTROL OVER TO FO AND ASKED HIM TO START PREPARING FOR AN ARR AND APPROACH INTO ZZZ WHILE I CHKED WITH DISPATCH AND THE FLT ATTENDANTS. WE CHKED OUR LNDG PERFORMANCE AND ARR PROCS. THOUGH WE WERE BIT ANNOYED BY ANOTHER ATC (TOWER CTLR) ASKING US OF NATURE OF THE EMER ON THE SHORT FINAL; WE GAVE HIM THE INFORMATION. WE SUSPECTED THAT LOSS OF XPDR CAUSED MORE THAN USUAL EMER ATC INQUIRIES DUE TO LOSS OF OUR RADAR CONTACT (THOUGH; WE WERE INFORMED THAT THE APCH CTLR WAS RECEIVING OUR TRANSPONDER CODE). STILL A BIT SUSPICIOUS OF AUTO FLT SYSTEM; I MADE A MANUAL CTRL APCH AND NORMAL LNDG.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT THE MULTIPLE FAILURES RESULTED IN HAND FLYING WHILE CONDUCTING AN EMER DSCNT IN THE VICINITY OF TSTMS. DEALING WITH THE NUMEROUS MALFUNCTIONS WAS A CHALLENGE WHILE WEARING OXYGEN MASKS. THE SOURCE OF THE NUMEROUS ANOMALIES WAS IDENTIFIED AS THE AIR/GND SENSING SYSTEM. INITIALLY; IT REPEATEDLY TRANSITIONED FROM THE AIR TO THE GND MODE. WHEN THE SENSOR FINALLY TRANSFERRED TO THE GND MODE; THE ACFT DEPRESSURIZED.

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.