Narrative:

The aircraft was then on the ground at mli while the 2 passenger and crew cleared customs and the aircraft was refueled with 500 gals jet. The aircraft was on the ground approximately 20-30 mins. The WX remained the same...+9 degrees C -- light rain. We departed mli and we cleared 'on course' direct to pia as filed. We climbed to FL330 without incident. Flight conditions during the climb were IMC until about FL250 (as I recall) with no visible icing encountered. As we approached the cold front the turbulence steadily increased until the 'ride' was becoming somewhat uncomfortable. Were informed by ZID that FL370 was reported as 'occasional light chop.' we requested FL370 as a cruise altitude, and were told to anticipate that clearance pending coordination by ATC. At that time the aircraft pressurization controller was reset by the reporter for FL370 and a positive rate of climb of the cabin was noted of 400-600 FPM. In about 60-90 seconds we received clearance to FL370. The altitude select indicator was reset to 37000 ft and en route climb power was set. A visual check of the windshield wiper arms showed frost had formed on the back side of them, but no ice was seen on the leading edge of them. At approximately 35500 ft the PF said 'there goes my airspeed.' I checked my airspeed indicator and noted it was indicating 210-220 KTS. I glanced at his on the left side and saw it read zero KTS with a red instrument failure 'flag.' also 'flagged' was his altimeter and vertical speed indicator. We agreed this was a failure of the 'captain's air data computer,' but the 'copilot's air data computer' appeared to be working normally. PF said 'I'm going to xfer the autoplt' and selected autoplt xfer. I noted the autoplt controller on my side of the cockpit was configured for altitude-select only and therefore pushed the FMS button to also couple the navigation system. As I was monitoring the autoplt annunciator I noticed the FMS couple and then the altitude-select couple. As I looked back to the flight instruments I noticed we were still climbing through 37200 ft and our rate of climb was about 1500-2000 FPM. As the altitude approached 37500 ft I noticed the 'master warning' annunciator light was flashing, the 'cabin altitude' annunciator was on and a warning horn was sounding. Control of the aircraft was reestablished and a descent back to FL370 was initiated. I then donned the oxygen mask and noted, at some point, the cabin altitude was 10000-11000 ft with a positive rate of climb of the cabin indicated. At that point the PF said something to the effect of 'we better get lower' and started an emergency descent. I then noticed him trying to put his oxygen mask on but was having trouble getting it on over his headset. After 'fumbling' with the radio I remembered to push the 'mask' button on the radio control panel, requested and was immediately given clearance to 15000 ft or 13000 ft. As we descended I noticed a second warning horn that was sounding intermittently. I recognized this as being an 'aircraft overspd' warning and I visually checked my airspeed indicator and observed we were in excess of the mmo indicator. I attempted to tell the PF but since we were both wearing the oxygen masks we found verbal communications were all but lost. During the descent we noted the 'cabin altitude' annunciator extinguished and the captain's air data computer came back on the line. After we leveled off we decided to go to springfield, il, (spi) for landing. Upon landing at spi an inspection of the aircraft was performed by the maintenance personnel. The aircraft pressurization system and air data computer system were found to both be normal. The pitot/static plumbing system was drained and 'only 1 or 2 drops of water' were found. The maintenance personnel stated to me 'that should not have been enough water to cause a problem.' as of this writing the problem remains a mystery, but it is my understanding that the air data computer and pressurization system share a common static port and the static system may have been blocked by a water or ice buildup. We normally (read that always) fly with headsets. Pilots also wear sunglasses, necessary from a comfort and health standpoint. While trying to put on his oxygen mask I noticed they both were in the way of the PF. The term 'quick donning' oxygen mask is, in my opinion, right in the same league as jumbo shrimp, military intelligence and fast food. We found another problem...no 'hot' intercom. Verbal communication between flight crew members had virtually been removed. We kept the headsets in place to hear ATC but could not hear each other...talk about poor crew coordination. Although we practice the emergency descent procedures in the simulator, we do so without wearing headsets or sunglasses. We will address this issue. We encountered 3 system abnormalities which my initial reaction was 'they can't be related' but seemed to be all happening at once. The air data computer failure resulted in using the 'autoplt xfer.' we do not normally use this as the aircraft is flown virtually 99% of the time from the left seat. When the master warning horn and cabin altitude activated, my initial reaction was one of disbelief, puzzlement and anxiety. I knew something was wrong, it seemed to be getting worse, I really had no idea what was causing it, and we were at FL370. The recent learjet accident in south dakota came to mind. Recurrent training focused my thinking as to the potential for hypoxia. I believe our decision to make an emergency descent was a valid one, but, in hindsight, also think perhaps we began to rush our actions when it may not have been necessary to do so. The donning of the oxygen masks resulted in very poor crew coordination at a time when it was, perhaps, most needed. It was a situation I had never thought of.

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Original NASA ASRS Text

Title: A HAWKER HS125-700 FLC EXPERIENCES A LOSS OF CABIN PRESSURE AFTER LOSING THEIR #1 AIR DATA COMPUTER AND HAVE TO DIVERT TO SPI, IN.

Narrative: THE ACFT WAS THEN ON THE GND AT MLI WHILE THE 2 PAX AND CREW CLRED CUSTOMS AND THE ACFT WAS REFUELED WITH 500 GALS JET. THE ACFT WAS ON THE GND APPROX 20-30 MINS. THE WX REMAINED THE SAME...+9 DEGS C -- LIGHT RAIN. WE DEPARTED MLI AND WE CLRED 'ON COURSE' DIRECT TO PIA AS FILED. WE CLBED TO FL330 WITHOUT INCIDENT. FLT CONDITIONS DURING THE CLB WERE IMC UNTIL ABOUT FL250 (AS I RECALL) WITH NO VISIBLE ICING ENCOUNTERED. AS WE APCHED THE COLD FRONT THE TURB STEADILY INCREASED UNTIL THE 'RIDE' WAS BECOMING SOMEWHAT UNCOMFORTABLE. WERE INFORMED BY ZID THAT FL370 WAS RPTED AS 'OCCASIONAL LIGHT CHOP.' WE REQUESTED FL370 AS A CRUISE ALT, AND WERE TOLD TO ANTICIPATE THAT CLRNC PENDING COORD BY ATC. AT THAT TIME THE ACFT PRESSURIZATION CTLR WAS RESET BY THE RPTR FOR FL370 AND A POSITIVE RATE OF CLB OF THE CABIN WAS NOTED OF 400-600 FPM. IN ABOUT 60-90 SECONDS WE RECEIVED CLRNC TO FL370. THE ALT SELECT INDICATOR WAS RESET TO 37000 FT AND ENRTE CLB PWR WAS SET. A VISUAL CHK OF THE WINDSHIELD WIPER ARMS SHOWED FROST HAD FORMED ON THE BACK SIDE OF THEM, BUT NO ICE WAS SEEN ON THE LEADING EDGE OF THEM. AT APPROX 35500 FT THE PF SAID 'THERE GOES MY AIRSPD.' I CHKED MY AIRSPD INDICATOR AND NOTED IT WAS INDICATING 210-220 KTS. I GLANCED AT HIS ON THE L SIDE AND SAW IT READ ZERO KTS WITH A RED INST FAILURE 'FLAG.' ALSO 'FLAGGED' WAS HIS ALTIMETER AND VERT SPD INDICATOR. WE AGREED THIS WAS A FAILURE OF THE 'CAPT'S AIR DATA COMPUTER,' BUT THE 'COPLT'S ADC' APPEARED TO BE WORKING NORMALLY. PF SAID 'I'M GOING TO XFER THE AUTOPLT' AND SELECTED AUTOPLT XFER. I NOTED THE AUTOPLT CTLR ON MY SIDE OF THE COCKPIT WAS CONFIGURED FOR ALT-SELECT ONLY AND THEREFORE PUSHED THE FMS BUTTON TO ALSO COUPLE THE NAV SYS. AS I WAS MONITORING THE AUTOPLT ANNUNCIATOR I NOTICED THE FMS COUPLE AND THEN THE ALT-SELECT COUPLE. AS I LOOKED BACK TO THE FLT INSTS I NOTICED WE WERE STILL CLBING THROUGH 37200 FT AND OUR RATE OF CLB WAS ABOUT 1500-2000 FPM. AS THE ALT APCHED 37500 FT I NOTICED THE 'MASTER WARNING' ANNUNCIATOR LIGHT WAS FLASHING, THE 'CABIN ALT' ANNUNCIATOR WAS ON AND A WARNING HORN WAS SOUNDING. CTL OF THE ACFT WAS REESTABLISHED AND A DSCNT BACK TO FL370 WAS INITIATED. I THEN DONNED THE OXYGEN MASK AND NOTED, AT SOME POINT, THE CABIN ALT WAS 10000-11000 FT WITH A POSITIVE RATE OF CLB OF THE CABIN INDICATED. AT THAT POINT THE PF SAID SOMETHING TO THE EFFECT OF 'WE BETTER GET LOWER' AND STARTED AN EMER DSCNT. I THEN NOTICED HIM TRYING TO PUT HIS OXYGEN MASK ON BUT WAS HAVING TROUBLE GETTING IT ON OVER HIS HEADSET. AFTER 'FUMBLING' WITH THE RADIO I REMEMBERED TO PUSH THE 'MASK' BUTTON ON THE RADIO CTL PANEL, REQUESTED AND WAS IMMEDIATELY GIVEN CLRNC TO 15000 FT OR 13000 FT. AS WE DSNDED I NOTICED A SECOND WARNING HORN THAT WAS SOUNDING INTERMITTENTLY. I RECOGNIZED THIS AS BEING AN 'ACFT OVERSPD' WARNING AND I VISUALLY CHKED MY AIRSPD INDICATOR AND OBSERVED WE WERE IN EXCESS OF THE MMO INDICATOR. I ATTEMPTED TO TELL THE PF BUT SINCE WE WERE BOTH WEARING THE OXYGEN MASKS WE FOUND VERBAL COMS WERE ALL BUT LOST. DURING THE DSCNT WE NOTED THE 'CABIN ALT' ANNUNCIATOR EXTINGUISHED AND THE CAPT'S ADC CAME BACK ON THE LINE. AFTER WE LEVELED OFF WE DECIDED TO GO TO SPRINGFIELD, IL, (SPI) FOR LNDG. UPON LNDG AT SPI AN INSPECTION OF THE ACFT WAS PERFORMED BY THE MAINT PERSONNEL. THE ACFT PRESSURIZATION SYS AND AIR DATA COMPUTER SYS WERE FOUND TO BOTH BE NORMAL. THE PITOT/STATIC PLUMBING SYS WAS DRAINED AND 'ONLY 1 OR 2 DROPS OF WATER' WERE FOUND. THE MAINT PERSONNEL STATED TO ME 'THAT SHOULD NOT HAVE BEEN ENOUGH WATER TO CAUSE A PROB.' AS OF THIS WRITING THE PROB REMAINS A MYSTERY, BUT IT IS MY UNDERSTANDING THAT THE ADC AND PRESSURIZATION SYS SHARE A COMMON STATIC PORT AND THE STATIC SYS MAY HAVE BEEN BLOCKED BY A WATER OR ICE BUILDUP. WE NORMALLY (READ THAT ALWAYS) FLY WITH HEADSETS. PLTS ALSO WEAR SUNGLASSES, NECESSARY FROM A COMFORT AND HEALTH STANDPOINT. WHILE TRYING TO PUT ON HIS OXYGEN MASK I NOTICED THEY BOTH WERE IN THE WAY OF THE PF. THE TERM 'QUICK DONNING' OXYGEN MASK IS, IN MY OPINION, RIGHT IN THE SAME LEAGUE AS JUMBO SHRIMP, MIL INTELLIGENCE AND FAST FOOD. WE FOUND ANOTHER PROB...NO 'HOT' INTERCOM. VERBAL COM BTWN FLC MEMBERS HAD VIRTUALLY BEEN REMOVED. WE KEPT THE HEADSETS IN PLACE TO HEAR ATC BUT COULD NOT HEAR EACH OTHER...TALK ABOUT POOR CREW COORD. ALTHOUGH WE PRACTICE THE EMER DSCNT PROCS IN THE SIMULATOR, WE DO SO WITHOUT WEARING HEADSETS OR SUNGLASSES. WE WILL ADDRESS THIS ISSUE. WE ENCOUNTERED 3 SYS ABNORMALITIES WHICH MY INITIAL REACTION WAS 'THEY CAN'T BE RELATED' BUT SEEMED TO BE ALL HAPPENING AT ONCE. THE ADC FAILURE RESULTED IN USING THE 'AUTOPLT XFER.' WE DO NOT NORMALLY USE THIS AS THE ACFT IS FLOWN VIRTUALLY 99% OF THE TIME FROM THE L SEAT. WHEN THE MASTER WARNING HORN AND CABIN ALT ACTIVATED, MY INITIAL REACTION WAS ONE OF DISBELIEF, PUZZLEMENT AND ANXIETY. I KNEW SOMETHING WAS WRONG, IT SEEMED TO BE GETTING WORSE, I REALLY HAD NO IDEA WHAT WAS CAUSING IT, AND WE WERE AT FL370. THE RECENT LEARJET ACCIDENT IN SOUTH DAKOTA CAME TO MIND. RECURRENT TRAINING FOCUSED MY THINKING AS TO THE POTENTIAL FOR HYPOXIA. I BELIEVE OUR DECISION TO MAKE AN EMER DSCNT WAS A VALID ONE, BUT, IN HINDSIGHT, ALSO THINK PERHAPS WE BEGAN TO RUSH OUR ACTIONS WHEN IT MAY NOT HAVE BEEN NECESSARY TO DO SO. THE DONNING OF THE OXYGEN MASKS RESULTED IN VERY POOR CREW COORD AT A TIME WHEN IT WAS, PERHAPS, MOST NEEDED. IT WAS A SIT I HAD NEVER THOUGHT OF.

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.