Narrative:

Flight departed san for den. Copilot was flying. Approaching the rockies west of denver, high clouds and rough air were encountered at FL330. I requested a lower altitude from ZDV and the first officer descended to FL290 in the clear. As we proceeded eastbound the clouds were lowering over the mountains and the turbulence remained. I requested lower and we descended to FL240 in the clouds. A short time later, while in the clouds, the ground proximity warning went off with 'terrain/pull up' warnings. I immediately checked the altimeters to verify that we were in fact at FL240, which both altimeters indicated. However, as the terrain warnings continued, I told the copilot to climb. We started climbing. I squawked identify on the transponder (there was radio traffic on center frequency) and the warnings stopped. As we approached FL260, we leveled at FL258, after approximately 15-20 seconds of terrain warning. After the warnings stopped, we both evaluated our position, rechked that the altimeters were correct and everything was normal, and started descending to our assigned altitude of FL240. I called ZDV to tell them of our action. They confirmed our position and altitude and replied that an aircraft had passed below us at FL230 that might have set off the alarm. Apparently no traffic conflict resulted from our actions and ZDV seemed unconcerned, but I was prepared to use my captain's emergency authority if necessary. We broke out of the clouds a short time later, well above all terrain. I believe several things caused my actions/reactions to the terrain warning. One is that we have become very dependent on various warning systems on modern jets. In the flight handbook, the GPWS warning is an emergency procedure requiring an immediate climb if in IMC. Our training center is in denver and these procedures are practiced during our annual check rides. Another reason is that although I 'knew' we were at FL240, it ran through my mind how many times altimeters have been misread, and of accidents caused by misread/misset altimeters. I didn't want to trust my eyes in this situation that required an immediate decision. The copilot said he felt the same way, and that although he 'thought' he was at FL240, he did not want to chance that he had made a mistake and was relieved when told to climb. He had no explanation as to why he did not initiate the climb himself. Why didn't I?

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Original NASA ASRS Text

Title: FLT CREW OF MLG AT FL240 EXPERIENCED GPWS ALERT. CAPT ORDERED CLIMB AND ADVISED ATC. AT FL250 GPWS ALERT WAS SILENCED AND FLT RETURNED TO FL240.

Narrative: FLT DEPARTED SAN FOR DEN. COPLT WAS FLYING. APCHING THE ROCKIES W OF DENVER, HIGH CLOUDS AND ROUGH AIR WERE ENCOUNTERED AT FL330. I REQUESTED A LOWER ALT FROM ZDV AND THE F/O DSNDED TO FL290 IN THE CLR. AS WE PROCEEDED EBND THE CLOUDS WERE LOWERING OVER THE MOUNTAINS AND THE TURB REMAINED. I REQUESTED LOWER AND WE DSNDED TO FL240 IN THE CLOUDS. A SHORT TIME LATER, WHILE IN THE CLOUDS, THE GND PROX WARNING WENT OFF WITH 'TERRAIN/PULL UP' WARNINGS. I IMMEDIATELY CHKED THE ALTIMETERS TO VERIFY THAT WE WERE IN FACT AT FL240, WHICH BOTH ALTIMETERS INDICATED. HOWEVER, AS THE TERRAIN WARNINGS CONTINUED, I TOLD THE COPLT TO CLB. WE STARTED CLBING. I SQUAWKED IDENT ON THE XPONDER (THERE WAS RADIO TFC ON CENTER FREQ) AND THE WARNINGS STOPPED. AS WE APCHED FL260, WE LEVELED AT FL258, AFTER APPROX 15-20 SECS OF TERRAIN WARNING. AFTER THE WARNINGS STOPPED, WE BOTH EVALUATED OUR POS, RECHKED THAT THE ALTIMETERS WERE CORRECT AND EVERYTHING WAS NORMAL, AND STARTED DSNDING TO OUR ASSIGNED ALT OF FL240. I CALLED ZDV TO TELL THEM OF OUR ACTION. THEY CONFIRMED OUR POS AND ALT AND REPLIED THAT AN ACFT HAD PASSED BELOW US AT FL230 THAT MIGHT HAVE SET OFF THE ALARM. APPARENTLY NO TFC CONFLICT RESULTED FROM OUR ACTIONS AND ZDV SEEMED UNCONCERNED, BUT I WAS PREPARED TO USE MY CAPT'S EMER AUTHORITY IF NECESSARY. WE BROKE OUT OF THE CLOUDS A SHORT TIME LATER, WELL ABOVE ALL TERRAIN. I BELIEVE SEVERAL THINGS CAUSED MY ACTIONS/REACTIONS TO THE TERRAIN WARNING. ONE IS THAT WE HAVE BECOME VERY DEPENDENT ON VARIOUS WARNING SYSTEMS ON MODERN JETS. IN THE FLT HANDBOOK, THE GPWS WARNING IS AN EMER PROC REQUIRING AN IMMEDIATE CLB IF IN IMC. OUR TRNING CENTER IS IN DENVER AND THESE PROCS ARE PRACTICED DURING OUR ANNUAL CHK RIDES. ANOTHER REASON IS THAT ALTHOUGH I 'KNEW' WE WERE AT FL240, IT RAN THROUGH MY MIND HOW MANY TIMES ALTIMETERS HAVE BEEN MISREAD, AND OF ACCIDENTS CAUSED BY MISREAD/MISSET ALTIMETERS. I DIDN'T WANT TO TRUST MY EYES IN THIS SITUATION THAT REQUIRED AN IMMEDIATE DECISION. THE COPLT SAID HE FELT THE SAME WAY, AND THAT ALTHOUGH HE 'THOUGHT' HE WAS AT FL240, HE DID NOT WANT TO CHANCE THAT HE HAD MADE A MISTAKE AND WAS RELIEVED WHEN TOLD TO CLB. HE HAD NO EXPLANATION AS TO WHY HE DID NOT INITIATE THE CLB HIMSELF. WHY DIDN'T I?

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.