Narrative:

On descent into spokane, wa in IFR conditions, we were being vectored for a visual approach runway 21 and cleared to 7000'. I read back 5000' and put 5000' in the altitude alert window. We descended to 5000' with no subsequent transmission from approach control. At approximately 30 NM ene from the geg VOR the ground proximity warning 2500' amber light came on and my attention went to the radar altimeter. The altitude needle started to descend and seconds later a pull up alert was given. We immediately started a climb and the alert stopped. I looked at the radar altimeter again and it once again started down quite rapidly and another 'pull up' was issued. Again we climbed leveling at about 6500' where the alert stopped. I asked approach what our terrain clearance was and his response was climb and maintain 7000'. That was the end of the incident as far as cockpit to ground communications was concerned. On the ground at spokane I called approach and asked for an explanation of what had occurred on descent and was advised the tapes would be pulled and investigated. Not being able to hear the tapes, all I can surmise is somehow I heard 7000' and converted it to 5000' in my mind. What is disconcerting is that 3 people missed my incorrect readback of the clearance, my copilot, the controller who issued it, and his supervisor who was looking over his shoulder during the incident. The encouraging point is the GPWS and my training using the system may have averted a disaster. Supplemental information from acn 127589: on initial contact, the air carrier medium large transport was turned to heading 270 and descended at pilot's discretion to maintain 7000'. The pilot acknowledged, but read back 5000'. The controller's attention was diverted by other aircraft. The aircraft descended to 5300', questioned the terrain clearance. The controller climbed the aircraft to 7000'.

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Original NASA ASRS Text

Title: ACR DESCENT BELOW ASSIGNED ALT. PLT DEVIATION. CTLR FAILED TO HEAR WRONG ALT READBACK. ACFT DESCENDED BELOW MVA. SYSTEM ERROR.

Narrative: ON DESCENT INTO SPOKANE, WA IN IFR CONDITIONS, WE WERE BEING VECTORED FOR A VISUAL APCH RWY 21 AND CLRED TO 7000'. I READ BACK 5000' AND PUT 5000' IN THE ALT ALERT WINDOW. WE DESCENDED TO 5000' WITH NO SUBSEQUENT XMISSION FROM APCH CTL. AT APPROX 30 NM ENE FROM THE GEG VOR THE GND PROX WARNING 2500' AMBER LIGHT CAME ON AND MY ATTN WENT TO THE RADAR ALTIMETER. THE ALTITUDE NEEDLE STARTED TO DSND AND SECONDS LATER A PULL UP ALERT WAS GIVEN. WE IMMEDIATELY STARTED A CLIMB AND THE ALERT STOPPED. I LOOKED AT THE RADAR ALTIMETER AGAIN AND IT ONCE AGAIN STARTED DOWN QUITE RAPIDLY AND ANOTHER 'PULL UP' WAS ISSUED. AGAIN WE CLIMBED LEVELING AT ABOUT 6500' WHERE THE ALERT STOPPED. I ASKED APCH WHAT OUR TERRAIN CLRNC WAS AND HIS RESPONSE WAS CLIMB AND MAINTAIN 7000'. THAT WAS THE END OF THE INCIDENT AS FAR AS COCKPIT TO GND COMS WAS CONCERNED. ON THE GND AT SPOKANE I CALLED APCH AND ASKED FOR AN EXPLANATION OF WHAT HAD OCCURRED ON DSCNT AND WAS ADVISED THE TAPES WOULD BE PULLED AND INVESTIGATED. NOT BEING ABLE TO HEAR THE TAPES, ALL I CAN SURMISE IS SOMEHOW I HEARD 7000' AND CONVERTED IT TO 5000' IN MY MIND. WHAT IS DISCONCERTING IS THAT 3 PEOPLE MISSED MY INCORRECT READBACK OF THE CLRNC, MY COPLT, THE CTLR WHO ISSUED IT, AND HIS SUPVR WHO WAS LOOKING OVER HIS SHOULDER DURING THE INCIDENT. THE ENCOURAGING POINT IS THE GPWS AND MY TRAINING USING THE SYSTEM MAY HAVE AVERTED A DISASTER. SUPPLEMENTAL INFORMATION FROM ACN 127589: ON INITIAL CONTACT, THE ACR MLG WAS TURNED TO HDG 270 AND DESCENDED AT PLT'S DISCRETION TO MAINTAIN 7000'. THE PLT ACKNOWLEDGED, BUT READ BACK 5000'. THE CTLR'S ATTN WAS DIVERTED BY OTHER ACFT. THE ACFT DESCENDED TO 5300', QUESTIONED THE TERRAIN CLRNC. THE CTLR CLIMBED THE ACFT TO 7000'.

Data retrieved from NASA's ASRS site as of August 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.