On north downwind vector to OAK ILS runway 11 by approach control, a Boeing 737-300 flight crew read back an incorrect descent altitude resulting in a GPWS alert climb.
Synopsis
On north downwind vector to OAK ILS runway 11 by approach control, a Boeing 737-300 flight crew read back an incorrect descent altitude resulting in a GPWS alert climb.
Narrative
Rewritten in plain English from NASA’s original all-caps telegraphic report — facts, figures, and terminology preserved.
We were being vectored to runway 11 at OAK. We were at 4000 feet on downwind to the ILS to runway 11 northeast of the field. We thought the controller said descent to 2100 feet. We were about abeam the outer marker for the approach.
The first officer read back 2100 feet and I started to descent to 2100 feet. The minimum altitude for that area is about 3800 feet, but we were about to turn toward the final approach fix and we were under his control. At 2100 feet we received a 'terrain, terrain, pull up.' I immediately executed a climb and was climbing through 3000 feet when the controller asked us to confirm that we were at '3100 feet.' We told him that we were leveling off from a climb from 2100 feet and we understood that he had given us a descent to 2100 feet and had read that back to him.
He told us to turn and descent to 2000 feet until localizer intercept and then intercept the glide slope at 1800 feet. Approach and landing was uneventful. This controller talked with a speech impediment and when he spoke fast it was difficult to understand him. Why he didn't correct us when we read back the wrong altitude, I don't know. He was busy. After we had our problem, he did slow his speech down and it was easier to understand. I should have questioned him further on the 2100 feet instruction because it was an odd altitude with that minimum sector altitude.
Callback conversation with reporter revealed the following info: Captain said that he waited a bit too long to challenge the descent clearance, especially knowing that he was heading for higher terrain. By the time it bothered him sufficiently to question ATC, the reporter advised the GPWS was alarming. The other principle problem was the controller's speech rate. It was very fast and appeared to have an impediment. After the controller questioned the flight crew's descent altitude and the captain's challenge response, the controller's speech rate slowed down sufficiently to be understood.
Show the original NASA report (verbatim, telegraphic)
WE WERE BEING VECTORED TO RWY 11 AT OAK. WE WERE AT 4000 FT ON DOWNWIND TO THE ILS TO RWY 11 NE OF THE FIELD. WE THOUGHT THE CTLR SAID DSND TO 2100 FT. WE WERE ABOUT ABEAM THE OM FOR THE APCH. THE FO READ BACK 2100 FT AND I STARTED TO DSND TO 2100 FT. THE MINIMUM ALT FOR THAT AREA IS ABOUT 3800 FT; BUT WE WERE ABOUT TO TURN TOWARD THE FAF AND WE WERE UNDER HIS CTL. AT 2100 FT WE RECEIVED A 'TERRAIN; TERRAIN; PULL UP.' I IMMEDIATELY EXECUTED A CLB AND WAS CLBING THROUGH 3000 FT WHEN THE CTLR ASKED US TO CONFIRM THAT WE WERE AT '3100 FT.' WE TOLD HIM THAT WE WERE LEVELING OFF FROM A CLB FROM 2100 FT AND WE UNDERSTOOD THAT HE HAD GIVEN US A DSCNT TO 2100 FT AND HAD READ THAT BACK TO HIM. HE TOLD US TO TURN AND DSND TO 2000 FT UNTIL LOC INTERCEPT AND THEN INTERCEPT THE GS AT 1800 FT. APCH AND LNDG WAS UNEVENTFUL. THIS CTLR TALKED WITH A SPEECH IMPEDIMENT AND WHEN HE SPOKE FAST IT WAS DIFFICULT TO UNDERSTAND HIM. WHY HE DIDN'T CORRECT US WHEN WE READ BACK THE WRONG ALT; I DON'T KNOW. HE WAS BUSY. AFTER WE HAD OUR PROB; HE DID SLOW HIS SPEECH DOWN AND IT WAS EASIER TO UNDERSTAND. I SHOULD HAVE QUESTIONED HIM FURTHER ON THE 2100 FT INSTRUCTION BECAUSE IT WAS AN ODD ALT WITH THAT MINIMUM SECTOR ALT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: CAPT SAID THAT HE WAITED A BIT TOO LONG TO CHALLENGE THE DSCNT CLRNC; ESPECIALLY KNOWING THAT HE WAS HEADING FOR HIGHER TERRAIN. BY THE TIME IT BOTHERED HIM SUFFICIENTLY TO QUESTION ATC; THE RPTR ADVISED THE GPWS WAS ALARMING. THE OTHER PRINCIPLE PROB WAS THE CTLR'S SPEECH RATE. IT WAS VERY FAST AND APPEARED TO HAVE AN IMPEDIMENT. AFTER THE CTLR QUESTIONED THE FLT CREW'S DSCNT ALT; AND THE CAPT'S CHALLENGE RESPONSE; THE CTLR'S SPEECH RATE SLOWED DOWN SUFFICIENTLY TO BE UNDERSTOOD.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.