Narrative:

Received and read back clearance from bay approach to descend and maintain 4000 ft out of 8000 ft. On vector for visual approach to runway 32 NAS moffett field. Passing 4300 ft for 4000 ft bay approach called and asked 'are you going to six (6000 ft)?' I replied 'negative, you cleared us to 4000 ft.' bay then issues 'climb and maintain 4500 ft.' both pilots, flight engineer, and observer on headset in cockpit at the time agree that initial clearance was in fact for 4000 ft. I, as pilot on radios at the time, also wrote down the number 4000 during the initial call up and noted the altitude again while executing the descent checklist. High terrain in the area is 3700 ft MSL. Due to the VFR conditions/good visibility at the time I elected to begin a slow descent to 4000 ft when cleared even though it seemed a low altitude. Terrain drops off rapidly into the bay area and considering this I felt a safe descent could be carried out while still complying with bay's 4000 ft instruction as long as the descent was gradual. I feel bay approach was preoccupied with conducting a UHF radio frequency check with us (bay's idea) and therefore issued an incorrect altitude clearance. Bay made no further comment on the incident and switched us to the next controller. Don't ask aircraft to do radio checks on other frequencys during high traffic density periods! Controllers are asking for mistakes.

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Original NASA ASRS Text

Title: MTR X ISSUED DESCENT CLRNC BELOW MVA.

Narrative: RECEIVED AND READ BACK CLRNC FROM BAY APCH TO DSND AND MAINTAIN 4000 FT OUT OF 8000 FT. ON VECTOR FOR VISUAL APCH TO RWY 32 NAS MOFFETT FIELD. PASSING 4300 FT FOR 4000 FT BAY APCH CALLED AND ASKED 'ARE YOU GOING TO SIX (6000 FT)?' I REPLIED 'NEGATIVE, YOU CLRED US TO 4000 FT.' BAY THEN ISSUES 'CLIMB AND MAINTAIN 4500 FT.' BOTH PLTS, FE, AND OBSERVER ON HEADSET IN COCKPIT AT THE TIME AGREE THAT INITIAL CLRNC WAS IN FACT FOR 4000 FT. I, AS PLT ON RADIOS AT THE TIME, ALSO WROTE DOWN THE NUMBER 4000 DURING THE INITIAL CALL UP AND NOTED THE ALT AGAIN WHILE EXECUTING THE DSCNT CHECKLIST. HIGH TERRAIN IN THE AREA IS 3700 FT MSL. DUE TO THE VFR CONDITIONS/GOOD VISIBILITY AT THE TIME I ELECTED TO BEGIN A SLOW DSCNT TO 4000 FT WHEN CLRED EVEN THOUGH IT SEEMED A LOW ALT. TERRAIN DROPS OFF RAPIDLY INTO THE BAY AREA AND CONSIDERING THIS I FELT A SAFE DSCNT COULD BE CARRIED OUT WHILE STILL COMPLYING WITH BAY'S 4000 FT INSTRUCTION AS LONG AS THE DSCNT WAS GRADUAL. I FEEL BAY APCH WAS PREOCCUPIED WITH CONDUCTING A UHF RADIO FREQ CHECK WITH US (BAY'S IDEA) AND THEREFORE ISSUED AN INCORRECT ALT CLRNC. BAY MADE NO FURTHER COMMENT ON THE INCIDENT AND SWITCHED US TO THE NEXT CTLR. DON'T ASK ACFT TO DO RADIO CHECKS ON OTHER FREQS DURING HIGH TFC DENSITY PERIODS! CTLRS ARE ASKING FOR MISTAKES.

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.