Narrative:

We were on approach to nantucket; ack. I was the pilot flying. Prior to the descent; the captain tuned in the ATIS and copied the data. My memory is unclear as to the exact timing of clearances and calls; but at some point I heard the altimeter setting of 30.17. When we descended through the transition level; FL180; I checked with the the captain to confirm that the altimeter setting was 30.17. I received confirmation from him; and we both set 30.17 in our altimeters. I think that the controller must have issued an altimeter setting when he issued our first clearance below the transition level; but I can't remember exactly what he said. I believe that there may have been a hearback/readback error between us and the controller; because we received no correction from the controller as to the altimeter setting. We were cleared to 1700 feet MSL to execute the ILS runway 6. We descended to 1700 feet on an altimeter setting of 30.17. Just as we reached that altitude; we broke out of the clouds. It was a very windy day at nantucket; and when I saw the whitecaps I knew that we were lower than 1700 feet. I looked at the radar altimeter reading; and it read 750 feet. I immediately began a climb. At about the same time; the controller called to alert us about about our altitude; and he gave us an altimeter setting of 29.11. We climbed to 1700 feet on the correct altimeter setting; completed the approach and landed. After we landed; I looked at the ATIS data that the captain had written down. The altimeter setting that he had written down from the ATIS was 29.18. It seems clear to me now that this was an error of memory; hearback and readback. The captain wrote down an ATIS altimeter setting off 29.18. But we (the crew) heard/misheard an altitude setting of 30.17. As a result; we descended too low. As for preventive measures; I can only suggest that we must very careful about altimeter setting readbacks.

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Original NASA ASRS Text

Title: A Cessna 560 Flight Crew suffered an altitude deviation when they set their altimeters to 30.17 vice 29.17 inches of Mercury. Their actual altitude was 750 MSL vice 1700 as cleared.

Narrative: We were on approach to Nantucket; ACK. I was the Pilot Flying. Prior to the descent; the Captain tuned in the ATIS and copied the data. My memory is unclear as to the exact timing of clearances and calls; but at some point I heard the altimeter setting of 30.17. When we descended through the transition level; FL180; I checked with the the Captain to confirm that the altimeter setting was 30.17. I received confirmation from him; and we both set 30.17 in our altimeters. I think that the controller must have issued an altimeter setting when he issued our first clearance below the transition level; but I can't remember exactly what he said. I believe that there may have been a hearback/readback error between us and the controller; because we received no correction from the controller as to the altimeter setting. We were cleared to 1700 feet MSL to execute the ILS Runway 6. We descended to 1700 feet on an altimeter setting of 30.17. Just as we reached that altitude; we broke out of the clouds. It was a very windy day at Nantucket; and when I saw the whitecaps I knew that we were lower than 1700 feet. I looked at the radar altimeter reading; and it read 750 feet. I immediately began a climb. At about the same time; the controller called to alert us about about our altitude; and he gave us an altimeter setting of 29.11. We climbed to 1700 feet on the correct altimeter setting; completed the approach and landed. After we landed; I looked at the ATIS data that the Captain had written down. The altimeter setting that he had written down from the ATIS was 29.18. It seems clear to me now that this was an error of memory; hearback and readback. The Captain wrote down an ATIS altimeter setting off 29.18. But we (the Crew) heard/misheard an altitude setting of 30.17. As a result; we descended too low. As for preventive measures; I can only suggest that we must very careful about altimeter setting readbacks.

Data retrieved from NASA's ASRS site as of April 2012 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.