Narrative:

I was approaching class B airspace and being vectored to my home airport. There was mild to moderate turbulence at the time. The controller called and instructed me to change heading; descend; and change frequencies. I repeated back the instructions. I was instructed to descend from 10000 feet to 9000 feet and I apparently acknowledged this in the read back; but in my head; I heard 7000 feet. I changed heading and began descending; and called the next frequency. On that call; I identified my aircraft; assigned heading and stated 'out of 9 point 8 for 7.' the new controller gave me a brief acknowledgement. It was busy. General aviation aircraft are frequently instructed to descend low in this area because of landing and departing commercial aircraft. I did notice I seemed lower than usual. When I reached about 7200 feet; I was called by a controller with a low altitude alert. I responded I was cleared to 7000 feet and asked what altitude he would like me at. He stated I was actually cleared to 9000 feet and to climb. I immediately did this; and shortly afterwards; asked to call the TRACON because of a possible pilot deviation (which I did after landing). The rest of the flight and landing were uneventful. Obviously; a miscommunication was the root of this problem. Because I received multiple instructions from the controller in significant turbulence; I misheard the assigned altitude; though I apparently initially acknowledged the correct altitude. When I called the next controller and stated the altitude I had 'heard;' which was incorrect; she did not catch the error. This two-part error chain caused the deviation. To prevent an error on my part like this again; writing down the instructions from ATC would allow me to check the instruction and verify it at the time and later. Also; when I noticed that I seemed lower than usual; I should have called ATC and verified the assigned altitude.

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Original NASA ASRS Text

Title: BE 35 pilot reported receiving a low altitude alert from ATC; after mistakenly descending to 7000 feet instead of ATC assigned 9000 feet.

Narrative: I was approaching Class B airspace and being vectored to my home airport. There was mild to moderate turbulence at the time. The controller called and instructed me to change heading; descend; and change frequencies. I repeated back the instructions. I was instructed to descend from 10000 feet to 9000 feet and I apparently acknowledged this in the read back; but in my head; I heard 7000 feet. I changed heading and began descending; and called the next frequency. On that call; I identified my aircraft; assigned heading and stated 'out of 9 point 8 for 7.' The new controller gave me a brief acknowledgement. It was busy. General Aviation aircraft are frequently instructed to descend low in this area because of landing and departing commercial aircraft. I did notice I seemed lower than usual. When I reached about 7200 feet; I was called by a controller with a low altitude alert. I responded I was cleared to 7000 feet and asked what altitude he would like me at. He stated I was actually cleared to 9000 feet and to climb. I immediately did this; and shortly afterwards; asked to call the TRACON because of a possible pilot deviation (which I did after landing). The rest of the flight and landing were uneventful. Obviously; a miscommunication was the root of this problem. Because I received multiple instructions from the controller in significant turbulence; I misheard the assigned altitude; though I apparently initially acknowledged the correct altitude. When I called the next controller and stated the altitude I had 'heard;' which was incorrect; she did not catch the error. This two-part error chain caused the deviation. To prevent an error on my part like this again; writing down the instructions from ATC would allow me to check the instruction and verify it at the time and later. Also; when I noticed that I seemed lower than usual; I should have called ATC and verified the assigned altitude.

Data retrieved from NASA's ASRS site 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.