Narrative:

I had just departed...after receiving my IFR clearance. I was given radar vectors after takeoff from...tower. I was initially assigned instructions to climb and maintain 2;000 feet and was given a vector heading to the east. I complied. I was then advised to contact departure on their assigned frequency. After contact was made with departure; I was instructed to climb and maintain 3000ft and was given a radar vector to the north. I complied. Shortly after that I was then given a vectors to the west and instructed to maintain 4;000 feet. I complied. I had activated my aircraft's autopilot during this climb out portion of the flight.as I leveled at 4;000 feet I was instructed that I was cleared direct [destination]. I then began entering that instruction into my GPS system when ATC advised that I had traffic 2 o'clock and more additional traffic (I can't remember for sure) at possibly my 10 o'clock. I replied that I was looking for traffic and that I had contacts on my traffic screen. Shortly after I advised ATC that I had the 2 o'clock traffic in sight. ATC was talking to other aircraft at that time and I believed he acknowledged me shortly after. I observed the 2 o'clock traffic as an [air carrier] aircraft that 'appeared' to be descending from my right to my left directly across what I felt was my flight path. I continued to watch that traffic as it became closer and at the same time scanning for the secondary traffic that I was told to look for. I then decided to deactivate my autopilot and fly the plane manually in case I needed to make any sudden changes in heading or altitude for traffic; and to prepare for any wake turbulence from the crossing jet that I had in sight in front of me.at this time I was listening to ATC give traffic alerts to a couple of other planes in my vicinity as I continued scanning the sky looking for the secondary traffic ATC advised me of. I was also waiting for ATC to finish speaking so that I could ask where the second aircraft was from my position if I couldn't locate it soon. As I was scanning I watched the [air carrier] aircraft pass over me from right to left and I inadvertently applied forward pressure to the yoke as I kept separation from it. I was then called up by ATC and asked what my altitude was. It was then that I noticed that I had descended below my assigned 4000ft and called back my altitude that I believe was between 3400-3600ft. I climbed back to 4000ft immediately. ATC then stated that he did not tell me to descend and that he had traffic above my position and below my position. ATC then asked me why I descended. I told him that I was watching the closing traffic in front of me crossing over me and I apologized for accidentally descending. I then heard ATC ask a pilot if he had to deviate his altitude to avoid me and he replied that he did not. I later reactivated my autopilot and continued my flight. I was given a phone number to call ATC upon lading.I take full responsibility for the incident. I am a new instrument rated pilot. During this situation I remembered from my training that during IFR flights in VMC conditions; the PIC is responsible for maintaining visual separation between other aircraft. In this situation; I observed the [air carrier] traffic in front of me and could hear ATC talking to other pilots about traffic and I instinctively applied too much forward pressure to the yoke as I had my eyes looking outside the plane for that traffic.

Related reports:

Google
 

Original NASA ASRS Text

Title: C182 pilot reported inadvertently descending below assigned altitude while monitoring traffic passing overhead.

Narrative: I had just departed...after receiving my IFR clearance. I was given radar vectors after takeoff from...tower. I was initially assigned instructions to climb and maintain 2;000 feet and was given a vector heading to the east. I complied. I was then advised to contact Departure on their assigned frequency. After contact was made with Departure; I was instructed to climb and maintain 3000ft and was given a radar vector to the north. I complied. Shortly after that I was then given a vectors to the west and instructed to maintain 4;000 feet. I complied. I had activated my aircraft's autopilot during this climb out portion of the flight.As I leveled at 4;000 feet I was instructed that I was cleared direct [destination]. I then began entering that instruction into my GPS system when ATC advised that I had traffic 2 o'clock and more additional traffic (I can't remember for sure) at possibly my 10 o'clock. I replied that I was looking for traffic and that I had contacts on my traffic screen. Shortly after I advised ATC that I had the 2 o'clock traffic in sight. ATC was talking to other aircraft at that time and I believed he acknowledged me shortly after. I observed the 2 o'clock traffic as an [air carrier] aircraft that 'appeared' to be descending from my right to my left directly across what I felt was my flight path. I continued to watch that traffic as it became closer and at the same time scanning for the secondary traffic that I was told to look for. I then decided to deactivate my autopilot and fly the plane manually in case I needed to make any sudden changes in heading or altitude for traffic; and to prepare for any wake turbulence from the crossing jet that I had in sight in front of me.At this time I was listening to ATC give traffic alerts to a couple of other planes in my vicinity as I continued scanning the sky looking for the secondary traffic ATC advised me of. I was also waiting for ATC to finish speaking so that I could ask where the second aircraft was from my position if I couldn't locate it soon. As I was scanning I watched the [air carrier] aircraft pass over me from right to left and I inadvertently applied forward pressure to the yoke as I kept separation from it. I was then called up by ATC and asked what my altitude was. It was then that I noticed that I had descended below my assigned 4000ft and called back my altitude that I believe was between 3400-3600ft. I climbed back to 4000ft immediately. ATC then stated that he did not tell me to descend and that he had traffic above my position and below my position. ATC then asked me why I descended. I told him that I was watching the closing traffic in front of me crossing over me and I apologized for accidentally descending. I then heard ATC ask a pilot if he had to deviate his altitude to avoid me and he replied that he did not. I later reactivated my autopilot and continued my flight. I was given a phone number to call ATC upon lading.I take full responsibility for the incident. I am a new instrument rated pilot. During this situation I remembered from my training that during IFR flights in VMC conditions; the PIC is responsible for maintaining visual separation between other aircraft. In this situation; I observed the [air carrier] traffic in front of me and could hear ATC talking to other pilots about traffic and I instinctively applied too much forward pressure to the yoke as I had my eyes looking outside the plane for that traffic.

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.