Narrative:

After departing; I was cleared direct to my destination; john C. Tune airport in nashville. I had filed for 8;000 ft but was cleared to 6;000 ft. At that altitude I was in solid IMC flight conditions and rain with very limited visibility in all directions. The departure controller advised that I would be at 6;000 ft for a while because of opposite direction traffic; 'one thousand feet above you'. I was handed off to atlanta center. The controller advised that there was aircraft Y descending to 7;000 ft at 11:30 and I don't remember the exact distance but I think it was about ten miles. I then heard a part of a conversation between the controller and a [regional] airline pilot. The [regional] asked the controller to verify the altimeter setting. The controller said the altimeter was 29.34. The pilot then said that the controller had initially said the altimeter was 30.34. The controller reiterated that the altimeter was 29.34. My airplane has a garmin GNS530 navigator and a garmin GTX330; mode south transponder which displays tis traffic on the GNS530. About this time I saw a target at my 11:30 to 12:00 o'clock with an altitude of +1;200 ft. I expected the target to level off at +1;000 ft but it continued to descend. When it descended to +800 ft I became concerned and tried to call the controller but he was busy talking to another airplane. When the target was +600 ft the controller asked the aircraft Y if he was level at 7;000 ft. Aircraft Y acknowledged that he was cleared to 7;000 ft and that he was descending through 7;600 ft. The controller told the aircraft Y pilot that the radar showed him at 6;600 ft. About this time my tis display showed the target at approximately +400 ft and the target turned yellow and I received an aural 'traffic' call. The controller finally told the aircraft Y pilot the altimeter setting was 29.34 and to check his altimeter. The aircraft Y pilot told the controller that when he checked on; he was told the altimeter was 30.34 and that was a lot different from what he had previously set. In the mean time the target indicator merged with my airplane symbol and the altitude was +200 ft and then +100 ft. When the target indicator was just past my airplane symbol (but still touching my airplane symbol) the target altitude showed +0 ft. The aircraft Y pilot then said he had reset his altimeter and was climbing back to 7;000 ft. Shortly thereafter; the controller cleared me to climb to 8;000 ft. I acknowledged the climb and then commented to the controller that I believed that the aircraft Y had passed over me at 200 ft. He didn't say anything. The whole evolution happened very quickly. I felt I needed to take corrective action but I could not decide what to do in the short time I had between recognition of a conflict and the merging of airplanes. Had I been equipped with TCAS I am certain I would have received a resolution advisory. In that case I would have been obligated to follow the resolution advisory and would have been authorized to deviate from my IFR clearance. But tis traffic calls are advisory and I consider them a VFR tool to avoid traffic. Tis traffic calls do not allow me to deviate from my clearance. I'm pretty certain that the controller gave the aircraft Y pilot an incorrect altimeter setting since he apparently also gave the incorrect setting to the [regional] pilot. Obviously that was an error that contributed to a near miss between two IFR airplanes. But I really fault the aircraft Y pilot. He received an altimeter setting that caused him to reset his altimeter by 1;000 ft and didn't question the altimeter setting in a timely manner.

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Original NASA ASRS Text

Title: A light aircraft pilot reported an NMAC in IMC conditions when another light aircraft pilot mis-set his altimeter by 1' HG; or 1;000 FT.

Narrative: After departing; I was cleared direct to my destination; John C. Tune airport in Nashville. I had filed for 8;000 FT but was cleared to 6;000 FT. At that altitude I was in solid IMC flight conditions and rain with very limited visibility in all directions. The Departure Controller advised that I would be at 6;000 FT for a while because of opposite direction traffic; 'one thousand feet above you'. I was handed off to Atlanta Center. The Controller advised that there was Aircraft Y descending to 7;000 FT at 11:30 and I don't remember the exact distance but I think it was about ten miles. I then heard a part of a conversation between the Controller and a [regional] airline pilot. The [regional] asked the Controller to verify the altimeter setting. The Controller said the altimeter was 29.34. The pilot then said that the Controller had initially said the altimeter was 30.34. The Controller reiterated that the altimeter was 29.34. My airplane has a Garmin GNS530 navigator and a Garmin GTX330; mode S transponder which displays TIS traffic on the GNS530. About this time I saw a target at my 11:30 to 12:00 o'clock with an altitude of +1;200 FT. I expected the target to level off at +1;000 FT but it continued to descend. When it descended to +800 FT I became concerned and tried to call the Controller but he was busy talking to another airplane. When the target was +600 FT the Controller asked the Aircraft Y if he was level at 7;000 FT. Aircraft Y acknowledged that he was cleared to 7;000 FT and that he was descending through 7;600 FT. The Controller told the Aircraft Y pilot that the radar showed him at 6;600 FT. About this time my TIS display showed the target at approximately +400 FT and the target turned yellow and I received an aural 'traffic' call. The Controller finally told the Aircraft Y pilot the altimeter setting was 29.34 and to check his altimeter. The Aircraft Y pilot told the Controller that when he checked on; he was told the altimeter was 30.34 and that was a lot different from what he had previously set. In the mean time the target indicator merged with my airplane symbol and the altitude was +200 FT and then +100 FT. When the target indicator was just past my airplane symbol (but still touching my airplane symbol) the target altitude showed +0 FT. The Aircraft Y pilot then said he had reset his altimeter and was climbing back to 7;000 FT. Shortly thereafter; the Controller cleared me to climb to 8;000 FT. I acknowledged the climb and then commented to the Controller that I believed that the Aircraft Y had passed over me at 200 FT. He didn't say anything. The whole evolution happened very quickly. I felt I needed to take corrective action but I could not decide what to do in the short time I had between recognition of a conflict and the merging of airplanes. Had I been equipped with TCAS I am certain I would have received a resolution advisory. In that case I would have been obligated to follow the resolution advisory and would have been authorized to deviate from my IFR clearance. But TIS traffic calls are advisory and I consider them a VFR tool to avoid traffic. TIS traffic calls do not allow me to deviate from my clearance. I'm pretty certain that the Controller gave the Aircraft Y pilot an incorrect altimeter setting since he apparently also gave the incorrect setting to the [regional] pilot. Obviously that was an error that contributed to a near miss between two IFR airplanes. But I really fault the Aircraft Y pilot. He received an altimeter setting that caused him to reset his altimeter by 1;000 FT and didn't question the altimeter setting in a timely manner.

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.