Narrative:

While being vectored for the ILS approach to runway 19 at teterboro; nj by new york approach; the TCAS was indicating traffic at our twelve o'clock position. The altitude on the target aircraft varied erratically from one thousand feet below us to four hundred feet below us. Sometimes there was no altitude readout at all. We could not spot the airplane visually. The vector that approach had us on was directly towards the greenwood lake airport (4n1). We spotted the airport and tried to find traffic in the vicinity. The first officer spotted a small aircraft well below us. The distance to this airplane coincided with the TCAS display. However there was no altitude information depicted at this time. Then we lost sight of the aircraft below the nose. I decided to start a left turn to get away from the target airplane since what altitude information that we were getting was erratic; and I wasn't certain that the airplane we saw was the one on the TCAS display; it appeared too low. As the TCAS display began to show the target aircraft begin to drift off to the right we saw the TCAS display a red box with a climb vector and altitude display of minus 400 ft. I put the gulfstream into a hard left climbing turn to get away from the target airplane. As I was doing this; we saw the other airplane just out side the right side 2:00 o'clock position; topping out of a loop; abreast of our aircraft not even a couple of hundred feet away. I firmly believe that had I not started the turn when I did; I would not be writing this narrative now. The turn and pull that I did caused us to climb past 5;000 ft MSL. The TCAS never gave us a verbal conflict warning; nor did we get any evasive maneuver instruction; such as climb. I believe that the airplane doing the loop; maneuvering; confused the TCAS computer. After we landed I called the greenwood lake airport to ask if they had a P51 doing aerobatics over their airport. The reason that we thought that it was a P51 was how big it looked out the windscreen and how fast it was climbing and descending; as well as how fast it was going. We were indicating 250 KTS. The person at the greenwood lake explained to me; that they didn't have any P-51's in the area; however they were conducting an aerobatic competition and that they had extra 300's and such. One of which matched the paint scheme that we saw. He also informed me that they had notified the TRACON of the aerobatic event; that they had an aerobatic box that went to 4;500 ft MSL; and that the event was NOTAM'ed. Even with all of that the word didn't get out; so shame on us for flying at 4;000 ft MSL over their airport. I have found that the TCAS on this particular aircraft is very accurate in terms of range and bearing; not so much on our other gulfstream. The accuracy of the TCAS display greatly helped in my choice of direction during my escape maneuver.

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Original NASA ASRS Text

Title: G550 Captain experienced a NMAC over 4N1 at 4;000 FT while being vectored for the ILS 19 at TEB. Upon landing it was learned that an aerobatic competition is being held and that the airspace had been NOTAM'ed.

Narrative: While being vectored for the ILS approach to Runway 19 at Teterboro; NJ by New York Approach; the TCAS was indicating traffic at our twelve o'clock position. The altitude on the target aircraft varied erratically from one thousand feet below us to four hundred feet below us. Sometimes there was no altitude readout at all. We could not spot the airplane visually. The vector that approach had us on was directly towards the Greenwood Lake Airport (4N1). We spotted the airport and tried to find traffic in the vicinity. The First Officer spotted a small aircraft well below us. The distance to this airplane coincided with the TCAS display. However there was no altitude information depicted at this time. Then we lost sight of the aircraft below the nose. I decided to start a left turn to get away from the target airplane since what altitude information that we were getting was erratic; and I wasn't certain that the airplane we saw was the one on the TCAS display; it appeared too low. As the TCAS display began to show the target aircraft begin to drift off to the right we saw the TCAS display a red box with a climb vector and altitude display of minus 400 FT. I put the Gulfstream into a hard left climbing turn to get away from the target airplane. As I was doing this; we saw the other airplane just out side the right side 2:00 o'clock position; topping out of a loop; abreast of our aircraft not even a couple of hundred feet away. I firmly believe that had I not started the turn when I did; I would not be writing this narrative now. The turn and pull that I did caused us to climb past 5;000 FT MSL. The TCAS never gave us a verbal conflict warning; nor did we get any evasive maneuver instruction; such as climb. I believe that the airplane doing the loop; maneuvering; confused the TCAS computer. After we landed I called the Greenwood Lake Airport to ask if they had a P51 doing aerobatics over their airport. The reason that we thought that it was a P51 was how big it looked out the windscreen and how fast it was climbing and descending; as well as how fast it was going. We were indicating 250 KTS. The person at the Greenwood Lake explained to me; that they didn't have any P-51's in the area; however they were conducting an aerobatic competition and that they had Extra 300's and such. One of which matched the paint scheme that we saw. He also informed me that they had notified the TRACON of the aerobatic event; that they had an aerobatic box that went to 4;500 FT MSL; and that the event was NOTAM'ed. Even with all of that the word didn't get out; so shame on us for flying at 4;000 FT MSL over their airport. I have found that the TCAS on this particular aircraft is very accurate in terms of range and bearing; not so much on our other Gulfstream. The accuracy of the TCAS display greatly helped in my choice of direction during my escape maneuver.

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.