A stemme sailplane took evasive action to avoid a collision with air carrier aircraft.

Attributes
ACN	716050
Time
Date	200611
Local Time Of Day	1201 To 1800
Place
Locale Reference	navaid : sjc.vor
State Reference	CA
Altitude	msl single value : 6000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Operator	general aviation : personal
Make Model Name	Ultralight
Operating Under FAR Part	Part 91
Flight Phase	descent : intermediate altitude
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Plan	IFR
Person 1
Affiliation	other
Function	flight crew : single pilot
Qualification	pilot : private
Experience	flight time last 90 days : 15 flight time total : 2456 flight time type : 1469
ASRS Report	716050
Person 2
Affiliation	company : air carrier
Events
Anomaly	conflict : airborne critical
Independent Detector	other flight crewa
Miss Distance	horizontal : 1000 vertical : 500
Supplementary
Problem Areas	Flight Crew Human Performance ATC Human Performance
Primary Problem	Ambiguous

Narrative:

While flying a westerly course approximately 15 NM from sjc on its 45 degree radial at 6000 ft MSL; I observed an airliner approaching me on what appeared to be a collision course from my 10 O'clock position. While the other aircraft appeared to be descending; since it was above my altitude; I turned right and dove. I believe the other pilot also took evasive action; but cannot be sure. The large size of the airliner may have made my distance and altitude separation estimates (1000 and 500 ft respectively) smaller than they were; but I suspect the separation was less than desired. My aircraft is a motor glider; I was in glider mode (engine off) at that point in time; and descending to set up for a landing at hwd. Maintaining a normal VFR cruising altitude was therefore not possible or appropriate. My transponder was set to 1200 and was being interrogated frequently. While 'see and avoid' is the rule in VMC; the high speed of jets makes TCAS and other collision avoidance aids highly useful. I am surprised that the airliner's TCAS did not pick me up sooner.

Original NASA ASRS Text

Title: A STEMME SAILPLANE TOOK EVASIVE ACTION TO AVOID A COLLISION WITH ACR ACFT.

Narrative: WHILE FLYING A WESTERLY COURSE APPROX 15 NM FROM SJC ON ITS 45 DEG RADIAL AT 6000 FT MSL; I OBSERVED AN AIRLINER APCHING ME ON WHAT APPEARED TO BE A COLLISION COURSE FROM MY 10 O'CLOCK POS. WHILE THE OTHER ACFT APPEARED TO BE DSNDING; SINCE IT WAS ABOVE MY ALT; I TURNED R AND DOVE. I BELIEVE THE OTHER PLT ALSO TOOK EVASIVE ACTION; BUT CANNOT BE SURE. THE LARGE SIZE OF THE AIRLINER MAY HAVE MADE MY DISTANCE AND ALT SEPARATION ESTIMATES (1000 AND 500 FT RESPECTIVELY) SMALLER THAN THEY WERE; BUT I SUSPECT THE SEPARATION WAS LESS THAN DESIRED. MY ACFT IS A MOTOR GLIDER; I WAS IN GLIDER MODE (ENG OFF) AT THAT POINT IN TIME; AND DSNDING TO SET UP FOR A LNDG AT HWD. MAINTAINING A NORMAL VFR CRUISING ALT WAS THEREFORE NOT POSSIBLE OR APPROPRIATE. MY XPONDER WAS SET TO 1200 AND WAS BEING INTERROGATED FREQUENTLY. WHILE 'SEE AND AVOID' IS THE RULE IN VMC; THE HIGH SPD OF JETS MAKES TCAS AND OTHER COLLISION AVOIDANCE AIDS HIGHLY USEFUL. I AM SURPRISED THAT THE AIRLINER'S TCAS DID NOT PICK ME UP SOONER.

Data retrieved from NASA's ASRS site as of January 2009 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.