Charter small transport had close encounter with unknown VFR light twin.

Attributes
ACN	89738
Time
Date	198806
Day	Tue
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : ptw
State Reference	PA
Altitude	msl bound lower : 4000 msl bound upper : 4000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : phl tracon : rdg
Operator	common carrier : air taxi
Make Model Name	Small Transport
Flight Phase	cruise other
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	cruise other
Flight Plan	VFR
Person 1
Affiliation	government : faa
Function	controller : approach
Qualification	controller : radar
Experience	controller military : 4 controller non radar : 1 controller radar : 6
ASRS Report	89738
Person 2
Affiliation	government : faa
Function	controller : approach
Qualification	controller : radar
Events
Anomaly	conflict : nmac
Independent Detector	other flight crewa
Resolutory Action	flight crew : took evasive action none taken : insufficient time
Consequence	faa : investigated
Miss Distance	horizontal : 200 vertical : 0
Supplementary
Primary Problem	Flight Crew Human Performance

Narrative:

Aircraft was handed off to reading approach control over pottstown, level at 4000'. After contact with reading, the pilot reported to me a near miss over pottstown. The other aircraft involved was unknown to me. The aircraft involved, were both in philadelphia's airspace and small transport X was IFR on the route. Small transport X reported a green and white twin, from the north and his right. Took evasive action and came within 200' of small transport X. No other information available. Reading's radar coverage in this area is extremely poor, aircraft must be at least 4000' or above to get a good return. Reading field elevation is 344. I don't have philadelphia's report, but this is a fairly congested area, too busy for traffic advisories or possibly no transponder, etc. Callback conversation with reporter revealed the following: the other aircraft involved in this near midair collision was an unreported VFR twin passing through the area. Phl approach control had no information on the aircraft but did see a 1200 squawk just after the change over was made on small transport X. Reporter's experience level was ascertained.

Original NASA ASRS Text

Title: CHARTER SMT HAD CLOSE ENCOUNTER WITH UNKNOWN VFR LIGHT TWIN.

Narrative: ACFT WAS HANDED OFF TO READING APCH CTL OVER POTTSTOWN, LEVEL AT 4000'. AFTER CONTACT WITH READING, THE PLT REPORTED TO ME A NEAR MISS OVER POTTSTOWN. THE OTHER ACFT INVOLVED WAS UNKNOWN TO ME. THE ACFT INVOLVED, WERE BOTH IN PHILADELPHIA'S AIRSPACE AND SMT X WAS IFR ON THE ROUTE. SMT X REPORTED A GREEN AND WHITE TWIN, FROM THE NORTH AND HIS RIGHT. TOOK EVASIVE ACTION AND CAME WITHIN 200' OF SMT X. NO OTHER INFORMATION AVAILABLE. READING'S RADAR COVERAGE IN THIS AREA IS EXTREMELY POOR, ACFT MUST BE AT LEAST 4000' OR ABOVE TO GET A GOOD RETURN. READING FIELD ELEVATION IS 344. I DON'T HAVE PHILADELPHIA'S REPORT, BUT THIS IS A FAIRLY CONGESTED AREA, TOO BUSY FOR TFC ADVISORIES OR POSSIBLY NO XPONDER, ETC. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: THE OTHER ACFT INVOLVED IN THIS NEAR MIDAIR COLLISION WAS AN UNREPORTED VFR TWIN PASSING THROUGH THE AREA. PHL APCH CTL HAD NO INFO ON THE ACFT BUT DID SEE A 1200 SQUAWK JUST AFTER THE CHANGE OVER WAS MADE ON SMT X. REPORTER'S EXPERIENCE LEVEL WAS ASCERTAINED.

Data retrieved from NASA's ASRS site as of August 2007 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.