Near midair collision. Flight crew of an medium large transport climbed in response to a TCASII RA and subsequent instructions from ATC due to the close proximity of a dehavilland twin otter DHC6. The reporting captain observed the twin otter cross by underneath their aircraft at 200-500 ft after the first officer made an immediate climb.

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Attributes
ACN	420560
Time
Date	199811
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : mxe airport : phl
State Reference	PA
Altitude	msl bound lower : 3000 msl bound upper : 3000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Flight Phase	descent other
Route In Use	arrival other arrival star : star enroute airway : phl
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Twin Otter DHC-6
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : flight engineer pilot : instrument
Experience	flight time last 90 days : 250 flight time total : 14000 flight time type : 6000
ASRS Report	420560
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : excursion from assigned altitude conflict : nmac other anomaly other
Independent Detector	other controllera other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course flight crew : took evasive action other
Consequence	Other
Miss Distance	horizontal : 0 vertical : 400
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

While on phl approach for arrival to phl at approximately time and position, crew received a TCASII TA (yellow) at 12 O'clock and minus 200 ft. Both crew members attempted to acquire visually on the target. At about the same time, a TA with warning to climb and a call from ATC, alerted us to begin a climb. Immediately the first officer (PF) disconnected the autoplt and applied climb power, rotated the aircraft, and began a climb (3500-4000 FPM). At the same instant, captain spotted an aircraft passing right to left at 200-500 ft below us. Aircraft was a twin otter. TCASII worked and may have avoided a midair. The controller's call was too late. We were already in the climb thanks to TCASII. At one time during the TCASII event during pullup, I noted an altitude difference of '0' and about 5 mi range setting. A very close call.

Original NASA ASRS Text

Title: NMAC. FLC OF AN MLG CLBED IN RESPONSE TO A TCASII RA AND SUBSEQUENT INSTRUCTIONS FROM ATC DUE TO THE CLOSE PROX OF A DEHAVILLAND TWIN OTTER DHC6. THE RPTING CAPT OBSERVED THE TWIN OTTER CROSS BY UNDERNEATH THEIR ACFT AT 200-500 FT AFTER THE FO MADE AN IMMEDIATE CLB.

Narrative: WHILE ON PHL APCH FOR ARR TO PHL AT APPROX TIME AND POS, CREW RECEIVED A TCASII TA (YELLOW) AT 12 O'CLOCK AND MINUS 200 FT. BOTH CREW MEMBERS ATTEMPTED TO ACQUIRE VISUALLY ON THE TARGET. AT ABOUT THE SAME TIME, A TA WITH WARNING TO CLB AND A CALL FROM ATC, ALERTED US TO BEGIN A CLB. IMMEDIATELY THE FO (PF) DISCONNECTED THE AUTOPLT AND APPLIED CLB PWR, ROTATED THE ACFT, AND BEGAN A CLB (3500-4000 FPM). AT THE SAME INSTANT, CAPT SPOTTED AN ACFT PASSING R TO L AT 200-500 FT BELOW US. ACFT WAS A TWIN OTTER. TCASII WORKED AND MAY HAVE AVOIDED A MIDAIR. THE CTLR'S CALL WAS TOO LATE. WE WERE ALREADY IN THE CLB THANKS TO TCASII. AT ONE TIME DURING THE TCASII EVENT DURING PULLUP, I NOTED AN ALT DIFFERENCE OF '0' AND ABOUT 5 MI RANGE SETTING. A VERY CLOSE CALL.

Data retrieved from NASA's ASRS site as of July 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.