B737-700
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 202
Confusion; Situational Awareness
ATC Issue All Types; Conflict Airborne Conflict
Flight Crew Became Reoriented
Procedure
This is to highlight a safety concern with our departure from PHL.  We were cleared for takeoff from Runway 9L; our departure instructions were to turn left Heading 081. After lining up on RWY 9L; I transferred aircraft control to the First Officer; the Pilot Flying for this leg.  Although takeoff roll and rotation were normal; during the takeoff roll I heard ATC (Philly Tower) clear another Carrier for takeoff from Runway 8; on departure his instructions were to fly Heading 050.  After our rotation and gear retraction; I noticed a TCAS Traffic Alert on our navigation multifunction display without an aural warning.  As I scanned to my left; I noticed the other Carrier (a Dash-8) about 400 feet below us; very slightly offset to our left.  Passing about 800 AGL we received a TCAS 'Maintain Vertical Speed' aural alert with a commanded climb of about 1500 fpm (our climb rate significantly exceeded this).  The FO called for the normal speed increase passing 1000 feet AGL; but I directed him to continue the climb as the Dash-8 was maintaining an approximately 400 feet vertical separation and approximately 100 feet lateral separation.  After a while the Dash-8 turned left and ceased to be a factor.  I'm surprised this is an accepted procedure with PHL departures. There should be more separation between aircraft departing RWY 9L and RWY 8 in PHL.  At the very least; RWY 9L departures should get a slight right turn (at least 090) and RWY 8 departures should get a left turn to provide lateral separation from the beginning of the departure phase.  Heading 081 after departing 9L actually places the aircraft closer to RWY 8.
B737 Captain believes that the practice by PHL Tower of departing aircraft simultaneously of Runway 9L and 8 is unsafe.