EMB145 CREW LEFT THE GATE BEFORE SCHEDULED DEP TIME.

Narrative

I OPERATED A SCHEDULED PART 121 PAX/CARGO FLT FROM PIT-EWR ON FEB/SAT/02. WE HAD ARRIVED TO PIT EARLY AT XA25 AND SHORTLY AFTER ARR; THE STATION AGENTS INFORMED ME THAT ALL 12 PAX BOOKED ON THE FLT HAD BEEN CHKED IN AND WERE AT THE GATE. THEY WANTED TO BOARD EARLY. I UNDERSTOOD THAT IN ORDER TO DEPART EARLY; DISPATCH NEEDED TO BE CONTACTED IN ORDER TO RECEIVE A NEW VALID TIME AMENDMENT. HOWEVER; IT WAS MY RECOLLECTION THAT IF ALL PAX WERE CHKED IN WE COULD DEPART WITH DISPATCH APPROVAL. THIS; HOWEVER; IS INCORRECT. WE DID DEPART AT XB00 WITH A SCHEDULED DEP TIME OF XB20. THEREFORE; WE LEFT 15 MINS PRIOR TO OUR VALID TIME OF THE DISPATCH RELEASE. I DID NOT CONTACT DISPATCH FOR A REVISED VALID TIME; RELYING ON MY INCORRECT RECOLLECTION OF THE FOM. THERE WAS A DELAYED TAXI TO THE ACTIVE RWY AND WE WERE GIVEN A WHEELS UP TIME OF XB22. WE DID TKOF AT XB22. THE REMAINDER OF THE FLT WAS UNEVENTFUL. I WAS CONTACTED BY THE OPS MGR ON MY DRIVE HOME TO INFORM ME OF THE ILLEGAL DEP AND SUBSEQUENT FLT. I THEN CONTACTED OUR CHIEF PLT AND DISCUSSED THE INCIDENT WITH HIM. HE ADVISED ME TO FILL OUT A NASA RPT. I WAS PRESSURED INTO AN EARLY AND ILLEGAL DEP DUE TO STATION AGENT'S DESIRE TO BOARD EARLY. I BELIEVE WE WERE THE LAST FLT OF THE NIGHT SO THEY WANTED TO GO HOME EARLY IF THEY COULD GET US OUT EARLY. FURTHERMORE; ANOTHER CREW MEMBER HAD WANTED TO MAKE HIS COMMUTE FLT HOME; ADDING ADDITIONAL PRESSURE TO DEPART EARLY. I RELIED ON MY RECOLLECTION OF FOM PROCS INSTEAD OF REFING THEM DIRECTLY AND ALLOWED EXTERNAL PRESSURES TO ALLOW ME TO INITIATE AN ILLEGAL FLT. WHEN ABNORMAL OR RARE CIRCUMSTANCES PRESENT THEMSELVES IN THE FUTURE; I WILL REF FOM/AOM MATERIALS.