EXCURSION FROM TAXIWAY.

Narrative

WHILE TAXIING TO RWY 8 HAD TO NEGOTIATE TWO 90 DEG TURNS ON A VERY NARROW TAXIWAY (50 FT). AS I STARTED THE LH TURN THE FO BEGAN A FLAP TEST (NORMAL PROC). THE NOSE WHEEL STARTED SHARPLY TOWARD THE L SO I ATTEMPTED TO APPLY THE BRAKES. (THE NOSE WHEEL STEERING; FLAPS AND BRAKES ARE ALL HYDRAULICALLY OPERATED). THE HYD ACTUATOR 'COUGHED' AS THE PRESSURE DROPPED TO THE BOTTOM OF THE INDICATOR ARC; THEN CYCLED ON. AS THE PRESSURE ARRIVED TO THE SYS; THE L BRAKE CAUGHT FIRST AND LAST 3 FT OF SKID MARKS ON THE TAXIWAY (LH WHEEL ONLY) AS THE LH MAIN GEAR DROPPED OFF THE TAXIWAY INTO A VERY MUDDY SURFACE THE MAIN GEAR SANK 14 INCHES INTO THE MUD AND THE ACFT CAME TO A STOP. THE FLT ATTENDANT WAS STANDING IN THE AISLE CONDUCTING A SAFETY CHK AND WAS THROWN TO THE FLOOR. THERE WAS NO ACFT DAMAGE; AND NO INJURIES. ALL CREW MEMBERS SUBMITTED TO A DRUG TEST WHICH ALL WERE 'NEGATIVE' AND ALL RPTED PROPER REST THE EVENING BEFORE. WHAT DID I LEARN? WHEN OPERATING ON NARROW TAXIWAYS AND WHEN OPERATING ON TAXIWAYS WHICH REQUIRED SHARP 90 DEG TURNS; SELECT HYD PUMP 'ON-OVERRIDE' SO THAT NO INTERRUPTION OF PRESSURE (COUGHING) CAN HAVE EFFECT ON STEERING OR BRAKING. ADDITIONALLY; I WILL INSTRUCT THE FO TO CONDUCT THE FLAP TEST ON STRAIGHT TAXIWAYS ONLY. I WILL TAXI AT A 'SNAILS PACE' ON ANY TAXIWAY LESS THAN 75 FT WIDE.