Excursion from taxiway.

Attributes
ACN	200713
Time
Date	199201
Day	Thu
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : cll
State Reference	TX
Altitude	agl bound lower : 0 agl bound upper : 0
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	Light Transport, Low Wing, 2 Turboprop Eng
Navigation In Use	Other Other
Flight Phase	ground other : taxi
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 63 flight time total : 75882 flight time type : 2217
ASRS Report	200713
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	aircraft equipment problem : critical other anomaly other anomaly other other anomaly other
Independent Detector	other flight crewa
Resolutory Action	none taken : unable
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation other

Narrative:

While taxiing to runway 8 had to negotiate two 90 degree turns on a very narrow taxiway (50 ft). As I started the lh turn the first officer began a flap test (normal procedure). The nose wheel started sharply toward the left so I attempted to apply the brakes. (The nose wheel steering, flaps and brakes are all hydraulically operated). The hydraulic actuator 'coughed' as the pressure dropped to the bottom of the indicator arc, then cycled on. As the pressure arrived to the system, the left 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 aircraft came to a stop. The flight attendant was standing in the aisle conducting a safety check and was thrown to the floor. There was no aircraft damage, and no injuries. All crew members submitted to a drug test which all were 'negative' and all reported proper rest the evening before. What did I learn? When operating on narrow taxiways and when operating on taxiways which required sharp 90 degree turns, select hydraulic pump 'on-override' so that no interruption of pressure (coughing) can have effect on steering or braking. Additionally, I will instruct the first officer to conduct the flap test on straight taxiways only. I will taxi at a 'snails pace' on any taxiway less than 75 ft wide.

Original NASA ASRS Text

Title: 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.

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.