Commercial Fixed Wing
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 240; Flight Crew Total 20509; Flight Crew Type 12000
Situational Awareness
Aircraft Equipment Problem Critical
General Maintenance Action
Aircraft
F/O conducted the walk-around and this was a night departure.  I was also on the ramp; returning from operations after picking up my required paperwork.  We met at the base of the jet bridge stairs; and he requested me to look at the left wing.  Approaching the wing; I saw nothing wrong.  After illuminating the flap track canoe; I was able to see a small diameter cable that appeared to be attached under the leading edge of the wing.  The aft end of the cable was tucked in to the aft end of the canoe; which caused an unsafe aircraft condition. Maintenance was notified; and in their actions; they lowered the leading edge slats.  It was then noted; the outboard lengthwise cable was broken in the slat asymmetry system.  Further; the inbound flight crew had no indication; due to the apparent lack of leading edge disagree light illuminating.  The discovery of this thin; broken cable during nighttime conditions was; in my opinion; an unlikely possibility.  This mechanical issue was attended to while the aircraft was still on the ground; and before it became an issue after departure or in in flight.
B737 Captain describes the discovery during preflight inspection of a cable broken in the slat asymmetry system.  The cable was difficult to detect in the darkness.