NASA - National Aeronautics and Space Administration

Overview: Also called the external tank gaseous hydrogen vent arm system, at the 167-foot level, the 48-foot-long arm allows mating of the external tank umbilicals as well as contingency access to the external tank intertank compartment.

Image to left: Flanked by a solid rocket booster and external tank at left, STS-88 crew members pose for a group photograph near the top of the fixed service structure at Launch Pad 39A. They are (front to back) Mission Commander Robert D. Cabana, Mission Specialist Nancy J. Currie, Pilot Frederick W. "Rick" Sturckow, Mission Specialists Jerry L. Ross, James H. Newman, and (at right) Sergei Krikalev, a Russian cosmonaut. The crew was at Kennedy Space Center to participate in the Terminal Countdown Demonstration Test (TCDT), a dress rehearsal for launch. The intertank access arm is shown on the lower left side edge of the image within the black half-circle. Credit: NASA/KSC

The arm rotates 210 degrees to its extended position. The arm is retracted after umbilical/vent line mating, typically at about T minus five days, leaving the umbilical vent line connected to the external tank to support tanking and launch. The umbilical vent line provides continuous venting of the external tank during and after loading of the volatile liquid hydrogen. The vent line is disconnected from the vehicle at first motion and retracts vertically downward to a stored position.
NASA's John F. Kennedy Space Center