In 2005, the problem of foam shed had not been fully cured; on STS-114, additional cameras mounted on the tank recorded a piece of foam separated from one of its Protuberance Air Load (PAL) ramps, which are designed to prevent unsteady air flow underneath the tank's cable trays and pressurization lines during ascent. The PAL ramps consist of manually sprayed layers of foam, and are more likely to become a source of debris. That piece of foam did not impact the orbiter. Reports published concurrent with the STS-114 mission suggest that excessive handling of the ET during modification and upgrade may have contributed to the foam loss on "Discovery" Return to Flight mission. However, three shuttle missions (STS-121, STS-115, and STS-116) have since been conducted, all with "acceptable" levels of foam loss. However, on STS-118 a piece of foam (and/or ice) about in diameter separated from a feedline attachment bracket on the tank, ricocheted off one of the aft struts and struck the underside of the wing, damaging two tiles. The damage was not considered dangerous. The external hardware, ET–orbiter attachment fittings, umbilical fittings, and electrical and range safety system weigh . Each propellant tank has a vent and relief valve at its forward end. This dual-function valve can be opened by ground support equipment for the vent function during prelaunch and can open during flight when the ullage (empty space) pressure of the liquid hydrogen tank reaches or the ullage pressure of the liquid oxygen tank reaches . On early flights, the liquid oxygen tank contained a separate, pyrotechnically operated, propulsive tumble vent valve at its forward end. At separation, the liquid oxygen tumble vent valve was opened, providing impulse to assist in the separation maneuver and more positive control of the entry aerodynamics of the ET. The last flight with the tumble valve active was STS-36. Each of the two aft external tank umbilical plates mate with a corresponding plate on the orbiter. The plates help maintain alignment among the umbilicals. Physical strength at the umbilical plates is provided by bolting corresponding umbilical plates together. When the orbiter GPCs command external tank separation, the bolts are severed by pyrotechnic devices. The ET has five propellant umbilical valves that interface with orbiter umbilicals: two for the liquid oxygen tank and three for the liquid hydrogen tank.