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(NSTS) Program is developing net type barriers to be added to a set of runways to provide additional rollout overrun protection.

XVIII. NOSEWHEEL STEERING

Nosewheel steering was provided in the basic program design. The as implemented system was found to have several single failure modes which could result in hardover failure. Lack of crew confidence in this system due to its failure modes, the apparent availability of redundant differential braking capability plus program funding limitations resulted in no improvements to the nosewheel steering system. The landing of STS 51-D pointed out that the brake stator lockup and resultant tire lockup without nosewheel steering could greatly compromise directional control and, hence, flight safety. The KSC landings were terminated until the nosewheel steering system could be upgraded.

A three-phase program was developed to eliminate the fundamental concerns over the use of nosewheel steering. The first was a switch modification to make the nosewheel steering engage switch easy to
locate by feel without having to look inside the cockpit. The second phase was an upgrade of the system itself to remove the hardover failure and to establish a failure path that would leave one of the nosewheel steering modes operational (direct or computer) or fail to a free caster condition. This does not provide a full redundant capability but does provide a capability that can be used without fear
of making the problem worse in the face of a single point failure.

In normal operation, the general purpose computer (GPC) is the primary control mode and should be active at nosewheel touchdown. As a design criteria, the system was built to provide sufficient directional
control for the two blown tire case with a 20-knot crosswind. Using the old tire model this was achieved. Using the new model, some severe cases can result in off runway situations; however, an update to the nosewheel steering (NWS) gains does compensate for this case.

Failure annunciation was provided with wraparound fault detection in the computer mode. The software in both the primary and backup flight computer system was modified to provide reliable control capability. Additionally, the direct mode command transducer was reshaped to remove a tendency to overcorrect.

Activation power is provided by the system 1 hydraulics system. Any failure of the system will result in fail over to the caster mode. The system design improvements were evaluated on the simulation facility at the Ames Research Center. Full-scale testing was performed for all new or modified components. Flight testing was accomplished on the EAFB lakebed on STS 61-A and the system was used operationally on the EAFB concrete runway on STS 61-C.

A possible long-term fix which, if funded and scheduled, will provide full system redundancy is referred to as phase III. This last phase would provide the full operations confidence that this directional

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