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4. Runway length margin (calculated runway length remaining after wheel stop) is reduced from 3000 to 2000 feet.

TAL abort may also be selected to cover major multiple or "non credible" systems failures during ascent to avoid going to orbit with problems that would seriously threaten Orbiter and crew safety. These failures would require the earliest available landing time or avoid going to orbit without the system capability to return. These failures are detailed below.

• Loss of RTLS aerodynamic separation capability resulting from certain RCS jet losses (continue ascent if performance available)

• Two external tank (ET) low level sensors fail dry in same tank

• Loss of two freon loops

• Impending loss of cabin pressure

• Impending loss of all cryo oxygen or hydrogen tanks

• Same OMS propellant tanks leaking/failed on both sides of Orbiter; different propellant also applies if OMS-1 cannot be supported

• Loss of both OMS helium tanks (when necessary)

• Aft reaction control system (RCS) propellant or pressurant leak that would lose entry attitude control

In general, energy downmoding is available at each TAL site in a manner similar to that available at KSC; i.e., both ends of the same runway can be used taking advantage of an overhead or straight-in approach plus using the HAC shrink and minimum energy aim point options. The capability to reach the threshold of a TAL runway should be comparable to that of reaching KSC for an RTLS (up to 9 to 22 nautical miles). Exceptions would include DAKAR where only one runway direction is available. Headwind/tailwind magnitude is a significant factor in TAL site energy downmoding since TAL runways are typically shorter than at KSC. It should also be noted that the landing aids at each TAL site may not be the same as at primary end-of-mission runways which could make the energy downmode options less viable.

Ferry operations are identical to those used for KSC returns from EAFB landings. The major problem is the preparation for ferry operations to reduce the SCA payload weight to a value acceptable for the range capability of the SCA. For landings at the western TAL site, Easter Island, there is no baseline Orbiter ferry plan to date. Program options include replacing engines on SCA, inflight refeuling, and developing a seagoing barge capability.

Each TAL site has its own climatology and presents its own forecasting problems. Observers are sent to each primary TAL site to augment the raw and forecast data available from foreign weather satellites and local forecasters. Tools to analyze, format/display, and forecast TAL

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