The AOA abort mode is
used in cases in which vehicle performance has been lost to such
an extent that either it is impossible to achieve a viable orbit
or not enough Orbital Maneuvering System (OMS) propellant is available
to accomplish the OMS thrusting maneuver to place the orbiter
on orbit and the deorbit thrusting maneuver. In addition, an AOA
is used in cases in which a major systems problem (cabin leak,
loss of cooling) makes it necessary to land quickly. In the AOA
abort mode, one OMS thrusting sequence is made to adjust the post-main
engine cutoff orbit so a second orbital maneuvering system thrusting
sequence will result in the vehicle deorbiting and landing at
the AOA landing site (White Sands, N.M.; Edwards AFB; or KSC).
Thus, an AOA results in the orbiter circling the Earth once and
landing approximately 90 minutes after liftoff.
After the deorbit thrusting
sequence has been executed, the flight crew flies to a landing
at the planned site much as it would for a nominal entry.