The test was a must-do task after technicians replaced a faulty power-drive unit (PDU) last week. The unit controls the shuttles rudder speed-brake used to slow the orbiter during landing. On Saturday afternoon, engineers began the process of testing the orbiters hydraulic systems by swiveling the main engines and moving the flight-control surfaces along with the speed-brake.

Already facing a time crunch, managers nixed a move back to the hangar and an inevitable launch delay as late as mid-May, and instead called for an unprecedented launch pad repair of the PDU.

Atlantis is scheduled to lift off from KSC at about 4:15 p.m. Eastern Daylight Time (20:15 GMT), April 24 on a critical mission to rendezvous with the International Space Station. While in orbit the crew will boost the station to a higher altitude, deliver supplies, replace faulty equipment and perform one spacewalk.

Increased solar activity and atmospheric drag is causing the ISS to drop about 1.75 miles (2.74 kilometers) per week.

Atlantis will boost the station about 20 miles (32 kilometers) so thrusters on the station can be saved for other purposes.

Engineers plan on "hot-firing," or running, one of the orbiters auxiliary power units (APU) about midnight EDT tonight with the test concluding early Monday. Technicians replaced a discharge hose on the APU last week.