Auxiliary Power Units (APUs) 2 and 3 were started (to provide flight control hydraulic pressure). The 160-second twin-engine OMS de-orbit burn took place during the 36th orbit over the southern Indian Ocean and changed the orbital parameters from to . This ensured atmospheric capture of the spacecraft close enough to the planned landing site to have sufficient energy for a controlled glide landing, but not so close that energy would have to be dissipated at a rate exceeding its structural capability. Young then slowly pitched "Columbia" up to the wings level nose high entry attitude. Both crew members armed their ejection seats during this pitch around. Nearly half an hour later APU 1 was started as planned. Shortly afterwards "Columbia" entered an approximately 21-minute long communications blackout. This was due to a combination of ionisation (16 minutes) and lack of ground station coverage between Guam and Buckhorn. Entry Interface (EI) was reached over the eastern Pacific Ocean from the landing site at a speed of around . EI is merely an arbitrarily defined geodetic altitude of employed by NASA for the purposes of trajectory computations and mission planning. Above this altitude, the spacecraft is considered to be outside the "sensable atmosphere".