The crew continued preparing for lunar module activation and undocking shortly after waking up to begin flight day five. The boom that extended the mass spectrometer out from the CSM's scientific instruments bay was stuck, semi-deployed. It was decided that Young and Duke would visually inspect the boom after undocking from the CSM in the LM. They entered the LM for activation and checkout of the spacecraft's systems. Despite entering the LM 40 minutes ahead of schedule, they completed preparations only 10 minutes early due to numerous delays in the process. With the preparations finished, they undocked in the LM "Orion" from Mattingly in the CSM "Casper" 96 hours, 13 minutes, 31 seconds into the mission. For the rest of the two crafts' passes over the near side of the Moon, Mattingly prepared to shift "Casper" to a circular orbit while Young and Duke prepared "Orion" for the descent to the lunar surface. At this point, during tests of the CSM's steerable rocket engine in preparation for the burn to modify the craft's orbit, a malfunction occurred in the engine's backup system. According to mission rules, "Orion" would have then re-docked with "Casper", in case Mission Control decided to abort the landing and use the lunar module's engines for the return trip to Earth. After several hours of analysis, however, mission controllers determined that the malfunction could be worked around and Young and Duke could proceed with the landing. As a result of this, powered descent to the lunar surface began about six hours behind schedule. Because of the delay, Young and Duke began their descent to the surface at an altitude higher than that of any previous mission, at . At an altitude of about , Young was able to view the landing site in its entirety. Throttle-down of the LM's landing engine occurred on time and the spacecraft tilted forward to its landing orientation at an altitude of . The LM landed north and west of the planned landing site at 104 hours, 29 minutes, and 35 seconds into the mission, at 2:23:35 UTC on April 21.