Buzz Aldrin is credited (or was tasked) with being the Apollo 11 Lunar Module pilot. However in fact it was Neil Armstrong who piloted the craft down when the LM guidance computer overflowed, with Aldrin reading out to navigation data.
Why didn't Aldrin, the LM pilot, "take the stick" when human intervention became necessary?
EDIT: Even though the question has been answered, I just found this mention that also on Apollo 12 was the LM piloted not by the pilot, but rather by the commander.
The question should really be, why did Armstrong pilot the Lunar Module (LM) manually, and the autopilot was switched off. According to Tales From The Lunar Module Guidance Computer by Don Eyles, Armstrong and Aldrin never switched places during the LM powered descent and landing phase. Armstrong was on the left side of the LM cockpit where the manual controls were, and Aldrin on the right and responsible for working the DSKY (Lunar Module Display and Keyboard Unit), essentially instructing the flight computer.
For reference, here's a photo of the Apollo Lunar Module's interior (source and credit: NASA HQ):
It seems that the 1202 Program Alarm that the LM was experiencing prior to the landing prompted the switch from AUTO to ATT HOLD flight mode. From mentioned source:
ATT HOLD meant the digital autopilot's Rate-Command Attitude-Hold mode, in which the astronaut could command an attitude rate by deflecting a joystick. After the stick was released the autopilot nulled rates to maintain the present attitude.
Armstrong, as the mission commander, had to make a decision whether to proceed with the landing or not, while dealing with it with the mission control in Houston. It turned out that:
Due to an error in the checklist manual, the rendezvous radar switch was placed in the wrong position. This caused it to send erroneous signals to the computer. The result was that the computer was being asked to perform all of its normal functions for landing while receiving an extra load of spurious data which used up 15% of its time. The computer (or rather the software in it) was smart enough to recognize that it was being asked to perform more tasks than it should be performing. It then sent out an alarm, which meant to the astronaut, I'm overloaded with more tasks than I should be doing at this time and I'm going to keep only the more important tasks; i.e., the ones needed for landing...
— Margaret Hamilton, lead Apollo flight software designer
And the computer was prompting Armstrong and Aldrin with other alarms, for all of which they received a go to proceed from Houston. According to Human and Machine in Spaceflight, David A. Mindell, 2008 Massachusetts Institute of Technology (PDF):
With the push of a button, the astronaut could abort the landing, an action practiced countless times in simulation. Yet he held off. Armstrong later explained himself as a mechanism:
‘‘In simulations we have a large number of failures and we are usually spring-loaded to the abort position. And in this case in the real flight, we are spring-loaded to the land position.’’
Houston checked out the problem. Young engineers recognized it from a recent simulation, and conferred with their support teams in the back room. They quickly found the cause. The computer was overloading and restarting but not shutting down. It was ignoring low-priority tasks, but these were not critical for the mission. ‘‘We're go on that alarm,’’ the ground controller replied, meaning the LM could proceed. For his role in clearing the landing, engineer Steven Bales later accepted a presidential award on behalf of the flight control team.
Armstrong surveyed the computer display; it had frozen. He checked the LM’s systems. The vehicle seemed to be responding to his commands, meaning the computer was still running. So he continued.
Emphasis mine. So it seems that the switch to manual was due to a set of circumstances prior to the landing. Armstrong, as the mission commander, continued piloting the LM in ATT HOLD flight mode after the 1202 Program Alarm, and all subsequent alarms they were in contact with Houston about. After mission control gave a go on the alarms, and having the control over the flight computer, he made a command decision and took The Eagle down to its landing spot — Tranquility Base at the Sea of Tranquility (Mare Tranquillitatis), the Moon.
I believe the best answer to this question is to deny its premise, "Buzz Aldrin is credited (or was tasked) with being the Apollo 11 Lunar Module pilot." A better phrasing is that he was billeted as being the LMP. But that was neither what he was credited for nor what he was tasked with doing. It was simply a great title for someone whose job in every non-contingency case was to assist.
Note that as a general principle if the CM or LM needed to be piloted and the commander was present then he would usually be the one to do it. A major exception was re-entry, since it was possible in the case where lunar rendezvous failed that the Command Module Pilot would then be the only person in the re-entering spacecraft; therefore it was his job to pilot the re-entry. Transposition and LM extraction was also the CMP's job, presumably because CM docking was necessarily his specialty.
Neil Armstrong was also very cool in the first USA in space mishap, tumbling after docking with the Agena
But a better explanation: All these astronauts were ultra high achieving, fighter pilots and test pilots with high ranks. The LMP was actually a co-pilot/flight engineer job (and needed said job very much along with a very competent main pilot). But none of these guys wanted to be called "co-pilots" so NASA smartly called them (Gemini) Command pilot and pilot and Apollo Cdr, CMP, LMP
I have mentioned in other answers that I was around as an adult when the Apollo programme was happening.
The following was "common knowledge' around the time of Apollo 11, as read in newspapers and books, so common I never thought that I would grow old enough to require references for what were "current affairs" for me.
NASA was formed with the early part-reason to reduce the sometimes bitter inter-service rivalries over which service would or could put a satellite in orbit. America needed all of its experts to work together.
It was always reported over the years when the Apollo programme was still current affairs for me, that Neil Armstrong was a civilian and that, alongside his technical abilities and academic training, was a prime reason why he was chosen to be the first man to set foot on the moon (and thus prevent an apparently ever-present worry of inter-service rivalry rearing its head sufficiently via "bragging rights" to compromise the high level of teamwork needed for NASA to operate efficiently).
I never heard of any other major reason why Neil Armstrong was chosen, and I did hear the "civilian" reason touted many times, for his choice among many others of equal competence.
However, although not publicised as such as a factor for his being chosen for his role, Neil Armstrong was known at NASA to be cool under threat of imminent death. He was seen - and the live video of the crash probably still exists on youtube although I haven't looked for a while - bailing out about one second before the crash of the out-of-control LLRV (lunar landing research vehicle) he was flying, impacted the ground. Later in the day, he apparently astonished his fellow astronauts by working quietly away at his normal tasks in his office like nothing had gone wrong with his day.
The question does raise the issue that by virtue of existing aeronautical tradition if nothing else, Buzz Aldrin should have been the lunar pilot and therefore should have been the first man to set foot on the moon. But he was a military man from a military family, and that would have counted against him, nothing to do with ability to do the job. Many articles were written around late 1960s-early 1970s of his prolonged efforts to be the LM pilot and thus be the first man to walk on the moon. Never once was his competency questioned, as I recall.
Edit: Addition to clarify that I answered the question "Why?" Point taken that there is more than one "why" about the choice of Neil Armstrong in the pilot's place in the LM.
I did dwell, above, on the "civilian" aspect why Neil Armstrong was chosen to fly the LM. It was consistently and widely publicised in international media. Could every reporter have gotten it so wrong for decades, without picking up the phone to NASA or having NASA call the major news feeds and say "you got the (civilian) reason wrong"?
But the question as put here here was fairly asked, and I was fairly reminded in a comment below, that my "why" answer, above, was maybe a bit limited in its breadth. Question: why was Neil Armstrong the LM pilot? Answer, worded in a different way so it covers more aspects of "why": because Neil Armstrong was in the pilot position, because he was to be first out the spacecraft hatch, because he and Buzz Aldrin couldn't swap places when suited up in the LM therefore Neil had to be nearest the hatch, because both Buzz and Neil were competent fliers of the LM so the question of who was the better pilot didn't arise, and in this most critical procedure of all time - the first landing on another world with no chance of rescue if things went wrong - it didn't hurt to know down at NASA that Neil was the one who had tangibly displayed incredible coolness under life-threatening danger with the LLRV crash.
So that is why Neil Armstrong flew the LM when Buzz Aldrin was the LM pilot.
I see that Deke Slayton has been quoted from his autobiography in a comment below, what he had felt in 1964 about how he chose astronauts. It's his autobio and one should respect that. However, the international news reports of US military rivalries and the damaging effect of a lack of co-operation between the services on a matter of international prestige, dated back to shortly after 1957 and Sputnik 1 - long before Deke's memories of his feelings in 1964. This was also a time of embarrassing American failures from different arms of the military around that time to get a "face-saving" satellite into orbit. NASA had to demonstrate that there was no place for egos of any kind as it settled down and focused on doing a job well, with eventual awe-inspiring success. That "unspoken" awareness had to have been in the NASA "psyche". The plentiful evidence of history strongly suggests why Neil Armstrong was to be given the ultimate prestige of all time rather than a military man, as he went on that journey carrying President Nixon's message, "We come in peace [not war] for all mankind."
In space missions, it is common to have two pilots. The "commander" is the more senior one, and does most of the piloting, while the "pilot" is the junior one, and may have some roles. Neil Armstrong, as commander, was the more senior pilot, and practiced the landing. Buzz, on the other hand, was primarily tasked to manage the spacecraft when on orbit, specifically the rendezvous operation.
You can see the specific definitions for each of the positions on Wikipedia.
I don't think these answers are correct. Think of the Captain of a ship. The pilot is one who provides the specialist navigation advice. To my way of thinking Aldrin was pilot and the equipment on his side of the LEM seems to support my hypothesis. Armstrong remained Captain. I know from naval days that when entering port etc as the navigating officer I would call the undisputed data (chains to wheel over, new heading etc) and then make recommendations to turn and degrees to set. Those recommendations may have been altered by half a second or so and a new heading recommendation accepted or altered by the captain. As a matter of pure pride and outrageous immodesty, it's a question I should have asked Apollo 12 Commander Pete Conrad with whom I spent a week in Sydney Australia flying MD 500 When he was VP Marketing for McDD in 1985. At one point we even flew to Palm Beach to pick up Qantas pioneer pilot Scotty Allan.
