At the end of the powered descent, when the lunar module was close to the lunar surface, the program P66 was running in the AGC.
Unlike the previous programs, this program was not entirely automatic, but only semi-automatic.
The role of the program P66 was to continuously adjust the thrust of the main engine, so that it would exactly counter the lunar attraction, and the lunar module would remain at a constant altitude as it moves over the lunar surface.
Indeed, as the lunar module burns propellant, it loses weight, and so the thrust must be adjusted so that it would continue to counter the lunar attraction, but without exceeding it (otherwise it would move up, for the force of the thrust would become over the lunar attraction with a lighter lunar module).
To do its calculations, the AGC uses the information of the weight, but also of the altitude and altitude rate.
If it was the astronaut who was controlling the thrust, with the handcontroller, he would not be as precise as the AGC, for he does not have a computer in his head, and he would alternately apply too must thrust or not enough, and the lunar module would not be as regular as with the control by the AGC; the lunar module would rather follow a trajectory looking like russian mountains than a stable trajectory.
During this phase, the Engine thrust control switch must remain on the position "Auto", which means that it is the LGC (and thence the AGC which commands the LGC) which entirely has the control of the thrust, and the handcontroller has no action on it.
But, if the astronaut does not have the control of the main engine, in this phase, he has on the other hand the control of the RCS, which means that he can move the lunar module over the lunar surface by using the lateral thrusters.
The RCS allows to move the RCS in all directions, and also to turn it in all directions; in absence of friction on the moon, even a moderate force can set the lunar module in motion, and at any speed.
So the program P66 is in fact a cooperation between the AGC, which takes in charge the control of the main engine, and the astronaut, who uses to RCS to move over the lunar surface.
The fact that the AGC takes care of the thrust of the main engine, and allows the lunar module to remain at a constant altitude over the moon, allows the astronaut to exclusively concentrate on the relief of the moon, and uniquely move the lunar module over the lunar surface.
When the astronaut has found the ideal spot to land the lunar module on, it cannot remain on the program P66, for this one would prevent from moving the lunar module down.
So the astronaut calls the last program of the descent, the program P67.
Unlike the program P66, and the previous programs, this last program is not automatic at all, it is entirely manual.
In fact, it just consists in instructions given to the astronauts to perform the descent, that they manually control themselves.
And, when they call a program on the AGC, it is not to ask it to help them monitor the descent, but only to get informations which are supposed to help them manually control the descent.
In particular, the program P67 specifies that they must call the function corresponding to the verb 06, noun 60.
This function corresponds to the periodical display of horizontal informations (Z is a horizontal axis of the lunar module, le vertical axis is called X).
Sure, horizontal informations are very useful to vertically control the descent!
You cannot compare the control of the lunar module with the one of a helicopter.
A helicopter counters earth's attraction passively, with the force of air, while the lunar module counters the lunar attraction actively, with the force of its engine, and the reactions are very different.
Then a helicopter has a much better vision of the ground underneath than the lunar module.
And don't think that it would be possible to use the gimballing system of the main engine to pitch the lunar module so that the astronauts would have a better vision of the ground under the lunar module.
The exclusive use of the gimballing system is to remake the alignment of the thrust with the center of mass, as this one gradually shifts with the level of the tanks lowering, as the lunar module burns propellant.
If the main engine was swivelled with the gimballing system so to pitch the lunar module, and allow the astronauts to have a better vision of the lunar ground, there would be a consistent shift between the line of thrust and the center of mass, which would create a misalignment torque, which would make the lunar module turn and crash on the moon.
And, anyway, the gimballing system cannot turn that much.
In this last phase of the landing, the Engine thrust control switch must be set on the position "Man", so to give the control of the thrust to the handcontroller instead of the LGC.
The problem is that, if the current position of the handcontroller does not correspond with the current thrust commanded by the LGC, which is little probable, the lunar module is going to either brutally move down ot up, according to the current position of the handcontroller.
The astronaut, with the poor vision of the lunar ground he has, is going to have a great difficulty to adjust the thrust of the main engine, and will aternately apply too much thrust or not enough, and the descent of the lunar module might look like the one of a yoyo.
And the lunar module might have too much speed when it is close to the lunar ground, which means that the touchdown would be a little brutal!
And controlling the thrust is not the only thing the astronauts had to worry about.
When the lunar probes detect the lunar ground (at 1.7 meter from the lunar surface), they don't automatically shut off the engine, they only give a visual indication, and it's up to the astronauts, when seeing this visual indication, to shuf off the engine by pressing the stop button.
Moreover, the way that this stop button was conceived was absurd, for it had to do a complete come and go to effectively stop the engine.
There was no guarantee that the astronauts would have shut off the engine (by pressing the stop button) before the lunar module would touch the ground.
For instance, in Apollo 11, Armstrong was a little late to shut off the engine, and this one was still running when the lunar module touched the ground.
Fortunately, the lunar module benefitted of a flat ground in Apollo 11, and the engine's nozzle didn't touch the ground when the lunar module rested on the lunar surface.
Such was not the case in Apollo 15, in which the lunar module landed on an uneven ground and the engine's nozzle touched the ground.
Fortunately, in this mission, David Scott shut off the engine before the lunar module touched the ground.
But, if David Scott had been as slow at shutting off the engine as Armstrong, and this one would still have been running when the lunar module touched the ground, it would have had a catastrophic effect in Apollo 15.
A helicopter has no such concern, as its blades don't touch the ground when it rests on the ground, and it can have its blades still turning as it lands.
The problem is that the astronauts have no warning sign that the lunar probe lights are going to be illuminated, no orange light, which means that, not knowing in advance when they will be illuminated, they have to constantly concentrate their attention on the dashboard, which might distract them from what they can see outside.
And, if they are distracted by what they see outside, they might not immediately notice the lunar probe lights.
And the problem is that, while this control is far from being obvious, the astronauts have not even been trained on earth to do it, it is the first time they do it.
Indeed, the training they have received on earth was much more basic, and not sophisticated enough to accustom them to correctly do this control.
Because of the difficulty of finding the right adjustment of the thrust to do this descent, the lunar module might arrive near the lunar ground a little too fast, and, because it is too fast, the astronauts might not have the time to press the stop button, after they have seen the lunar probe lights go on, before the lunar module touches the ground, a ground which is unknown, and which might be uneven.
In other words, the manual program P67 is very far from being safe!
Rationally, the program P67 should not have been manual, but entirely automatic instead.
The Engine thrust control switch would have remained on the position "Auto", to give the control of the thrust to the LGC (and so to the AGC).
The AGC would have programmed the descent so that it would have been very regular and slow; the lunar module would have arrived near the lunar ground with a moderate speed; when the AGC would have received the signal from the lunar probes, it would have known that the lunar module was at 1.7 meter from the lunar surface, and it could have stopped the engine at the right moment.
The lunar module would have landed on the moon smoothly, and safely, with the engine shut off as the lunar module touched the ground.
The landing would have been totally safe, without any human error possible.
for, if the astronauts could manually start and stop the engine (indicated in green), The AGC could also automatically do it (indicated in blue)!
But the engineers illogically preferred a manual procedure, which was far from being safe, and which could eventually end fatally.
In a movie, the actors can take any risk, as insane as it can be, for it is written in the script that everything will end well.
But, in a real mission, nothing is written in the script, and an unnecessary risk can lead to a catastrophy.
Now, I know what some smart minds are going to tell me: It was working fine with the manual descent, for it perfectly succeeded six times in a row; remember, these astronauts were real aces!
Oh yes, it perfectly worked six times in a row...on the fake moon set, it can never fail!
