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« Last post by JayUtah on November 07, 2024, 04:18:09 PM »
I still use "Tindallgrams" as prime examples of how to write in a technical context so that your communication is clear, accurate, and jargon-minimal.
As I wrote above, P64 originally transitioned to P65 when it reached a prescribed altitude. P65 is an automatic descent program: it nulled the lateral motion rates and lowered the LM straight down to the surface by managing the throttle to match preprogrammed height and descent-rate criteria. But the crews generally used P66, which is misleadingly described as "manual control."
In P66, the PGNS mode is switched to ATT HOLD mode. This means the hand controller dictates the LM attitude. You move the hand controller as usual, and you let it go back to the center detent when the LM is in the attitude you want, which may be pitched forward, backward, or to either side. Obviously this means the DPS is giving you horizontal acceleration as well as controlling the descent. Armstrong basically pitched Eagle foward considerably in order to speed past the boulder field. When the controller enters detent, the DAP automatically nulls any rotation rates and then holds the LM in that commanded attitude until told differently.
Incidentally this is why you hear Armstrong say, "ACA out of detent." A consequence of landing on the surface in ATT HOLD mode is that the surface orientation may not be the commanded LM attitude. As the landing gear settles and the ship lands, the DAP will fire the RCS jets furiously trying to put the LM back in the commanded attitude, only to fail because the ship can't rotate while landed. The immediate solution is just to wiggle the controller out of detent and let it snap back. The DAP then accepts the landed attitude as the commanded attitude. The permanent solution is Aldrin's indication, "Four-thirteen is in." Erasable memory location 0413 in the AGC is the "We have landed" variable. If the DAP sees anything non-zero in that memory location, it doesn't do anything autopiloty. The pilot manually sets that to non-zero on the DSKY to confirm to the computer that the ship is on the ground.
What happened on Apollo 12 is that when the crew switched into P66, they started veering to the right. Not much at first, but the LM had just enough right roll in the commanded attitude to build up quite a bit of ground-track error. By the time Pete Conrad noticed it, he had to steer sharply to the left to return to the nominal landing track. Imagine your teenager pulling into the garage for the first time and paying so much attention to one side of the car that he nearly shears off the wing mirror on the other side—he'll wrench the wheel sharply in the other direction once he notices.
The concern was that there was too much mental workload in P66. The pilot had to manage horizontal rates in two dimensions as well as descent rate, all while keeping an eye on the intended landing site. This is why Armstrong had Aldrin watching those figures and calling them out when they were important. Armstrong didn't want his attention shifting from cockpit to window and back. And that's a legitimate concern: it's why we have heads-up displays where possible.
The analysis between NASA and MIT was that some of the functions of P65 needed to be duplicated in P66: notably the code to null out the lateral rates. This was then tied to the PGNS mode selector so that ATT HOLD would continue to behave as before, but AUTO would engage the automatic lateral control. Then there were a few refinements to prevent this new mode from doing stupid stuff like lurching violently in order to null high lateral rates, and cutting out at a certain low altitude when the radar data would get noisy and cause misbehavior.
The final change was to have P64 transition to P66 instead of P65.
This whole episode is a great example of how things look at first clean and elegant on paper—separating mostly-automatic and mostly-manual modes—but then become appropriately baroque as the initial operators say, "Hey, now that I think about it..." I always quip that there are two stages to every design: (1) too early to tell, and (2) too late to do anything about it.
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