This is a related question about the words said during Apollo 11 as soon as the LM touched the ground.

My question is specific to

ACA out of detent

As soon as the LM touched down on the surface it wouldn't be in the same attitude just before landing, causing DAP to fire the thrusters to correct for the attitude. By nudging the ACA (the hand controller) the DAP would interpret the then current attitude as a new request to be held and thus as now current attitude is equal to requested attitude, DAP would stop firing.

My question is then, how did the code implement this functionality? If in attitude hold mode, the pilot nudged the hand controller, would the then current attitude always be copied as requested attitude? How then did it accept the new attitude from the pilot, if all it did was copy the attitude, or after copying, a small delta attitude was added in the direction where the hand controller was nudged?


  • ACA = Attitude Control Assembly, the joystick that the pilot uses to manually control attitude. It has springs to return it to a centered position and a detent to keep it there.
  • DAP = Digital Autopilot, a program in the guidance computer.
  • LM = Lunar Module, the spacecraft that landed on the Moon.

The "attitude hold" mode holds whatever the current attitude is at the moment when the ACA returns into the neutral position. The DAP does not interfere while the crew is making attitude changes using the ACA, but as soon as they're done with adjustments, the DAP will record and keep the current attitude:


(<”MODE CONTROL”> Switch in <"ATT HOLD"> position)

The ACA Electric Proportional output enters the LGC "A" Interface Circuit for processing. The Attitude-Hold feature engages whenever the ACA Stick is returned to the Detent position.

(From "Lunar Module Attitude Controller Assembly Input Processing" by J. R. Portillo Lugo, emphasis mine.)

So the idea behind "out of detent" is actually to make a dummy manual attitude adjustment and then let go of ACA again, so the post-landing attitude gets interpreted by the DAP as the desired state.

  • $\begingroup$ So is it like this, as hand controller is moved, the LM rotates as per request. But as soon as it comes back to detent position(like capture attitude on rising edge of the signal), the current attitude becomes the reference one? $\endgroup$
    – zephyr0110
    Jun 21 '21 at 11:49
  • $\begingroup$ @Prakhar Yes, that's exactly how it works. $\endgroup$
    – TooTea
    Jun 21 '21 at 12:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.