What was the correct procedure to stop the descent engine on the Apollo 11 LEM?

When the LEM had a contact light on Apollo 11, there was some terminology spoken that I'm assuming is important (and which I find quite interesting regardless).


However, what of this was actually required to stop the engine? As an aside, I think it is highly relevant to ask for a clarification for the following terminology without making a separate question for each phrase. In particular the meaning of:

"ENGINE STOP": Does this actually stop the engine, or is it simply a part of the steps that must be taken? If not, then what is the meaning of the other phrases spoken?

"ACA -- out of detent": What is this? A mechanical fuel pump that is stopped? What is ACA? Attitude Control Assembly (https://www.hq.nasa.gov/alsj/apollo.glossary.html)?

"MODE CONTROL—both AUTO": I don't know what this is.

"DESCENT ENGINE COMMAND OVERRIDE—OFF": I'm assuming this is to prevent the abort stage from firing automatically.

"ENGINE ARM—OFF": Perhaps this is the actual step of stopping the engine, and the "ENGINE STOP" is merely announcing that they now start the procedure of stopping the engine, and this is the final step?

As the LEM descent engine uses hypergolic fuel with a pintle injector, it doesn't seem like a mechanical pump is required.

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    $\begingroup$ Engine arm off means that an additional action (arming) will be necessary, to start the engine again. This is to prevent accidental activation. Any device is typically completely inactive at the time when the arming is turned off. $\endgroup$ Commented Jul 28, 2015 at 21:55

2 Answers 2


The annotated transcripts are great for this stuff. Also O'Brien's AGC book.

Engine Stop: The engine stop button is pressed, which I believe would set throttle immediately to zero. In this picture, the stop button assembly is visible between the yellow handle and the start button, pointed leftward. Presumably the placement of it is chosen to avoid accidental shutdown.

enter image description here

Once the contact light came on (about 1.6m above the surface), the commander would hit the button, the engine would stop, and landing would be inevitable.

ACA out of detent: The control system was in "attitude hold" mode; as long as the commander wasn't actively trying to rotate the ship, the system would try to maintain the LM in fixed orientation. At touchdown, the attitude control thrusters would be trying to hold the last commanded attitude. By pushing the Attitude Controller Assembly just barely out of its neutral (detented) position, the system is instructed that the current attitude is the desired attitude, stopping the RCS rockets from firing.

Mode control both auto: I assume this is related to the guidance mode control -- automatic landing (LM commander designates a landing spot and the LM computer does the rest) versus attitude hold (LM computer holds the LM's orientation stable and the commander does the rest). I imagine you'd want it back in automatic mode in case you needed to do an emergency ascent. Not sure what "both" refers to; best guess would be that the PGNS (primary guidance) and AGS (abort guidance) each had mode controls.

Descent engine command override: Again from the annotated transcript:

Command Override is a shorthand for 'Descent Engine Command Override', which will allow either pilot to assume control of the descent engine's thrust using the Thrust/Translational Hand Controllers, if required.

Engine arm off: I think this is just a master switch for the descent engine, independent of the throttle. In the transcript, Aldrin arms the descent engine at 102:32:34, but ignition occurs some 30 seconds later.

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    $\begingroup$ Yes, the mode control switches are MODE CONTROL PGNS [AUTO / ATT HOLD / OFF] and MODE CONTROL AGS [AUTO / ATT HOLD / OFF]. AUTO is the program controlled mode, ATT HOLD is rate-command / attitude-hold, and OFF disconnects the given system from the attitude thrusters. As you say, the switches would have been in ATT HOLD prior to landing. $\endgroup$
    – hobbs
    Commented Jul 17, 2017 at 23:29
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    $\begingroup$ eaglelander3d.com/documentation/images/instruments1complete.jpg is a fake (rendered) version of that bit of switch panel $\endgroup$
    – hobbs
    Commented Jul 17, 2017 at 23:32
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    $\begingroup$ Just to expand on "ACA out of detent" a little bit - I always found this one a bit humorous. Russell is correct about what it means, but there's one more aspect of "why" to consider. The LM is in ATT HOLD when it is about to land, but the AGC doesn't really "know" about touchdown; all it knows is that the LM has just had its attitude knocked out of whack - and that's because its four legs have just settled onto terrain. So the CDR has to push the ACA out of its center detent just to give the RCS an arbitrary command to change attitude, at which point the AGC stops caring about maintaining its $\endgroup$ Commented Mar 14, 2020 at 19:35
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    $\begingroup$ old attitude and accepts sitting on the ground as the new attitude. When you think about it, you can understand why one wouldn't just code the AGC to stop maintaining attitude at contact light. First, what if contact light got triggered accidentally by an electrical or mechanical flaw? You'd hate for that to happen during powered descent at any point. Second, what if the CDR decided at the last moment to change his landing position after contact light? $\endgroup$ Commented Mar 14, 2020 at 19:35
  • $\begingroup$ this answer is wrong: the "START" button is NOT the "STOP" button; the "STOP" putton os poorly documented everywhere, and even pictures of it are very rare; it's "hidden" behind the STOP button, orthogonal to it, pointed to the left, behind the yellow handle in the picture in the answer. It's slightly better visible in this image: miro.medium.com/max/700/0*2d0-pFwojzdANswg Button was also duplicated near the DEDA, where it is alone, without START button: i0.wp.com/www.americasuncommonsense.com/wp-content/uploads/… $\endgroup$
    – jumpjack
    Commented Feb 18, 2022 at 12:01

Checklist for descent engine after contact light is lit:

  • ENG STOP: Push (Note: at first keypress, STOP button remains pressed)
  • ENG ARM: Off - Disarm (=disable) descent engine (Possible values: Ascent, Descent, Off: same switch arms descent engine OR ascent engine OR none)

In case of no-stay:

  • ENG ARM: Asc - Arm ascent engine (=enable START button)
  • ENG STOP: reset - Press ENGINE STOP again to unlock the button and allow engine start
  • ENG START: push - The ascent engine is started

Some sources: STOP button (from LM-10 Handbook vol.1)

Lunar contact checklist (from "Apollo 11 LM TimelineBook excerpts")

STOP button schematic (from LM-10 Handbook vol.1)

There is one START button in the LM, but two STOP buttons, one for commander and one for pilot; both STOP buttons are hidden behind yellow handles (note: they look different in different version of the Lunar Modules):

START/STOP buttons location

We have 3 checklists: Lunar contact, stay, no stay:

Lunar contact:

  • ACA OUT OF DETENT (put Attitude Control Assembly joystick away from central position, to disable auto-hold)
  • DES ENG CMD OVRD OFF (Disable override of autopilot)
  • ENG ARM OFF (disable START button)
  • Input 10000 in register 413 using DEDA: 413+1 (Turns on the "surface flag", i.e. the bit 8 of FLAGWORD8; this is read by Ground Control as "Eagle has landed" before Armostrong announces it by voice)
  • Input 20000 in register 414 using DEDA: 414+2
  • ASC FEED 2 (2) CLOSE

For "413+1" and "414+2":

413+1 https://www.hq.nasa.gov/alsj/LMA790-3-LM-4.4.pdf

413+1 https://www.hq.nasa.gov/alsj/FP6_OperatingManual.pdf


  • THRUSTER PAIR ISOL VLV(8) OPEN (Isolation valves of thrusters)
  • MAIN SOV (2) OPEN (Shut off valve)
  • CRSFD CLOSE (Crossfeed)
  • ASC FEED 1 (2) OPEN
  • DES VENT FIRE (Vent fuel)
  • OXID VENT OPEN (Venmt oxidizer)

(numbers in brackets should indicate the count of switches to be moved)

"Des vent" and "Oxid vent" are controlled by explosive valves, i.e. they are irreversible:

explosive devices

Lunar  module valves

No stay


I collected all the info in one single gigantic image.... unfortunately too large for SE, so I can only post a "miniature"; you should anyway be able to follow all the lines to the various controls, and have a closer look at them in the original image:

All controls


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