During the launch of Apollo 12 a major malfunction was caused by 2 lightning strikes. See http://www.realclearscience.com/blog/2013/02/apollo-12-struck-by-lightning.html for more info.

The solution to the issue was to 'Switch SCE to AUX', which is pretty clearly to switch 'something' to Auxiliary.

What was switched to auxiliary? And why was this solution so unexpected?

I would also appreciate a link to a more complete story of this event if possible.

  • $\begingroup$ I know I've seen this question on stackexchange somewhere before; but I'm not having any luck finding it. $\endgroup$ Commented Aug 16, 2016 at 19:20
  • $\begingroup$ I had the same feeling. I see a couple of questions about lightning that mention A12, but not "SCE to Aux". $\endgroup$ Commented Aug 17, 2016 at 3:21

1 Answer 1


It stands for Signal Conditioning Equipment. From Wikipedia

The loss of all three fuel cells put the CSM entirely on batteries, which were unable to maintain normal 75-ampere launch loads on the 28-volt DC bus. One of the AC inverters dropped offline. These power supply problems lit nearly every warning light on the control panel and caused much of the instrumentation to malfunction.

Electrical, Environmental and Consumables Manager (EECOM) John Aaron remembered the telemetry failure pattern from an earlier test when a power supply malfunctioned in the CSM Signal Conditioning Equipment (SCE), which converted raw signals from instrumentation to standard voltages for the spacecraft instrument displays and telemetry encoders.[3]

Aaron made a call, "Try SCE to aux," which switched the SCE to a backup power supply. The switch was fairly obscure, and neither Flight Director Gerald Griffin, CAPCOM Gerald Carr, nor Mission Commander Pete Conrad immediately recognized it. Lunar Module Pilot Alan Bean, flying in the right seat as the spacecraft systems engineer, remembered the SCE switch from a training incident a year earlier when the same failure had been simulated. Aaron's quick thinking and Bean's memory saved what could have been an aborted mission, and earned Aaron the reputation of a "steely-eyed missile man".[4] Bean put the fuel cells back on line, and with telemetry restored, the launch continued successfully. Once in Earth parking orbit, the crew carefully checked out their spacecraft before re-igniting the S-IVB third stage for trans-lunar injection. The lightning strikes had caused no serious permanent damage.

And here is another good version of the story.


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