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.
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.
In addition to kim holder's answer, let me fill in some more technical details.
The three position SCE switch (NORMAL, OFF, AUX) does not select the power bus from which the SCE power module receives power! It has a different function than other "NORMAL, OFF, AUX" switches.
During ground drills and tests, there may be an auxiliary power source connected to the capsule. During flight, this AUX power bus is connected to the NORMAL power bus. No chance of switching to an non-existent power source mid-flight. Some have theorized that going though OFF on the SCE switch just "turned it off and on again", based on the assumption that the switch selects between power busses. It doesn't, so simply forget about these details.
Instead, the switch selects which internal switching power supply (regulator) inside the SCE is used. Switching power supplies were still kind of exotic back in the days, but the SCE had one (two) for efficiency. Two for redundancy, with one out of these two active at any time (unless powered down, OFF).
The SCE power supply module (with its two internal regulators) has a failover logic, which will switch to the other when ever the currently active module fails to deliver at least one its voltages (+5V, +10V, -20V, +20V) for longer than 200ms. This logic can be overridden by the AUX setting. Switching to AUX switches from the currently selected regulator to the currently inactive regulator (this "toggle" action is different from other systems where a similar switch directly directly selects the primary or the secondary power supply). The regulators are named "NO 1 POWER SUPPLY" and "NO 2 POWER SUPPLY". Whenever I talk about the two "NO x POWER SUPPLY" units, I'll call them "regulators", and when I talk about the whole unit with failover logic and the two regulators inside, I'll say "supply module" (which is technically part of the SCE as a whole).
The voltage on the power bus which supplied the SCE with power dropped from 28V to 18V when the fuel cells went offline, and back to 24V when the batteries took over.
The SCE power supply module also has an undervoltage and overvoltage detector. In case of over- or undervoltage detection, the automatic failover will be prevented, and both switching regulators will be shut down. However, if the SCE witch is set to AUX, the over- and undervoltage detector will be disabled (which makes total sense, as this detector could be a single point of failure). The SCE can work with less than the nominal 28V on the power bus, so it resumed work after the undervoltage detection was overruled. It would also have resumed work after the fuel cells were brought back online; so switching to AUX just sped things up a bit.
John Aaron remembered a similar situation from a ground test or drill (IIRC after the drill/test as officially done, but I could be wrong), where someone set the SCE switch to AUX and data resumed to pour in. This was not common practice written down anywhere.
I guess it was "unexpected" since the switch normally stays in the NORMAL position for the whole flight. Manually selecting the backup switching regulator was probably never envisioned (an automatic failover was present, and the SCE was not mission-critical!).
Whether the undervoltage detection was a bit "too strict", or whether it was intentional to relieve the power bus from unneeded non-mission critical consumers in low voltage conditions, remains an open question.
Schematics (block diagrams) are here: https://www.righto.com/2022/05/talking-with-moon-inside-apollos.html
