In reporting about the unsuccessful green run of the SLS core stage, Ars Technica mentions

About 50 seconds into what was supposed to be an 8-minute test firing, the flight control center called out, “We did get an MCF on Engine 4.” This means there was a “major component failure” with the fourth engine on the vehicle.

What is the Major Component Failure (MCF) referred to?

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    $\begingroup$ but could it be a minor component which fails in a major way @RonJohn $\endgroup$
    – user20636
    Commented Jan 18, 2021 at 23:04

1 Answer 1



Each engine reports a self-test status to the vehicle it's attached to. "MCF" is one of the possible statuses and indicates that the engine controller has detected a serious - but not catastrophic - failure within the engine.


The Space Shuttle Main Engines (SSMEs) aka RS-25s transmit data to the vehicle they are attached to using a format called the Vehicle Data Table (VDT). One word in the VDT is the Engine Status Word (ESW), a 16 bit word containing much important information about the engine packed into it.

This graphic shows what information is contained in the ESW. I added a green arrow showing the MCF in the Engine Self-Test Status bits.

enter image description here

The SSME controller (SSMEC) sets the Self-Test Status based on the results of extensive tests that it continuously performs. The three levels of the Self-Test Status have the following meanings

  • Engine OK: No failures detected
  • Major Component Failure: Failure detected in a redundant component or other failure that does not necessarily preclude continued engine operation
  • Engine Limit Exceeded: A potentially catastrophic failure has been detected and the engine should be shut down ASAP. (On shuttle, there was a crew station switch that controlled whether or not the engine would shut down; I do not know how this works on the Space Launch System (SLS))

Examples of failures that set MCF:

  • Valve servoactuator command / position miscompare
  • Fuel flowmeter sensor qualification
  • Redundant computer or electronics unit failure
  • Receipt of certain commands
  • Failure of purge/ancillary system components
  • Pneumatic shutdown entered
  • Chamber pressure or fuel density sensor qualification
  • Limit monitor sensor reasonableness failure
  • Command voting channel failure
  • Thrust Limiting mode entered

So just knowing that the engine set an MCF does not tell much about what failed. However, another word in the VDT provides information about the specifics of the failure that set the MCF.

  • 7 bits of this 16-bit word encoded an octal number corresponding to the particular type of device that failed (for example, 015 indicated a servoactuator failure).
  • The other 9 bits encoded an octal number corresponding to the specific subcomponent or failure cause (for example, an 001 appended to the previous 015 indicated Channel A on the Main Fuel Valve).
  • The first octal number was called the Failure ID (FID) and the second one was called the Delimiter (DLM), although in practice the combination (for example 015 001) was referred to as a FID.

It would be very interesting to know what FID(s) accompanied the MCF on the SLS green run. I hope this information is forthcoming.

Additional notes as I learn stuff:

  1. This NASA article says the shutdown was initiated by onboard software.
  2. In this Space News article a manager mentions a "failure ID" but does not say what it is, and from the quote, may be confused about the difference between a MCF and a FID.
  3. All technical info in this answer is derived from shuttle sources and may not be completely relevant to the SLS.


  • $\begingroup$ doesn't the octal break down into 015 and 001? or does the way you've written it make more sense to normal people? $\endgroup$
    – user20636
    Commented Jan 18, 2021 at 23:10
  • $\begingroup$ @JRCM I'm not totally sure what you're asking but all the FIDs were a basic octal number like 1000, 2000...up to 111000 (skipping some) to which an octal "delimiter" like 1,2, 27,101, etc got added to produce the final FID as octal 15001, 111031, etc, etc. I have no idea why they picked octal. $\endgroup$ Commented Jan 18, 2021 at 23:33
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    $\begingroup$ because octal is lovely, and the obvious base to use when using three bits for things. Just wondering why you said 15001 broke down to 15000 and 001, when I would have said 015001 breaks down to 015 and 001 -- to me the way you wrote it looks really weird, and the way I write it looks correct, but I'm not a lay person, so your way may well be the better way to write it. $\endgroup$
    – user20636
    Commented Jan 18, 2021 at 23:41
  • 1
    $\begingroup$ If it was referred to as a fifteen-thousand error, then the way you wrote it makes sense. I guess in the Apollo days it would have been noun 15 verb 1 (except it would have been noun 13. I should probably go to bed now) $\endgroup$
    – user20636
    Commented Jan 18, 2021 at 23:49
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    $\begingroup$ Not sure if it's applicable to this answer, but blogs.nasa.gov/artemis/2021/01/19/… mentions the MCF appears to have been some sort of instrumentation issue. $\endgroup$
    – DylanSp
    Commented Jan 19, 2021 at 16:14

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