NTO's are very common in Engineering.
Work that is accomplished IAW the MM, the SRM or an SB is usually FAA approved. (a few SB's are not but none in recent years.
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For work not currently in any manuals, like a repair that is outside the SRM, or deviation from a mod, many Airworthiness Authorities require the approval to come from the FAA and if this is the case it then requires an FAA form 8110.
The 8110 is actually certified by a Designated FAA engineer (DER) and are dealt with in a more involved way. If the Authority accepts an NTO then this is the easier route for the manufacturer.
It means that the manufacturer is happy but has not put it through the full FAA papermill procedure.
