AD Overruns - in general
I have been in the industry (UK) for over thirty years in the maintenance field and have occasionally been involved in dealing with AD overruns after they have occurred.
The overruns that I have seen have usually occurred through either the interpretation of the AD or the actioning of the AD once interpreted.
When an AD is issued it needs to be interpreted ADs can be very complex as they are usually written to cover the whole world fleet. There will often be many variables to consider such as Aircraft build date (serial number), Aircraft Series, Aircraft Flight Hours/Cycles, and Modification status (for each Airframe).
The variables will determine when the initial inspections/modifications will take place and further ongoing repeat inspections (which are often not the same frequency as the initial). This will need to be determined for each and every Airframe in your fleet which will likely not be all the same series or standard. Additionally there are often modifications which can be performed in part(s) or in full to either vary the inspection frequency or to terminate it.
Once an assessment of your particular fleet has been made, this data needs to be inputted into your maintenance control system (usually a computer system) where the system will flag up impeding inspections/modifications which can be planned to be performed at the appropriate downtime.
The interpretation and data input process needs to be very robust to prevent mistakes being made, with constant audits to ensure compliance. However, mistakes do get made (Swiss Cheese Human Factors) and occasional overruns do occur. I doubt whether there is an Airline flying (with a sizeable fleet) which has never experienced an AD overrun.
It gets even more complex when an Airline takes on Aircraft from another operator (rather than new aircraft) where the new Operator not only has to make sure that the technical records data transfer is taken across accurately, but also that the previous operator had actually interpreted the ADs correctly in the first place!
But what do you do if you find an overrun ground the affected Aircraft? The certificate of Airworthiness has been invalidated but this does not mean that the Aircraft(s) is un-airworthy (legally yes, but actually not necessarily). As an example if you were mandated to look for a crack at 6000 flight hours and you perform it at 7000 hours due to a data input error and you find no crack was the Aircraft un-airworthy during the overrun?
I have seen it where a large European carrier of which I am familiar, experienced an AD overrun due to an error in the records where a risk assessment was conducted between the Airline, MRO, OEM and the Airworthiness Authority in this case the Aircraft were allowed to continue in operation for a period of time as they were considered to be still Airworthy despite not being in compliance with the AD.
The easy answer is to ground Aircraft but that may not necessarily be in the best interest of all parties while there is obviously the finger pointing about it being a commercial decision, you also need to consider the Passengers (both safety and travel disruption) Airline operational disruption (confidence in the Airline), confidence in the Aircraft type itself (OEM), and confidence in Airworthiness oversight.
There are numerous ADs per Aircraft type and they can be incredibly complex an overrun should not dam the whole Airline or industry where an honest mistake has been made, especially where a risk assessment has been performed coupled with an assessment of the implications of a finding, and an expeditious recovery plan put in place. I am not familiar with the SW case but I suspect they performed a risk assessment with the full blessing of the OEM.
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