Hi - the first thing that came to mind for your Item/MEL Code question was to create another table that has two fields: (1) Items & (2) MEL Codes.
My understanding is that there are only 4 possible MEL Codes: A, B, C, or D.
Accordingly, in this new table you could list all of your items in the Items field, and the corresponding MEL Codes for each item in the MEL Codes field.
Then create a relationship between your original table & the new table based on items.
In your layout about, change the field showing your MEL Codes to the related table's MEL Codes field and that should take care of it - the related MEL Codes will automatically be filled whenever an item from your original table matches an item in the new table.
Attached is the FMP file I made & of which I took the pics. If you type one of the existing items in _kp_Item field, it will auto-populate its related MEL Code in the Related MEL Codes field.
For example, if you type "Cat" in the item field & hit tab, the MEL Code "A" will appear.
MEL Code.fmp12.zip 65.7 K
I'm assuming that this data is one table with fields for aircraft, chapter/code, item and MEL code with the MEL code being a unique value ...like a part number. One way to do it would be to setup conditional value lists based on user choice ...that simulates drilling down to get to the MEL code by creating relationships in the relationships graph to this table.
Example: TOG is table on the relationship graph :
(1) Main Table (cartesian join) 'x' Repair Code Table ..to lookup all aircraft choices. Value List is "use values form field from the TOG (Repair Code Table) you created
(2) Make another TOG Main Table (aircraft) '=' Repair Code Table (aircraft) ... to lookup Chapter/Code options for the aircraft chosen *make sure that in your value list for this field references the TOG you created and "include only related values starting from the Main Table (the LOOKUP Table) is chosen
(3) Make another TOG > Main Table (aircraft) '=' Repair Code Table (aircraft) 'AND' Main Table (Chapter/Code) '=' Repair Code Table (Chapter/Code) *again make sure that in your value list for this field references the TOG you created and "include only related values starting from the Main Table is chosen
(4) Make another TOG>Main Table (aircraft) '=' Repair Code Table (aircraft) 'AND' Main Table (Chapter/Code) '=' Repair Code Table (Chapter/Code) 'AND' Main Table (Item) '=' Repair Code Table (Item) ......the MEL Code unique value will show without user input after the other fields have values