FAP5021

I know what the technical answer is but I want to know why it will not work.

I have a Simrad AP21 that has a broken screen and it is not replaceable due to no parts. Can the FAP5021 take the place of the AP21 heldheld. Just by wiring in in direct to the simrad controller?

The only answer is technical. The software and hardware are different. Although the outside looks like the same design that is where the similarity ends.

I am not aware of any manufacturers autopilot system that can use mix and match control heads with another. You might try to post on a few forums like "The Hull Truth" and see if any of there members has an AP21 for sale.