Airworthy or Serviceable?
Oh dear, oh dear! I think the post from JP perfectly illustrates the point made by Tuc - many people cannot distinguish airworthiness from maintenance. My day job is airworthiness, acting as a Type Certificate Holder for some 15 Type Certificates with a worldwide fleet of 1,200 active aircraft (light civvy of course, but I did do 34 years in the mob).
Here is a scenario. We are contacted by the owner of a light single who has had his maintenance organisation fit an additional electric fuel pump to his engine to provide a back up should the mechanical engine-driven pump fail. He thinks this is a good idea. We are sent some details of the work carried out on the aircraft and a load of photographs and we are asked to "put a mod into the CAA - I would like to fly tomorrow". The system as fitted by the maintenance organisation worked - ie, it was serviceable - but there was no supporting design investigation, no certification plan, no electrical load analysis, no flight manual supplement, no maintenance manual supplement etc etc.
The new system fit might have been serviceable (ie the pump worked when the pilot ackled the switch), but it was not airworthy. A quick look at the schematic showed that a failure of any of the new flexible hoses would have the engine driven pump dump all its fuel overboard, presenting the pilot with at best an engine failure, at worst an airborne fire.
Many will say that the process of approving this 'good idea' is time consuming and expensive. It is, but better that than the installation being found to be unairworthy when it counts most - when there has been an airborne failure.