The point yr right is trying to make is this:
An STC is based on an aircraft that meets the original type design and, in some cases, specified modifications that are already the subject of other STCs.
Almost all GA aircraft that have been around for a decade or more will almost certainly be the subject of numerous avionics modifications that have been approved outside the STC process. For example, pull up the current TCDS for e.g. a Bonanza V35 and you won’t find anything in there for an intercom or two VHFs or a GPS or an EDM 700 etc etc. All of those modifications can affect electrical load, W&B, environmental considerations like ventilation and cooling etc etc. Some of those modifications may be the subject of an STC, but the holder of the STC for e.g. an EDM 700 cannot possibly anticipate all of the other modifications that might already have been made to the aircraft to which it can be fitted.
If an aircraft has e.g. a transponder and there’s an STC for the rack-mounted box to be replaced with another box, of a different part number, that nonetheless: (1) slots into the same rack as the previous box; (2) weighs substantially the same as the previous box; (3) consumes the same or less power than the previous box; (4) does not have any different ventilation/clearance/environmental requirements than the previous box; (4) does not have any different antenna cable and antenna characteristics than the existing system, there’s almost certainly no need for an EO.
However, very few avionics ‘upgrades’ are a simple black box ‘swap and play’.
Last edited by Creampuff; 25th Aug 2014 at 05:24.