Aircraft cargo Structure

[ottawa]

Airframes converted to cargo configuration may have their passenger window transparencies removed and replaced by mechanically fastened sheet metal panels. Piercing and forced entry both remain restricted by the heavy construction zone around the window openings. Do aircraft that are purpose built as freighters still have the window frames or are they deleted to reduce weight and fabrication complexity? If windows frames and their surrounding substructure (skin doublers and triplers) are not installed, does the window belt skin sheet thickness remain the same or is it thinner? Thinner skins and the elimination of built-up or unit window frames and their surrounding substructure would positively affect maindeck piercing and forced entry considerations on freighter airframes.

[CV880]

Using the 747 as an example-
1/ The cabin window frames are not installed on a Boeing built freighter
2/ Only 2 passenger doors are installed; the other 8 and their supporting structure are not fitted
3/ Purpose built freighters are often beefed up to allow higher max landing weights and zero fuel weights to make them more useful as freighters eg a 747-200F has a max landing weight 36,000 lb higher than a pax version of comparable vintage converted to freighter
4/ Areas recommended for cutting into the cabin are usually above the window belt to avoid trying to chop through the reinforced zone

[grounded27]

Yes the window frames remain, you still have to make up for the structural integrity of the hole. I suppose the frame could be removed but this would require the whole side of the aircraft to be re-skinned.

[CV880]

A Boeing built 747 freighter (not a converted aircraft) does not have main deck cabin window openings (holes). It uses different skin panels to those used on a pax aircraft. Also it only has openings for 2 main deck doors thus the reinforcing that goes around the windows and the "missing" 8 doors openings is unnecessary.
On converted freighters you can leave the cabin windows installed or replace them with metal plugs (customer choice). The unused doors are simply locked closed and placarded accordingly.
I do not know enough about other freight aircraft to comment on them.