飛機上的國旗圖樣左右倒轉?

如以一般國營的民航機而言 (舉例如 中國國航,新加坡航空)
機體上都有該國的國旗圖樣,在左邊都是「正常」的,但在右
側卻是左右倒轉的. 只要到搜尋飛機照片的網頁去找,就發現
所有國家的國旗在飛機右側都是左右倒轉的.

是否只是一個常規,還是有特別意義?

yes, it's usually have its right side inverted.
It's because it shows which direction is the plane flying and just like you holding a flag.
if you look from one side which is normal, then another side is an inverted flag.

那叫 "順風旗"。