Displaying the Aircraft Registration Certificate in the Aircraft
If you have an aircraft registration certificate for an aircraft, but you don't display it in the aircraft, what is the fine?
FAR 91.203(a)(2) states that no person may operate
a civil aircraft unless it has within it
an effective U.S. registration certificate issued to its owner or, for operation within the United States, the second duplicate copy (pink) of the Aircraft Registration Application as provided for in FAR 47.31(b), or a registration certificate issued under the laws of a foreign country.
It doesn’t say the aircraft registration certificate has to be displayed, but it must always be kept within the aircraft when operated.
If an FAA inspector asks to see the registration, and you do not have it available within the aircraft, then it would be a violation. Each circumstance is different, and I would not want to speculate how much the fine would be.