Mason,
Control surfaces are critical surfaces, generally needing rebalancing when even a coat of paint is applied. Depending on the aircraft, until the surface is rebalanced following maintenance, speed restrictions or other flight limitations may apply. On some aircraft, even removing and putting back the same part may require flight testing per the manufacturer (the learjet, for example, requires a test flight by a factory test pilot when the leading edge has been removed and replaced...using the same parts).
So far as removing a skin surface and replacing it with the same part, in general one may say tha the flight characteristics haven't been appreciably changed as you've replaced it with an identical part. Except here, you're talking about a control surface, and if you refer to the definition of a major repair, it states a repair in which the control chracteristics *
might* have been changed if the repair is done incorrectly. This is a strong hint that even though the work has been inspected, it really hasn't been tested until it's been flown.
In accordance with 14 CFR 1.1, a major repair is one that if improperly done
might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness. Most certainly reskinning a control surface, if improperly done, may affect the airworthiness of the aircraft.
A ground inspection will show that the work has been properly done, but you're not going to be able to determine if the flight characteristics have been changed unless you fly the aircraft. In this case, you must sign the logs, as it's you returning the aircraft to service. Many pilots don't appreciate this fine point; the mechanic can only approve the aircraft for return to service; the pilot actually returns it to service by flying it.
91.407(b) specifically states that the pilot conducting this flight must sign the aircraft logs:
§ 91.407 Operation after maintenance, preventive maintenance, rebuilding, or alteration.
(b) No person may carry any person (other than crewmembers) in an aircraft that has been maintained, rebuilt, or altered in a manner that may have appreciably changed its flight characteristics or substantially affected its operation in flight until an appropriately rated pilot with at least a private pilot certificate flies the aircraft, makes an operational check of the maintenance performed or alteration made, and logs the flight in the aircraft records.
Note that the maintenance test flight isn't just a flight; it's an operational check of the maintenance performed. In this case, you need to check the effect of the elevator at various deflection angles, it's travel, it's response, any changes in trim at varying airspeeds, angles of attack, and even power settings (as power settings will change airflow over the elevator/stab).
Any time a control surface has received a repair, it's operation in flight may have been affected. This is the languge of 91.407(b), which includes the word "may." Note that the regulation doesn't stipulate a test flight when work has been done that
has changed the characteristics of the aircraft...only work that
may have changed the characteristics of the aircraft.
The most serious change that may have taken place will be flutter, and this is tested in the theoretical world by a flutter analysis, and in the real world by a dive test to determine that the aircraft is free from flutter. Flutter sounds harmless; it sounds like what a butterfly does. In reality, however, a fluttering contol surface can depart the aircraft in a fration of a second and take major portions of the aircraft with it. Flutter is extremly dangerous.
If skin only has been replaced, with he same weight and thickness using the same techniques, fasteners, and everything else called for by the manufacturer...the characteristics shouldn't have changed appreciably. If everything was done properly. If the control surface was properly balanced. If, if, if. Remember, however, that the regulation also says "may," and the definition of the repair itself involves the world "might."
"Appreciable" is a favorite FAA term. Throughout all the FAA publications, the word appears 87 times, and is never once defined. It therefore becomes a subjective and qualitative term, rather than a quantitative one. Does "appreciably" mean a little or a lot, and if it's little, how much? We don't know. We do know that flutter is very serious, and that's definitely appreciable, and we know that work improperly done might cause such a condition...which should be enough to cause us to make the test flight.
Part 43, Appendix A, specifies what constitutes a major repair, and states:
43xA.b Major repairs
(1) Airframe major repairs. Repairs to the following parts of an airframe and repairs of the following types, involving the strengthening, reinforcing, splicing, and manufacturing of primary structural members or their replacement, when replacement is by fabrication such as riveting or welding, are airframe major repairs.
(ii) Monocoque or semimonocoque wings or control surfaces.
Again, that definition is further refined, as previously described, by the definition found in 1.1, which states:
Major repair means a repair: (1) That, if improperly done, might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness; or (2) That is not done according to accepted practices or cannot be done by elementary operations.
The concept that the repair might impact the above mentioned characteristics if improperly done is determiend prospectively, or in advance. One can't merely perform a major repair then looking back decide it was done well, and therefore didn't really impact anything...if it might, it's a major repair and as such, being that it might impact the discussed criteria if done improperly, should be flight tested.
You asked if a ground test would suffice. If it involved something ground testable, such as swinging landing gear, or shutting a door, or even a pressurization check, then yes..and ground check might suffice. In this case, you're interested in the changes in the flight characteristics of the aircraft...and a flight check is in order.
