Suppose the max crosswind rated for my C-182 is 15kts, but the wind is directly across and gusting to 20kts. Is it legal for me to go? I've heard plenty hangar flying stories of many an operation that exceeds the design specifications of certain planes that end successfully.
The 15kt crosswind for the 182 is not a design limit at all. If it were a design limit, it would be specifically stated in the POH. To be honest, I think there's no such thing as a design limit for crosswind. It all comes down to how much rudder authority there's left at a specific crosswind strength. And even that depends on many factors like approach speed, flap settings, weight, CG, etc.
This is what the POH for the C182T says:
Demonstrated Crosswind Velocity is the velocity of the crosswind component for which adequate control of the airplane during takeoff and landing was actually demonstrated during certification tests. The value shown is not considered to be limiting.
The maximum allowable crosswind velocity is dependent upon pilot capability as well as airplane limitations. Operation in direct crosswinds of 15 knots has been demonstrated
So it is perfectly legal to land and takeoff with a stronger crosswind component. It's up to the pilot to determine if it's safe to do so or not. It all depends what the pilot's capabilities are.
Not if it is actually a design limit. In the case of the 182 it probably isn't actually a limit though.
With regard to crosswind information provided by the manufacturer, there are two different types. For example, here are excerpts from the Falcon 50 AFM (which I happen to have handy):
Demonstrated Crosswind Component
Satisfactory controllability during take-off and landing has been demonstrated with 90 degrees crosswind component up to 30 kt.
A demonstrated crosswind component is just that. Demonstrated. It is the highest amount of crosswind that was encountered during the certification process and is put into the AFM as a guide. It is not a limiting factor though. This is what the Falcon 50 has in its POH.
- Inoperative engine on lee side: 23 kt.
- Inoperative engine on weather side: 5 kt.
- Center inoperative engine: 23 kt.
In this case, we have an actual crosswind limitation which can not be legally exceeded. (Note that in this case this is for a one engine inoperative ferry.)
In the case of the Falcon, it is certified under 14 CFR Part 25. The requirements are similar (but not the same) for Part 23 airplanes. In particular, we need to look at:
(a) For land planes and amphibians, the following applies:
(1) A 90-degree cross component of wind velocity, demonstrated to be safe for takeoff and landing, must be established for dry runways and must be at least 20 knots or 0.2 VSR0, whichever is greater, except that it need not exceed 25 knots.
(2) The crosswind component for takeoff established without ice accretions is valid in icing conditions.
(3) The landing crosswind component must be established for:
(i) Non-icing conditions, and
(ii) Icing conditions with the landing ice accretion defined in appendix C.
(b) For seaplanes and amphibians, the following applies:
(1) A 90-degree cross component of wind velocity, up to which takeoff and landing is safe under all water conditions that may reasonably be expected in normal operation, must be established and must be at least 20 knots or 0.2 VSR0, whichever is greater, except that it need not exceed 25 knots.
(2) A wind velocity, for which taxiing is safe in any direction under all water conditions that may reasonably be expected in normal operation, must be established and must be at least 20 knots or 0.2 VSR0, whichever is greater, except that it need not exceed 25 knots.
This basically says that they need to test it up to 25 knots. They can choose to test it higher if they want to, but it isn't required. If they find that there is a speed less than 25 knots where the airplane isn't "safe", then they need to impose an actual operating limitation. Otherwise, they just publish the value that it was tested to for reference and it is not a limiting value.
The controlling regulation for Part 91 flights is:
91.9 — Civil aircraft flight manual, marking, and placard requirements.
(a) Except as provided in paragraph (d) of this section, no person may operate a civil aircraft without complying with the operating limitations specified in the approved Airplane or Rotorcraft Flight Manual, markings, and placards, or as otherwise prescribed by the certificating authority of the country of registry.
the word "limitation" in this section refers specifically to SECTION 4 LIMITATIONS in the POH.
In the 182 you mentioned, the demonstrated crosswind number comes from SECTION 1 INTRODUCTION.
So you are not prohibited from taking off with a 16kt cross-wind.
If you have an accident with a 16kt crosswind where the crosswind is a factor in the accident, you will still be subject to 91.13:
Sec. 91.13 — Careless or reckless operation.
(a) Aircraft operations for the purpose of air navigation. No person may operate an aircraft in a careless or reckless manner so as to endanger the life or property of another.
(b) Aircraft operations other than for the purpose of air navigation. No person may operate an aircraft, other than for the purpose of air navigation, on any part of the surface of an airport used by aircraft for air commerce (including areas used by those aircraft for receiving or discharging persons or cargo), in a careless or reckless manner so as to endanger the life or property of another.