Originally Posted by
Scurvy.D.Dog
… Airbus composite fins and rudders in general … or this particular (repaired) fin?
This specific fin did not fail until in excess of twice design load, a margin of 33% over the requirement. So even if the repair did weaken the fin, it was still much stronger than it had to be. Which is precisely the basis on which ANY aircraft repair - whether in build or in service - is assessed - "Is it still strong enough to do what's required?"
Originally Posted by
Scurvy.D.Dog
… in any event … are the ‘regulatory/certification requirements’ reasonable in that case??
The only means whereby the regulations could be changed to directly address this accident would be to require active envelope protection for all airliners. On a conventional aircraft, where the pilot is the "Flight Control Computer", we rely on his training and airmanship to prevent inputs which risk the airframe structure. The design load manoeuvres required for certification are an ARBITRARY set of cases which have, in practice, proven adequate to provide sufficient structural margin for operations. Since there has been, basically, one event of this type, in who-knows-how-many billion flight hours, it would appear that the regulations are appropriate (the intent of the regulations is not to eliminate risk - that's impossible - but to reduce it to a level consistent with what's perceived as "safe")
surely one must consider ‘possible’ rudder induced side load/s on the fin and attach points when paired with side load from inertial yaw forces + the H-M interface of 1.2 inches of pedal travel at 250kt??
How many such inputs, and at what frequency? Make enough full rudder inputs at close to an aircraft response mode frequency and you WILL break any aircraft of conventional design.
.. common sense suggests the formular in a crude sense is:-
.
- provide enough surface and structural strength to enable management of required/possible flight envelope (then add margins)-
- limit control input parameters to protect the structure whilst providing necessary control authority to the pilots for 'unusual' encounters!
This is
exactly what the A300 has: structure that didn't fail until it exceeded the regs by 33% (there's your margin), an RTL to protect the structure from excessive inputs AND enough rudder to meet the various regs for trim requirements.
The simple fact is that the only way to restrict control authority such that no-one can break their aircraft, whatever they do, is to either detach the pilot entirely from the controls, or to so limit authority as to make the aircraft almost unmanoeuvrable. Or make the structural design requirements so stringent it's too heavy to take off.