Quote:
What's the max "g" loading that an A320 (or indeed any commercial acft) can have on landing without structural damage?
Airliner airframes structural mininums are:
Up 3G
Fwd. 9G
Down 6G
Laterally 1.5G
Just a small clarification on this:
The above quote is referring to § 25.561 of the FARs. That requirement is about aircraft crashworthiness. Aircraft structure is
not designed to withstand those levels without damage. The requirement is that the aircraft structure be designed to give each occupant every reasonable chance of escaping serious injury in a minor crash landing. The acceleration levels quoted above are those felt by the occupants. In addition, the attachments for items of mass in the aircraft which cannot be positioned so that, if they break loose, they do not injure occupants, pierce fuel tanks, or block emergency egress, must also be designed for those acceleration levels.
So, for example, the mounting system of the APU must be designed to withstand the loads produced by those inertia levels applied to the mass of the APU. Otherwise, in a "minor" crash landing, there is a risk that the APU breaks loose and causes hazards for the occupants of the aircraft.
I always understood that the design landing limit equated to a touch down at 1000ft per/min at max weight + proof factor.
§ 25.473 "Landing load conditions and assumptions" specifies 10 fps at design landing weight, or 600 fpm, and 6 fps at maximum take-off weight, or 360 fpm. Aircraft structure is designed so that no permanent deformation occurs at the maximum loads induced by these landing conditions, over a range of aircraft mass distributions, landing attitudes and landing speeds. The structure is designed not to fail (although permanent deformation can occur) for loads up to these maximum (limit) loads mulitiplied by an ultimate factor of 1.5. In addition, the landing gear itself is tested to reserve energy conditions (§ 25.723), a 12 fps descent rate at design landing weight, which corresponds roughly to the ultimate factor of 1.5 (12^2/10^2 = 144/100 = 1.44). The landing gear structure is designed not to fail up to the reserve energy condition.
It's important to note that the FARs allow the manufacturer to make the assumption of 1g wing lift in the load calculations. Therefore, the 10fps and 6fps landing descent rates are steady-state conditions. In bounce situations, with lift dump deployed, the aircraft can land considerably harder than it was designed for, due to the lack of lift on the wings.
It's also important to note that not all of the aircraft structure will be designed by landing loads. Many flight load cases are more critical.