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Hard Landing Records Forces ‘Off the Page’

Wed, Oct 28, 2009 — David Evans

Articles

For a good illustration of what the term “ultimate load” means, look no further than the 4 August hard landing of an Azores-based carrier SATA airplane, in which the landing gear and associated structure of the Airbus A320 was damaged.

The SATA A320.

The SATA A320.

An interim report, in Portuguese, was just issued by the Portuguese accident investigation board, GPIAA (Gabinete de Prevenção e Investigação de Acidentes Com Aeronaves), with graphs and photographs documenting the incident at João Paulo II Airport in the Azores. The airplane was flying from Lisbon.

First, a brief discourse on ultimate load. Recall that aircraft designers focus on making sure the structure can withstand without damage what is called a “limit load.” At limit load, a structure must be able to withstand the heaviest loads expected in the airplane’s service life without any deformation. To this limit load a 50% safety factor is applied. This is a required safety factor where deformation may occur but the structure will not fail – ultimate load. The safety factor between limit load and ultimate load is applied to account for small defects, manufacturing tolerance and uncertainty on the stress calculations.

Airbus testing of the A380 wing to limit load and ultimate load.

Airbus testing of the A380 wing to limit load and ultimate load.

Some engineers believe the 50% safety factor used in aviation design is too low. “I’m not trying to be picky, but I’m used to much bigger factors,” says one. “On lifting cables, a factor of 12 (reserve) is quite usual.”

Another says:

“In cars we use ‘safety factors’ of about two, on worst case loads, which in themselves are about double what any reasonable user would see. You can’t afford to do that on all parts, just those that would be immediately dangerous if they failed.”

And finally, from another engineer:

“The safety margins are very thin in the aircraft business, when compared to what are used in most other industries. But there are several points to consider when comparing aircraft to other machines:

“– The loads on aircraft structures are applied under strictly controlled operating conditions (pilots must be conscientious).

“– Aircraft are very weight sensitive. To design an aircraft with the safety factors used by automotive engineers would make it incapable of flight (and cars are weight sensitive, too).

“– Aircraft wings and fuselages penetrate through air, a very forgiving medium, unlike, for example, a bulldozer through gravel…

“Each industry has developed standards that suit their normal practices. Hoisting cables, for example, must be able to absorb shocks from dropped loads, friction on sheaves, sharp bends, etc., and all with a covering of rust from the salty sea air. No wonder a safety factor of 12 is imposed.”

Nonetheless, a safety factor of 2.0 or 2.5 seems reasonable for aviation. Such a load would not prevent damage during the SATA A320’s second bounce, but would have assured that breakage would not occur on the first bounce.

The Airbus clearly went beyond the 1.5 limit load in the hard landing, as the structure was permanently deformed.

The interim report contains a graph, based on the flight data recorder, that tells the story. The airplane touched down once at a force of 2.13G, followed by a bounce (in part, because the throttle was not closed and so the spoilers did not deploy).

sata-graph

The throttle was then closed, the spoilers duly deployed, and the airplane contacted the tarmac with much greater force. The second impact went up to 4.88G.

The resulting damage photographs illustrate an exceedance of limit load and beyond, to ultimate load and permanent deformation of structure. As the GPIAA report indicates, the incident was a “severe hard landing.”

Main landing gear with damage circled in red.

Main landing gear with damage circled in red.Inside damage, clearly beyond ultimate load.


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