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Faro
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Safety Margins Over Ultimate Design Loads

Thu Oct 22, 2009 4:01 pm

In a related thread Why Don't Flaps Fly Off? (by Faro Oct 20 2009 in Tech Ops), it is mentioned that flaps are designed to withstand 200%-300% of their ultimate design load. For wings, I believe the figure is generally 150% and for landing gear struts, no load-related safety margin is designed into the components as these are fatigue-limited and not supposed to snap at all, even in catastrophic impacts.

What about other structural items like the fuselage, fin, stabiliser, pylons, etc. What safety margin is designed into these items as a % of ultimate design load? Are some of these also tested to destruction like with wings?

Faro
The chalice not my son
 
roseflyer
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Joined: Fri Feb 13, 2004 9:34 am

RE: Safety Margins Over Ultimate Design Loads

Thu Oct 22, 2009 4:27 pm

Boeing uses a design guide to determine what every safety margin is. I can't share everything, but the safety margin is affected by many criteria including:

  • Inspection criteria during manufacturing processes (penetrant/radiographic/magnetic particle inspection)
  • Inspections during in service operation
  • Failure modes
  • Criticality of failure (will there be a negligent/minor/severe/catestrophic effect from failure?)
  • Material quality
  • Accuracy of load calculations
  • Manfacturing process (casting factors/machining factors/temperature factors)
  • Impact loads (a flap has to take a birdstrike)
  • Abuse loads (what if someone steps on the cover for something not designed to take 200lbs?)


There is a lot of analysis and the science is usually done through FMEA (Failure Modes and Effects Analysis). Safety margins can range from about 1.5 to 4. Sometimes there are multiple margins built in. For example a maximum pilot load might be 200 pounds of force which makes everything adequate with a safety factor of 1. However other areas have a more predicted or measured force and the factors are built up from experience.

Many times improving the manufacturing process can allow the safety factor to be lowered. It might cost more to make something cast out of aluminum than magnesium, but the safety factor could be lower and the overall weight could be less. It takes highly skilled and experienced engineers to be able to make the correct decisions. In aerospace there are always outside engineers that focus just on materials or stress so that everything is thoroughly reviewed.

As far as testing goes, almost everything is tested. There use to be more destruction testing than there is now as models have approved. However the FAA almost always likes to see things proved by test rather than analysis. Every part from a stowbin latch to the landing gear actuators go through qualification testing. They are tested at as high as 4x max load. Fatigue and ultimate load tests are done as well as potentially a myriad of other tests. Boeing has a huge lab that has been testing every actuator/component for the 787 for years. They have everything imaginable being tested including a full sized mockup of the landing gear that gets cycled a hundred thousand times to look for possible errors. In general you always want to find a few things during qualification testing because if you do not find issues, you probably overdesigned your system and it is too robust/heavy/inefficient.

Hope that helps!

[Edited 2009-10-22 10:22:10]
If you have never designed an airplane part before, let the real designers do the work!
 
tdscanuck
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RE: Safety Margins Over Ultimate Design Loads

Sat Oct 24, 2009 5:04 am



Quoting Faro (Thread starter):
For wings, I believe the figure is generally 150% and for landing gear struts, no load-related safety margin is designed into the components as these are fatigue-limited and not supposed to snap at all, even in catastrophic impacts.

The 150% on the wings comes straight from the FAR's. This applies to all primary structure. However, the 150% criteria may not be the critical case, so you may end up with parts that are considerably stronger than that.

It's not exactly correct to say that the landing gear has no load-related safety margin...the number still exists, it's just unlikely that the static load is the limiting margin.

Tom.
 
aeroweanie
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RE: Safety Margins Over Ultimate Design Loads

Sun Oct 25, 2009 3:57 pm

The Ultimate Design Load = 150% of Design Limit Load. Beyond this, there might be some structural margin, but usually there is not much safety margin beyond Ultimate Design Load.

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