part 23 emergency landing load factors

[johnchance]

I am confused about emergency landing static inertia load factors. My confusion centers on FAR paragraph 23.562(d)(1), which I read to require a fairly large increase in the 23.561 load factors for those airplanes that weigh less than 6,000 LB, Vso>61.0 kts, that do not comply with 23.67(a)(1). I happen to be sizing cockpit structures in a two-engine jet weighing less than 6,000 LB with a Vso>79 kts, an airplane that seems to slide right into this paragraph in the FAR! I hesitate to use the paragraph since it equates to an additional factor of 1.67 for emergency landing loads, applied to both the seated occupants and items of mass within the cockpit. It seems especially odd that the factor ceases to appy if my airplane goes over 6,000 LB! Has anyone else had to deal with this paragraph in the FAR?

-Thanks

 

[Miper]

You have to do what the regulations require -- either as stated in the FAR, or you can propose and defend something else that gives equivalent safety.

It is not uncommon for rules to change with weight. 6000 lb is one of the classic break points, although the rationale for it is a little vague.

The single engine, 5000 ft climb gradient requirement of FAR 23.67 is a gentle hint that any twin engine airplane that can't make a 1.5 % gradient with one engine out isn't getting much additional safety value from the 2nd engine.

The 1.5% value is used because as an airplane ages it will be barely able to maintain altitude on one engine if all it could do when new was 1.5%. Every business jet in production exceeds the 1.5% requirement. Much of the outstanding safety record of current twin, turbine-engined airplanes is related to their excellent single-engine performance.

The higher load factor for stall speeds greater than 61 KT is required because of the much higher crash energy involved when an airplane with higher stall speed hits the ground.

Finally, anything you do that appears to be an attempt to avoid or sidestep these crash safety requirements will put you on the wrong side of the argument when the lawyers start picking apart your design after a crash. All the explanations in the world won't convince a "jury of your peers" (hand picked to be both dumber than stumps and tender-hearted toward grieving widows) that your approach is safer (even if it is); because the plaintiff’s lawyer will, with calculated malice, willfully misrepresent that you "failed" to follow the FAA's safety rules.

 

[johnchance]

Miper,

Your description of the reason for the increased emergency landing loads makes it clear. It is the 1.5% climb gradient the FAA is concerned about. Since the aircraft I am working on is a twin turbo, 23.67(a) does not apply to me, 23.67(b) does. 23.67(b) doesn't allow any exceptions, the 1.5% climb gradient has to be met. So no increased emergency landing loads are necessary.

-Thanks