Expert: Paul Soderman - 3/27/2010

Question
Hi.
I fly an airplane designed for water bombing. The water tanks are located under the wings.
This airplane can be used also for carrying cargo and few passengers.

This airplane can fly under 2 categories: Utility and restricted.
In the AFM, you can find in the performance section that:

for the Utility category, the structural weight limits are 41000 lbs for take off from a runway and 37000 lbs for landing.

for the Restricted  category, the structural weight limits are 44000 lbs for take off from a runway, and 37000 lbs for landing.

What applies to structural limits and why there is such a difference?

Thank you very much for answering.

Chris Weber

Answer
Chris
I am not an aircraft structures expert, but my understanding is that structural limits of a aircraft are often described in terms of load factor n = L/W  (lift of weight ratio).  As weight goes up, the g loading of the aircraft goes up during maneuver.  And the aerodynamic stall limit is affected also.  The idea is that you don't want any structural component to bend such that it cannot return to the original shape.  In some cases, aircraft are designed so that there is a 50% chance that a component will fail at loads of 1.5 times the ultimate load limit.  So, I don't know what is actually setting the limits for your aircraft, but I assume the design and load tests during certification showed that the load factor would be acceptable for a weight of 41,000 lbs on takeoff (utility) because a heavier weight would overstress the wing, for example.  The heavier the load, the more the wing bends.  Landing is even harder on the aircraft because of the ground impact, so the weight limit is lower.  If your aircraft was designed to carry water tanks under the wing, it is likely that the wing root section is the critical structure that would fail if the load was too great.
Paul