FAA Aviation Mechanic Airframe (AMA)Airframe StructuresHard
A technician is statically balancing a repaired aileron and determines that 18 inch-pounds of nose-down balancing moment must be added forward of the hinge line to meet the manufacturer's balance requirement. If the balance weight will be installed 3 inches forward of the hinge line, what weight must be added?
- A9 pounds
- B6 pounds
- C15 pounds
- D3 pounds
Show answer & explanationAnswer & explanation
Correct answer: B. 6 pounds
Moment equals weight multiplied by arm distance, so weight equals moment divided by arm: 18 inch-pounds ÷ 3 inches = 6 pounds. Mass balancing of control surfaces prevents flutter by ensuring the center of gravity of the surface is at or ahead of the hinge line as specified by the manufacturer.
Why the other options are wrong
- A. 9 pounds would produce 27 inch-pounds of moment, exceeding the requirement.
- C. 15 pounds would produce 45 inch-pounds of moment, far too much.
- D. 3 pounds would only produce 9 inch-pounds of moment at this arm, not 18.
Control Surface Mass Balance
The process of adding weight forward of a control surface's hinge line to shift its center of gravity and prevent aerodynamic flutter.
- Moment = Weight x Arm distance from hinge line
- Weight = Moment ÷ Arm distance
- Improper balance can lead to dangerous flutter conditions
Memory trick: 'Moment over arm gives you the weight to disarm flutter.'