Airframe Mechanic Certification Flight Control Systems 4 — Questions and Answers
Question 1: What is the purpose of a servo tab (floating tab) on a large aircraft's control surface?
- To aerodynamically assist control surface movement and reduce pilot effort (Correct answer)
- To limit control surface travel to the maximum structural angle
- To lock the control surface during ground operations
- To provide backup pitch trim in case of elevator failure
Correct answer: To aerodynamically assist control surface movement and reduce pilot effort
A servo tab deflects opposite to the desired surface direction, using aerodynamic force to drive the main surface, which reduces the hinge moment the pilot must overcome.
Question 2: Aircraft control cables are typically fabricated from which material?
- Aluminum alloy 2024-T3
- Galvanized or corrosion-resistant steel (CRES) (Correct answer)
- Titanium alloy wire
- Carbon fiber strands
Correct answer: Galvanized or corrosion-resistant steel (CRES)
Flight control cables are most commonly made from extra-flexible 7×19 or flexible 7×7 carbon steel or corrosion-resistant steel wire rope.
Question 3: What is the maximum recommended bend angle for a flight control push-pull rod when routing around an obstruction?
- No bends are permitted; push-pull rods must be straight (Correct answer)
- Up to 3 degrees
- Up to 15 degrees
- Up to 45 degrees
Correct answer: No bends are permitted; push-pull rods must be straight
Push-pull rods must remain straight between attachment points because bending introduces bending loads the tube is not designed to carry; pulleys and bellcranks are used to change direction.
Question 4: Which locking method is approved for safetying a completed turnbuckle in a flight control cable system?
- Adhesive threadlock (Loctite)
- Safety wire using the double-wrap method per AC 43.13 (Correct answer)
- A nylon self-locking nut on the barrel
- Cotter pin through the barrel
Correct answer: Safety wire using the double-wrap method per AC 43.13
AC 43.13-1B specifies the double-wrap safety wire method (or clip-locking) as the approved means of safetying turnbuckles in flight control systems.
Question 5: On a hydromechanical flight control system, what happens if hydraulic pressure is lost to the power control unit (PCU)?
- The PCU locks in its last position permanently
- Mechanical inputs bypass or lock out and the surface is immovable
- The system typically reverts to direct manual cable control (Correct answer)
- The autopilot automatically takes over surface control
Correct answer: The system typically reverts to direct manual cable control
Many hydromechanical PCUs are designed so that loss of hydraulic power allows the unit to be manually back-driven, reverting to manual reversion for pilot control.
Question 6: What is 'aerodynamic balance' as applied to flight control surfaces?
- Equal weight distribution about the hinge line to prevent flutter
- Placing part of the surface area ahead of the hinge line to reduce hinge moment (Correct answer)
- Balancing cable tensions on both sides of the control circuit
- Matching the aerodynamic profile of all movable surfaces
Correct answer: Placing part of the surface area ahead of the hinge line to reduce hinge moment
Aerodynamic balance uses surface area forward of the hinge centerline to generate a moment that opposes the hinge moment, reducing the force the pilot must apply.
Question 7: When inspecting a cable swaged terminal, which defect would require immediate replacement?
- Minor surface oxidation on the terminal barrel
- Any visible crack in the swaged portion of the terminal (Correct answer)
- Slight discoloration from age
- One loose strand visible at the cable end entry
Correct answer: Any visible crack in the swaged portion of the terminal
Any crack in a swaged terminal indicates structural failure of the fitting and requires immediate replacement before further flight.
