When laying out a sheet metal bend, what is the bend allowance?
ACS code: AM.II.A
Correct answer: The length of material in the curved section of the bend
Rationale: Bend allowance is the length of material actually consumed in the curved portion of a bend. It must be added to the flat dimensions when computing the total developed (flat-pattern) length of a part, because the metal in the radius is neither a straight flange nor part of the setback. Setback is a separate value (the distance from the bend tangent line to the mold-point intersection), and the bend radius is the inside curvature, so those describe different quantities.
The neutral axis (neutral line) of a piece of sheet metal that is being bent is the location where the metal
ACS code: AM.II.A
Correct answer: is neither compressed nor stretched
Rationale: During bending, metal on the inside of the radius is compressed and metal on the outside is stretched. Between these zones lies the neutral axis, where the material is neither compressed nor stretched and its length does not change. Bend-allowance formulas are based on the length of this neutral line, which for practical purposes lies near 0.445 of the material thickness from the inside surface.
What is the primary purpose of an aircraft sheet metal stop drill at the end of a stress crack?
ACS code: AM.II.A
Correct answer: To prevent the crack from spreading farther
Rationale: A crack acts as a stress concentrator with a very sharp tip. Drilling a small hole precisely at the end (tip) of the crack blunts that sharp point and spreads the load over the larger radius of the hole, which arrests the crack's progress. It is a temporary measure to stop further growth until a proper repair is made; it does not relieve panel-wide stress or serve as a rivet pilot.
The diameter of a rivet shank should generally be about
ACS code: AM.II.A
Correct answer: three times the thickness of the thickest sheet being joined
Rationale: A common rule for selecting rivet diameter is to use a rivet whose shank diameter is approximately three times the thickness of the thickest sheet in the joint. A rivet that is too small will not carry the shear load; one that is too large can cause excessive bearing stress and crack the sheet. This three-times rule produces a rivet sized to develop adequate strength in the joint.
After a solid rivet is properly driven, the formed (bucked) head should have a height of approximately
ACS code: AM.II.A
Correct answer: one-half the shank diameter
Rationale: Standards for an acceptable driven rivet specify that the formed head should be about one-half the shank diameter in height and about one and one-half times the shank diameter across its width (diameter). These proportions ensure the rivet has been upset enough to fill the hole and clamp the sheets without being over- or under-driven. A formed head that is too thin or too small indicates insufficient driving.
In a riveted lap joint, the minimum edge distance from the center of a rivet to the edge of the sheet is normally
ACS code: AM.II.A
Correct answer: two times the rivet diameter
Rationale: Edge distance is measured from the center of the rivet hole to the nearest edge of the sheet. The minimum acceptable edge distance is generally two times the rivet diameter (2D) for universal-head rivets. Too little edge distance allows the sheet to tear out under load, while excessive edge distance lets the joint buckle. Rivet spacing (pitch) is a separate dimension typically ranging from about 3D to 12D.
Which type of corrosion appears as a powdery deposit and a roughened, pitted surface on aluminum?
ACS code: AM.II.A
Correct answer: Surface (uniform) corrosion
Rationale: Surface, or uniform, corrosion on aluminum shows up as a general roughening, etching, or pitting of the surface, often accompanied by a white or gray powdery oxide deposit. It is the most common form on aluminum structures. Stress corrosion cracking is intergranular cracking under sustained tensile load, and fretting corrosion occurs at the interface of close-fitting parts subject to slight relative motion, so each presents differently.
When a countersunk (flush) rivet is to be installed in sheet metal that is too thin to machine countersink safely, the preferred preparation method is to
ACS code: AM.II.A
Correct answer: dimple the sheet to receive the rivet head
Rationale: Machine (countersink) cutting removes material, so on thin sheet it would cut through and leave a knife-edge with no bearing surface for the rivet head. Instead, the sheet is dimpled, which forms a recess by pressing or coining the metal without removing it, preserving the sheet's strength and giving the flush head full support. Machine countersinking is reserved for material thick enough to retain adequate bearing.
Alclad aluminum sheet is produced by
ACS code: AM.II.A
Correct answer: coating a strong alloy core with pure aluminum on both surfaces
Rationale: Alclad is a clad aluminum product in which a high-strength alloy core (such as 2024) is metallurgically bonded with a thin layer of nearly pure aluminum (about 5 percent of total thickness per side) on each surface. The pure aluminum cladding resists corrosion and protects the core sacrificially. Anodizing and chemical conversion coating are separate surface treatments applied to existing metal, not the rolled cladding process that defines Alclad.
Which major transverse, vertical, load-bearing member of the fuselage can serve as a wall or partial wall that divides sections of the structure?
ACS code: AM.II.A
Correct answer: Bulkhead
Rationale: A bulkhead is a major transverse, vertical, load-bearing member that closes off or divides sections of the fuselage and can act as a wall or partial wall, such as a pressure bulkhead. Frames give shape around the section and stringers run longitudinally, so neither performs the wall function of a bulkhead.
A vertical structural member that forms a partial or full wall across the fuselage is called a
ACS code: AM.II.A
Correct answer: Bulkhead
Rationale: A bulkhead is a vertical structural member that forms a partial or full wall across the fuselage, dividing it into compartments and carrying loads. Longerons run longitudinally along the fuselage, and frames give the fuselage its cross-sectional shape, so neither describes a wall-forming vertical member.
When an antenna opening is cut into a metal skin, structural strength is restored by adding:
ACS code: AM.II.A
Correct answer: an internal doubler
Rationale: Cutting an opening removes load-carrying material and creates a stress concentration. An internal doubler is riveted in to reinforce the skin and redistribute loads around the cutout, restoring strength while keeping the external aerodynamic surface clean and flush.
A semi-monocoque structure is one which has
ACS code: AM.II.A
Correct answer: a load-bearing skin supported by an internal load-bearing framework
Rationale: A semi-monocoque structure carries loads in both the stressed outer skin and an internal framework of frames, formers, and stringers. This differs from a pure monocoque (skin only, no load-bearing internal members) and from a fabric-covered tubular truss design.
A structural member designed primarily to resist a compressive load is called a
ACS code: AM.II.A
Correct answer: strut
Rationale: A strut is a structural member designed primarily to resist compressive (squeezing) loads. A tie carries tension and a beam resists bending, so a member intended to carry compression is correctly called a strut.
A modern large transport aircraft fuselage is normally constructed using
ACS code: AM.II.A
Correct answer: semi-monocoque construction
Rationale: Modern large transport fuselages use semi-monocoque construction, in which a stressed skin is supported by internal frames and stringers to share loads efficiently. Warren-truss and geodetic constructions are older, heavier methods not used for modern pressurized transport fuselages.
Access panels that are attached to the airframe only by quick-release fasteners and carry no significant flight or pressure loads are classified as
ACS code: AM.II.A
Correct answer: tertiary structure
Rationale: Per FAA-H-8083-31, panels held only by quick-release fasteners carry no significant flight or pressure loads, so they are classed as tertiary structure whose failure would not endanger the aircraft. Primary and secondary structure carry essential loads and are permanently fastened, not designed for routine quick removal.
An extruded or rolled aluminum channel section that runs lengthwise along the fuselage to stiffen the skin is known as a
ACS code: AM.II.A
Correct answer: stringer
Rationale: A stringer is the light extruded or rolled section that runs lengthwise along the fuselage, stiffening the skin and carrying axial loads between frames. An intercostal runs in short lengths between frames, and a cleat is a small angle bracket used to join members, not a continuous longitudinal member.
A point identified as Fuselage Station +88, Water Line -12 is located
ACS code: AM.II.A
Correct answer: 88 inches rearward of the datum and 12 inches below the water line reference
Rationale: Fuselage station numbers increase rearward from the aircraft datum, so station +88 is 88 inches aft of the datum, and a negative water line value (WL -12) places the point 12 inches below the water line reference. Forward or above-the-line readings would require the opposite signs.
Cantilever construction describes a structure that is supported at
ACS code: AM.II.A
Correct answer: one end only
Rationale: Cantilever construction is fixed and supported at one end only, with the remainder projecting unsupported, so all bending loads are reacted at that single built-in root. Structures held at both ends, or at the ends and the center, are simply supported beams rather than cantilevers.
A fuselage construction in which the skin carries all of the loads is known as
ACS code: AM.II.A
Correct answer: monocoque
Rationale: In a pure monocoque fuselage the skin (shell) carries all of the loads with no internal load-bearing framework. A semi-monocoque shares the loads between the skin and an internal structure of frames and stringers, so it is not a skin-only design.
A part of the airframe whose failure would jeopardize the safety of the aircraft and could cause it to crash is classified as
ACS code: AM.II.A
Correct answer: primary structure
Rationale: Primary structure carries flight, ground, or pressurization loads and is essential to the safety of the aircraft; its failure could result in loss of the aircraft. Secondary structure carries lighter loads, and its failure would not be catastrophic. The term that describes a structure whose failure could cause a crash is therefore primary structure.
The progressive loss of strength and eventual cracking of a metal structure resulting from repeated cyclic loading is called
ACS code: AM.II.A
Correct answer: fatigue
Rationale: Fatigue is the progressive weakening and eventual cracking of a material caused by repeated cyclic loading, even when each individual load remains below the material's static strength. Age hardening is a strengthening heat-treatment process, and fail-safe is a design philosophy, so the failure mechanism produced by cyclic loading is fatigue.
An engine mount is designed to allow for which type of engine movement as the engine heats up and cools down?
ACS code: AM.II.A
Correct answer: Longitudinal (fore-and-aft) movement
Rationale: An engine mount supports the engine and transmits thrust to the airframe while accommodating the lengthwise (fore-and-aft) expansion and contraction of the engine as it heats and cools. The mount allows this longitudinal movement without imposing damaging stresses on the engine or structure (FAA-H-8083-31, Metallic Structures).
What is the purpose of a Dynafocal engine mount?
ACS code: AM.II.A
Correct answer: It reduces engine vibration transmitted to the airframe
Rationale: A Dynafocal engine mount uses resilient rubber isolators arranged about the engine's center of gravity to absorb and damp engine vibration before it reaches the airframe. It reduces the vibration transmitted to the structure but cannot eliminate it entirely, and it is not made of plastic (FAA-H-8083-31, Metallic Structures).
Tension is the stress that tends to
ACS code: AM.II.A
Correct answer: elongate or stretch the material
Rationale: Tension is the stress that tends to pull a material apart, stretching or elongating it along the line of the applied load. Compression crushes the material and torsion twists it; the stress that stretches is tension (FAA-H-8083-31, Metallic Structures).
A fuselage station identifies a
ACS code: AM.II.A
Correct answer: longitudinal (fore-and-aft) point on the fuselage
Rationale: Fuselage stations are measured fore-and-aft in inches from a reference datum, locating a point along the longitudinal axis of the fuselage. Lateral positions across the fuselage or wing are defined instead by buttock lines and wing stations (FAA-H-8083-31, Metallic Structures).
In the ATA 100 zonal numbering system, major zone 100 designates the
ACS code: AM.II.A
Correct answer: lower half of the fuselage
Rationale: In the ATA 100 zonal system, major zone 100 designates the lower half of the fuselage while zone 200 covers the upper fuselage. The left and right wings are zones 500 and 600, so zone 100 is the lower fuselage (FAA-H-8083-31, Metallic Structures).
What force is an I-beam primarily designed to resist?
ACS code: AM.II.A
Correct answer: Bending
Rationale: An I-beam is shaped to resist bending: its top and bottom flanges carry the tension and compression produced by a bending moment while the web carries the shear. The geometry concentrates material where bending stresses are greatest, so it is designed primarily to resist bending (FAA-H-8083-31, Metallic Structures).
Precise locations on an aircraft structure are defined by a system of
ACS code: AM.II.A
Correct answer: fuselage stations, water lines, and buttock lines
Rationale: Aircraft locations are defined by three orthogonal references: fuselage stations measured fore-and-aft, water lines measured vertically, and buttock lines measured laterally from the centerline. Together these three systems pinpoint any precise location on the structure (FAA-H-8083-31, Metallic Structures).
Which of the following is an example of a fail-safe (multiple-load-path) structural member?
ACS code: AM.II.A
Correct answer: A stringer
Rationale: A fail-safe structure relies on multiple load paths so that if one member fails the load is redistributed to others. Stringers are numerous and share skin loads, so the failure of one is carried by adjacent members. A spar or longeron is a primary single load path whose failure is more critical (FAA-H-8083-31, Metallic Structures).
Damage-tolerant structural design
ACS code: AM.II.A
Correct answer: allows certain damage to remain un-repaired between scheduled inspections
Rationale: Damage-tolerant design assumes that flaws or cracks may exist and ensures the structure retains adequate residual strength so the damage can be found at scheduled inspections before it becomes critical. It therefore permits certain damage to remain between inspections and applies to primary structure, not only secondary (FAA-H-8083-31, Metallic Structures).
In the ATA 100 zonal system, a passenger entry door is identified by which major zone designation?
ACS code: AM.II.A
Correct answer: 800
Rationale: In the ATA 100 zonal system, zone 800 is allocated to the doors, including passenger entry doors. The 400 and 600 series designate other major zones (powerplant and right wing), so a passenger entry door carries an 800-series designation (FAA-H-8083-31, Metallic Structures).
Which area of an aircraft is subject to hoop stress?
ACS code: AM.II.A
Correct answer: The pressurized cabin
Rationale: Hoop stress is the circumferential tension that acts to split a pressurized cylinder along its length. The pressurized fuselage cabin behaves like such a cylinder, so cabin pressurization loads the skin in hoop tension. Wings and control surfaces are loaded mainly in bending and shear, not hoop stress (FAA-H-8083-31, Metallic Structures).
Shear stress is best described as a stress that tends to make adjacent layers of material
ACS code: AM.II.A
Correct answer: slide or slip past one another
Rationale: Shear stress arises when two opposing forces act parallel to a section, tending to make one layer of material slide or slip relative to the adjacent one. Compression pushes material together and tension pulls it apart; the stress that makes parts slide past each other is shear (FAA-H-8083-31, Metallic Structures).
An aircraft location given as FS 245, RWS 45 refers to a point that is
ACS code: AM.II.A
Correct answer: 245 inches aft of the datum and 45 inches outboard from the right wing centerline
Rationale: Fuselage stations are measured from the aircraft reference datum, not from the nose, so FS 245 means 245 inches aft of that datum. RWS 45 means 45 inches outboard along the right wing from the wing centerline, so the position is referenced to the datum and the wing centerline (FAA-H-8083-31, Metallic Structures).
The permanent deformation a material undergoes after being loaded beyond its elastic limit is described in terms of
ACS code: AM.II.A
Correct answer: strain
Rationale: Strain is the deformation a material experiences under load. When a material is stressed beyond its elastic limit, the strain becomes permanent. Shear and bending describe types of loading, not the resulting deformation itself.
A characteristic of semi-monocoque fuselage construction is that it
ACS code: AM.II.A
Correct answer: offers good damage resistance
Rationale: Semi-monocoque construction carries loads through a stressed skin supported by formers, frames, bulkheads, and stringers, providing multiple load paths. If one member is damaged, adjacent members can carry the load, giving it good damage resistance. It is used for more than just the fuselage.
Most radio antennas on an aircraft are
ACS code: AM.II.A
Correct answer: bonded to the airframe
Rationale: Radio antennas are electrically bonded to the airframe so they share a common ground potential and have a low-resistance path that protects the antenna and radio equipment from static buildup and lightning. An unbonded or insulated antenna would allow charge accumulation and degrade radio performance.
Water lines (WL) are measured reference points located along a
ACS code: AM.II.A
Correct answer: vertical line
Rationale: Water lines measure height, locating points along a vertical reference above (or below) a horizontal datum plane. They form the vertical leg of the station system, complementing fuselage stations (fore-and-aft) and buttock lines (lateral).
The separate parts of an aircraft airframe are maintained at the same electrical potential by
ACS code: AM.II.A
Correct answer: bonding
Rationale: Bonding connects the separate parts of the airframe together with low-resistance straps so they are held at the same electrical potential, preventing dangerous voltage differences and providing a path for static and lightning currents. Static wicks discharge accumulated charge to the atmosphere; equalizing potential between structural parts is bonding.
The basic stresses acting on an aircraft structure are
ACS code: AM.II.A
Correct answer: tension, compression, torsion, and shear
Rationale: The five basic stresses acting on aircraft structure are tension (stretching), compression (crushing), torsion (twisting), and shear (sliding), with bending being a combination of tension and compression rather than a separate basic stress. The correct grouping of the main forces is tension, compression, torsion, and shear.
A structural member designed to carry a compression load is called a
ACS code: AM.II.A
Correct answer: strut
Rationale: A strut is a structural member designed to carry compression (push) loads along its length. A cable can only take tension, and a beam is designed to resist bending, so the member that takes a compression load is a strut.
Stringers are longitudinal stiffeners used in which type of fuselage construction?
ACS code: AM.II.A
Correct answer: semi-monocoque
Rationale: Stringers are longitudinal stiffeners fitted in semi-monocoque construction, where the skin, frames, and stringers share the loads. Pure monocoque relies on the skin alone with no stringers, and truss-type fuselages use welded tube members, so stringers belong to semi-monocoque design.
Wing station numbers are measured
ACS code: AM.II.A
Correct answer: outboard from the fuselage centerline
Rationale: Wing station numbers increase outboard from the aircraft's longitudinal centerline, giving a spanwise coordinate. They are not taken from the upper surface, which is a vertical reference, so wing stations are measured outboard from the fuselage centerline.
What type of load is a tie rod designed to accept?
ACS code: AM.II.A
Correct answer: tensile
Rationale: A tie rod is a slender member that carries only tensile (pulling) loads, holding two points together. It is not designed for bending or torsion, which require stiffer sections, so a tie rod is intended to accept a tensile load.
Primary structure carries the principal flight, ground, and pressurization loads. Which of the following is classified as primary structure?
ACS code: AM.II.A
Correct answer: the stressed skin
Rationale: Primary structure carries the principal flight, ground, and pressurization loads, and its failure would endanger the aircraft. The stressed skin of a semi-monocoque airframe carries a major share of these loads and is therefore classified as primary structure.
Fuselage station numbers are measured aft from a forward datum and are conventionally expressed in
ACS code: AM.II.A
Correct answer: inches
Rationale: Fuselage station numbers are linear distances measured aft from a forward datum and, by convention, are expressed in inches. Using a single consistent unit of inches keeps station, water-line, and buttock-line coordinates uniform.
Which statement best describes secondary structure on an aircraft?
ACS code: AM.II.A
Correct answer: It is highly stressed, but if damaged it will not by itself cause failure of the aircraft
Rationale: Secondary structure is still highly stressed and load-bearing, but its failure alone will not cause loss of the aircraft because loads can be redistributed. This distinguishes it from primary structure, whose failure would be catastrophic, and from lightly loaded fairings and minor brackets.
A structure built with redundancy so that more than one member shares a load is described as
ACS code: AM.II.A
Correct answer: fail-safe
Rationale: A fail-safe structure has built-in redundancy: multiple load paths so that if one member fails, the remaining members carry the load until the defect is found and repaired. Safe-life relies on retiring a part before failure rather than on redundancy, so a redundant design is termed fail-safe.
A structural component that must be replaced at a specified number of cycles, flight hours, or calendar years regardless of its physical condition is what type of item?
ACS code: AM.II.A
Correct answer: Safe-life
Rationale: A safe-life item is removed from service at a predetermined number of cycles, flight hours, or calendar time regardless of apparent condition, because fatigue failure cannot be reliably detected before it occurs. Fail-safe and condition-monitored items remain in service as long as inspections show them serviceable.