18 Rotorcraft Fundamentals Practice Questions & Answers

1. 1. A helicopter is reported to have a high-frequency vibration. Consistent with the FAA vibration frequency classifications, where should the mechanic begin the inspection?

   • A.The spanwise and chordwise balance of the main rotor blades
   • B.The main rotor blade track and pitch change link settings
   • C.The main transmission mounts and their dampening elements
   • D.The tail rotor, its gearbox, and the tail rotor drive shaft✓ Answer

   High frequency is defined as vibration at or above tail rotor speed, so its source is anything rotating at or above that speed: the tail rotor, tail rotor gearbox, drive shaft, hanger bearings, or an engine-driven fan. Low frequency (1 and 2 per revolution) points to the main rotor instead, and an extreme low frequency of 2 to 3 cycles per second is pylon rock at the transmission mounts.

   Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Helicopter Vibration AnalysisReport a problem with this question

2. 2. A main rotor produces a low-frequency vibration that is felt vertically, at one per revolution. What condition does this indicate, and how is it normally corrected?

   • A.A drive shaft defect, corrected by replacing the hanger bearings
   • B.An out-of-balance condition, corrected with hub weights and sweeping
   • C.A loose airframe component, corrected by securing the skid hardware
   • D.An out-of-track condition, corrected with pitch links and trim tabs✓ Answer

   A vertical one-per-rev vibration means one blade is developing more lift than the other at the same point in the disc, which is an out-of-track condition caused by chord profile variation, pitch link misadjustment, or trim tab setting. Track is corrected with the pitch change links for ground and hover and with the trim tabs for forward flight; lateral vibration, by contrast, is the out-of-balance symptom.

   Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Rotor Blade Tracking and BalancingReport a problem with this question

3. 3. An extreme low frequency vibration of about 2 to 3 cycles per second is felt in a helicopter. What is this vibration called, and what controls it?

   • A.Pylon rock, controlled by the dampening of the transmission mounts✓ Answer
   • B.High frequency, controlled by balancing the tail rotor drive shaft
   • C.Medium frequency, controlled by tightening loose airframe parts
   • D.One-per-rev lateral, controlled by hub weights added at the head

   Extreme low frequency vibration, below one per revolution, is pylon rock at roughly 2 to 3 cycles per second. It is inherent to the rotor, mast, and transmission acting as a mass on flexible mounts, so it is controlled by the dampening designed into the transmission mounts, and deteriorated mounts are a legitimate finding when it increases.

   Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Helicopter Vibration AnalysisReport a problem with this question

4. 4. A two-bladed semirigid main rotor head has no vertical drag hinge. How are the lead and lag forces on the blades accommodated in that design?

   • A.By hydraulic dampers installed between each blade and the hub
   • B.By bending of the blades and by the underslung mounting of the hub✓ Answer
   • C.By the feathering bearings, which allow fore-and-aft blade travel
   • D.By the teetering hinge, which lets one blade lead as the other lags

   The semirigid head has a teetering hinge for flapping and feathering bearings for pitch change, but no vertical drag hinge and no lead-lag dampers, so hunting forces are absorbed by bending of the blades themselves. The underslung design places the blades below the plane of rotation so the blade center of gravity stays aligned with the flapping hinge, which minimizes the geometric imbalance that drives lead and lag.

   Source: FAA-H-8083-21 Helicopter Flying Handbook, Semirigid Rotor SystemReport a problem with this question

5. 5. Mast bumping is a hazard associated with which main rotor system, and what physically contacts the mast?

   • A.The semirigid system, when a pitch link strikes the mast
   • B.The semirigid system, when a static stop strikes the mast✓ Answer
   • C.The rigid system, when a flexing blade root strikes the mast
   • D.The articulated system, when a drag hinge strikes the mast

   Mast bumping is unique to the semirigid two-bladed head because the whole hub teeters on a single hinge; if flapping exceeds the design limit, the static stop contacts the mast. The resulting damage can shear or separate the mast, so any suspected mast bumping requires inspection of the mast, stops, and hub before further flight.

   Source: FAA-H-8083-21 Helicopter Flying Handbook, Mast BumpingReport a problem with this question

6. 6. A rigid (hingeless) main rotor system has no flapping or lead-lag hinges. How are those blade forces handled, and what is the design's main drawback?

   • A.They are absorbed by elastomeric dampers, and control response is slower
   • B.They are absorbed by hub weights, and blade track drifts more quickly
   • C.They are absorbed by blade flexibility, and vibration levels are higher✓ Answer
   • D.They are absorbed by the teetering hinge, and mast bumping can occur

   In a rigid system the blades are attached to the hub without flapping or drag hinges, so those forces are taken up by the flexibility of the blade itself, while feathering bearings still allow pitch change. The design is simple and gives good control response and aerodynamic efficiency, but because the hinges are absent it transmits more vibration to the airframe than the other two types.

   Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Main Rotor SystemsReport a problem with this question

7. 7. In forward flight the advancing blade meets a higher relative wind than the retreating blade. What automatically compensates for the resulting difference in lift?

   • A.Increased tail rotor thrust applied through the pedals
   • B.Blade lead and lag about the vertical drag hinges
   • C.Blade flapping, aided by cyclic feathering of each blade✓ Answer
   • D.Blade coning produced by the increased rotor loading

   Dissymmetry of lift is corrected by blade flapping: the advancing blade flaps up, which reduces its angle of attack and lift, while the retreating blade flaps down, increasing its angle of attack and lift, and cyclic feathering supplements this. Lead and lag is the response to Coriolis effect, not to dissymmetry of lift, and the tail rotor plays no part in balancing lift across the disc.

   Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Dissymmetry of LiftReport a problem with this question

8. 8. Because of gyroscopic precession, where does the maximum deflection of a main rotor blade occur relative to the point where the pitch change force is applied?

   • A.Approximately 180 degrees later in the direction of rotation
   • B.Approximately 90 degrees earlier, opposite the rotation
   • C.Approximately 90 degrees later in the direction of rotation✓ Answer
   • D.At the same point, with no displacement around the disc

   A spinning rotor behaves as a gyroscope, so the reaction to an applied force shows up approximately 90 degrees later in the direction of rotation. That is why maximum blade deflection lags the maximum pitch change by a quarter turn and why the control linkage must apply the cyclic pitch change ahead of the point where the disc is meant to tilt.

   Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Gyroscopic PrecessionReport a problem with this question

9. 9. Coriolis effect causes a main rotor blade to speed up as it cones upward and slow down as the disc flattens. What blade motion does this produce, and where is it accommodated on a fully articulated head?

   • A.Lead and lag, accommodated by the vertical drag hinge and its damper✓ Answer
   • B.Flapping, accommodated by the horizontal hinge and its droop stop
   • C.Coning, accommodated by the blade grip and its retention bolt
   • D.Feathering, accommodated by the spanwise bearing and its pitch horn

   Coriolis effect is the conservation of angular momentum: as coning moves blade mass inboard the blade speeds up, and as the disc flattens it slows down, producing the fore-and-aft hunting motion called lead and lag. On a fully articulated head that motion takes place about the vertical drag hinge and is controlled by a damper, while flapping is the separate response to dissymmetry of lift.

   Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Coriolis EffectReport a problem with this question

10. 10. Retreating blade stall limits which of the following in a helicopter, and what protects against it?

    • A.Maximum forward airspeed, protected by observing the placarded VNE✓ Answer
    • B.Maximum hover ceiling, protected by observing the density altitude
    • C.Maximum gross weight, protected by observing the loading placard
    • D.Maximum rotor rpm, protected by observing the tachometer red line

    As forward speed increases, the retreating blade must fly at a higher angle of attack in a lower relative wind, and eventually it stalls; that is the aerodynamic limit on maximum forward airspeed. Protection is the never-exceed speed, shown as a red line on the airspeed indicator and stated on a placard, and the symptoms are a nose-up pitch, vibration, and a rolling tendency.

    Source: FAA-H-8083-21 Helicopter Flying Handbook, Retreating Blade StallReport a problem with this question

11. 11. How does a collective pitch input differ from a cyclic pitch input at the main rotor blades?

    • A.Collective changes rotor speed; cyclic changes the pitch of all blades equally
    • B.Collective changes all blade pitch equally; cyclic changes it once per revolution✓ Answer
    • C.Collective changes tail rotor pitch; cyclic changes main rotor pitch equally
    • D.Collective changes blade pitch once per revolution; cyclic changes it equally

    The collective changes the pitch angle of all main rotor blades simultaneously and equally, raising or lowering total lift and moving the helicopter vertically. The cyclic changes each blade's pitch differentially once per revolution, so the tip-path plane tilts and the helicopter moves horizontally; at the swash plate a collective input slides the assembly while a cyclic input tilts it.

    Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Helicopter Flight Control SystemsReport a problem with this question

12. 12. On a conventional swash plate assembly, which component keeps the stationary swash plate from turning with the mast?

    • A.The antidrive link attached to the stationary swash plate✓ Answer
    • B.The pitch links connecting the rotating plate to the horns
    • C.The uniball sleeve joining the rotating plate to the fixed plate
    • D.The drive links connecting the mast to the rotating plate

    The swash plate has two halves that tilt and slide as one unit: the stationary plate is held from rotating by an antidrive link while remaining free to tilt and slide on the mast, and the rotating plate is turned by drive links from the mast. The uniball sleeve joins the two plates and the pitch links carry the motion from the rotating plate to the blade pitch horns.

    Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Swash Plate AssemblyReport a problem with this question

13. 13. What do the antitorque pedals of a single main rotor helicopter actually change?

    • A.The angle of a movable rudder on the tail fin
    • B.The pitch angle of the tail rotor blades✓ Answer
    • C.The rotational speed of the tail rotor blades
    • D.The tilt of the main rotor tip-path plane

    The pedals are connected to the tail rotor pitch change mechanism, so they vary the pitch of the tail rotor blades and therefore the antitorque thrust, controlling heading about the vertical axis. Tail rotor speed is fixed by the drive system and does not change with pedal input, and the pedals are not a rudder even though a fin may be fitted.

    Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Antitorque PedalsReport a problem with this question

14. 14. What does the freewheeling unit do when engine rpm drops below main rotor rpm?

    • A.It disengages the engine so the rotors can continue turning freely✓ Answer
    • B.It locks the engine to the rotor so the engine is turned by the rotor
    • C.It applies the rotor brake so rotor rpm decays in a controlled way
    • D.It disengages the tail rotor drive so main rotor rpm is preserved

    The freewheeling unit, usually a one-way sprag clutch, automatically disengages the engine whenever engine rpm falls below main rotor rpm, which is what makes autorotation possible: air then flows up through the disc from below and keeps the rotor turning. Because the tail rotor is driven from the transmission, it keeps turning as well, so directional control is retained after an engine failure.

    Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Freewheeling UnitReport a problem with this question

15. 15. A transmission chip detector warning light illuminates and does not clear after the detector's burn-off function operates. What does this oblige?

    • A.Adding oil at the sight gauge and rechecking the detector after next flight
    • B.Replacing the detector plug and continuing to the next scheduled inspection
    • C.Resetting the light and returning the aircraft to service with a log entry
    • D.Following the model's emergency procedure and investigating internal failure✓ Answer

    The chip detector sits in the gearbox sump and is wired to a cockpit warning light; the burn-off feature is designed only to clear harmless fuzz, so an indication that survives burn-off means real metal is present. That points to an internal failure of a highly loaded dynamic component, and the manufacturer's emergency procedure for the model must be followed and the cause found before further flight.

    Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Transmission Chip DetectorsReport a problem with this question

16. 16. During rigging of a rotary-wing aircraft, what is the purpose of installing pins, clamps, or jigs in the control system?

    • A.To relieve cable tension so the control surfaces can move freely
    • B.To measure the angular difference between a surface and the airframe
    • C.To limit the maximum range of travel once the linkages are adjusted
    • D.To hold the system in a specific position while linkages are adjusted✓ Answer

    Rigging follows three major steps, and the first is to place the control system in a specific position and immobilize it with pins, clamps, or jigs so the linkages can be adjusted to fit the held component. The second step positions the control surfaces against a reference using a jig, bubble protractor, or spirit level, and the third sets the maximum range of travel; a functional check follows before the aircraft is returned to service.

    Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Helicopter RiggingReport a problem with this question

17. 17. What is the function of the skid shoes on a skid-equipped helicopter, and how is the wear limit determined?

    • A.They are corrosion barriers; the limit is any visible loss of the coating
    • B.They are structural stiffeners; the limit is one-half the original thickness
    • C.They are vibration dampers; the limit is set by the tolerances of 14 CFR 43.13
    • D.They are sacrificial wear strips; the limit comes from the manufacturer's manual✓ Answer

    Skid shoes are replaceable wear strips bolted or clamped to the underside of each skid tube so that abrasion from landings and ground contact consumes the shoe rather than the tube, which is far cheaper to replace. The FAA handbooks publish no universal wear dimension, so the allowable wear and the replacement criteria are taken from the specific manufacturer's maintenance manual.

    Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Helicopter Landing Gear; 14 CFR 43.13(a)Report a problem with this question

18. 18. Before a main rotor blade is placed in its shipping and storage container, why are holes in the blade taped over?

    • A.To keep moisture out of the blade interior and prevent corrosion✓ Answer
    • B.To keep internal pressure in the sealed blade spar from bleeding down
    • C.To keep the blade's static balance within the master blade tolerance
    • D.To keep the damage visible so the repair station can locate it quickly

    Blade preservation calls for taping all holes, such as those from a tree strike or foreign object damage, specifically to protect the blade interior from moisture and the corrosion it causes while the blade is in storage or transit. The same procedure has the blade cleaned with mild soap and water, bare metal and bushings coated with corrosion preventive, and the blade secured on a shock-mounted support in the container.

    Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Rotor Blade Preservation and StorageReport a problem with this question

Practice questions based on the Aviation Mechanic Airman Certification Standards (FAA-S-ACS-1), 14 CFR parts 43, 65, and 91, and the FAA Aviation Maintenance Technician Handbook—Airframe (FAA-H-8083-31). This site is not affiliated with or endorsed by the Federal Aviation Administration. This bank covers the AIRFRAME written test only; the General and Powerplant written tests and the oral and practical tests are separate. Torque values, cable tensions, inflation pressures, servicing quantities, wear limits, and repair criteria always come from the manufacturer's current maintenance data, the structural repair manual, and the applicable airworthiness directives — never from a practice test. Confirm current eligibility and testing requirements with the FAA before you test. About the A&P mechanic certificate →