FAA Certified Flight Instructor – Instrument (CFII)

Correct: Oleo struts rely on a specific balance of hydraulic fluid and compressed nitrogen to support the aircraft; a fully retracted piston indicates the system cannot support the static load and is therefore unairworthy.

Incorrect: The strategy of assuming the strut should be compressed under static load ignores the design requirement for a cushion of gas to be present at all times. Attributing the lack of extension to a mechanical wind lock is incorrect because oleo struts do not utilize such locks for ground operations. Opting to shift weight to the rear of the aircraft merely masks a mechanical failure rather than addressing the underlying loss of pressure or fluid.

Takeaway: A fully compressed oleo strut indicates a loss of internal pressure or fluid and must be repaired before flight.

Correct: Bernoulli’s Principle states that as the velocity of a moving fluid increases, the pressure within that fluid decreases. In aviation, the shape of an airfoil causes air to travel faster over the upper cambered surface than the lower surface. This increase in velocity results in a lower static pressure on top of the wing compared to the bottom, creating the pressure differential necessary for the production of lift.

Incorrect: The strategy of suggesting that total pressure increases with velocity is incorrect because total pressure remains constant in a closed system; rather, static pressure decreases as dynamic pressure increases. Focusing only on the impact of air molecules or downward deflection ignores the significant role of pressure differentials described by fluid dynamics. Choosing to believe that velocity decreases on the upper surface to create lift is a fundamental misunderstanding of airfoil design, which is specifically shaped to accelerate airflow over the top.

Takeaway: Bernoulli’s Principle explains that higher airflow velocity over the upper wing surface results in lower static pressure, generating lift.

Correct: Newton’s Third Law of Motion states that for every action, there is an equal and opposite reaction. In the context of aerodynamics, as the airfoil moves through the air, it deflects a mass of air downward (the action). The reaction to this downward deflection is an upward force exerted by the air on the wing, which contributes to the total lift generated by the aircraft.

Incorrect: Focusing on the velocity of air over the upper surface and the resulting pressure differential describes Bernoulli’s Principle, which is a different physical explanation for lift than Newton’s Third Law. Describing the tendency of the aircraft to remain in a state of uniform motion refers to Newton’s First Law, also known as the Law of Inertia. Suggesting that lift is simply the product of aircraft mass and particle acceleration misapplies Newton’s Second Law and fails to account for the action-reaction pair required by the Third Law.

Takeaway: Newton’s Third Law explains lift as the upward reaction resulting from the wing’s downward deflection of the air mass. aviation physics relies on this action-reaction principle for flight.

Correct: The Angle of Attack is defined as the acute angle between the chord line of the airfoil and the direction of the relative wind. This relationship is fundamental to aerodynamics because lift is generated based on this angle. As the pilot increases back-pressure, the chord line tilts further away from the flight path, increasing the AOA until the critical angle is exceeded and the wing stalls.

Incorrect: Relying on the relationship between the aircraft’s nose and the horizon describes pitch attitude, which is a visual reference rather than an aerodynamic angle. The strategy of measuring the angle between the wing and the fuselage defines the angle of incidence, which is a fixed design characteristic. Focusing only on the flight path relative to the ground describes the climb or descent gradient, which does not account for the wing’s orientation to the airflow.

Takeaway: Angle of Attack is the specific angle between the wing’s chord line and the oncoming relative wind.

Correct: Lift is a complex phenomenon involving both pressure changes and momentum transfer. Bernoulli’s Principle explains the lower pressure on the upper surface due to higher velocity, while Newton’s Third Law explains the upward force resulting from the downward deflection of the air.

Incorrect: Relying on the theory that air molecules must meet at the trailing edge at the same time is a common misconception not supported by fluid dynamics. Focusing only on the impact of air on the bottom of the wing fails to account for the significant lift generated by the low-pressure area on top. Choosing to describe lift as a localized vacuum ignores the continuous flow field and the necessity of air deflection for sustained flight.

Takeaway: Lift results from both the pressure differential across the airfoil and the downward deflection of the airflow.

Correct: For most asymmetrical or cambered airfoils used in general aviation, an increase in the angle of attack shifts the pressure distribution forward, causing the Center of Pressure to move toward the leading edge.

Incorrect: The approach of suggesting the point moves aft is factually incorrect as that movement occurs when the angle of attack is decreased. Simply assuming the point stays at a fixed percentage of the chord confuses the Center of Pressure with the Aerodynamic Center. Choosing to describe a movement toward the wingtip incorrectly applies spanwise pressure concepts to the longitudinal movement of the resultant force.

Takeaway: On a cambered airfoil, the Center of Pressure moves forward as the angle of attack increases.

Correct: When the aileron on the outside of the turn moves downward to increase lift, it simultaneously increases the induced drag on that wing. This drag creates a yawing moment that pulls the nose of the aircraft in the opposite direction of the roll. Pilots must use the rudder to counteract this effect and maintain coordinated flight.

Correct: The phugoid oscillation is a long-period longitudinal mode where the aircraft exchanges altitude for airspeed. In this state, the angle of attack remains nearly constant while the pitch attitude varies slowly. This represents a continuous trade-off between kinetic and potential energy as the aircraft seeks its trimmed equilibrium.

Incorrect: Focusing on rapid, heavily damped oscillations in pitch and angle of attack describes the short period mode, which occurs much faster than the phugoid. The strategy of identifying coupled oscillations between the roll and yaw axes refers to Dutch roll, which is a lateral-directional stability characteristic. Choosing to describe a non-oscillatory tendency for the bank angle to increase over time identifies spiral instability, which involves a divergence from level flight rather than a rhythmic cycle.

Takeaway: Phugoid oscillations are long-period longitudinal cycles characterized by varying airspeed and altitude at a nearly constant angle of attack.

Correct: Induced drag is a byproduct of lift and is inversely proportional to the square of the airspeed. As the aircraft slows down, a higher angle of attack is required to maintain the same amount of lift, which increases the strength of wingtip vortices and induced drag. Parasite drag, which includes skin friction and form drag, is directly proportional to the square of the airspeed and therefore decreases significantly as the aircraft decelerates.

Incorrect: The strategy of suggesting parasite drag increases at lower speeds is incorrect because parasite drag is a function of velocity and always diminishes as the aircraft slows down. Simply assuming induced drag remains constant ignores the fundamental aerodynamic principle that slower speeds require higher angles of attack to maintain altitude. Focusing only on a total decrease in drag fails to account for the drag curve where induced drag becomes the dominant force at low airspeeds. Choosing to believe induced drag decreases at low speeds contradicts the fact that induced drag is highest when the aircraft is slow and the wing is at a high angle of attack.

Takeaway: Induced drag increases as airspeed decreases, while parasite drag decreases as airspeed decreases.

Correct: Reducing the angle of attack is the primary requirement to restore laminar flow over the wing and break the stall. Applying power assists in the recovery and minimizes altitude loss, while rudder is used for directional control to avoid the adverse effects of aileron use at high angles of attack.

Incorrect: Increasing back pressure on the control column is counterproductive because it further increases the angle of attack, which is the root cause of the stall. Relying on ailerons for roll control during a stall can be dangerous as it may induce a spin or worsen the wing drop due to increased drag on the descending wing. Choosing to retract flaps immediately is incorrect because the sudden loss of lift can cause the aircraft to sink rapidly or increase the stall speed before the recovery is complete.

Takeaway: The most critical step in stall recovery is reducing the angle of attack to restore lift.

Correct: The Critical Mach Number is defined by the Federal Aviation Administration as the lowest flight Mach number at which the speed of the air over any part of the aircraft reaches the speed of sound. At this specific speed, local supersonic flow begins, marking the onset of transonic aerodynamic characteristics and the potential for shock wave formation.

Incorrect: Focusing on the point where total aircraft drag begins to rise sharply describes the drag divergence Mach number, which typically occurs after the initial sonic flow is established. Describing the aerodynamic phenomenon where an aircraft exhibits a nose-down pitching tendency due to shock wave formation on the wing refers to Mach tuck. Attributing the condition to a loss of lift caused by airflow separation behind a shock wave describes a shock stall, which is a result of exceeding the critical speed rather than the definition of the speed itself.

Takeaway: Critical Mach Number marks the transition where local airflow first reaches supersonic speeds on an aircraft.

Correct: Positive static stability is defined as the initial tendency of an aircraft to return to its original equilibrium position after being disturbed. In the United States, aircraft certified in the normal category must demonstrate this restorative force to ensure predictable handling for pilots.

Incorrect: Choosing to believe the aircraft stays in the new position describes neutral static stability where no restorative force exists. Opting for the idea that the nose continues to move away describes negative static stability which is an unstable condition. Focusing only on the dampening of oscillations over time confuses dynamic stability with the initial tendency of static stability. Simply conducting a recovery based on the assumption of neutral stability ignores the inherent aerodynamic design of most certified aircraft.

Correct: Parasite drag, which includes form and skin friction, increases with the square of the airspeed. Induced drag is a byproduct of lift generation; as the aircraft speeds up, it requires a lower angle of attack to maintain level flight. This reduces the strength of wingtip vortices and the associated induced drag.

Incorrect: The strategy of suggesting induced drag increases at high speeds ignores the inverse relationship between speed and the angle of attack required for lift. Simply stating that both drag types increase fails to account for the aerodynamic efficiency gained at higher speeds regarding lift-induced pressure differentials. The notion that parasite drag remains constant or decreases contradicts the basic physical principle that fluid resistance increases as velocity increases. Choosing to assume induced drag increases with the square of the speed misidentifies the relationship between lift production and airspeed.

Correct: Extending the landing gear adds significant non-lifting surface area to the airflow, which increases parasitic drag. Trailing-edge flaps increase the effective camber of the wing, which raises the maximum lift coefficient and allows the aircraft to fly at slower speeds. This change in wing geometry also increases drag and typically shifts the center of pressure aft, which induces a nose-down pitching moment in most general aviation aircraft.

Incorrect: The strategy of claiming that flaps increase the lift-to-drag ratio is incorrect because the drag penalty of flaps, especially at larger settings, reduces the overall aerodynamic efficiency. Focusing only on the critical angle of attack is misleading, as flaps actually decrease the stall speed rather than increasing the speed for approach. Choosing to believe that landing gear reduces drag or that flaps move the center of pressure forward contradicts the physical reality of increased frontal area and the aft pressure shift.

Takeaway: Flap and gear extension increases lift and drag while lowering stall speed and typically inducing a nose-down pitching moment.

Correct: Positive static stability is defined by the initial tendency of the aircraft to return to its original equilibrium position after being disturbed. Negative dynamic stability occurs when the resulting oscillations increase in amplitude over time, showing that the aircraft is moving further away from the equilibrium state with each cycle.

Incorrect: The strategy of identifying negative static stability is incorrect because the aircraft initially attempted to return to its original attitude rather than continuing to diverge immediately. Choosing to classify the aircraft as having neutral stability is inaccurate because the aircraft did not remain in the disturbed position or maintain a constant oscillation amplitude. Opting for positive dynamic stability is incorrect because that would require the oscillations to dampen and eventually cease as the aircraft returns to level flight.

Takeaway: Static stability determines the initial corrective tendency, while dynamic stability describes the behavior of oscillations over time.

Correct: The wing spars are the principal structural members of the wing, running spanwise from the fuselage to the tip. They are engineered to withstand the heavy bending moments and shear loads that occur when the wings generate lift, providing the necessary longitudinal strength for the assembly.

Incorrect: Suggesting that the wing ribs carry the primary bending load is inaccurate because their main function is to give the wing its aerodynamic shape and transfer skin loads to the spar. Attributing the primary longitudinal strength to the stressed skin is a characteristic of pure monocoque designs, whereas in semi-monocoque wings, the skin primarily handles tension and torsion while the spar handles bending. Identifying formers as the primary load-bearing members is incorrect as formers are typically used in the fuselage to maintain shape rather than as the primary longitudinal load-bearing components of a wing.

Takeaway: Wing spars are the primary structural components designed to resist longitudinal bending moments in semi-monocoque wing construction.

Correct: In an oleo-pneumatic strut, the compressed nitrogen gas acts as a variable-rate spring. This gas supports the static weight of the aircraft and provides cushioning for the airframe during taxiing and ground operations. While the hydraulic fluid handles the high-energy dissipation of a landing, the gas provides the necessary suspension for the aircraft’s weight.

Incorrect: Attributing the primary damping force to the gas is incorrect because damping is actually provided by the hydraulic fluid being forced through a restricted orifice or metering pin. Suggesting the gas is used to prevent fluid foaming is a misunderstanding of the strut’s internal physics and fluid dynamics. Claiming the nitrogen charge is responsible for centering the nose wheel is inaccurate, as centering is typically achieved through mechanical cams or steering linkages rather than the gas pressure itself.

Takeaway: Oleo struts utilize compressed nitrogen as a spring for ground loads and hydraulic fluid for landing impact damping.

Correct: Bernoulli’s Principle states that an increase in the velocity of a moving fluid is accompanied by a simultaneous decrease in the fluid’s internal static pressure. On an airfoil, the upper camber forces air to travel at a higher velocity, resulting in lower static pressure compared to the ambient air, which contributes to the total lift force.

Incorrect: Choosing to believe that air velocity decreases over the upper surface fails to account for the acceleration caused by the wing’s curvature. The strategy of assuming that pressure increases alongside velocity contradicts the fundamental laws of fluid dynamics. Opting for the idea that static pressure remains constant despite changes in velocity ignores the physical mechanism that generates the pressure differential necessary for lift.

Correct: Newton’s Third Law states that for every action, there is an equal and opposite reaction. In the context of an airfoil, the wing exerts a downward force on the air, known as downwash. Consequently, the air exerts an equal and opposite upward force on the wing, which is a fundamental component of lift generation as recognized by the Federal Aviation Administration (FAA).

Incorrect: Relying on the equal transit time theory incorrectly suggests that air parcels must meet at the trailing edge, which is a common aerodynamic myth. The strategy of describing a high-pressure cushion without air deflection fails to account for the necessary change in momentum of the fluid. Focusing only on thermal layers and friction ignores the fundamental mechanical interactions between the airfoil shape and the relative wind.

Takeaway: Lift is generated as a reaction to the downward deflection of air by the airfoil, consistent with Newton’s Third Law.

Correct: Positive static longitudinal stability is the initial tendency of the aircraft to return to its original state of equilibrium after being disturbed. Positive dynamic longitudinal stability is demonstrated when the amplitude of the resulting oscillations decreases over time, eventually returning the aircraft to its original trimmed condition.

Incorrect: Suggesting the aircraft exhibits neutral dynamic stability is incorrect because that would require the pitch oscillations to continue at a constant amplitude without dampening. Identifying the initial reaction as neutral static stability is a misconception because a neutral aircraft would simply remain in the new pitch attitude rather than attempting to return to the horizon. Classifying the dampening motion as negative dynamic stability is wrong because negative dynamic stability involves oscillations that grow larger in magnitude with each cycle.

Takeaway: Positive static stability provides the initial corrective tendency, while positive dynamic stability ensures the aircraft eventually returns to equilibrium.