Study guide
Understanding why an airplane flies, why it sometimes stops flying, and how its systems and instruments tell you what is happening is the technical foundation beneath every maneuver you will fly. This chapter covers the four forces, stalls and load factor, key powerplant and electrical systems, and the six flight instruments along with their characteristic errors, all described in plain language since you will not have diagrams on the knowledge test.
The Four Forces and Stability
Four forces act on an airplane in flight: lift, weight, thrust, and drag. Lift is generated primarily by the wing as air flows over and under it, and by Newton's third law and pressure differences the wing deflects air downward while an equal and opposite force pushes the wing up; lift increases with airspeed squared, angle of attack, wing area, and air density. Weight is the constant downward pull of gravity acting through the aircraft's center of gravity. Thrust is the forward force produced by the propeller, and drag is the rearward-acting resistance from air moving over the aircraft, split into parasite drag, which increases with speed, and induced drag, which is a byproduct of generating lift and decreases with speed. In steady, unaccelerated straight-and-level flight, lift equals weight and thrust equals drag. Stability describes how the airplane responds after being disturbed from equilibrium; positive static stability means the airplane initially tends to return toward its original condition, and dynamic stability describes whether those returning oscillations dampen out over time or grow. Designers build in stability through features such as dihedral, the upward angle of the wings that helps the airplane roll back toward wings-level after a bank disturbance, and a properly sized horizontal stabilizer that provides pitch stability. A student pilot named Priya feels this directly the first time she releases the controls briefly in smooth air and the trainer gently returns toward its trimmed attitude rather than diverging further from it.
Stalls, Spins, and Load Factor
A stall occurs when the wing exceeds its critical angle of attack, the angle at which smooth airflow over the wing separates and lift decreases abruptly; a stall can happen at any airspeed or attitude, because it is fundamentally a function of angle of attack, not speed alone. Recovery from a stall requires reducing the angle of attack first, typically by relaxing back pressure or pushing the nose down, then adding power to regain energy and minimize altitude loss, while maintaining coordinated flight with the rudder. An uncoordinated stall, especially one with excessive rudder input, can develop into a spin, in which one wing stalls more deeply than the other and the airplane rotates around a vertical axis while descending; spin recovery generally requires opposite rudder to stop the rotation, forward elevator to break the stall, and a controlled pullout, though procedures vary by aircraft and pilots should follow the specific airplane's flight manual. Load factor is the ratio of the load supported by the wings to the aircraft's weight, expressed in G units, and it increases sharply in a level turn as bank angle increases: at a 60-degree bank, load factor reaches 2 G, meaning the wings must support twice the airplane's weight, and the stall speed increases correspondingly. This relationship matters because a stall in a steep turn happens at a higher indicated airspeed than a stall in level flight, a fact tested frequently as the accelerated stall. A pilot named Tomas practicing steep turns should expect to feel and hear the increased load well before any buffet, and should recognize why an overbanked turn close to the ground is dangerous.
Powerplant, Fuel, and Electrical Systems
Most training aircraft use a reciprocating engine with either a carbureted or fuel-injected induction system. Carburetor icing occurs when fuel vaporizing in the carburetor throat cools the surrounding air, sometimes by 60 to 70 degrees Fahrenheit, and can form ice even in above-freezing outside air with visible moisture present, most insidiously at low power settings such as during a descent; the carburetor heat control diverts unfiltered, heated air into the induction system to melt existing ice, and using it causes a rougher-sounding engine and a slight power loss as expected side effects, not a malfunction. Fuel-injected engines meter fuel directly into the cylinders or intake ports and are far less prone to icing, but are more susceptible to vapor lock in hot conditions. The mixture control adjusts the fuel-to-air ratio to compensate for the reduced air density at altitude; leaning the mixture as altitude increases keeps combustion efficient and, if left unleaned, an overly rich mixture wastes fuel and can foul spark plugs. The fuel system typically includes tanks, a selector valve, a primer, and gauges, and pilots must understand their aircraft's specific fuel management procedure, including any required tank-switching schedule. The electrical system, usually powered by an engine-driven alternator or generator charging a battery, supplies the radios, lights, and many flight instruments; an ammeter or loadmeter shows whether the alternator is charging the system or the battery is discharging it, and a failed alternator leaves a pilot flying on battery power alone until it is depleted.
The Pitot-Static System and Its Three Instruments
Three of the six basic flight instruments rely on the pitot-static system: the airspeed indicator, the altimeter, and the vertical speed indicator. The pitot tube faces forward into the relative wind and senses ram air pressure, while static ports, usually flush with the fuselage, sense ambient still air pressure; the airspeed indicator compares these two pressures, the altimeter and vertical speed indicator use static pressure alone. A blocked pitot tube, often from insect nests or ice, affects only the airspeed indicator: if the ram inlet is blocked but the drain hole remains open, the indicated airspeed drops toward zero; if both the ram inlet and the drain hole are blocked, trapped pressure makes the airspeed indicator act like an altimeter, giving false high readings in a climb and false low readings in a descent while remaining frozen in level flight. A blocked static port causes the altimeter to freeze at the last correct reading, the vertical speed indicator to read zero continuously, and the airspeed indicator to become unreliable in a way that depends on whether the current altitude is above or below the altitude at which the blockage occurred. Most aircraft include an alternate static source that can be opened if the primary port is blocked, though selecting it typically causes the altimeter to read slightly high and the airspeed indicator to read slightly high in unpressurized cabins due to the different pressure environment inside the cabin. The altimeter itself must be set to the current reported altimeter setting so its internal aneroid wafers reflect local pressure; flying with an outdated setting after departing a region of lower pressure without adjusting can cause an airplane to fly lower than the altimeter indicates, an error pilots learn as 'high to low, look out below.'
Gyroscopic Instruments and the Magnetic Compass
The remaining three basic flight instruments rely on gyroscopic principles or, in the case of the compass, magnetism. The attitude indicator shows pitch and bank by referencing a gyroscope spinning on a horizontal axis, displaying a miniature airplane against an artificial horizon; it can be powered by engine-driven vacuum or electrically, and a failure often shows as a slow, subtle tumble rather than an obvious flag. The heading indicator, also gyroscopically stabilized, shows aircraft heading and must be periodically realigned with the magnetic compass because its gyroscope is subject to precession, a gradual drift caused by internal friction and normal operation. The turn coordinator shows rate of turn and, through its inclinometer ball, whether the turn is coordinated; unlike the attitude and heading indicators, it is typically powered electrically as a backup source independent of the vacuum system. The magnetic compass, the one instrument that needs no external power and always seeks magnetic north, nonetheless suffers from its own quirks: in the Northern Hemisphere, it lags behind the turn when turning toward or through north (so pilots roll out early, or undershoot) and leads the turn when turning toward or through south (so pilots roll out late, or overshoot), an effect known as northerly turning error and remembered by the acronym UNOS, undershoot north, overshoot south, and during acceleration on an east or west heading it shows a false turn indication toward north, while during deceleration it shows a false indication toward south, remembered by pilots through the acronym ANDS, accelerate north, decelerate south. Compass readings are also accurate only in straight, level, unaccelerated flight.
Key terms
- Angle of attack
- — The angle between the wing's chord line and the relative wind, the primary factor that determines whether a wing stalls.
- Load factor
- — The ratio of the load on the wings to the aircraft's weight, expressed in G units, which rises as bank angle increases in a turn.
- Carburetor icing
- — Ice formation inside a carburetor caused by the cooling effect of vaporizing fuel, possible even in warm, humid air at low power.
- Mixture control
- — The cockpit control that adjusts the fuel-to-air ratio to compensate for decreasing air density as altitude increases.
- Pitot-static system
- — The system of forward-facing ram air and ambient static pressure ports that feeds the airspeed indicator, altimeter, and vertical speed indicator.
- Static port blockage
- — A pitot-static malfunction that freezes the altimeter, zeroes the vertical speed indicator, and distorts the airspeed indicator.
- Attitude indicator
- — A gyroscopic instrument that displays pitch and bank against an artificial horizon.
- Precession
- — The gradual, predictable drift of a gyroscopic instrument caused by internal friction, requiring periodic realignment with the magnetic compass.
- Northerly turning error
- — A magnetic compass error in which the compass lags the turn approaching a north heading and leads approaching a south heading (UNOS: undershoot north, overshoot south).
- ANDS (Accelerate North, Decelerate South)
- — A memory aid for magnetic compass acceleration error on east or west headings, where accelerating shows a false turn toward north and decelerating shows a false turn toward south.
- Vacuum system
- — An engine-driven pneumatic system that historically powers the attitude and heading indicators, independent of the electrical system.
- Load meter / ammeter
- — An electrical system instrument showing whether the alternator is charging the system or the battery alone is supplying current.
Exam tips
- Remember a stall depends on angle of attack, not airspeed — a question describing a stall at high speed in a steep turn is testing accelerated stall knowledge.
- Memorize the 60-degree bank equals 2 G relationship exactly, since load factor calculation questions commonly use this benchmark.
- Separate the pitot-static instruments (airspeed, altimeter, vertical speed) from the gyroscopic and magnetic instruments (attitude, heading, turn coordinator, compass) when diagnosing an instrument failure scenario.
- For compass error questions, work out whether the heading is north, south, east, or west before applying turning error or acceleration error rules — they do not both apply on every heading.
- When a question describes carburetor heat use, expect the 'correct' answer to include a rougher engine sound and slight power loss as the normal, expected result.