Airplane systems

This element is about the airplane as a system of systems — the chains of dependency that turn "the engine is running" into the gyros spinning, the radios talking, and the airspeed indicator reading. On the checkride the DPE will hand you the airplane and ask what each instrument depends on, and what fails when one piece breaks. The per-system detail lives in the browser below; the framework prose here is about the dependencies that connect them all.

A single engine drives the vacuum pump that drives the gyros, the alternator that feeds the bus that powers the avionics, and the oil pump that keeps it all turning. Map the chains, and the failure tree draws itself.

why systems knowledge matters

Three angles converge on the same point.

The regulatory floor: under 14 CFR 91.7, the pilot in command is responsible for determining whether the airplane is in an airworthy condition for the flight — and "airworthy" is a system-by-system question, not a single bit. Per 14 CFR 91.9, you may not operate outside the limitations in the POH, and most limitations are system-specific. Knowing what each system does is the prerequisite for both judgments §91.7 §91.9.

The practical floor: failures don't announce themselves with a label. An ammeter trending negative, a suction gauge drifting low, an oil temperature creeping up, a faint roughness in the engine — each is just a needle on a dial until you can connect it back to a system. The faster you can name what changed and why, the more time you have to do something about it.

The checkride floor: the DPE will hand you the airplane and probe. "What drives the attitude indicator?" expects "the vacuum pump" — not "the gyro." "Where does the alternator feed?" expects "the main bus, then everything off it." The questions sound mechanical because the answers must be: each one tests whether you carry a working mental model of how the airplane is wired together.

how the systems hang together

The airplane is a network, not a list. Five chains carry power, fuel, air, fire, and pressure from where they start to where they're needed; understand the chains and most failures stop being surprises.

The electrical chain runs engine → alternator → main bus → avionics bus, with the battery in parallel as a reserve. Every electrical load taps the bus through a circuit breaker. Lose the alternator and the bus is now on battery alone, with a finite budget that load shedding extends but can't replace.

The vacuum chain (steam-gauge airplanes) runs engine → vacuum pump → suction lines → the attitude and heading gyros. Lose the pump and both AI and HI quietly become unreliable as the gyros spin down. The turn coordinator survives because it is electric — that is the entire architectural reason it lives next to the vacuum-driven instruments.

The fuel chain on the C172S in the plates runs tanks → selector → reservoir → shutoff valve → strainer → engine-driven pump → fuel/air metering unit → injector nozzles at each cylinder, all by gravity with an electric auxiliary pump as backup. The selector is the chokepoint; BOTH is the normal setting, LEFT and RIGHT isolate a tank, and the shutoff valve is what actually stops fuel. On a carbureted trainer the metering unit and nozzles are replaced by a carburetor — same chain up to the strainer, different last link, and that last link is the one that can make ice.

The ignition chain runs magnetos → spark plugs → cylinders, and it deliberately bypasses the rest. Magnetos are engine-driven and self-powered, so the engine keeps firing even if the battery dies and the alternator never recovers. That independence is by design — and it is why the mag check exists at every runup.

The pitot-static chain runs pitot tube and static port → ASI, ALT, and VSI on a 6-pack, or pitot and static → air-data computer → PFD tapes on a glass panel. The sensors are the same; only the downstream differs.

Now invert each chain and you have the failure tree. Alternator fails: the bus runs on battery, you shed non-essential loads and land at the nearest suitable airport before the battery quits. Vacuum pump fails: AI and HI go dark, you transition to partial panel (turn coordinator + airspeed + altimeter + VSI + compass). Static port blocks: altimeter freezes, VSI reads zero, ASI errs — open the alternate static and accept the small bias. Engine quits: vacuum pump and alternator stop with it, but the battery, the electric turn coordinator, and the pitot-static instruments keep working long enough to fly a forced landing. The Airplane Flying Handbook walks each of these procedurally FAA-H-8083-3.

The lesson is that almost every failure that isn't the engine itself is recoverable when you can name the chain it broke.

exploring the systems

The browser below is the per-system reference. Select a system on the left, and the panel updates with the diagram and the operational facts — battery and alternator on the bus for electrical, tanks through the selector to the engine for fuel, surfaces and what they command for flight controls, and so on through the seven systems a private applicant should know cold.

Three of them — pitot-static, vacuum, and the instrument panel itself — show a 6-pack / G1000 toggle in the header. That toggle isn't cosmetic. It swaps the architecture, not the styling: in 6-pack mode a vacuum pump drives mechanical gyros and three mechanical gauges live on the pitot-static lines; in G1000 mode a solid-state AHRS replaces the vacuum system entirely and an air-data computer feeds tapes on the PFD. The instruments tell you the same things; the failure modes are different. You almost certainly fly only one of the two — but the DPE expects you to know both.

Use the diagrams to anchor the mental model the rest of this task builds on. The actual failure-response procedures — memory items, abnormal checklists, decision points — live in the failure-management element (K2). The risk-management synthesis (detection, decision, automation discipline) lives in R1.

6-pack and g1000 architectures

The glass cockpit isn't an improved steam panel; it's a different architecture. The trade is concrete and worth stating cleanly.

On a 6-pack airplane, attitude and heading are vacuum-driven. The pump can fail, the gyros spin down, and the partial-panel response uses the electric turn coordinator and the pitot-static gauges as the backup set. Pitot-static feeds three separate mechanical gauges. Engine instruments are individual round dials.

On a G1000 airplane, there is no vacuum pump — a solid-state AHRS feeds the PFD electrically. Pitot-static still feeds physical sensors, but they go to an air-data computer rather than mechanical gauges, and the ADC drives the airspeed, altitude, and VSI tapes on the PFD. Engine instruments live on the MFD. The vacuum-failure mode is gone — and the electrical bus is now load-bearing for the flight instruments themselves. The fallback is the small standby cluster (typically a mechanical ASI, ALT, and AI plus a magnetic compass).

The same engine fails the same way on both. What changes is which downstream systems quietly degrade with it, and which fallback you reach for. Toggle the browser above and watch each system swap its downstream picture — that is the entire point.

worked examples

Scenario 1 — alternator failure at night, IFR.

Cruising in IMC after dark, you notice the ammeter has been steady at zero for a while and the low-voltage annunciator flickers on. The bus is on the battery now, and the clock is running.

Walk the chain: alternator → bus → everything. Battery capacity is finite. Shed non-essential loads first — landing light, taxi light, an extra radio, pitot heat if not in icing, and any nice-to-have avionics. Preserve the radios you need to fly the approach and the lights you'll need on short final. Declare to ATC, ask for the nearest suitable airport with an approach you can fly, and accept vectors. On a 6-pack airplane you'll lose pitot heat and instrument backlighting first; on a G1000 the PFD itself is on the bus, so reversion to the standby instruments is a real possibility — fly the standbys and the compass if it comes to it. Decision: land at the closest suitable airport that has an approach you're prepared to fly, sooner rather than later. This isn't the kind of failure to fly through.

Scenario 2 — carb ice in cruise.

Day VFR cruise in cool, humid air — in a carbureted trainer this time, not the fuel-injected C172S the plates draw. (Injection is why the C172S has no carb-heat knob; carbureted 172s and most legacy trainers still do, and the ACS expects you to know both.) RPM ticks down 50, then another 50, with no throttle change. The engine begins to roughen.

The chain is induction → carburetor → cylinders. Ice in the venturi restricts airflow; the engine starves; the tach reflects it before the ear does. Apply carb heat — full ON. Expect an initial further RPM drop as the ice and warmer, less dense air both pass through, then a slow recovery as the venturi clears. Leave carb heat on through the descent and approach, and return to OFF only when the conditions and the POH say so. Decision: continue the flight only if the recovery is clean and the conditions that caused it have changed; otherwise divert before the next encounter. The failure was telegraphed by an instrument you were already scanning — the win was naming it as carb ice quickly enough to act.