Major Aircraft Systems

Methods of flight control system redundancy

- ailerons (usually mechanical with hydraulic assist) are connected to one Pilot's yoke. Roll spoilers are connected to the other. In the event of a jammed or inoperable system, a clutch connecting the two systems is released, allowing one pilot to fly via the operating system - splitting the elevator controls (one for each side of the elevator) between yokes - alternate power sources for the elevators

Fuselage tanks are normally constructed of...

Aluminum

How is the water caught by the water separator kept from freezing?

An anti-ice valve opens to send bleed air in the ACM that has been cooled slightly by the primary heat exchanger (before it has been cooled by the secondary heat exchanger) to the water separator

Flap or slat asymmetry protection

Automatic asymmetry detectors monies lap and slat movements by counting the revolutions a each drive gear, and constantly comparing them to the drive-gear controlling corresponding surfaces on the opposite wing. As little as one revolution differena between them will cause the flap motor to stop, and control brake to lock the malfunctioning flaps or slas in their current positions-pretty amazing since it may take hundreds of revolutions to fully extend or retract a flap or slat. Once a flap or slat is locked out, it can only be reset on the ground.

Auxiliary fuel pumps

Aux pumps have various purposes depending on specific aircraft fuel system design. They are often used to transfer fuel from one tank to another and to provide emergency backup for the engine-driven low-pressure pump. In large air-craft, boost pumps operate continuously in tanks being used, since suction from low-pressure engine pumps is not sufficient to draw fuel at altitude. In most systems, aux pumps earn glory for crossfeed-ing fuel in the event of engine failure.

Why is jet fuel measures in pounds?

Jet fuel changes volume measurably with changes in temperature. The volume of fuel in a tank may change, but it's weight won't

LED

Leading edge device

Does the pressure vessel occupy the entire fuselage?

No; it uses pressure bulkheads and the aircraft's skin to contain the passenger cabin and some or all cargo areas

Roll spoilers

Roll spoilers are used only in flight, to assist ailerons in banking for increased roll response.

Positive pressure relief valve

Safety valve — releases excess pressure to prevent overpressurization in the event of the malfunction of controller or outflow valves

Ram air vent

Scoop ram air in flight to pressurize the tanks and enhance fuel flow

Wing area and ailerons

Since flaps take up a lot of the wing and ailerons have less space, many turbine airplanes are equipped with roll spoilers to supplement the ailerons in imparting roll

Header tank

Small separate tanks that are the same thing as collector bays

When do passengers use oxygen?

Smoke in the cabin or loss of cabin pressure

Spoilers vs speed brakes

Spoilers disturb lift and therefore increase descent rate Speed brakes increase drag and therefor increase descent rate for a constant airspeed. Found on the fuselage and tailcone in addition to the wing

Diluter-demand

Supplies oxygen only when the user inhales through the mask. Diluter-demand oxygen systems are the most commonly found type on turbine aircraft. Diluter-demand oxygen regulators dilute the oxygen supplied to the mask with predetermined levels of air from the cockpit when the aircraft is flying at lower altitudes. At sea level the mask supplies virtually no supplemental oxygen unless the pilot manually selects 100 percent oxygen (as when there is smoke in the cockpit). As cabin altitude increases, so does the percentage of oxygen supplied to the mask, with the amount of cockpit air mixed with oxygen diminishing until about FL 350 (35,000 feet), above which the pilots receive 100 percent oxygen. Some diluter-demand systems include an emergency switch for delivering a continuous flow of 100 percent oxygen under positive pressure to the mask. This is designed for use primarily during loss of cabin pressure at high altitudes, above FL 350, where positive pressure is required to force oxygen into the lungs and bloodstream.

Continuous flow oxygen system

System that supplies a constant supply of pure oxygen to a rebreather bag that dilutes the pure oxygen with exhaled gases and thus supplies a healthy mix of oxygen and ambient air to the mask, or the oxygen is just mixed with ambient air from the cockpit. Primarily used in passenger cabins of commercial airliners. Also used for pilots in GA and turboprops.

VCM

Vapor cycle machine

When are low-speed emergency descents used?

When a structural failure has occurred, like a cracked window or missing door. Also, if pressurization has been lost in areas of severe turbulence

When are fuel dump valves installed?

When maximum allowable takeoff weight is significantly greater than maximum allowable landing weight

ECU

Electronic (fuel) control unit

How much does jet fuel weigh?

6.7 lbs / gal on average. Fuel trucks are calibrated in gallons, so when accuracy is critical, a fuel conversion chart may be used

Negative pressure relief valve and when does it help

Ensures that cabin pressure never falls below outside atmospheric pressure. They pome into play when the pilots forget, upon initiating descent, to set the controller for landing. Say that the airplane is cruising at 25,000 feet, with a cabin altitude of 4,000 feet. (We'll assume a sea-level destina-fion.) When properly set for descent, the controller gradually descends the cabin at a few hundred feet per minute, while the plane itself may be descending at several thousand feet per minute. However, if the controller is not reset for landing, the cabin will stay at 4,000 feet until the airplane reaches the matching 4.000-foot pressure altitude. From that point on, the negative pressure relief valves will vent the cabin so as to prevent cabin pressure from becoming lower than outside air pressure. This situation, known as "catching the cabin," is not particularly serious, except that the cabin's descent rate now matches that of the airplane. In a fast airplane, high descent rates are required to get down in the same distance as a slower plane. (Descent rates of 3,000 pm are common in turbine aircraft.) Catching the cabin, therefore, makes it difficult to keep both ATC and the passengers' ears happy.

Landing gear squat switch (pressurization system)

Ensures that pressurization is dumped or at preset value when the plane is on the ground. Comes into play when the pilot sets the wrong destination pressurization setting. For example, if the controller thinks the pilot is descending down to sea level when he is actually descending to 5,000 feet, when the plane lands, the cabin will be at an altitude close to sea level, and the squat switch will activate to dump the pressurized air from the cabin

In many airplanes, tanks must be burned off in a specific order during flight, or...

Excess fuel returning from the engine high-pressure pump may be indirectly pumped overboard

Different types of FCUs

FADECs and ECUs

For redundancy, some aircraft have mechanical ___ backing up ___ in the same fuel system

FCUs, ECUs

Flap overspeed protection system

Flap overspeed protection or flap load relief systems protect the aircraft wing flaps from excessive air loads created by flying too fast for a given flap setting. Typically, these systems operate only in the landing-flap range, since those are the farthest-extended settings most prone to damage created by excessive air loads. If the placarded airspeed limit for a given flap setting is exceeded, the overspeed protection system automatically retracts the flaps to their next-lower set-ting. (Note that the cockpit flap-selection lever does not move.) Once the airspeed is reduced back to within normal limits, the flaps automatically re-extend to the selected flap position.

Flight spoilers

Flight spoilers may be used in flight to increase descent rate. On most aircraft they also deploy automatically after touchdown, as part of the ground spoiler system.

Fly-by-wire

Fly-by-wire control systems electronically transmit all control inputs from pilots to control surfaces via wire

Why might you physically measure the exact amount of fuel in a given tank?

For calibrating the fuel quantity measuring system or to permit dispatch when a fuel gauge is out of service

FCU

Fuel Control Unit

Fuel heaters

Fuel Heaters Jet fuel has the unfortunate characteristic of absorbing water. When a turbine aircraft is flying at high altitudes, low outside air temperatures can cause this absorbed water to crystalize in the fuel system. As you might imagine, ice crystals clog the fuel filters and can accumulate to the point of causing engine flameout. (Flameout means that combustion ceases in the engine: "the fire goes out.") Fuel heaters are used to warm the fuel and prevent this from happening. There are two common ways of warming the fuel: fuel-oil and fuel-air heat exchangers. Fuel-oil heat exchangers transfer heat from engine oil to warm the fuel. Fuel-air heat exchangers use hot bleed air for the same purpose.

Unporting

Fuel interruption caused by fuel in a tank sloshing away from the intake that draw it in

How many packs are installed and why?

Two for capacity and redundancy

Solid chemical generator

Typically found only on larger turbine aircraft having one hundred or more seats. Eliminate the need to route oxygen lines throughout the cabin to every passenger seat. Instead, each row is equipped with a separate self-contained oxygen generating unit.

What could occur in turbine fuel systems without proper design?

Unporting because the wings are so long

Heated vents

Used for protection from structural ice

Fuel trim tank

Used to keep the aircraft CG at an optimum location. Fuel is transferred back to the trim tank to move the aircraft CG aft, and out of the trim tank to move the aircraft CG forward. As you remember from your commercial pilot training, aft CG loading improves aircraft performance by reducing aerodynamic download on the horizontal stabilizer. Transferring fuel to modify aircraft CG takes advantage of that phenomenon to reduce tail down-force and trim drag, allowing the aircraft to fly more efficiently. A fuel management and quantity computer controls the necessary transfers

Fuel quantity measuring stick

Usually built into the fuel tank These handy devices are used to measure fuel quantity from beneath the wings and are located in one or more hollow tubes that run vertically through each fuel tank. Under normal circumstances, the measuring sticks are locked in place in each fuel tank. To manually measure fuel quantity, each stick is unlocked and lowered until its top is level with the fuel surface. On older "dripsticks" the fuel level was identified when fuel began dripping out of the bottom of the hollow stick. Newer magnetic models are located in sealed tubes. A magnetic float rides along each tube's outer surface on top of the fuel. When the "Magnastick" is unlocked, it slides freely out of the wing's bottom until another magnet atop the stick aligns itself with the magnetic float. Both dripsticks and Magnasticks are calibrated so that fuel quantity can be read directly, based upon how far each stick projects out of the wing.

Power-assisted controls

A further complicating factor on large aireraft is control forces. The elevators on a 757, for example, are heavy and a heck of a long way back from the cockpit. Therefore, medium to large aircraft often require power-assisted controls so the pilots can handle them. This is usually accomplished through hydraulic boost and is carefully designed to give the pilot normal control feel. These power-assisted control systems, of course, are double or triple redundant.

In many turbine aircraft, out-of-service environmental cooling equipment is...

A no go (because the hot breed air needs to be cooled down before it is safe for it to enter the cabin)

Slat

A secondary airfoil mounted on the leading edge of the wing, which extends hydraulically and automativally when the flaps are extended, usually during the first few flap extension settings. They form a slotted leading esce which allows high pressure air from below the wing to flow over the upper portion of the wing, delaying airflow separatjon

Capacitance Fuel Quantity indicator system

A single capacitance probe is basically a hollow metal tube with another tube in the middle. They are held in place by insulators to ensure they do not touch. A hole at the bottom allows fuel to flow inside; a hole at the top allows air to enter as well. Small, known electrical charges are placed on these tubes and the capacitance is measured. The value changes with the ratio of fuel to air When the fuel tanks are full, the capacitance of the probes is greater than the probe capacitance when the fuel tanks are empty. Changes in fuel level cause a change in probe capacitance. In practice, several capacitance probes are placed throughout the tank so a better sampling can be taken. Then a computer checks these against known values to come up with a fuel quantity. The probes can be fooled by the density of the fuel and some aircraft include density measuring devices to adjust the reported figure

What two devices are commonly used to cool hot bleed air before it enters the cabin?

Air cycle machines and vapor cycle machines

How is cabin temperature controlled?

By mixing hot bleed air with the cooled air of the ACM/VCM using temperature mixing valves

How is temperature that is output by the PACK controlled by the temperature controller?

By modulating the amount of bleed air going through the ACM and modulating the amount of bleed air that is being mixed with the cold ACM air

Dump valve

Can be opened by the pilots to rapidly depressurize the plane. May be used in the event of a pressurization malfunction or to "dump" the cabin in the event of smoke or other cabin air contamination

What position should the negative pressure relief doors be in on your walk around?

Closed

What position should the positive pressure relief doors be in on your walk around?

Closed

Where does the air that comes out of the cabin air vents come from?

Cold air cooled by the ACM before it is mixed with bleed air

Three types of cockpit oxygen breathing systems

Continuous-flow, diluter-demand, and pressure-demand

Conventional flight control system

Conventional flight control systems, while often hydraulically boosted, mechanically connect pilot controls to control surfaces using cables and/or pushrods

From the standpoint of weight and balance, it is possible to...

Create an unbalanced situation through poor fuel management

How does pressurization climb/descent planning work?

Either the computer or the pilot times the cabin altitude climb/descent rate to be proportional to the aircraft's climb/descent rate. For example, if the airplane is starting at sea level and going up to 15,000 feet, and the cabin is climbing to 5,000 feet, the cabin will need to climb at a third of the rate of the airplane

To ensure a steady supply of fuel to the engines in all flight attitudes...

Fuel tanks are often divided or interconnected

Types of passenger oxygen systems

Gaseous oxygen systems and solid chemical generator systems

Control tabs

Giant trim tabs that the yoke controls that then move the main controls to their desired position. Used on older turbine planes that don't have power assisted controls

Flight control surface position indicating system

Gives the crew a continuous visual display of primary control surface and spoiler positions. As you might imagine, this display is especially valuable when performing pre-takeoff control checks

ground spoilers

Ground spoilers are used only on landing rollout. Ties to the landing gear safety switch (squat switch) ensure that deployment occurs only on the ground.

Common categories of fuel pumps

High-pressure, low-pressure, auxiliary, jet

Two types of emergency descents

High-speed and low-speeds

How are spoilers usually powered?

Hydrualically

Why are there more than one set of ailerons?

If there were only ailerons on the inboard section, control wouldn't be adequate at lower speeds. For example, on the 767, the outboard ailerons operate below 240 knots

Basic operating principle of air and vapor cycle machines

If you start with a liter of gas at a given temperature and compress it to a smaller volume, the compressed gas will be hotter than it was originally. Now if you remove some of the heat from that compressed gas by blowing some cool air past it (say, through a heat exchanger) and then expand it back to its original volume again, it will be cooler than it was to begin with.

How does an air cycle machine work?

In air cycle machines, high-pressure bleed air from the engines is first passed through a compressor, further squeezing the already hot gas. It is then routed through a heat exchanger or two to remove heat. The now cooler but still highly compressed air then passes through an expansion turbine into a larger chamber. The combined effects of driving the turbine and expanding into a larger chamber dramatically cools the air (usually down close to freezing; water traps are critical in the system to prevent freeze-up). The expansion turbine is connected by shaft to the ACM's compressor, so expanding air works to compress upstream bleed air similar to the way a turbine engine or a piston engine turbocharger works. This cycle may be repeated several times, with the end result that system air temperature is cooled far below ambient temperature This cooled air is then mixed with recirculated cabin air and sent to the cabin

In addition to passenger seats, where are passenger oxygen masks stored?

In every lavatory and flight attendant station

NACA vent

Installed in some locations due to low-drag characteristics and resistance to icing

For many pressurized aircraft, certified maximum operating altitude...

Is determined not by the airplane's service ceiling but by the ability of the pressurization to meet supplemental oxygen requirements of the FARs (10,000 feet under part 135)

Jet pump

It is desirable to collect fuel from several places in each tank in order to prevent unporting and to back up any clogged pickups. Rather than installing lots of mechanically driven pumps all over the tank, these venturi devices, or jet pumps, draw fuel into the lines by creating low-pressure areas in the passing fuel moving through the lines. Therefore, jet pumps cannot pump fuel by themselves. Rather, they act as localized secondary pumps (sometimes called scavenge pumps), effectively powered by the main pump on the line. (Usually it's the low-pressure engine-driven pump or an electric auxiliary pump.)

Configuration for high-speed emergency descents

Mmo/Vmo with throttles at idle, gear up, speed brakes deployed

What are the control surfaces on fly-by-wire systems moved by?

Motors

Lateral fuel balance

On the ground when the wings aren't producing lift and the tanks are full, the wings may droop downward. In the air when the wings are flexing upward due to the generation of lift, it's helpful to have fuel concentrated outboard to reduce the upward bending moment. Depending on aircraft type, this is accomplished by some combination of burning off "center" fuselage tanks before accessing wing-tank fuel, and selectively drawing or transferring between inboard and outboard wing tanks.

What position should the outflow valve be in on the ground during your walk around?

Open

Manual reversion

Operating the controls with only human strength

Where does the air used to cool the bleed air as part of the ACM come from?

Outside ram air

Passenger Gaseous oxygen system

Oxygen supplied from a tank

Solid chemical generator system

Oxygen supplied through chemical reaction

Oxygen system check

PRICE check

Regulator

Powers the pressure of the supplied bleed air to a level suitable for the cabin

PRICE

Pressure (O2 pressure and quantity) Regulator (regulator set for normal use) Indicator (flow indicator operates normally) Connections (O2 lines are secure) Emergency (availability of masks for each flight station, and that the microphone operates in each mask)

Why does maximum differential pressure vary with aircraft type?

Pressure vessel design, engine bleed capacity, and aircraft weight and power considerations

Pressure-demand oxygen system

Pressure-demand oxygen systems are found on a number of high-performance aircraft that regularly fly at altitudes above FL 350. Below this altitude, the oxygen system operates like a diluter-demand system. Above FL 350, the pressure-demand regulator automatically furnishes pilot oxygen masks continuously with 100 percent oxygen under positive pressure in the event of lost cabin pressure.

Why is it important that the aircraft land unpressurized or at a low predetermined pressurization value?

Pressurization stresses the fuselage significantly, and a hard landing while pressurized could cause structural failure Also, opening and closing the passenger doors while pressurized can be nearly impossible, and it would not be good for the inner ears of the passengers to suddenly go from pressurized to unpressurized

Flame arrestor vent

Protect fuel tanks from ignition by hot exhaust

Pack

The environmental heating/cooling system; "Package" of ACM, bleed air source, and the associated mixing valves

What operates the firewall fuel shutoff valves?

The fire handles

Most fuel pumps cool and lubricate themselves with...

The fuel they pump (you could burn an aux pump up by operating it from an empty tank)

Pressure vessel

The portion of the airplane that is pressurized

Wet wing

The wing is the fuel tank — part of the wing structure is sealed off

Auto-slat stall protection system

This system is designed to automatically extend the leading edge slats to the full-down position (without pilot input), anytime the stall-warning computer detects the aircraft is at an excessively high angle of attack. In short, the auto-slat stall-protection system enhances the aircraft's stall characteristics by automatically lowering the wing's angle-of-attack. The slats automatically retract back to their default midrange position once the angle of attack is decreased back within normal limits.

Configuration for low-speed emergency descents

Throttles at idle, airspeed as slow as possible to minimize structural loads that might further damage the airframe, landing gear extended, flaps extended a notch or two

When are high-speed emergency descents used?

When the flight crew must descend from high altitudes without cabin pressure, but the aircraft is otherwise structurally sound

Fuel control unit

a precise hydromechan-ical or computerized electronic device that delivers fuel to the engine.

Collector bay

a separated segment of a larger fuel tank. It is located near the engine and directly supplies it with fuel. Each collector bay is in turn fed by the regular selected tanks. In the event of a momentary fuel supply fluctuation from the regular tank, the collector bay takes up the slack to provide continuous flow. Flapper valves (spring-loaded one-way doors) are sometimes installed between the collector bay and the rest of the fuel tank in order to prevent reverse flow back to the main tanks

Roll spoilers

flat panels mounted on the upper wing surfaces and are used to assist the ailerons in roll control. In a turn, they work by deploying, one-at-a-time, up into the slipstream on the down wing, disturbing lift and thereby aiding the down wing aileron in effecting the turn

Fuel vents and thermal expansion

fuel vents must also provide for relief of fuel tank overpressure due to thermal expansion, but they must be designed to prevent fuel from venting overboard under normal conditions.

Low-pressure fuel pump

is usually also engine driven. It draws fuel from the tanks and supplies it to the high-pressure pump, often through a fuel filter and heater.

Surge tanks

located near the wing tips of large aircraft to manage fuel movement, overflow, and venting.

High-pressure fuel pump

supplies fuel to each engine at pressures in the neighborhood of 900-1,000 psi. Use of high pressure for this purpose is important for several reasons. First, the amount of fuel required on takeoff may be two or three times greater than that needed at normal cruise. Therefore, both pressure and flow capacity must be available to rapidly provide large increases in fuel delivery. (Excess fuel delivered by the pump, at any given time, is routed back to the tanks via return line.)

Why might crossfeed be required?

to maintain lateral balance and thereby improve aircraft control in the event of uneven fuel flow, fuel leaks, or uneven fuel burn due to engine failure