The Aircraft: Exterior and Safety: Airplane and Airline FAQ

WHAT THE MECHANIC DOES

If the flight crew detected any type of mechanical mal function while in the air—from a generator tripping off line to the movie being improperly projected—they will have radioed the problem to the maintenance’ coordinator via central dispatch. The maintenance coordinator will in turn contact the local mechanics, who will meet the flight with the tools needed to replace the malfunctioning part. The airlines have found it more timely to replace parts than repair them. After removal from the plane, the faulty part can be shipped to a central maintenance hangar and rebuilt like-new. Once repaired, it can be returned to the “on line” inventory.

If a problem occurs closer to the airport—on approach, for instance—there isn’t time to radio the maintenance coordinator and discuss it. In this case, when the mechanic meets the inbound plane, he’ll find out about the problem by plugging his headset into an interphone jack located near the nose wheel and asking the flight crew, “How’s the aircraft?”

Whenever there is something wrong with the plane, it must be legally noted in an aircraft log book. The log book is kept on the airplane at all times, and it contains that plane’s mechanical history. This enables the crew to anticipate a problem before it happens. Also, if something recurs frequently, the plane will be taken out of service and repaired.

Most frequently, nothing is wrong and the mechanic will begin the first of two walkarounds—or inspections—of the airplane. A member of the flight crew conducts the other one. The mechanic begins the exterior inspection while the passengers are still deplaning, then comes to the cockpit to check the pressure and quantities of oil, hydraulics, fuel, and emergency oxygen.

On the newer aircraft the mechanic can directly access the aircraft’s “status” computers. These computers record transient malfunctions, discrepancies that are not displayed on the cockpit instruments and engine exceedences. With these status computers, troubleshooting becomes more efficient and cost-effective.

When the mechanic is satisfied the plane is airworthy, he will sign the log book, deeming the plane okay for its next flight.

FLIGHT CREW WALKAROUND

The pilots are required to arrive at the airplane thirty minutes prior to departure. Responsibility for the flight crews’ walkaround, which follows on the heels of the mechanic’s inspection, will usually fall on the junior member—the co-pilot if it’s a two-man crew, the flight engineer if there are three.

WHAT IS CHECKED DURING THE WALKAROUND INSPECTION?

The exterior inspection of an aircraft is a general condition check and an inspection of more than 100 specific items. The airplane’s exterior surface is checked for dents and peeling paint, or any possible signs of damage or wear. If some damage is found, for instance a dent, an airframe mechanic would inspect the area to determine if the plane can remain in service or if a more detailed inspection and repair is necessary.

Specific checks include a visual inspection of the exterior components (discussed later in this section) with extra attention given to the high-use items such as the landing gear, brakes, and tires. The multiple hydraulic systems are checked for minute leaks, particularly around the pumps and actuators, where a drip can be detected before it registers on the cockpit gauges. The engines are checked for visible wear and the presence of oil in non-normal locations. The fuel lines, which are encased in a protective sleeve, are checked for leaks at these sleeve drain ports. Flight controls are checked and their position is noted. They are later verified with the cockpit flight-control position gauges. Pressurization valves are inspected. Ram-air and static-air vents are examined to ensure that they are open and clear. Service and inspection panels are all checked to be properly closed and locked.

ICE AND FROST

During the cold winter months a thorough preflight inspection must include checking for frost or ice on the wings. If there is any, or if the pilot thinks there’s a possibility it might form, then the airplane must be de-iced. Dc-icing is done with dc-icing fluids. The Type One fluid, the original type solution, is good for fifteen minutes. The 1 Two fluid, a newer one, is good for thirty minutes or more. Ice or frost on the wings reduces the lifting capability and adds weight, and therefore must be removed before takeoff.

The aircraft has its own anti-ice system on board. The engine anti-ice system can be used full-time, and when needed can be turned on immediately after engine start. The wing anti-ice system gets its hot-air supply from the engine compressors, and if used on the ground, would reduce the power available for takeoff. Consequently, this system is not used until the plane is at least 400 feet above ground and climbing.

If a plane is dc-iced at the gate and, for some reason, there is a lengthy delay at the runway, it is necessary for the wings to be re-inspected for ice and frost. If there is a renewed buildup, the plane must return to the gate again.

Airlines are now beginning to experiment with a new “car wash” approach to dc-icing. Instead of dc-icing at the gate, a specific dc-icing facility located on a taxiway near the active runway provides dc-icing closer to departure time. This system requires the aircraft climate-control system must be turned off for up to five minutes to prevent the smell of the dc-icing fluid from entering the cabin. The disadvantages are: (1) The plane’s interior would get cold; and (2) dc-icing away from the gate doesn’t allow the flight crew to completely inspect their own aircraft, since not all of the plane can be seen from inside the cockpit and cabin.

WHAT ELSE IS GOING ON DURING THE WALKAROUND?

As the crew member conducts the walkaround, he is surrounded by a flurry of activity. A fuel truck. A catering truck. Conveyor belts used to load the individual luggage, mechanized loaders for the container luggage and cargo, plus all the tugs and carts needed to haul everything. A water-servicing truck to refill the drinking water tanks. A waste truck, which suctions out the holding tanks. And any kind of maintenance vehicles necessary to work on the plane. The aircraft are designed so that the various vehicles can all work at the same time.

EXTERIOR AIRCRAFT COMPONENTS

Looking out at a plane from the terminal does not always do justice to the size of the aircraft. Take one of the newer jets, the Boeing 767-300. The airplane is 180 feet long, which is about two-thirds of a football field. Its highest point measures 52 feet, about five stories. The wing span, from tip to tip, is 156 feet, a length that would extend across more than ten lanes of an interstate. At 24 feet, 4 inches, the cockpit is almost two stories above the ground. The diameter of the airplane itself is 16 feet, 6 inches.

Some airlines paint their planes, others don’t. The basic choice comes down to personal preferences, but there are advantages to painting the craft. Paint helps prevent corrosion and can help cool the fuselage during hot summer months. The major disadvantage of paint is its weight. An L-101 1, for instance, has over 8,632 square feet of painted area, which requires roughly 58 gallons of paint weighing almost 700 pounds.

THE ENGINES

The powerful engines are perhaps the most ominous- looking feature of an airplane. The two engines on a 767 are 106 inches in diameter—that’s 8 feet, 10 inches, taller than many of the rooms in your home, and larger than the diameter of the fuselage of the original DC-3 manufactured in the 1930s and 1940s. Each of those engines weighs more than 10,000 pounds. That’s five tons. The 767 uses two of these engines; the MD-II, a heavier airplane, three; and the 747-400, still bigger, four.

Not surprisingly, the engines are as powerful as they are large. Each engine is capable of generating 60,000 pounds of thrust, which is 24,000 horsepower. An older 727 has three engines, each developing 15,500 pounds of thrust. For comparison, one 767 engine is more powerful than all three of a 727’s combined.

ENGINE RELIABILITY

Two assumptions are usually made about engine reliability, and both are false. One, that as engines get bigger and more powerful, they become more susceptible to failure. Two, as engines become older, they are more likely to have breakdowns. Neither is true.

Engines in service are meticulously maintained and regularly overhauled back to like-new status. And unlike your automobile, as advancements are made in engine technology, some of these improvements can be incorporated on an older engine during overhaul. In reality, some of the older engines, after modifications and refinements, are just now achieving the level of reliability of the newer turbofan engines. Statistically speaking, the chances of an engine shutdown—whether as a precaution or actual mal function, are approximately one per 50,000 hours of flight time. (Though this is extremely remote, we will discuss later how all aircraft are designed and all pilots are trained and retrained for the worst-case scenario.)

FUEL

To most people, wings appear thin and narrow. Nothing could be further from the truth. Most of the airplane’s fuel is stored inside the wings. Why? It adds extra strength. If you squeeze a full carton of milk, it is less compressible than a carton that’s empty. Likewise, aircraft wings filled with fuel are stronger.

How much fuel is contained? A 767, for example, holds 24,000 gallons of fuel. That may not sound like a lot, but there is enough fuel on board a 767 to operate your car for 32 years. The 747—400, the largest plane in commercial use, holds 60,000 gallons of fuel, an amount that would enable you to run the family car for 80 years.

THE WINGS

Passengers often ask if the wings are large and strong enough to support the entire airframe through all types of weather over years of service. The answer is yes.

The size of a wing is directly proportional to the maxi mum weight of the airplane. The wing of the 747—400, which has a surface area greater than a basketball court, has to support exactly the same weight per square foot (100 pounds) as the wing of a smaller airplane with a smaller wing. The wings are made out of two giant steel girders, the same type of girders used in high-rise buildings. Two of these girders extend from tip to tip through a wing box that is attached to the fuselage. Essentially, that’s the structure of the wing itself. They are incredibly strong, but also incredibly flexible—like buildings that have the strength and flexibility to sway and bend and not crumble during earthquakes or extremely high winds.

In fact, in normal flight the wings flex 3 to 6 feet. Maximum flexibility tolerance greatly exceeds that. In the case of a 747, the wings are stressed to flex 29 feet up and down without damage. The strength of the wing and fuselage are equally impressive. Aircraft are built to with stand a minimum of 2½ g’s—or more than 2½ times the force of gravity. What does this mean? One g of gravity is the force we feel normally here on earth. If we could fly into space, we would feel zero g’s, or be weightless. A 767, for example, has a maximum gross weight of 407,000 pounds. When there is turbulence putting stress on the airframe and wings, the plane is constructed to withstand 2 1/2 times that 407,000 pounds. The manufacturers are also required to build the aircraft to withstand an additional 50 percent safety overload above the already conservative maximum stress limit. What this means is that the wings of a plane with a gross weight of 407,000 pounds can support well over one million pounds of weight in normal flight, and 1½ million pounds in a maximum-stress condition.

SLATS/FLAPS

Slats are a series of panels on the front side—or leading edge—of the wing. They can be symmetrically moved forward and down. The flaps are two sets of panels located along the backside or trailing edge of the wings. They can move outward and downward. At the gate on most aircraft the flaps and slats are positioned in the fully up, or retracted, position. Both slats and flaps are operated hydraulically but locked (held in position) mechanically. On the underside of the wings you can see the cover for the jack screws that turn to move the flaps.

When the flaps and slats are fully extended they increase the surface area approximately 20 percent and the lifting capability of the wings 80 percent. What this does, in simple terms, is make it possible for the plane to fly slower, which reduces the amount of runway needed for takeoff and landing.

If for some reason there was a malfunction and the flight crew couldn’t get the flaps and slats out hydraulically, they would turn to a backup electrical system. If those failed, there are further redundant systems that would enable them to bypass the faulty portion of the system and extend some of the flaps and slats. But in the extremely rare situation where nothing could be done, the plane could still land. The only difference is that the crew would have to land at a higher speed, requiring a longer runway and utilizing greater braking power after touchdown.

COMPASSES, GYROS, AND STROBES

All aircraft have compasses on board, even though the old-fashioned Boy Scout navigational tools are rarely used. Their shortcoming is that they are susceptible to magnetic and radio interference common in an airplane. Instead, directional gyroscopes or heading indicators are used. This directional system uses sensors that are located in the wing tips as far from any interference as possible, with their compass display located in the cockpit. To further reduce the interference, the wing tips are made of a nonmetal graphite epoxy, which is a slightly different color than the rest of the plane. They are secured with demagnetized screws.

Also found on the wing tips are position lights. The left wing tip has a red light, the right one a green light. Mounted on the rear of the wing tips are white lights. They are on continually, day and night, and assist in helping the flight crew tell if a plane is coming toward them or flying away.

Also on the wing tips are white strobe lights. The strobes flash 48 times a minute, and they’re used as an anti-collision light. When flying through clouds, their reflection into the passenger cabin can sometimes be mistaken for lightning.

Additionally, on the top and bottom of the fuselage not directly visible from the cabin, there is a flashing red rotating beacon—also an anti-collision light. Though not as bright as a strobe in clouds, the red reflection is sometimes visible.

AILERONS

These are one or two sets of movable surfaces on the trailing edge of the wings, used to control the angle of bank when making turns. When one aileron goes up during a turn, the other simultaneously goes down. The result is unequal lift on the two wings, causing the aircraft to bank, and thus turn.

SPEED BRAKES AND SPOILERS

These are the multiple large panels that rise from the trailing-edge portion of the top of the wings. In the air they’re called speed brakes, and on the ground the same panels function as spoilers. The left-wing and right-wing panels can be moved independently from the cockpit to augment the roll or turning capability of the aircraft. Using a speed brake handle located between the captain and co-pilot, both the left and right set of panels can be used simultaneously to slow the plane down.

When Air Traffic Control asks a pilot to descend and slow down at the same time, the speed brakes are deployed. If you were going downhill in your car and just took your foot off the gas, there wouldn’t be a huge difference in speed. Same thing with an airplane. So these panels actually rise above the wing, causing an increase in drag and enabling the plane to slow down at the same time it descends.

In the air they come up in increments to a maximum of 40 degrees; on the ground they go up rapidly to 60 degrees, instantly spoiling lift, decreasing the chances of a bounce on landing, and applying extra weight on the landing gear to increase braking effectiveness.

THE LANDING GEAR—TIRES

The piece of equipment on an airplane that receives the most wear and tear are the tires. Aircraft tires do not rotate in the air, therefore, at touchdown the tires spin up from zero to the landing speed of almost 150 mph instantly. Watching aircraft land, you can see the puff of smoke generated when touchdown and simultaneous wheel spin- up occurs.

The tires are extremely durable. The tires on a 767 are up to 32 ply, a drastic increase over the two- or four-ply tires of a family car. If there is a single nick or tear in just an outermost layer, the 32nd ply, the tire will be replaced. Under normal wear and tear, tires are changed every 200 or so landings—about every month and a half.

The tire pressure is 220 pounds per square inch, and during the walkaround, if pressure is ever found to be as little as 20 pounds low, instead of troubleshooting the cause, the tire is automatically replaced.

Replacing a tire is no small task. A sixty-ton (120,000 pound) hydraulic jack lifts the landing gear section of the tire being replaced off the ground, and a one-ton hydraulic lift is used just to lift the tires into position. A tire change can be completed on a fully loaded aircraft.

One hundred eighty-five pounds of tire pressure seems high to those of us used to filling our auto tires with 25 pounds of air. But it’s quite important when you consider hydroplaning, a condition in rainy weather when a film of moisture can build up between the runway and the tires, reducing the frictional contact with the pavement.

There’s a specific formula used to figure out when a tire will hydroplane. It’s nine times the square root of the tire pressure. If a car tire is inflated to 25 pounds per square inch, the speed at which the car will hydroplane is 45 mph.

For an airplane, whose tires are filled to 220 psi, it’s 133 mph. The chances of hydroplaning on a wet runway are further reduced by use of the ground spoilers to spoil lift and increase the effective weight on the tires. By crowning and grooving the runway—higher in the center, with channels every several inches to enhance water runoff— even during heavy rain, water buildup is kept to a mini mum.

The tires themselves are inflated with dry nitrogen—not air—to reduce their flammability in case the tires got excessively hot. The tires are also equipped with heat- sensitive fuse plugs, which blow out when the tire’s internal temperature rises above a certain point—usually greater than 350 degrees. When these plugs blow out, they release pressure slowly, preventing a sudden blowout that could effect directional control at high speeds on the runways or be a safety hazard to personnel in the ramp area.

The crew monitors these temperatures in the cockpit on a brake temperature gauge that warns of hot brakes causing hot tires.

The tires have a maximum speed of 253 mph, far above what’s required for normal takeoff and landings.

THE BRAKES

Each tire has its own set of brakes. But unlike a car’s brakes—one disc per tire—a plane will usually have five or more sets of discs for each tire. Therefore, on one wheel assembly with four tires, there will be four sets of brakes with twenty stopping discs.

The amount of energy absorbed by the brakes during the deceleration after landing is enormous. Heat can build up rapidly. On a warm summer day, after a landing with only moderate braking, it can take 30 to 45 minutes for the brakes to cool down to the ambient outside temperature. Since most flights are scheduled for at least a 45-minute ground turnaround time, this brake cooling period does not usually cause a delay.

Each brake also has a brake-wear pin, which is checked on the walkaround. The pin protrudes from the brake assembly. When the brake wears down, the brake pad thins and causes the pin to retract. When it gets to a certain point, the brakes are changed. Under a new FAA rule, all aircraft stopping distances are calculated assuming only 10 percent of each brake’s pad is remaining. So even as the brake pads wear, the stopping capability of your airplane will be better than the test aircraft.

Older type brakes are made of metal. Though extremely reliable, they could under extreme heat conditions lose some effectiveness. They could also heat up to a point that if you applied your parking brake once stopped at the gate, the brakes could “lock-up,” which requires a complete brake change. All new aircraft are equipped with carbon disc brakes, which are able to dissipate the heat faster, reducing brake cooling times and the risk of locked brakes at the gate.

Looking at an aircraft at the gate, you will see large rubber or metal “chocks” in front and behind all the tires. Though there is little chance an aircraft would roll on level ground, by using these chocks as a safety against ground movement, the flight crew does not have to leave the parking brake on at the gate. Brake cooling is much more effective with the parking brake off.

In addition, each brake has its own antiskid system similar to an automobile’s antilock system. If the brakes were applied and the antiskid system sensed the wheels were not turning, due possibly to a skid, the system would automatically release the brakes, allow the tire to spin up and instantaneously reapply brake pressure. In the case where one individual brake on one tire—say, the left side—was grabbing, the antiskid system would release that brake as well as the one brake on the right-side tire assembly simultaneously, to prevent directional control problems. All other individual brakes would function normally.

AUXILIARY POWER UNIT

The Auxiliary Power Unit (APU) is located in the tail of most aircraft. The APU is a jet engine, about the size of an engine found on most commuter aircraft. It provides for ground electricity and air for ground air conditioning, but no thrust.

If you ever walk the ramp area of a commercial airport, you will hear the tremendous jet engine noise generated by the APU.

FLIGHT AND VOICE RECORDERS

The flight and voice recorders that are always mentioned any time there is an accident are also located in the tail. Though commonly called the black boxes, they are housed in bright orange containers, which are able to withstand incredible forces — 200 g’s or more — and incredibly high temperatures — + 2000 degrees Fahrenheit. They are located in the tail for maintenance accessibility.

Inside the flight recorder there is a 25-hour tape that records all the parameters of the flight, including the time, altitude, airspeed, vertical acceleration, heading, the time of each radio transmission, the pitch and roll of the plane, the position of the flight controls, position of the thrust levers and the thrust (power) of each engine, the flap position, and so on.

The voice recorder is separate and records all cockpit conversations and radio transmissions with ATC and the company.

Both recording devices are automatically activated when ever the aircraft is operating and has electrical power. If an airplane was forced to ditch at sea, the flight recorder contains a water-activated transmitter that will emit signals for thirty days to aid in its recovery.

NOSE WHEEL ASSEMBLY

The nose wheels, the two front tires, are the only steer able wheels of an aircraft. Using a tiller, or side-mounted steering wheel in the cockpit, the nose gear can turn approximately 75 degrees left or right of center, allowing for almost perfect corner turns. Though not a landing gear per Se, the nose wheel is extremely strong and is the point at which the ground service tug attaches to the aircraft for pushback from the gate.

Also located on or near the nose-wheel assembly is a ground-alert system. Some of the same horns and buzzers that sound in the cockpit—indicating, for example, the electronic equipment bay is not being cooled or the APU is overheating—will sound at the nose-wheel area, alerting the ground crew to investigate a problem, if the flight crew hasn’t already arrived.

NOSE

The nose of the aircraft is called the radome. Painted a non-reflective color, usually black, in order to reduce glare in the cockpit, the radome is made of nonmetallic plexiglass and houses the weather radar. The radar scans up to 320 miles in front of the aircraft, left and right almost 90 degrees, up and down nearly 15 degrees, and it can detect rain showers, thunderstorms, and their associated turbulence.

COCKPIT WINDOWS

From the outside, the cockpit windows appear small, especially in relation to the airplane’s overall size. But when you’re sitting in the cockpit, you’re close to the windows and the visibility is excellent. As technology for window strength improves, the cockpit windows on newer airplanes are getting larger.

The windshield is obviously much stronger than a car’s. It’s built of three layers of glass and separated by two layers of vinyl. It’s almost three-quarters of an inch thick. The windows are electrically heated, which prevents buildups of fog and ice, and also reduces brittleness in the very cold climates of high altitude.

The windows are individually heated by two systems. If the heating systems all failed, the airplane’s cruise speed would be reduced until a repair could be made at the next station. Any imperfections in the windshield affecting its strength would dictate windshield replacement immediately.

Although there’s quite a bit of pressure pushing on the window from the outside when traveling 550 mph, there is also 8.6 psi of internal pressure pushing from the inside out. A balance is struck. This enhances window strength. Window strength is tested by the manufacturers with a five-inch bore-gun. Objects are shot at the forward fuselage and windows; the only acceptable mark is no damage at all.

PASSENGER WINDOWS

The cabin windows are double-paned for extra strength. The plastic shield you can touch from the cabin is not part of the window structure but simply a temperature and moisture barrier. The size and location of the cabin windows are dictated by the structure of the aircraft itself. Windows are rounded at the corners to reduce points of structural stress.

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