Maintenance: Airplane and Airline FAQ

MAINTENANCE HISTORY

Aircraft manufacturers certify all the individual parts of an airplane for a useful life of a specific number of cycles and/or hours of service. Aircraft cycles are the number of flights the airplane has made; hours of service is the total flight time. Short-haul aircraft may fly eight flights a day accumulating 12 hours in service. A long-haul widebody aircraft can fly 12 hours in just one flight. Both parameters are needed to define the limits of the individual aircraft components, because cycles cause more wear and tear than actual hours. To arrive at these “to be overhauled” or “to be replaced” limits, the manufacturers test the critical parts individually, even sacrificing one entire airplane to a total stress-to-destruction test so the strength of the plane can be accurately analyzed. Airplanes have been tested to the equivalent of thirty years in service and 100,000 cycles with no critical failures.

Once a plane enters service, the airlines become legally responsible to maintain that plane to exacting standards, and document in a permanent record its maintenance history. Each individual part on each individual airplane has its own unique number and reliability record. Each part can be tracked by computer, so its exact number of cycles and/or hours can be recorded.

The maintenance history of an airplane develops with each flight. Flight crews are responsible for completing an aircraft log for every flight. Nearly 100 percent of the time no entry is made, signifying every component of the aircraft worked perfectly. If something were to malfunction, an aircraft log entry would be made to document the problem; a mechanic would replace the malfunctioning part, and the maintenance and engineering department will record the “write-up” in the central computer and compare the data with information gathered from other planes. Patterns and trends can become apparent before they become a problem. Engine performance is also documented on each and every flight approximately once an hour. The incredible reliability of modem turbofan engines can be attributed to three things: (1) the manufacturers are building state-of-the-art engines, (2) the power-plant mechanics who maintain the engines do a perfect job; and (3) regular engine-performance data is gathered and analyzed on a daily basis.

After a flight crew records the exact performance of all the engines, the people behind the scenes can computer analyze the data and determine if an engine is not running perfectly even though it is producing full power. An in crease in oil or fuel consumption, slightly higher oil or exhaust gas temperatures, slightly lower power output at the same throttle position, etc. can all be signals. Long before the flight crew has an engine problem that would affect the flight, this slightly out of tune engine can be removed from the aircraft, replaced with a spare, and taken to the maintenance base for inspection and overhaul.

The newer aircraft that have the EICAS (Engine Indicating and Crew Alert System) computers are capable of automatically recording engine data, and faxing that information to a central maintenance base independent of any flight crew action while the airplane is still in-flight. It promises to further increase engine reliability.

PREVENTIVE MAINTENANCE

Preventive maintenance occurs before each flight. The exterior walkaround inspection, the interior safety inspection, and the cockpit acceptance check, are all examples of a routine trip check. If something is found marginally out of tolerance, the mechanics will replace that part from the parts inventory. Furthermore, all the newer aircraft have “status” computers that automatically alert the pilots and mechanics to a malfunction. Accessing this computer can simplify the troubleshooting and repair of a faulty system.

Once a day a more comprehensive preventive maintenance layover check is performed. Taking between two and five hours, depending on the size of the aircraft, this layover check is a detailed functional check of the aircraft. Even though everything may be normal, all the systems are operationally checked. The engine cowling—exterior chassis of the engine—is opened so a close inspection can be made. Inspection panels can be removed so the hard-to-see hidden components can be examined.

Service checks are performed every 100 hours of flight, on average every 10 days. This is similar to a layover check, but one step more thorough. Trip checks, layover checks, and service checks are performed each flight, daily, and every 100 hours respectively, for all make and models of aircraft.

Periodic checks—commonly referred to as letter checks because the required inspections are catalogued as A, B, C, etc. —are different. Specifically tailored to each different type of aircraft, this preventive maintenance program is designed by the manufacturers, airlines, and FAA jointly. This letter-check schedule requires progressive maintenance. The number of components and systems of the aircraft that must be inspected, repaired, or replaced each subsequent check increases with each trip to the hangar. For example, some aircraft require an A letter check after 300 hours of flight time. One item that must be checked is the landing gear. The B check might be required at 600 hours; requiring the inspection of the landing gear again, but also the landing gear doors and mechanisms. The C letter check will continue to increase the list. At approximately 18,000 hours of flight time, or 48 months, this preventive maintenance letter-check program culminates in a required complete overhaul of the aircraft.

OVERHAUL

An aircraft completing its required 18,000-hour overhaul is essentially a new airplane. During the four to six weeks— working twenty-four hours a day, seven days a week—that it takes to overhaul a commercial airliner, the plane is almost completely gutted and rebuilt. The airplane is disassembled, including removing the engines and landing gear. All the flight controls, cockpit instrumentation, and the passenger cabin are stripped to the bare metal fuselage. After everything (including the paint) is removed, the components are sent to the various specialty maintenance shops—hydraulic, electric, landing gear, metal, etc.— where all the parts are further disassembled, cleaned, inspected, repaired, reassembled, tested to the same tolerances and limits of a new item, and returned to the parts MAINTENANCE

inventory if they pass, or are discarded. The remaining structure—basically the empty fuselage, wings, and tail— are themselves thoroughly tested. Something as minute as one faulty rivet will be replaced. Upon completion of all the checks and testing, the airplane is reassembled almost from the ground up. Except for the date on the original airworthiness certificate, it is sometimes difficult to tell a new aircraft from one just out of overhaul.

Any time an airplane is overhauled, it will be flown on a test flight with only pilots and mechanics on board. This two to four hour flight involves shutting down and restarting the engines in-flight; shutting down the hydraulics, turning off the electrics; basically checking all the systems and their primary, secondary, and backup features discussed throughout this guide. After verifying that every thing is 100 percent, the aircraft is certified fit to return to passenger service.

TYPES OF INSPECTIONS

Visiting an airline maintenance facility is more like visiting a hospital than the common misconception of a greasy machine shop. Some of the modern equipment used to maintain aircraft are very similar to the diagnostic equipment used by medical technicians and doctors. After visually checking the nuts and bolts and the other metal structural components, X-ray equipment is used to check the internal integrity and verify no hidden cracks are present. A small amount of electricity, called eddy currents, can be applied to the aluminum skin of the fuselage to ascertain that the structure is flawless. A tiny imperfection hidden beneath a rivet head, for example, will disrupt the even flow of electricity and register on the test equipment. Gyroscope equipment allows a magnified look at the inside of an engine without requiring any disassembly. Radio isotopes and dye penetrants can be injected into the engines and other systems to X-ray the nonmetal structures to obtain an even clearer picture of their “internal health.”

Ultrasonic and acoustic emissions tests can be used to listen to the almost imperceptible “moans and groans” of the plane as it is physically twisted and stressed. Flaws as small as 50 one-thousandths of an inch can be detected long before they become any sort of hazard.

AIRWORTHINESS DIRECTIVES (A.D.’s)

As a further double-check that the commercial airline fleet is meticulously maintained, the FAA has set up an information system where maintenance reliability data can be shared by the airlines. If certain important components of an aircraft such as the engines, flight controls, hydraulic system, brakes, etc. should fail during flight, the individual airline must notify the FAA, which forwards the data to all airlines. This serves as an alert to the airlines so their inspection schedule can be altered as necessary. If another failure should occur with that same component, the FAA can order a mandatory change in the inspection timetable by issuing an Airworthiness Directive. Regardless of how soon the next preventive maintenance or overhaul is scheduled, an A.D. requires immediate action in the time allowed by the FAA, even if it requires removing an airplane from service. If an unsafe condition exists, an A.D. will be issued that will ground an entire fleet of airplanes, until a safe fix can be found.

All these maintenance inspections, and checks and balances within the system, are impressive. Regardless of cost, safety is and will always be the number-one goal of every responsible airline.

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