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Mitigation of AOG status: When an aircraft "goes AOG" and materials required are not on hand, parts and personnel must be driven, flown, or sailed to the location of the "grounded A/C". Usually the problem is escalated through an internal AOG Desk, then the Manufacturer's AOG Desk, and finally competitors' AOG desks. All major air carriers have an "'AOG Desk". This desk is manned 24/7 by personnel trained in purchasing, hazardous materials shipping, and parts manufacturing / acquisition processes.
AOG personnel are trained to "loan" or "borrow" spare parts from other air carriers, per FAA/EASA etc. regulations. AOG personnel work in conjunction with their carriers' maintenance operations department, supporting aircraft maintenance with all parts or material requests very rapidly. There are two ways of achieving this: by local engineering support or by support flown out base.
One 747-200 had accumulated 36,000 hours in eight years when it hit the hangar for a “heavy” checkup, also called a “D” inspection. The big D is a really big deal that can take up to a month or more and cost upwards of $2 million. See what it takes for this Boeing behemoth to get a clean bill of health.
What Does It Take To Keep Them Flying?
Ladies and gentlemen, welcome to New York City's John F. Kennedy Internation al Airport." That announcement to arriving passengers marks the beginning of a flurry of activity in and around the aircraft as its occupants leave. Have you ever wondered what happens to the plane at this point?
Commercial aircraft make money only while flying passengers or cargo, not while sitting on the ground. Therefore, airlines aim for the highest possible utilization of their fleets. As passengers wait for their baggage, the aircraft is being swiftly prepared for the next flight. Mechanics swing into action by reviewing the aircraft log for any mechanical problems noted by the last flight crew. Any matters affecting the safe operation of the plane, also called nogo items, are corrected.
The aircraft's wheels, tires, brakes, and engine oil levels are checked. Cleaning crews tidy up the passenger cabin. The kitchen units, or galleys, are resupplied with food and beverages. Fuel is pumped into the wing tanks. Before the aircraft is again ready for departure, the flight crew performs an exterior walkaround inspection, checking for any conditions that might compromise safety.
This turnaround service and immediate maintenance is performed on thousands of aircraft every day. But that is only a tiny fraction of what it takes to keep a large passenger plane safe to fly. Just as automobiles need periodic servicing, airplanes regularly require a series of extensive and expensive maintenance checks. Who perform these aircraft maintenance services? How is the work carried out?
How the Planes Are Kept Airworthy
According to the U.S. Air Transport Association, member airlines carry more than 95 percent of the air traffic, both passenger and freight, in the United States. In 1997 those airlines had about 65,500 aircraft mechanics on the job. Along with engineers and other maintenance personnel, the aircraft mechanics' mission is to keep the aircraft airworthy and to ensure passenger comfort. That means inspecting, repairing, and overhauling the multitude of specialized parts —the machines within the machine—that make an airplane fly.* Such scheduled maintenance includes everything from overhauling jet engines weighing over four tons to replacing wornout cabin carpets.
* A 747-400 has six million parts, half of which are fasteners (rivets and bolts), and 171 miles of electrical wiring.
Most mechanical problems get immediate attention. However, the aircraft maintenance program schedules other maintenance on the basis of the number of months the aircraft has been in use or the number of cycles # and the number of hours each aircraft has flown, not on the total number of miles it has flown. The program begins with maintenance recommendations made by the aircraft manufacturer to the airplane operators, which must be acceptable to government aviation authorities. Each aircraft has its own tailored maintenance program, from light to intermediate to heavy checks. These checks are designated by letters, such as A, B, C, D, L, or Q.
# A cycle equals one take-off and landing
One 747-200 took about eight years to accumulate some 36,000 hours of flying time. When it did, it was time to head to the hangar for a heavy check, sometimes called a D check. Commenting on this complex and time consuming check, Overhaul & Maintenance, an aviation management magazine, says: "The goal . . . is to, as much as possible, return an entire airframe to its original condition… A D check takes between 15,000 and 35,000 hrs. of labor, and can put a plane out of service for 15 to 30 days, or more. The total cost averages between $1 million and $2 million." "A typical D check is 70% labor and 30% material" said Hal Chrisman of The Canaan Group, an aerospace management consulting firm. Of course, some of that cost is included in your airline ticket.
What a D Check Involves
Once the aircraft is parked inside the hangar—a huge complex of aircraft service areas, support shops, and warehouses—the maintenance team goes to work. Worktables, platforms, and scaffolds are rolled into position for access to otherwise unreachable areas of the plane. Seats, floors, walls, ceiling panels, galleys, lavatories, and other equipment are opened or removed from the aircraft to permit close inspection. The aircraft is essentially gutted. Following step-by-step instructions, workers examine the aircraft for signs of metal cracks and corrosion. Whole sections of the aircraft’s landing gear, hydraulic system, and engines may be replaced. The D check requires the skills of engineers, technical writers, quality control inspectors, avionics technicians, * sheet-metal workers, and airframe and power-plant mechanics, ** most of whom are government licensed. When cabin equipment mechanics, painters, and cleaners are added, the number of personnel swells to well over 100 per day. Scores of others provide essential equipment, parts, and logistics support.
* “Avionics” is an abbreviation for aviation electronics.
**The airframe and power-plant certificate allows mechanics to approve flight work that he or she has performed on airplane structures, systems and engines.
Over time, inflight vibrations, fuselage pressurization cycles, and the jolts of thousands of takeoffs and landings cause cracks in the metal structure of the aircraft. To address this problem, aviation employs diagnostic principles similar to those used in the field of medicine. Both use such tools as radiology, ultrasonics, and endoscopy to detect what the human eye cannot see.
For a conventional medical X ray, the patient is placed between a sheet of film and an Xray beam. To Xray the landing gear, wings, and engines, maintenance inspectors use similar methods. For example, a sheet of Xray film is placed at a desired point on the engine exterior. Next, a long metal tube is placed inside the hollow shaft that runs the length of the engine. Finally, a pill of radioactive iridium 192—a powerful isotope—no bigger than a pencil eraser, is cranked into the tube to expose the Xray film. The developed film helps to reveal cracks and other flaws that may require that the engine be repaired or replaced.
During the D check, samples of the aircraft's fuel and its hydraulic fluids are sent for laboratory analysis. If microorganisms are found in the fuel sample, antibiotics are prescribed. To kill jet fuel bugs—fungi and bacteria that can get into fuel tanks through the air, water, and fuel—the tanks are treated with a biocide, a form of antibiotic. This treatment is important because the byproducts of microbial growth can corrode the protective coatings on the surface of the tanks. Fuel probes in the tanks can also be affected and thus cause the pilots to receive inaccurate fuel gauge readings.
As a result of normal wear, vibrations, and internal seal damage, fuel tanks can develop leaks. A supervisor asks his assembled D check crew, "Does anyone want to be a 'frogman'?" The joyless but necessary chore falls to John. Looking somewhat like a scuba diver without flippers, he dons special cotton coveralls, puts on a respirator connected to a fresh air supply, and takes tools, sealant, and a safety light with him. Through a small opening in the bottom of the wing, he squeezes his way into the defueled wing tank, locates the source of the fuel tank leak, and seals it.
Built into the wings of the plane, the fuel tanks of a 747 are a maze of walled compartments connected by small openings. Fuel tanks are no place for the claustrophobic. A 747-400 can hold more than 57,000 gallons of fuel. This fuel capacity makes it possible to fly extremely long routes nonstop, such as from San Francisco, California, U.S.A., to Sydney, Australia—a distance of 7,400 miles.
Three stories above the ground on the flight deck, an avionics technician inspects a built-in testpattern display on the TVlike weather radar indicator screen. Pilots use this instrument to detect and avoid thunderstorms and turbulence that may be as far as 300 miles ahead of the airplane. So when the pilot turns on the "Fasten Seat Belt" sign, he may have seen turbulence on his radar screen. However, to prevent injuries, many airlines request that when seated, passengers keep their seat belts fastened at all times, even if the captain turns off the sign. Atmospheric changes in the form of clear air turbulence are often encountered before pilots have time to turn it on.
During the D check, safety equipment, such as life vests and emergency lighting, is checked or replaced. When a check of the passenger emergency oxygen system is under way, oxygen masks dangle like oranges on branches. Jet airplanes routinely cruise at altitudes of four to seven miles above the earth, where the oxygen content and the atmospheric pressure are insufficient to sustain life. How is this problem solved? The aircraft's pressurization system draws in outside air and then compresses it. This air is finally supplied to the cabin at an acceptable temperature. If the air pressure in the cabin falls below safe levels, oxygen masks automatically drop from overhead compartments. The emergency oxygen is supplied to the passengers until the aircraft descends to an altitude where the emergency oxygen is no longer needed. On some airplanes, oxygen masks are stowed in passenger seatback compartments, not in overhead compartments. That is why it is important to pay attention to preflight passenger briefings, which identify the location of the oxygen masks.
A heavy maintenance check is also the time to install new cabin walls and ceiling panels as well as to replace carpets, curtains, and seat cushion covers. Galley equipment is disassembled, cleaned, and sanitized.
Ready to Fly
After 56 days of inspections, checks, repairs, and maintenance, the aircraft is ready to leave the hangar and resume flying passengers and cargo. Only a small fraction of the maintenance operations have been mentioned here. But before flying again, the aircraft may be test flown by a special crew to ensure that all systems function properly. It is reassuring to consider briefly how much expertise and technology go into keeping the aircraft that you fly in mechanically sound.
However, the best single tool in aircraft maintenance is said to be the human element —sharp eyes and alert minds. The trained personnel take their jobs very seriously. They know that poor maintenance can cause big problems. Their goal is to provide reliable aircraft that will speed you to your destination safely and comfortably.
(originally appeared in the September 8, 1999, edition of AWAKE! magazine)
Aircraft maintenance checks are periodic inspections that have to be done on all commercial/civil aircraft after a certain amount of time or usage. Airlines and other commercial operators follow a continuous inspection program approved by the Federal Aviation Administration (FAA) in the United States or by other airworthiness authorities such as the European Aviation Safety Agency (EASA). The maintenance program includes both routine and detailed inspections. Airlines and airworthiness authorities casually refer to the detailed inspections as "checks", commonly one of the following: A-check, B-check, C-check or D-check. A- and B-checks are lighter while C- and D- are considered heavier checks.
The maintenance of aircraft is differentiated into line maintenance and overhaul (base maintenance). The work that has to be done in the short term is called line maintenance. This means that the aircraft keeps to its usual schedule. The daily, weekly and monthly checks are carried out overnight, the next morning the aircraft goes back into scheduled service. Merely the C-Check that is carried out about every one and a half years takes somewhat more than one day. For an overhaul, let it be an IL (intermediate layover check) or D-Check, the aircraft is taken out of service for several weeks.
Pre-flight Check / Trip Check:
The lowest-level maintenance event is the pre-flight check that precedes every flight and involves a visual inspection of the aircraft by the cockpit crew (and mechanics if present) at the gate. The aim of this check is to detect obvious external damage, wear and oil or hydraulic leaks. Depending on the aircraft type it lasts for 15 to 60 minutes.
The next maintenance event in the hierarchy is the ramp check, in which mechanics test individual functions of the aircraft, check tire pressure and brakes and replenish fluids such as hydraulic, oil and water. A visual inspection of the aircraft is also carried out, both externally and in the cabin. Such a check, carried out on a daily basis, requires between 6 and 35 man-hours.
Next in size is the weekly service check, a combination of the work performed in the ramp check with tasks such as topping up the water, air and oil and thorough cleaning of the cabin, which takes between 10 to 55 man-hours.
This is performed approximately every one to two months, after 300-600 flight hours or 200–300 cycles, depending on aircraft type. (Maintenance checks are the same for long haul and short haul aircraft because the maintenance schedule is based on airframe hours and landings (known as cycles). Aircraft flying several short trips a day may have maintenance due earlier than aircraft flying long-haul because they have more cycles attributed to them.)
As well as general inspections of the interior and the aircraft hull, it also covers service checks as well as engine and function checks. At the same time the technicians replenish consumables such as oil, water and air and eliminate defects whose rectification has been postponed on the grounds that they did not impair flight safety. If any extensive seat repairs are required, these are also carried out in this interval inspection. It needs about 20–60 man-hours and is usually performed overnight at an airport gate. The occurrence can be delayed by the airline if certain predetermined conditions are met.
Examples of A-check items include:
• General external visual inspection of aircraft structure for evidence of damage, deformation, corrosion, missing parts
• Check crew oxygen system pressure
• Operationally check emergency lights
• Lubricate nose gear retract actuator
• Check parking brake accumulator pressure
• Perform Built-in Test Equipment (BITE) test of Flap/Slat Electronics Unit
Contemporary maintenance programs do not use this interval. The B-check, which comes into play only in some types of aircraft such as the Boeing 747-200 or the B737-200, corresponds to an enhanced A-Check with a slightly more detailed check of components and systems. Special equipment and tests may be required but it does not involve, however, detailed disassembly or removal of components. It needs about 120-150 man-hours, depending on the aircraft, and is usually completed within a day at an airport hangar. This kind of check is performed approximately every 5-8 months or after 1000 hours of flight time. A similar occurrence schedule applies to the B-check as to the A-check. However, B-checks may also be incorporated into successive A-checks, i.e.: Checks A-1 through A-10 complete all the B-check items.
The following two checks are traditionally known as heavy checks. They are normally accomplished at the main maintenance base of the airline where specialized manpower, materials, tooling, and hangar facilities are available.
|C-Check on Crystal Holidays' leased charter B737|
This is an intensive review carried out approx. every 18–24 months or after about 5000 hours of flight time. The schedule of occurrence has many factors and components thus varies by aircraft category and type. It is a high-level check that involves extensive tooling, test equipment, and special skill levels. This maintenance check requires a large majority of the aircraft's components to be inspected therefore the aircraft will be partially disassembled in a hangar at a maintenance base. This entails thorough inspections inside and outside, along with meticulous examination of structures (load-bearing components on the fuselage and wings) and functions. The time needed to complete such a check is generally up to five days and the effort involved can require up to 1500 and 2000 man-hours.
The C-check includes the lower checks, i.e. A-, B-, and Daily checks.
Examples of C-check items:
• Visually check flight compartment escape ropes for condition and security
• Check operation of DC bus tie control unit
• Visually check the condition of entry door seals
• Operationally check flap asymmetry system
• Pressure decay check APU fuel line shroud
• Inspect engine inlet TAI ducting for cracks
• Operationally check RAT deployment and system
IL-Check (Intermediate Layover Check):
The classical IL-Check [or HMV (heavy maintenance visit)] is carried out every four to six years, about halfway between the D-Checks and lasts between 2 to 4 weeks. Recently lost its significance as the lifespan and reliability of the systems are continually improving. Therefore it is now possible to distribute the elements of the IL-Check over several C-Checks or to delay it to the next D-Check.
To obtain easier access to the fuselage and wing structure for inspection purposes, a number of large assemblies such as the high-lift devices are dismantled. At the same time numerous items of equipment and systems are tested and repaired as necessary. Cabin components such as seats, galleys and toilets are also completely overhauled and, if necessary, the aircraft will be repainted.
This check can also be referred to as Structural check which is due every 6 to 10 years or after about 25.000 hours of flight time.
|Completely empty cabin: wall and ceiling panels all removed|
During a D-check the entire structure is inspected down to the smallest detail. The engines, the landing gears and the high-lift devices are dismantled, along with the cabin interior and the wall and ceiling panels. The instruments, the electrical systems, the electronics, hydraulic and pneumatic equipment is also removed. All equipment dismantled and taken apart is closely scrutinized and any necessary repair work is carried out.
Every component is given a docket that states when and whereto it has to be delivered in a serviceable condition in order for the aircraft to be finished on time. Once the aircraft has been stripped to its skeleton, the outer skin with ten of thousands rivets and the load-bearing structure of the aircraft can be scrutinized for damage and corrosion. Here, non-destructive testing methods such as eddy-current, ultrasonic and x-raying are used to detect the tiniest hairline cracks in the aircraft structure or its retaining elements.
Given the time requirements of this check, many airlines use the opportunity in order to also make major cabin modifications on the aircraft, which would otherwise require an amount of time that would have to put the aircraft out of service without the need for an inspection. This may include new seats, entertainment system, carpeting, etc.
Such a check can generally take up to 50.000 man-hours and 4-6 weeks to complete, depending on the aircraft and the number of technicians involved. It also requires the most space of all maintenance checks, and as such must be performed at a suitable maintenance base.
|Celebrating a succesful D-check on a B747 at HAITEC|
The requirements and the tremendous effort involved in this maintenance check make it by far the most expensive with a total cost up to 8 to 10 million dollars. Because of the nature and cost of such a check, most airlines — especially those with a large fleet — have to plan D-checks for their aircraft years in advance. Often older aircraft being phased out of a particular airline's fleet are either stored or scrapped upon reaching their next D-check due to the high costs involved in comparison to the aircraft's value. On average a commercial aircraft undergoes three D-checks before being retired.
The D-check includes the lower checks, i.e. A-, B-, C- and Daily checks.
Examples of D-check items include:
• Inspect stabilizer attach bolts
• Inspect floor beams
• Detailed inspection of wing box structure
Maintaining an aircraft and ensuring that it is constantly airworthy. Airplanes have to be in perfect shape so we need technicians who are always prepared, skilled and up-to-date in order to operate both routine and extraordinary maintenance. You have to keep studying but you won’t get bored...