Metal working – Method of mechanical manufacture – Repairing
Reexamination Certificate
2001-05-09
2004-01-27
Hong, John C. (Department: 3726)
Metal working
Method of mechanical manufacture
Repairing
C029S402060, C029S402080, C029S402090, C029S402110, C029S402150
Reexamination Certificate
active
06681466
ABSTRACT:
BACKGROUND OF THE INVENTION
Aircraft are very expensive capital goods, whether used in military, commercial, or private sectors. It is not unusual for serviceable aircraft to be flying and earning revenue or performing their mission for 30 or 40 years after manufacture. Examples may be KC-135 tanker aircraft, Boeing 707 and 727 commercial aircraft, and Douglas DC-3 and DC-8 aircraft. Such refurbished aircraft may be re-engined, they may receive completely new avionics, and they may be older than their pilots when returned to service. The important point is that an aircraft and its fuselage may have no inherent life limitation, so long as the owners take steps to maintain the aircraft, prevent corrosion, and insure its serviceability and safety.
One limitation on an aircraft is that its structure undergoes stress and strain every time it takes off and lands. An aircraft also experiences a pressure cycle every time it flies to a relatively high altitude and then returns to earth. The atmospheric pressure at sea level is about 14.7 psia, about 11 psia at 8000 ft, and about 3.5 psia at 35,000 ft. In a passenger aircraft having a pressurized cabin, the fuselage maintains a pressure equivalent to 8000 ft at all altitudes above 8000 ft., or about 11 psia. Thus, the fuselage maintains a delta pressure of about 7.5 psid when it flies at 35,000 ft., about 7.5 lbs of force (higher pressure) inside the cabin pushing against each square inch of the aircraft skin and its fasteners. An aircraft that travels several legs each day goes through one pressure cycle on each leg, as its external atmosphere goes from normal to partial vacuum during its highest point in flight, and back to normal. These cycles lead to cumulative wear and tear on the aircraft, and in particular, may result in stress cracking of skins by the time 50,000 cycles are experienced.
An important element in preserving aircraft and in lengthening their service life is the repair of these skins. Aircraft are periodically inspected to determine the quantity, location, and size of cracks in their skins. These inspections may be visual and they may also make use of non-destructive testing (NDT) means, such as ultrasonic or eddy current inspections. When an inspector determines that an aircraft skin has too many cracks, or when a schedule determines that it is time, the aircraft skin itself may be repaired or replaced. Repair mechanics typically use a grinder or other portable hand tool to cut away portions of skin, similar to auto body shop techniques. The use of a grinder is very time consuming and could have the potential to damage aircraft structural members supporting the skin from below. There is need for a tool and a method that quickly and reliably removes aircraft skin portions without damaging other structural members.
BRIEF SUMMARY OF THE INVENTION
One aspect of the present invention is a special router apparatus for removing aircraft skin laps, and a method for using the apparatus to remove and replace aircraft skin laps. The apparatus includes a guide, fastened to the skin of the aircraft, to guide the router in its path. A platform or trolley sits atop the guide, fitting snugly and interfacing with roller bearings for ease of movement along the guide. A router then mounts atop the platform, the router having a vertical adjustment so as to adjust the depth-of-cut of the skin without penetrating too far and damaging structural elements below the skin. As the router moves along the guide, it makes a linear cut and removes a desired portion of skin from the aircraft. These portions of the skin are those which are overlapped, and in which both the overlap and underlap portions are to be removed. As the router moves along the guide or track, it generates debris as it cuts the metal, typically aluminum, and typically in the form of small chips. A vacuum hose mounts to the platform to collect chips and debris as the router tool generates them.
Another aspect of the invention is a method for using the router apparatus in a skin lap replacement method for an aircraft skin. The method is put into use when an inspection or schedule determines that replacement is needed. In one embodiment, the method includes installing a skin lap router apparatus on the aircraft. A first skin portion, the overlap, is removed along with a second skin portion, the underlap, using the router apparatus. The removed portions are then replaced and fastened into place. The installed skin portions are then inspected. The apparatus and method may also be used for removing portions of sheet metal from other structures
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Product Catalog, Sioux Tools, Inc., Aug. 2000.
David Bruce R.
Rushin William J.
Brinks Hofer Gilson & Lione
Hong John C.
United Air Lines, Inc.
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