Aircraft Aluminum Polishing Part II
Posted by Jay Schneider. Filed under Aviation.
January 18, 2011
As an Aircraft Air Frame & Power Plant Engineer, (or AME), our bible reference guide is known as AC43.13-1B, or Advisory Circular 43.13-1B Acceptable Methods, Techniques, and Practices - Aircraft Inspection and Repair. Please keep in mind, the following is an excerpt from this FAA publication on correct procedures for aluminum corrosion repair and is a guide to perform the task of polishing/restoring aluminum. Our goal here at Lake Country Manufacturing is bring to you factually accurate information for any restoration project wherever and whenever we can:
Note! If performing work on an aircraft, it is advisable to consult a rated AME to oversee your work.
6-113. GENERAL. When active corrosion is found, a positive inspection and rework program is necessary to prevent any further deterioration. The following methods of assessing corrosion damage and procedures for rework of corroded areas could be used during cleanup programs. In general, any rework would involve the cleaning and stripping of all finish from the corroded area, removal of corrosion products, and restoration of surface protective film.
- Repair of corrosion damage includes removal of all corrosion and corrosion products. When the corrosion damage is severe and exceeds the damage limits set by the aircraft or parts manufacturer, the part must be replaced.
6-114. PREPARATIONS FOR REWORK.
All corrosion products should be removed completely when corroded structures are
reworked. Before starting rework of corroded areas, carry out the following:
a.Document corrosion damage.
b. Position the aircraft in a wash rack or provide washing apparatus for rapid rinsing of all surfaces.
c. Connect a static ground line from the aircraft to a grounding point.
d. Prepare the aircraft for safe ground maintenance.
(1) Remove battery(s), liquid oxygen generator container (if installed), and external
hydraulic and electric power.
(2) Install all applicable safety pins, flags, and jury struts.
e. Protect the pitot-static ports, louvers, airscoops, engine opening, wheels, tires,
magnesium skin panels, and airplane interior from moisture and chemical brightening agents.
f. Protect the surfaces adjacent to rework areas from chemical paint strippers, corrosion
removal agents, and surface treatment materials.
6-115. FAIRING OR BLENDING REWORKED AREAS.
All depressions resulting from corrosion rework should be faired or blended with the surrounding surface. Fairing can be accomplished as follows:
a. Remove rough edges and all corrosion from the damaged area. All dish-outs should be elliptically shaped with the major axis running spanwise on wings and horizontal stabilizers, longitudinally on fuselages, and vertically on vertical stabilizers. (Select the proper abrasive for fairing operations from table 6-1.)
b. In critical and highly stressed areas, all pits remaining after the removal of corrosion
products should be blended out to prevent stress risers that may cause stress corrosion cracking. (See figure 6-14.) On a non-critical structure, it is not necessary to blend out pits remaining after removal of corrosion products by abrasive blasting, since this results in unnecessary metal removal.
c. Rework depressions by forming smoothly blended dish-outs, using a ratio of 20:1, length to depth. (See figure 6-15.) In areas having closely spaced multiple pits, intervening material should be removed to minimize surface irregularity or
waviness. (See figure 6-16.) Steel nut-plates and steel fasteners should be removed before blending corrosion out of aluminum structure. Steel or copper particles embedded in aluminum can become a point of future corrosion. All corrosion products must be removed during blending to prevent reoccurrence of corrosion.
6-116. CORROSION REMOVAL BY BLASTING. Abrasive blasting is a process for
cleaning or finishing ferrous metals by directing a stream of abrasive particles against the surface of the parts. Abrasive blasting is used for the removal of rust and corrosion and for cleaning prior to painting or plating. The following standard blast-cleaning practices should be adopted.
a.The part to be blast-cleaned should be removed from the aircraft, if possible. Otherwise, areas adjacent to the part should be masked or protected from abrasive impingement and system (hydraulic, oil, fuel, etc.) contamination.
b. Parts should be dry and clean of oil, grease, or dirt, prior to blast cleaning.
c. Close-tolerance surfaces, such as bushings and bearing shafts, should be masked.
d. Blast-clean only enough to remove corrosion coating. Proceed immediately with
surface treatments as required.
6-117. CLEANERS, POLISHES, AND BRIGHTENERS. It is important that aircraft be kept thoroughly clean of contaminating deposits such as oil, grease, dirt, and other foreign materials.
Next article – going to work!
Keep the Blue Skies Up and Happy Motoring!
Scott S. Mc Lain (F.A.A. A&P Certification # 2748259)
Lake Country Mfg.