Table of Contents systems are found nose to tail on  commercial and military aircraft, as well as on jet engines, industrial gas turbines, automobiles, commercial transportation vehicles, wind turbines, solar power systems, and construction and industrial equipment . Engineered Structures Forged Wheels Engineered Structures produces titanium ingots and mill products for aerospace and defense applications and is vertically integrated to produce titanium forgings, extrusions forming and machining services for airframe, wing, aero-engine, and landing gear components. Engineered Structures also produces aluminum forgings, nickel forgings, and aluminum machined components and assemblies for aerospace and defense applications. The principal end markets served by Engineered Structures are commercial aerospace, defense aerospace, and land and sea defense. Forged Wheels manufactures forged aluminum truck, bus, and trailer wheels and related products for the commercial transportation end market globally. The Company’s portfolio of wheels is sold under the product brand name Alcoa® Wheels. Its Ultra ONE® Wheel with MagnaForce® alloy is the lightest portfolio of wheels on the market. The Company’s proprietary Dura-Bright® surface treatment is unmatched in appearance and corrosion protection. For additional discussion of each segment's business, see “Results of Operations—Segment Information” in  Part II, Item 7  (Management’s Discussion and Analysis of Financial Condition and Results of Operations) and Note  D  to the Consolidated Financial Statements in Part II, Item 8. Sales by End Market and Significant Customer Revenue Sales by end markets for the years ended December 31, 2020, 2019, and 2018, were: For the Year Ended December 31, 2020 2019 2018 Aerospace - Commercial 50  % 59  % 59  % Aerospace - Defense 19  % 13  % 11  % Commercial Transportation 16  % 17  % 18  % Industrial and Other 15  % 11  % 12  % In 2020, General Electric Company, Raytheon Technologies Corporation and The Boeing Company represented approximately 11%,  9 %  and 8%, respectively, of the Company’s third-party sales. The loss of any such significant customer could have a material adverse effect on such businesses. See  Part I, Item 1A  (Risk Factors). 4