On Apr 12, Zacks Investment Research upgraded Spirit AeroSystems Holdings, Inc. SPR to a Zacks Rank #1 (Strong Buy).Why the Upgrade?Spirit AeroSystems Holdings has been witnessing a rise in earnings estimates on the back of strong fourth-quarter 2015 results. This commercial aero structures supplier has been a consistent performer, reporting positive earnings surprises in three out of the last four quarters, with an average beat of 4.20%. The long-term earnings growth of the company is pegged at 10.83%.The company is a key supplier of important parts for the best-selling aircraft of The Boeing Company BA and Airbus Group EADSY. Record deliveries from these two commercial airplane manufacturers have in turn boosted the performance of Spirit AeroSystems.Thanks to consistent demand for aero structures, Spirit AeroSystems' backlog at the end of the fourth quarter of 2015 increased 1.4% from the previous quarter to $47 billion.Spirit AeroSystems expects 2016 earnings in the range of $4.15–$4.35 per share on revenues of $6.6–$6 .7 billion.The company was able to generate free cash flow of nearly $930 million in 2015. Following the completion of $350 million of share repurchases in 2015, the board of directors has authorized a new share repurchase program of $600 million under which repurchases may be made intermittently through Dec 31, 2017. This will boost shareholder value.The Zacks Consensus Estimate has moved up 0.5% to $4.30 per share for 2016 over the last 60 days.Another Favorable StockApart from Spirit AeroSystems Holdings, Rockwell Collins Inc. COL also sports a Zacks Rank #1 (Strong Buy).Want the latest recommendations from Zacks Investment Research? Today, you can download 7 Best Stocks for the Next 30 Days. Click to get this free report >>Want the latest recommendations from Zacks Investment Research? Today, you can download 7 Best Stocks for the Next 30 Days. Click to get this free report BOEING CO (BA): Free Stock Analysis Report ROCKWELL COLLIN (COL): Free Stock Analysis Report SPIRIT AEROSYS (SPR): Free Stock Analysis Report AIRBUS GROUP NV (EADSY): Free Stock Analysis Report To read this article on Zacks.com click here. Zacks Investment Research