UPDATE 2-Spirit AeroSystems tops profit estimate, shares fall on forecast

* Sees 2018 EPS of $6.25-$6.50 vs est $5.85

* Sees 2018 free cash flow $550 mln-$600 mln vs est $572.85 mln

* Q4 adj. profit $1.32 vs est $1.21

* Shares fall as much as 5.2 pct (Adds shares, analyst comment, background)

Feb 2 (Reuters) - Aircraft parts maker Spirit AeroSystems Holdings Inc on Friday reported better-than-expected quarterly results, helped by higher deliveries to its biggest customers, Boeing Co and Airbus SE.

But Spirit Aero's shares fell in morning trade as its 2018 free cash flow (FCF) forecast of $550 million to $600 million was largely in line with Wall Street estimate of $572.85 million.

Spirit Aero Chief Executive Tom Gentile said the company intended to reinvest substantially all of its savings from recent changes to the U.S. tax code into "high-return" capital expenditures and research and development activities.

"It takes away the possibility of a near-term increase in the FCF margin target (currently 7-9 percent of revenue), which we had seen as a possibility, given the existence of pre-tax reform internal target of 10 percent," J.P. Morgan analyst Seth Seifman wrote in a note.

The Wichita, Kansas-based company's shares fell as much as 5.2 percent to $97.56, posting their biggest intraday percentage loss in nine months.

Spirit Aero forecast 2018 earnings in the range of $6.25 to $6.50 per share. Analysts on average were expecting a profit of $5.85, according to Thomson Reuters I/B/E/S.

The company predicted revenue of $7.1 billion to $7.2 billion, compared with analysts' expectation of $7.13 billion.

In August, Spirit Aero reached an agreement with Boeing on pricing terms related to parts it supplies for aircraft such as the 787 Dreamliner.

The deal reduced uncertainty over pricing that has long existed with Boeing and preserved the company's ability to meet long-term cash flow goals.

Spirit Aero, which on Friday increased its share repurchase authorization up to $1 billion, earned $1.32 per share on an adjusted basis in the fourth quarter ended Dec. 31, beating the average estimate of $1.21. nCNWVTPBVa 3/8

Total revenue rose 9.2 percent to $1.71 billion and was above the $1.66 billion expected by analysts.

Up to Thursday's close, Spirit Aero's stock had risen about 85 percent in the past 12 months, compared with a 65.6 percent increase in the Dow Jones U.S. Aerospace index. (Reporting by Ankit Ajmera in Bengaluru; Editing by Maju Samuel)