plans for eligible employees and retirees. These plans are closed to new entrants and are frozen for future benefit accruals. Our funding obligations for these plans are governed by the Employee Retirement Income Security Act, as modified by the Pension Protection Act of 2006. In 2015, we contributed $1.2 billion to our qualified defined benefit pension plans . As a result of these contributions, we satisfied, on an accelerated basis, our 2015 required contributions for our defined benefit plans including nearly $600 million above the minimum funding requirements. We contributed $917 million and $914 million in 2014 and 2013, respectively, including $250 million above the minimum funding levels in each of those years. We estimate our funding under these plans will total at least $1.0 billion in 2016 , including $500 million above the minimum funding requirements.

Profit Sharing. Our broad-based employee profit sharing program provides that, for each year in which we have an annual pre-tax profit, as defined by the terms of the program, we will pay a specified portion of that profit to employees. In determining the amount of profit sharing, the program defines profit as pre-tax profit adjusted for profit sharing and certain other items. For years prior to 2016, our profit sharing program paid 10% to employees for the first $2.5 billion of annual profit and 20% of annual profit above $2.5 billion.

We made profit sharing payments of $756 million, $821 million and $372 million in 2015, 2014 and 2013, respectively, in recognition of our employees' contributions toward meeting our financial goals. During the year ended December 2015, we recorded $1.5 billion in profit sharing expense based on 2015 pre-tax profit, which will be paid to employees in February 2016.

Beginning with 2016 pre-tax profit (to be paid out in 2017), the profit sharing formula will be adjusted to pay 10% of annual pre-tax profit (as defined by the terms of the program) and, if we exceed our prior-year results, the program will pay 20% of the year-over-year increase in pre-tax profit to eligible employees. The profit sharing program for pilots remains unchanged and will continue in 2016 under its terms.

Investing Activities

Capital Expenditures. Our capital expenditures were $2.9 billion in 2015 , $2.2 billion in 2014 and $2.5 billion in 2013 . Our capital expenditures during 2015 were primarily related to the purchase of B-737-900ER aircraft to replace a portion of our older B-757-200 aircraft, purchases of A330-300 aircraft, advance deposit payments on A321-200 aircraft and seat density projects for our domestic fleet. Our capital expenditures during 2014 and 2013 were primarily for the purchase of aircraft and modifications to upgrade aircraft interiors that enhance our product offering.

We have committed to future aircraft purchases that will require significant capital investment and have obtained long-term financing commitments for a substantial portion of the purchase price of these aircraft. We expect that we will invest approximately $3 billion in 2016 primarily for (1) aircraft, including deliveries of B-737-900ERs, A321-200s and A330-300s, along with advance