FIXED ASSETS



(dollars in millions)	Estimated Useful Lives	2018			2017
Land		$	425		$	412
Buildings and improvements	12-40 years	6,486			5,727
Machinery, tools and equipment	3-20 years	15,119			13,476
Other, including assets under construction		2,054			1,749
		24,084			21,364
Accumulated depreciation		(11,787		)	(11,178		)
		$	12,297		$	10,186

The increase in fixed assets is primarily driven by the acquisition of Rockwell Collins as described in Note 2 to the Consolidated Financial Statements. Depreciation expense was $1,240 million in 2018, $1,178 million in 2017 and $1,105 million in 2016.