PROPERTY AND EQUIPMENT. Expenditures for major additions, improvements and flight equipment modifications are capitalized when such costs are determined to extend the useful life of the asset or are part of the cost of acquiring the asset. Expenditures for equipment overhaul costs of engines or airframes prior to their operational use are capitalized as part of the cost of such assets as they are costs required to ready the asset for its intended use. Maintenance and repairs are charged to expense as incurred. We capitalize certain direct internal and external costs associated with the development of internal-use software. Gains and losses on sales of property used in operations are classified within operating expenses.
For financial reporting purposes, we record depreciation and amortization of property and equipment on a straight-line basis over the asset's service life or related lease term, if shorter. For income tax purposes, depreciation is computed using accelerated methods when applicable. The depreciable lives and net book value of our property and equipment are as follows (dollars in millions):
| Net Book Value at May 31, | |||||||
| Range | 2013 | 2012 | |||||
| Wide-body aircraft and related equipment | 15 to 30 years | $ | 7,191 | $ | 7,161 | ||
| Narrow-body and feeder aircraft and related equipment | 5 to 18 years | 2,284 | 1,881 | ||||
| Package handling and ground support equipment | 5 to 30 years | 676 | 598 | ||||
| Vehicles | 3 to 12 years | 798 | 558 | ||||
| Computer and electronic equipment | 3 to 10 years | 224 | 189 | ||||
| Facilities and other | 2 to 30 years | 1,673 | 1,482 | ||||
Substantially all property and equipment have no material residual values. The majority of aircraft costs are depreciated on a straight-line basis over 15 to 30 years. We periodically evaluate the estimated service lives and residual values used to depreciate our property and equipment. This evaluation may result in changes in the estimated lives and residual values as it did in 2013 and 2012 with certain aircraft. In May 2013, we made the decision to accelerate the retirement of 76 aircraft and related engines to aid in our fleet modernization and improve our global network. In May 2012, we shortened the depreciable lives for 54 aircraft and related engines to accelerate the retirement of these aircraft, resulting in a depreciation expense increase of $69 million in 2013. As a result of these accelerated retirements, we expect an additional $74 million in year-over-year depreciation expense in 2014.
Depreciation expense, excluding gains and losses on sales of property and equipment used in operations, was $1.3 billion in 2013, $1.2 billion in 2012 and $1.0 billion in 2011. Depreciation and amortization expense includes amortization of assets under capital lease.