Six months ended June 30, 2019, compared to the six months ended June 30, 2018
Rental revenue
As of June 30, 2019, we owned 297 aircraft with a net book value of $17.8 billion and recorded $919.6 million in rental revenue for the six months then ended, which included overhaul revenue, net of amortization expense related to initial direct costs, of $15.9 million. In the prior year, as of June 30, 2018, we owned 271 aircraft with a net book value of $14.9 billion and recorded $771.3 million in rental revenue for the six months ended June 30, 2018, which included overhaul revenue, net of amortization of initial direct costs, of $2.7 million. The increase in rental revenue was primarily due to the increase in net book value of our operating lease portfolio to $17.8 billion as of June 30, 2019 from $14.9 billion as of June 30, 2018.
Aircraft sales, trading and other revenue
Aircraft sales, trading and other revenue totaled $17.8 million for the six months ended June 30, 2019 compared to $7.7 million for the six months ended June 30, 2018. During the six months ended June 30, 2019, we recorded $1.6 million in gains from the sale of six aircraft from our operating lease portfolio. During the six months ended June 30, 2018, we did not sell any aircraft from our operating lease portfolio.
Interest expense
Interest expense totaled $203.3 million for the six months ended June 30, 2019 compared to $158.4 million for the six months ended June 30, 2018. The increase was primarily due to an increase in our aggregate debt balance. We expect that our interest expense will increase as our average debt balance outstanding continues to increase. Interest expense will also be impacted by changes in our composite cost of funds.
Depreciation expense
We recorded $331.2 million in depreciation expense of flight equipment for the six months ended June 30, 2019 compared to $278.7 million for the six months ended June 30, 2018. The increase in depreciation expense for the six months ended June 30, 2019, compared to the six months ended June 30, 2018, is primarily attributable to the acquisition of additional aircraft during the last twelve months.
Selling, general and administrative expenses
We recorded selling, general and administrative expenses of $57.5 million for the six months ended June 30, 2019 compared to $44.8 million for the six months ended June 30, 2018. Selling, general and administrative expense as a percentage of total revenue increased to 6.1% for the six months ended June 30, 2019 compared to 5.8% for the six months ended June 30, 2018. Selling, general and administrative expenses increased due in part to increased transactional expenses incurred during the period. As we continue to add new aircraft to our portfolio, we expect over the long-term, selling, general and administrative expense to decrease as a percentage of our revenue.
Taxes
The effective tax rate was 20.6% and 21.8% for the six months ended June 30, 2019 and 2018, respectively. Changes in the tax rate were primarily driven by variances in permanent items.
Net income available to common stockholders
For the six months ended June 30, 2019, we reported consolidated net income available to common stockholders of $262.1 million, or $2.33 per diluted share, compared to a consolidated net income available to common stockholders of $225.9 million, or $2.04 per diluted share, for the six months ended June 30, 2018. Net income available to common stockholders increased in the second quarter of 2019 as compared to 2018, primarily due to the continued growth of our fleet and an increase in our aircraft sales, trading and other activity, partially offset by increases in our interest expense and selling, general and administrative expenses.