Aircraft fuel. The $3.2 million, or 8.3%, increase in fuel cost was primarily due to an increase in the number of flights we operated under our prorate agreements and SWC and the corresponding increase in gallons of fuel we purchased, offset by a decrease in our average fuel cost per gallon from $3.65 for the six months ended June 30, 2023, to $3.42 for the six months ended June 30, 2024. We purchase and incur expense for all fuel on flights operated under our prorate agreements and SWC. All fuel costs incurred under our capacity purchase agreements are either purchased directly by our major airline partner, or if purchased by us, we record the direct reimbursement as a reduction to our fuel expense. The following table summarizes the gallons of fuel we purchased under our prorate agreements and SWC, for the periods indicated:
| | | | | | | | | |
| | For the six months ended June 30, |
(in thousands) | | 2024 | | 2023 | | % Change |
Fuel gallons purchased | | | 12,440 | | | 10,766 | | 15.5 | % |
Fuel expense | | $ | 42,492 | | $ | 39,243 | | 8.3 | % |
Airport-related expenses. Airport-related expenses include airport-related customer service costs such as outsourced airport gate and ramp agent services, airport security fees, passenger interruption costs, deicing, landing fees and station rents. For clarity, our employee airport customer service labor costs are reflected in salaries, wages and benefits and customer service labor costs we outsource to third parties are included in airport-related expenses. The $3.2 million, or 9.0%, increase in airport-related expenses for the six months ended June 30, 2024, compared to the six months ended June 30, 2023, was primarily due to an increase in subcontracted airport services and landing fees as a result of an increase in the number of flights we operated under our prorate agreements.
Aircraft rentals. The $19.4 million, or 88.2%, decrease in aircraft rentals was primarily related to a decrease in our leased aircraft since the six months ended June 30, 2023. During 2023, we acquired 26 CRJ700 aircraft, eight CRJ200 aircraft and one CRJ900 aircraft under early lease buyouts.
Other operating expenses. Other operating expenses primarily consist of property taxes, hull and liability insurance, simulator costs, crew per diem and crew hotel costs. The $0.5 million, or 0.4%, increase was primarily related to an increase in other operating costs as a result of the higher number of flights we operated during the six months ended June 30, 2024, compared to the six months ended June 30, 2023, such as crew per diem and crew hotel costs, offset by the timing of training events and other non-direct operating expenses for the six months ended June 30, 2023, compared to the six months ended June 30, 2024.
Summary of interest expense, interest income, other income (loss), net and provision for income taxes
Interest Expense. The $8.5 million, or 12.7%, decrease in interest expense was primarily related to a decrease in outstanding debt. At June 30, 2024 we had $2.8 billion of outstanding debt, compared to $3.2 billion at June 30, 2023.
Interest income. Interest income increased $3.1 million, from $20.5 million for the six months ended June 30, 2023, to $23.6 million for the six months ended June 30, 2024. The increase in interest income was primarily related to an increase in average interest rates attributed to our marketable securities for the six months ended June 30, 2024, compared to the six months ended June 30, 2023.
Other income (loss), net. Other income (loss), net decreased $12.9 million during the six months ended June 30, 2024, compared to the six months ended June 30, 2023. Other income (loss), net primarily consists of the unrealized and realized gains and losses on our investments in other companies, income or loss related to our equity method investments and gains or losses on the sale of assets. The decrease in other income (loss), net was primarily a result of a decrease in the fair value of our investments in other companies for the six months ended June 30, 2024, compared to the six months ended June 30, 2023.
Provision (benefit) for income taxes. For the six months ended June 30, 2024 and 2023, our effective income tax rates were 25.5% and 21.6%, respectively, which included the statutory federal income tax rate of 21% and other reconciling income tax items, including state income taxes, the impact of non-deductible expenses and a discrete tax benefit or expense on employee equity transactions. The increase in the effective tax rate primarily related to the impact of non-deductible expenses relative to the pre-tax earnings for the six months ended June 30, 2024, compared to pre-tax loss for the six months ended June 30, 2023, and the impact of a discrete tax benefit from excess tax deductions