Salaries, wages and benefits. The $92.8 million, or 9.4%, increase in salaries, wages and benefits was primarily due to an increase in direct labor costs that resulted from the higher number of flights we operated during the nine months ended September 30, 2024, compared to the nine months ended September 30, 2023.
Aircraft maintenance, materials and repairs. The $27.2 million, or 5.6%, increase in aircraft maintenance expense was primarily due to higher flight volume, which increased maintenance activity and related expenses, for the nine months ended September 30, 2024, compared to the nine months ended September 30, 2023.
Depreciation and amortization. The $1.5 million, or 0.5%, increase in depreciation and amortization expense was primarily due to an increase in depreciation expense related to the acquisition of three new E175 aircraft and spare engines since September 30, 2023.
Aircraft fuel. The $2.6 million, or 4.2%, 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 $3.71 for the nine months ended September 30, 2023, to $3.30 for the nine months ended September 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 nine months ended September 30, |
(in thousands) | | 2024 | | 2023 | | % Change |
Fuel gallons purchased | | | 19,776 | | | 16,863 | | 17.3 | % |
Fuel expense | | $ | 65,216 | | $ | 62,573 | | 4.2 | % |
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 $7.4 million, or 13.8%, increase in airport-related expenses for the nine months ended September 30, 2024, compared to the nine months ended September 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 $20.1 million, or 83.7%, decrease in aircraft rentals was primarily related to a decrease in our leased aircraft since the nine months ended September 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 $14.4 million, or 7.0%, increase was primarily related to an increase in other operating costs as a result of the higher number of flights we operated during the nine months ended September 30, 2024, compared to the nine months ended September 30, 2023, such as crew per diem and crew hotel costs, offset by the timing and the higher number of training events during the nine months ended September 30, 2023, compared to the nine months ended September 30, 2024.
Summary of interest expense, interest income, other income (loss), net and provision for income taxes
Interest Expense. The $13.3 million, or 13.3%, decrease in interest expense was primarily related to a decrease in outstanding debt. At September 30, 2024 we had $2.7 billion of outstanding debt, compared to $3.1 billion at September 30, 2023.
Interest income. Interest income increased $4.3 million, from $31.8 million for the nine months ended September 30, 2023, to $36.1 million for the nine months ended September 30, 2024. The increase in interest income was primarily related to an increase in average interest rates attributed to our marketable securities for the nine months ended September 30, 2024, compared to the nine months ended September 30, 2023.