Aircraft rentals. The $0.5 million, or 3.3%, increase in aircraft rentals was primarily related to an additional aircraft leased from a third party since March 31, 2021.
Payroll support grant. In January 2021, we entered into an agreement with U.S. Treasury and received $233.1 million in emergency relief through the 2021 Appropriations Act payroll support program, of which $193.2 million was in the form of payroll support grants that were recognized as a reduction in labor expense over the periods the grants intended to compensate. We recognized $193.2 million in payroll support grant proceeds we received as a reduction to our operating expenses for the three months ended March 31, 2021. We did not have a comparable payroll support grant during the three months ended March 31, 2022.
Other operating expenses. Other operating expenses primarily consist of property taxes, hull and liability insurance, simulator costs, crew per diem, crew hotel costs and credit loss reserves. The $17.1 million, or 31.4%, increase in other operating expenses was primarily related to an increase in other operating costs that correspond to the higher number of flights we operated during the three months ended March 31, 2022, compared to the three months ended March 31, 2021, such as crew per diem, crew hotel costs and simulator costs.
Interest Expense. The $2.8 million, or 8.8%, decrease in interest expense was primarily related to an overall lower effective interest rate during the three months ended March 31, 2022, compared to the three months ended March 31, 2021.
Total airline expenses. Our total airline expenses, comprised of our total operating expenses and interest expense, increased $226.6 million, or 46.7%, primarily due the payroll support program grant reflected as an offset to operating expenses for the three months ended March 31, 2021 and an increase in direct operating costs attributed to the higher number of completed flights during the three months ended March 31, 2022, compared to the three months ended March 31, 2021.
As our interest expense is primarily attributed to debt associated with financing aircraft under our capacity purchase agreements and as revenue earned under our capacity purchase agreements is intended to compensate us for our aircraft ownership costs, including interest expense, we believe our total airline expense is a meaningful expense measure for management discussion and analysis purposes.
Summary of interest income, other income (expense) and provision for income taxes:
Interest income. Interest income increased $0.1 million, or 49.6%, during the three months ended March 31, 2022, compared to the three months ended March 31, 2021. The increase in interest income was primarily related to an increase in average interest rates attributed to our marketable securities subsequent to March 31, 2021.
Other income (expense), net. Other income primarily consisted of income related to our investment in a joint venture with a third party.
Provision for income taxes. For the three months ended March 31, 2022 and 2021, our effective income tax rates were 28.4% and 28.2%, respectively, which include the statutory federal income tax rate of 21% and other reconciling income tax items, including state income taxes and the impact of non-deductible expenses.
Net income. Primarily due to the factors described above, we generated net income of $17.7 million, or $0.35 per diluted share, for the three months ended March 31, 2022, compared to net income of $35.9 million, or $0.71 per diluted share, for the three months ended March 31, 2021.
Our Business Segments
Three Months Ended March 31, 2022 and 2021
For the three months ended March 31, 2022, we had two reportable segments, which were the basis of our internal financial reporting: SkyWest Airlines and SkyWest Leasing. Our segment disclosure relates to components of our business for which separate financial information is available to, and regularly evaluated by, our chief operating decision maker.