The Airport and Airway Trust Fund (AATF), sometimes referred to as the aviation trust fund, is the major funding source for federal aviation programs. The trust fund finances Federal Aviation Administration (FAA) capital investments in the airport and airway system as well as supports FAA research and operations costs. In order to for avoid disruptions, both the authority to collect aviation excise taxes and to spend from the trust fund must be reauthorized periodically by Congress. The most recent reauthorization, the FAA Extension, Safety, and Security Act of 2016 ( P.L. 114-190 ), expires on September 30, 2017. If FAA's authorization were to expire without a reauthorization or extension, there would be lapses in aviation excise tax collection authority, and the agency would be unable to spend any revenues allocated from the trust fund. Congress established the AATF in the Airport and Airway Revenue Act of 1970 (P.L. 91-258) to provide a dedicated source of federal funding for the aviation system in the United States. Trust fund revenue is derived principally from a variety of taxes paid by users of the national aviation system. Revenue sources for the trust fund include taxes on airline passenger ticket sales, segment fees, air cargo fees, and aviation fuel taxes paid by both commercial and general aviation aircraft (see Table 1 ). The trust fund also accrues interest on its cash balance. In FY2016, the AATF received revenues of over $14.4 billion, with nearly 70% coming from taxes and fees levied on transportation of passengers. Figure 1 illustrates the composition of AATF excise tax revenues collected in FY2016. Trust fund revenue can be volatile, as external factors affect demand for air travel. For example, when the terrorist attacks of September 11, 2001, substantially reduced demand for air travel, trust fund revenues plummeted. The Severe Acute Respiratory Syndrome (SARS) epidemic, which caused passengers to curtail travel to Asia in 2003, had a similar effect. During the last recession, the airline industry responded to a weak economy, lower consumer demand for air travel, and high fuel prices by reducing domestic capacity in 2008 and 2009, which resulted in a decrease in fuel consumption and lower fuel and gasoline tax receipts. Although the stream of trust fund revenues has been relatively stable in recent years, the long-term vitality of the AATF remains a concern for many. AATF revenue is largely dependent on airlines' ticket sales and, therefore, is affected by air travel demand. However, the spread of low-cost air carrier models may have held down ticket prices and, subsequently, AATF receipts. AATF revenues have also been adversely affected by the recent trend among airlines to impose fees for a variety of add-on services and amenities such as checked bags, onboard Wi-Fi access, or seats with additional legroom. Generally, fees not included in the base ticket price are not subject to the federal excise taxes. Air carriers generated over $3.8 billion in baggage fees alone in 2015, which translates into more than $285 million that the trust fund could have received had those fees been subject to the 7.5% excise tax. If airlines continue to seek additional revenue from ancillary fees as an alternative to increasing base ticket prices, federal aviation programs and activities may become more dependent on contributions from the general fund. Because authority to collect taxes for the trust fund must be authorized, any lapse in aviation tax authority would halt the flow of tax revenues into the trust fund. This has happened three times over the past two decades: in 1996, 1997, and 2011. On January 1, 1996, authority to collect taxes for the AATF expired. Spending from the trust fund, however, continued. The lapse continued until August 27, 1996, when the tax authority was extended to the end of the calendar year by the Small Business Job Protection Act of 1996 ( P.L. 104-188 ). Tax-collection authority then lapsed again on January 1, 1997, until it was authorized by the Airport and Airways Trust Fund Reinstatement Act of 1997 ( P.L. 105-2 ), enacted on February 28, 1997. The trust did not receive an estimated $4 billion in forgone tax collections during the first lapse and another $1 billion during the second. The 1996 and 1997 lapses in the aviation tax authorization and the approaching possibility of another lapse increased support in Congress for a longer-term resolution that would ensure a stable funding source for the trust fund. The Taxpayer Relief Act (TRA) of 1997 ( P.L. 105-34 ) extended the aviation trust fund taxes for 10 years, through FY2007. It also modified the taxes to form the aviation tax structure in place today. These changes included a reduction of the ticket tax to 7.5% , imposition of a flat fee on each domestic flight segment, a tax on frequent flyer awards, and a tax on passengers departing or arriving in the United States on international flights. Another lapse in FAA's authority to collect trust fund revenues and expend money from the trust fund occurred in 2011 and lasted for two weeks, from July 23 to August 5. FAA estimated the 2011 lapse to have resulted in approximately $200 million of forgone trust fund revenues each week, or about $400 million for the entire lapse. Although the Airport and Airway Extension Act of 2011 ( P.L. 112-27 ), signed into law on August 5, 2011, retroactively reinstated the taxes as though they never expired, the Internal Revenue Service granted relief to airlines and to taxpayers who purchased tickets during those two weeks. Airlines have long contended that general aviation operators, particularly corporate jets, should provide a larger share of the revenues supporting the trust fund. General aviation interests dispute this, arguing that the air traffic system mainly supports the airlines and that nonairline users pay a reasonable share given the relatively small incremental costs arising from their flights. Until 2015, the Obama Administration proposed a per-flight user charge of $100 on commercial and general aviation jets and turboprops that fly in controlled airspace as an additional revenue source for the AATF. The proposal, estimated to generate roughly $1.1 billion annually, was opposed by general aviation interests, which saw this as a first step toward funding the air traffic control system wholly or substantially through user charges. The Administration's budgets for FY2016 and FY2017 did not include such a proposal. Proposals by the Clinton Administration and the George W. Bush Administration to establish user charges for air traffic services also failed to gain congressional support. A 2007 Senate proposal (the Aviation Investment and Modernization Act of 2007; S. 1300 ; S.Rept. 110-144 ) to finance air traffic system modernization with a $25-per-flight user charge faced similar opposition from business and general aviation interests, and failed to gain traction in Congress. The concept of user-fee funding for air traffic services has resurfaced in current debate over proposals to transfer FAA's air traffic system to a private entity or an independent government-owned corporation. Under some proposals, the air traffic control organization would fund its capital and operating costs from user fees that would not flow into the aviation trust fund. Federal aviation taxes would remain in place to fund other FAA operations. The U.S. aviation system has historically been funded in part from the AATF and in part from the Treasury general fund ( Table 2 ). The general fund share of FAA appropriations has varied widely, ranging from 0% in FY2000 to 38% in FY1997. The general fund share tends to follow a cyclical trend, typically rising when trust fund revenues fall during economic slowdowns. The only FAA program that can receive funding from general fund contributions is the Operations and Maintenance (O&M) account, which covers air traffic services and FAA safety oversight. General revenues have funded 12% to 48% of annual O&M appropriations since FY2012. Between FY2012 and FY2016, the general fund provided between 7.2% (FY2015) and 28.9% (FY2012) of FAA's total annual funding. The philosophical basis for a general fund contribution to FAA costs is the claim that aviation safety is a public good. Some elements of the aviation industry, especially general aviation, have long maintained that the airways system is a pure public good and should therefore be paid for exclusively by government rather than by users of the system. At the other extreme, critics might assert that many Americans make little or no use of civil aviation while others use it intensively, and that the costs of air traffic control and aviation safety are properly borne by those who use the system rather than by taxpayers at large. Conceptually, that portion of the cost of operating the airway system that is appropriated from the general fund is supposed to equate to the amount military, government, and untaxed beneficiaries of the aviation system might have contributed to the trust fund through the payment of user fees, if there were user fees to be paid. This is sometimes referred to as a "public interest" contribution. Having both general government revenue and AATF revenue to fund FAA provides congressional appropriators with the flexibility of raising or lowering the contribution from the general fund to respond to changing needs or economic conditions. In addition, having some general fund revenues available allows certain FAA operations, such as air traffic control, to continue during a lapse in AATF reauthorization. Annual FAA expenditures have been in the range of $15 billion to $17 billion in recent years.