Today's U.S. Air Force is the smallest and oldest since its inception. Of its roughly 5,000 aircraft, the average age is 25.

America’s Air Force is quickly shrinking before the nation’s eyes. Optimistic aircraft-purchase quantities are unlikely to materialize in the near-term, and the service’s upcoming “bow wave” of aircraft buys will come at the worst possible time, in the early 2020’s, when all other federal spending will squeeze defense budgets further and faster.

All this in a period when the Air Force is the smallest and oldest since its inception in 1947. Of its roughly 5,000 aircraft, the average age is twenty-five years old.

Today’s Air Force is already struggling, and tomorrow’s is entirely at risk.

In light of the latest budget proposal, Congress must step back and look at the collective impact of recent capacity and capability cuts on purchases of aircraft in particular. They will find that not only is there virtually no slack left in America’s current Air Force to meet global peacetime and war plan demands, the historically most innovative service is now left to incrementally upgrade existing capabilities while abandoning transformational and leap-ahead investments.

Additionally, Congress must understand several trends underway in recent years that have it buying fewer and fewer planes, both in absolute and relative terms, while at the same time proposing hundreds for retirement.

Of the three military departments, the U.S. Air Force is buying the fewest total amount of new aircraft, purchasing the fewest types of aircraft, and retiring the most airplanes

Since defense budgets peaked in 2010, and continuing through the 2015 budget request, the U.S. Navy is on a path to have acquired 1,133 new aircraft while the Air Force will have bought 824. Of these planes, the Navy will acquire 264 fighters ( including the EA-18G Growler electronic attack aircraft ) to the Air Force’s 117.

Excluding Remotely Piloted Vehicles (RPVs)—or drones—the Navy is on a path to have purchased 1,039 new aircraft, and the Air Force just 400 airplanes between 2010 and 2015.

This shows just how much the Air Force aircraft replacement rate is slowing, which means the already smaller force is getting older, faster.

Granted, numerical comparisons alone only tell part of the story. The Air Force is all-in on the newest fifth-generation Joint Strike Fighter (JSF) program, which costs more than any other platform, particularly in its early stages. And even when the JSF is in full production, the Air Force will not replace legacy fighters on a one-to-one basis given the extra capability that will come with investment in the F-35.

But policymakers cannot assume continued U.S. dominance of the skies—even with this investment in JSF. Air power allows leaders the ability to wage mobile and adaptive campaigns that maximize economy of force in wars based on attrition and occupation. But the Air Force has too many recent investments in limited “silver bullet” fleets that try to perform too many missions with only a few select aircraft. The bottom line is that numbers are down at the precise time when qualitative and quantitative advantages are critical for U.S. military forces as traditional margins of superiority erode or are at risk .

The Air Force’s investment in fifth-generation fighters is what, in part, allows the Navy to continue to buy new fourth-generation F/A-18E/F Super Hornets for its fleet. Purchases of more advanced fighters by the Air Force—complete with their stealthy signature and sophisticated sensors, which contribute technological and material prowess to the fight, especially in a conflict’s opening stages—fulfills a necessary role that allows other branches to carry out their missions in a lower-threat environment later.

Still, Congress has taken disproportionate care of the Navy’s investment in the Super Hornet purchases, including adding jets above budget requests and granting generous, multi-year procurement authorities, while showing far less support of the Air Force’s need for new bomber and fighter fleets that help the entire military gain global access and accomplish its objectives.

Navy investment in new fourth-generation fighters has also allowed that service to make plans much earlier than the U.S. Air Force for a so-called “sixth-generation” fighter or family of systems. The Navy’s thirty-year aviation plan in 2012 called for work to begin on a Next Generation Air Dominance Aircraft after 2019 . The Air Force had no similar plans at the time. While there is a very small sum of money for capability beyond the F-22 in the President’s latest defense budget, this work should have begun much sooner and reflects the zero-sum nature of all financing decisions under the Air Force topline.

The lack of robust investment in large quantities of new airframes is even more worrisome when accounting for the proposed retirements of legacy Air Force aircraft. Compared to global demand, the Air Force’s supply is simply outmatched.

Excluding remotely-piloted aircraft, the Air Force has proposed divesting about 634 aircraft from 2010 through 2015—nearly 160 percent more aircraft than it bought over the same period. This harsh reality demonstrates the intense pressure on Air Force modernization accounts as the service struggles to allocate shrinking resources to buy newer and more expensive airframes.

Of those aircraft proposed for divestment, just under 400 were combat aircraft, including F-15s, A-10s, F-16s and B-1s. Even more are on the chopping block to be let go early if Congress cannot compromise with the Pentagon and agree to allow some fleets of aircraft to retire entirely .

But even if full sequestration does not continue throughout the decade, “sequestration-lite” is here to stay. Congressional and Pentagon leaders have a variety of tools at their disposal to help ease the budget crunch, including requests for additional funds, the war budget, reprogramming authorities and acquisition tools like block buys and multi-year contracts. Air Force leaders have done their part, and should be commended for carefully thinking through sequestration along with their robust outreach to the aerospace industry to help bring the costs of systems down while fielding capability much sooner. It’s time for Congress and this administration to do theirs.

All efforts, including more creative ones, will have to be employed to keep America’s Air Force dominant for the next fight. The Air Force needs to begin robust investment now for a new cargo aircraft, a sixth-generation fighter aircraft or family of systems, a new tanker, a trainer jet, a combat search and rescue helicopter, and recapitalization of select intelligence, surveillance and reconnaissance fleets.

To maintain the same level of service to the nation as has come to be expected in recent decades, the Air Force needs an unwavering partner in Congress, the White House and, by extension, the American people.

Mackenzie Eaglen is a resident fellow in the Marilyn Ware Center for Security Studies at the American Enterprise Institute. You can follow her on Twitter: @MEaglen .

Image: Flickr/Official U.S. Air Force