Marine Maj. Sterling Norton, 36, was killed when his F/A-18 Hornet crashed on July 28 during a live-fire nighttime training accident in Southern California.

Less than a week later, another F/A-18 from the same squadron crashed outside Naval Air Station Fallon, Nev. The pilot ejected safely – but it was the squadron’s third F/A-18 crash since October - two of which were fatal.

The Marine Corps, in response, conducted a one-day safety stand-down.

But such accidents are becoming more frequent – amid concerns that insufficient training and an aging fleet hobbled by a shortage of spare parts are contributing factors. A Fox News investigation reveals that, overall, the entire U.S. military saw a 48 percent increase in non-combat aviation crashes in 2014 and 2015 compared with the two prior years, based on press reports.

“They are going up partly because they are not getting the training they should get. They're going up because maintenance is harder and harder to accomplish. They are going up because the airplanes are getting older and older,” said House Armed Services Committee Chairman Mac Thornberry, R-Texas, in an interview with Fox News.

Maj. Norton deployed in combat to Afghanistan in 2012. His commanding officer called him one of his best pilots. According to the Washington Post, a Marine who witnessed the crash said Norton’s jet “broke apart in midair” while in a dive preparing to fire weapons.

"I want to wait for the investigation report. However, these jets are too old and should not be flown anymore," Norton's mother, Mary Anne Vanderhoof, told Fox News.

She added that her son was an elite Top Gun graduate and weapons tactics instructor.

On Capitol Hill in July, the head of Marine Corps aviation seemed to share her concern.

“I worry about my young aviators that aren't getting the number of hours they need to. And so it's the mishaps that loom over the bow that we don't see coming just now … Will they have the experience to keep that bad thing from happening?" said Lt. Gen. Jon "Dog" Davis.

So far in 2016, there have been nine military aircraft crashes. Four involved Navy F/A-18 Hornet jets. There were 33 total across all branches in 2014 and 2015 – up from just 23 in 2012 and 2013.

The admiral in charge of Navy aviation denies a link between the crashes and the age and readiness of their planes.

"I wouldn't characterize it as a crisis. I get the question a lot of, do you tie it to readiness or a lack of proficiency … and in review of those mishaps, I can't make that connection," said Vice Adm. Mike Shoemaker, speaking in Washington last month.

According to statistics provided by the U.S. Navy, only 21 percent of its early model Navy F-18 Hornets can fly -- and only half of its newer Super Hornets can as well. Over 100 Super Hornets are not flying due to shortages in critical spare parts.

The Navy’s fleet of MH-60 helicopters is not much better. Only 57 percent of its 412 helicopters can fly.

The Navy, like the Marines, is having a hard time finding available jets for its pilots to fly and train in – amid more than $100 billion in defense cuts since 2009, a steady tempo of combat missions, and a delay of the F-18’s replacement, the F-35 Joint Strike Fighter.

Training is another concern. Right now, the Navy is averaging 12-14 flight hours a month for its pilots, according to Shoemaker. The Navy is buying more Super Hornets and hopes to increase that average to 15 hours by December 2017, according to a Navy official who shared a forecast model with Fox News.

Following a Fox News investigation into Marine Corps aviation in April, an interview with the head of Marine Corps aviation reveals some gains, but many problems persist.

Today, only two of the Marine's 12 F-18 Hornet squadrons meet their flying hours, Davis told Fox News. He said they are averaging 11.1 flight hours a month per Marine pilot right now. While it was 8.8 back in March, he said his pilots should average 16 hours a month.

“Our model is all squadrons ready to go,” he said. When asked why his pilots were not getting enough hours in the air he replied, “Not enough airplanes to fly, it’s a simple physics problem.”

Right now, of the Marines' 273 F-18s, only 91 can fly; 88 are waiting for parts.

Thornberry said President Obama has effectively sent more U.S. troops into harm’s way without paying for the increase in costs.

“When the president sends more people to Afghanistan more people to Iraq, he doesn't ask for more money. The costs just come out of the training, the maintenance and the readiness of our force. The problem is getting worse,” he told Fox News.

The Pentagon disagrees.

Lt. Col. James B. Brindle, a Defense Department spokesman, said in a statement, "Ensuring that the force is well-equipped and well-trained to execute critical missions is a priority. We have looked at our data and have not observed an overall trend in the increase of mishaps due to reduced training hours. Annual mishap totals vary for a variety of reasons. Any increases we have observed are too small, and over too short a duration, to categorize."

Marine and Navy F-18s were originally designed for 6,000 flight hours, but they were refurbished and extended to 8,000 hours while waiting for the new Joint Strike Fighter. Some jets may even reach 10,000 hours, according to Navy and Marine Corps officials.

In 2015, the Marine Corps’ aviation mishap rate was three times the Navy’s.

The Air Force, while not suffering from the same shortage of parts, is short 700 pilots, and the secretary of the Air Force said last month it will grow to 1,000 “in just a couple of years from now.”

When asked how quickly the Marine Corps can get more Joint Strike Fighters into the fleet to replace 24-year old F-18s, Davis replied, “I am buying as many as we can afford. The money is not there.”

Fox News’ Leonard Balducci contributed to this report.