MOSCOW and WASHINGTON — Russia's Air Force is falling from the sky.

As the Kremlin continues to assert its air power in a bid to intimidate NATO allies in Europe and North America, its mostly Soviet-built aircraft are being pushed to their limits — a fact experts point to when attempting to explain the loss of five aircraft of different designs in just the past month.

The latest in the string of crashes came July 6, when a two-seat Su-24 strike fighter crashed at an air base outside of Khabarovsk, in Russia's Far East, while trying to take off for a training exercise.

This follows the crashes of two MiG-29s, an Su-34 and a Tu-95, all in the last month — part of a larger trend of Russian aviation failures over the last several years as the Soviet-era fleets have fallen victim to age and substandard sustainment.

A source close to the Defense Ministry said on condition of anonymity that the crashes are the result of two key trends dogging Russia's Air Force today — the overuse of old aircraft and a lack of qualified pilots.

It is easy to understand the first point. The pace of Russian activities has shot up dramatically since its invasion of Ukrainian territory in March of 2014.

According to data provided by NATO, the alliance intercepted over 400 Russian aircraft near its airspace in 2014 following the start of the Ukraine Crisis, a 50 percent increase over the previous years and a rate that harkens back to the antagonistic posturing of the Cold War.

× Fear of missing out? Sign up for the Early Bird Brief, the defense industry's most comprehensive news and information, straight to your inbox. Thanks for signing up. By giving us your email, you are opting in to the Early Bird Brief.

The regular nature of these activities is one of the factors that led US Gen. Joe Dunford, expected to easily be confirmed as the next chairman of the Joint Chiefs of Staff, to identify Russia as the "greatest existential threat" to the US during a confirmation hearing July 9.

Paul Schwartz, a nonresident senior associate who focuses on Russia with the Center for Strategic and International Studies, notes that it is not unusual for Russian aircraft to experience a "spate of incidents," but that the current rate of aircraft loss is beyond the norm.

The numbers bear that out. Since 2010, when the Russian government began putting its Air Force back into regular action under former-Defense Minister Anatoly Serdyukov, it has experienced over 30 crashes, hardly ever breaking more than one a month, according to Russian news reports — a stark contrast to the last month of aircraft losses.

There is "clearly a linkage between the increased tempo of military operations undertaken over the last year and a half and the increased spate of accidents for military aircraft," Schwartz said.

"It's clearly taking a turn upwards."

The Russian source pointed to the stress placed on the Russian fleets by demanding patrol and exercise rates over the past several years — and since the start of the Ukraine crisis in particular.

"The majority of the equipment, apart from the [recent crash] of a newer Su-34, is very old. Under [Defense Ministers] Anatoly Serdyukov and Sergei Shoigu, the planes are being used very extensively, especially during these so-to-speak famous snap inspections," the source said.

"If you start to extensively use equipment made many years ago, even if the equipment is certified [in good shape], the percentage of failure becomes higher," he added.

The issue of aging gear is not one faced just by Russia, points out Richard Aboulafia, an analyst for the Teal Group. In fact, it has some similarities to the situation faced by the US Air Force, where service leaders have explicitly said they need to recapitalize aging fleets.

But the situation in Russia, where sustainment and upkeep have never been strong suits, is worse.

"It's exactly like us, except for a couple very big differences — we take sustainment seriously and we build robust systems," he said. "They don't."

Schwartz notes that while the Russians have made changes since 2008 in how they do sustainment and maintenance, historically "the amount they expand on maintenance has been substandard when compared with Western approaches."

The poor state of Russia's defense industry is also contributing to the state of Russia's aircraft fleet, said Vadim Kozyulin, a military expert at the Moscow-based PIR Center think tank.

"These old aircraft require a lot of maintenance, and the spare parts currently in stock are old," Kozyulin said, noting that when it comes to maintenance personnel, the older ones are experienced while the younger ones are not qualified.

"Many manufacturers of military components went bankrupt, converted to civilian production, or were left abroad — like in Ukraine — after the Soviet collapse," Kozyulin said. "Large numbers of existing producers of military components do not have military quality control inspectors on site to ensure the quality of components, as was done in Soviet times."

Sanctions from Western countries are having an impact in that regard, Schwartz said, as many of the high-end components that would help keep the fleets in top shape are no longer available to Putin's government.

"They've been especially dependent on electronic components from abroad," Schwartz said. "With sanctions taking effect that reduce their ability to purchase some of the components they use in their aircraft, they have to look for substitutes or look to buy from intermediaries."

Another issue the Russian source identified is the lack of qualified pilots to fly the kinds of missions the Defense Ministry is asking of the Air Force as Moscow tries to flex its muscles in the face of NATO.

"There are less pilots [in Russia] than there are aircraft, and they gave young pilots missions that are supposed to be given to experienced pilots," the source said.

These young pilots are lacking in basic skills such as midair refueling, the source said, noting "today, air refueling in Russia is, I dare to say, almost something exceptional."

Schwartz concurred that pilot training has been an issue for Russia since the end of the Soviet Union.

"They had pilots who flew so infrequently following the collapse that flight time for pilots was down to 20-30 hours a year, in some cases," he said.

Most of Russia's qualified pilots have retired or taken up better jobs flying for commercial airlines such as Aeroflot, Transaero and foreign companies, the source added.

Grounded Fleets

Russia is not ignoring the problem of lost planes, and has not hesitated to ground whole fleets while they attempt to find the sources of the problems.

"They're pretty good about grounding a particular category of aircraft when there's been an incident so they can trace the cause of the incident and prevent additional incidents from happening," Schwartz said.

Shortly after the crash of the Su-24 on July 6, the head of Russia's Air Force, Col. Gen. Viktor Bondarev, ordered the grounding of all Su-24 planes until the cause of the crash is identified — making the fleet Russia's third to be grounded over the last month.

On July 3, Russia's force of over 200 MiG-29 air superiority fighters were grounded after one crashed near its base in Krasnodar in southern Russia — the fourth loss of a MiG-29 over the past year.

Another MiG-29 crashed on June 4 in nearby Astrakhan during training. Just two and a half hours after that MiG-29 crash, one of Russia's newer Su-34 fighter-bomber aircraft flipped over while trying to land at its base in Voronezh region, about 500 kilometers south of Moscow.

An unidentified Defense Ministry source, quoted by state news agency RIA Novosti that day, said the plane's drag chute failed to deploy upon landing.

The Russian Air Force was also forced to ground its fleet of 61 Tu-95 "Bear" long-range strategic bombers temporarily last month after an engine fire during takeoff led one to run off its runway during takeoff from the Ukrainka Air Base in Russia's Far East.

The Bears are Soviet-era staples that form the mainstay of Russia's strategic bomber force. The planes have been spotted repeatedly buzzing along the fringes of NATO airspace over the past year. However, their engines are old and a modernization program is progressing at a snail's pace.

There is, for example, a production bottleneck in engines for the Tu-95s. Russia can only produce about 10 Bear engines a year for a fleet of over 60 of the four-engined airplanes.

After last month's incident with the Bear at Ukrainka Airbase, it is not clear how long the bombers were grounded, or if it impacted the rate of Russia's provocative patrols near NATO borders.

However, two Tu-95s were intercepted by US Air Force F-22 aircraft off the California coast on July 4.

The Russian Defense Ministry did not respond to a request for comment on the reasons behind the crashes, or clarify the status of its temporarily grounded fleets of Su-24s, MiG-29s and Tu-95 long-range strategic bombers.

The rate of attrition could begin to impact Russia's ability to fly patrols with regularity. While noting that five losses in a month likely won't happen on a regular basis, Schwartz said he expects the fleets to continue to grind down for several more years.

Eventually, however, the influx of newer aircraft into those fleets will help balance out the losses. The Su-24, for instance, is planned to be replaced by the Su-34 by around 2020. But replacing the aging fleets could take longer than desired if sanctions continue to impact Russia's fiscal situation.

In the meantime, the mix of older planes, lack of experienced pilots and sustainment issues will likely continue to be a problem for Putin's government.