The UK's newest aircraft carrier has already encountered a century-old undersea threat

• British aircraft carrier HMS Queen Elizabeth was trailed by Chinese submarines as it sailed through the South China Sea this fall.
• That encounter, during the carrier's maiden deployment, was an introduction to the challenge that subs pose to aircraft carriers.

During its maiden deployment this fall, the British Royal Navy's newest aircraft carrier launched into anti-submarine-warfare training when it detected Chinese submarines nearby.

The encounter was a taste of what Western navies can expect in an era of renewed great-power competition with peer adversaries — namely, China and Russia.

A UK carrier strike group led by the HMS Queen Elizabeth detected Chinese subs as it sailed through the Indo-Pacific region and was ready to intercept any incoming aircraft with fighter jets aboard the carrier, Sky News reported in November.

Royal Navy officials told the outlet how the carrier's accompanying frigates and helicopters, equipped for anti-submarine warfare (ASW), held the Chinese boats at bay until the carrier could "literally side-step" them.

The run-in served as training for both the Chinese submarines — which without a doubt intended to gather reams of data on the Queen Elizabeth — and for the British-led multinational carrier strike group, comprising eight supporting ships, including a Dutch frigate and a US guided-missile destroyer, along with a British Astute-class submarine and five air squadrons.

The carrier sailed with a blended air wing of US Marine Corps and Royal Air Force F-35Bs, along with British Merlin helicopters, during the seven-month deployment that featured interactions and exercises with over 40 countries.

The deployment was an opportunity for the United Kingdom's newest naval asset to venture into the Indo-Pacific — a region of growing interest and activity for many of the world's militaries.

It also tested the navy's ASW acumen after many years of atrophy and provided an opportunity for the fleet to understand "the issues involved in deploying a task force of this size and capability, at this range, into that region, and how it fits in with working with allies and partners," said Nick Childs, a senior naval analyst at the International Institute for Strategic Studies in London.

The navies that took part in the deployment have had limited occasion to practice real-world ASW since the end of the Cold War. Some, including the US Navy, have reemphasized ASW in recent years, but it remains a challenging discipline.

The different platforms helped the strike group detect the subs nearby, but unmanned systems and more varied sonar techniques could help fill gaps in Western navies' anti-submarine warfare strategies.

Old threat, new methods

Submarines have posed an undersea threat to surface fleets for over a century, but recent technological advancements make them particularly menacing for aircraft carriers.

Many navies, including China's and Russia's, have introduced newer generations of submarines that are much quieter than their predecessors. Others, such as North Korea and Iran, are using subs to "level the playing field" with regional and Western competitors, according to a report by the Hudson Institute, a US think tank.

For over a decade, Beijing's submarines have shadowed US carrier deployments for training purposes. Their presence, in turn, becomes a training opportunity for the carrier strike group to "hunt a sub," said Bryan Clark, a senior fellow at the Hudson Institute and a coauthor of the report.

There are plenty of ways to respond to an identified submarine threat, but actually detecting it in the first place is the tough part, Clark said. "Finding it means you've got to use sonar. Sonars have a limited range, and they can only cover certain sectors. So you've really got to have multiple platforms out there … to be able to cover the 360 degrees around the carrier."

The Royal Navy likely anticipated that it would encounter some Chinese submarines over the course of this deployment and approached that as its own training mission, Clark told Insider.

Beijing "will want to get as much data as possible on the UK strike group, particularly the Queen Elizabeth," Clark said. That includes acoustic and electromagnetic data, along with information on different radars and radio systems.

While submarines themselves have become more technologically sophisticated, the main way navies find subs — using receivers to detect vibrations and sound waves in the water via passive sonar — is largely unchanged since the Cold War.

But new undersea vessels are so quiet that the range on a passive sonar system is too restricted to detect them, Clark said. The Merlin helicopters aboard the Queen Elizabeth have dipping sonar systems, but those are only really effective at a short range, he added.

European navies are shifting back to "active sonar" technologies that send sound waves through the water in order to find those boats, a method that will probably be "the future of anti-submarine warfare for the next couple of decades," Clark said.

Filling the gaps

The arrival of HMS Queen Elizabeth in 2017 was a renewal of the Royal Navy's carrier capability after its last carrier left service in 2014. As with its carriers, the Royal Navy is renewing its focus on ASW, which it neglected for more than a decade.

In 2016, the UK committed to buying nine P-8 Poseidon maritime patrol aircraft from the US, and it received the last of them in January. The P-8 is considered the most capable sub-hunting aircraft in operation.

The Type 26 City-class frigate in production for the Royal Navy "will probably be the best anti-submarine frigate in the world, when it finally arrives," Childs said. The first, HMS Glasgow, is being built and is scheduled to enter service in the mid-2020s.

The next-generation frigate will host multiple ASW features, including an acoustically quiet hull and multiple sonar systems. It will have space to accommodate unmanned aerial systems and be capable of carrying anti-submarine rockets.

Unmanned systems could also fill in the gaps in ASW coverage for Western navies.

The Royal Navy has selected the maritime variant of the General Atomics-built MQ-9 Reaper drone. The MQ-9, which can stay aloft for more than 18 hours, has a dedicated ASW package that it can use to conduct an eight-hour patrol over a radius of 1,200 nautical miles.

With that kind of range, a drone like the MQ-9 could easily deploy from an airfield on shore, fly down to the ship, and operate from there, Clark noted.

The US navy is also developing an unmanned surface vessel that could eventually counter the submarine threat.

"Those are two unmanned systems that could help a lot in terms of giving [navies] more capacity for searching for submarines," Clark said.

Childs said that with renewed concern about great-power competition and with "the challenges of complex naval warfare," ASW is "back on the agenda in a way that it hadn't been in the last two or three decades."

"Everyone, from the Americans downwards, in terms of particularly carrier operations, is having to adapt to that," Childs said.

Vivienne Machi is an award-winning reporter based in Stuttgart, Germany. Her writing has appeared in outlets including Foreign Policy, Defense News, the Counter, and Via Satellite. Twitter: @VivienneMachi