The challenge

A three-seat, single engine torpedo bomber, it was launched from aircraft carrier decks during World War II, carrying their lethal load to drop on to targets. Despite the numbers that were built, none remain in the UK today, at least not in complete form. However, restoration engineers at the Fleet Air Arm Museum (FAAM) in Yeovilton are looking to change that and a chance find in the English Solent is helping them on their way.

It’s the wreck of a Mk II Fairey Barracuda, discovered in 2018 by James Fisher Marine Services (JFMS) during a UXO survey for a new 204 km long power interconnector between the UK and France as part of the Interconnexion France-Angleterre 2 (IFA2) project.

IFA2 is National Grid’s second electricity subsea interconnector to France and is a joint venture with French System Operator RTE.

The wreck is believed to be one of two Barracuda aircraft which were based at Lee-On-Solent, Gosport. Both planes suffered forced landings in the Solent during WW2, shortly after take-off from HMS Daedalus airfield. While each pilot survived and made it through the remainder of WW2, their planes remained at rest on the seabed.

A challenging acoustic environment for USBL systems

Recovery of the wreck offered a great opportunity to the Fairey Barracuda restoration effort. But, it also posed a number of challenges, not least the water depth – or rather lack of it. Lying in just 5 m, Robin Fidler, who was then Survey Operations Manager at JFMS, expected to encounter acoustic interference problems tracking his divers due to signals bouncing off the seafloor and sea surface – often referred to as multipath.

Multipath can cause a USBL transceiver at the surface to falsely detect (or completely miss) a genuine reply signal from a transponder, leading to unstable tracking performance. Previous generation USBLs were particularly susceptible to multipath and needed careful setup to overcome the problem – not always successfully.

The solution

JFMS chose our Mini-Ranger 2 USBL system for the project. Mini-Ranger 2 is our mid-level USBL target tracking system. It provides shallow water performance without the cost and complexity of a deep water USBL solution. It’s also portable and quick to mobilise; a great choice for small survey vessels, moored barges and uncrewed vessels.

Six divers were used on the three-week project from the Stour jack up barge, with one diver in the water at any one time. The barge itself was fitted with an HPT 3000 transceiver mounted to the side, cabled back to a survey shack where the diving operations were controlled from.

WSM 6+ transponders fitted to each diver’s cylinder enabled the HPT to track every moment of their dive, providing a valuable layer of safety to the operation. Each diver also carried one of our Nano transponders in their pocket, this was used to place directly on top of any archaeological finds, so that precise waypoints for each artefact they discovered could be logged (and individually named) in the Mini-Ranger 2 USBL software. This information is then available for offline analysis.

The Mini-Ranger 2 USBL uses Wideband 2 for digital signal processing, so multipath never became an issue for the recovery operations, regardless of the depth of the tide. This had the additional benefit of freeing up users to deploy our USBLs virtually anywhere.

The results

The crash site was heavily silted so it needed to be cleared away for sections of the aircraft to be lifted out of the water. Artefacts retrieved included one of the pilot’s boots, a boost gauge and the underwing pitot head and mounting bracket – a delicate instrument that was used for recording the aircraft’s airspeed. The fact that this was found intact implies that the Barracuda was almost at stalling speed by the time it reached the water, says Wessex Archaeology’s Senior Project Manager Euan Mc Neill.

“We were really impressed with just how Mini-Ranger 2 operated,” says Fidler. “We thought we were going to have to use a (Fanbeam) laser radar system, tracking a reflective buoy attached to the diver to give us a range and bearing to the diver. We didn’t have to use it once; we could do it all with USBL, no matter what the tide, which made our lives much easier and that’s all we could ask. The USBL didn’t miss a beat. We were up and running with it quickly meaning that we were able to maximise the three week window we had on site.”