TIM ASHCOM
G3, Accident Investigations, Reporting and Tracking
U.S. Army Combat Readiness Center
Fort Rucker, Alabama

The U.S. Army Combat Readiness Center has investigated mishaps that occurred over water. The lessons learned from these investigations provide an overview of the unusual issues related to underwater location and recovery operations as well as the expertise, procedures and equipment needed to mount an effective response to such an accident. It is intended for use by all who might find it helpful — in particular air accident investigation authorities who might find themselves faced with the task of investigating the loss of an aircraft in these challenging circumstances.

Fatal aircraft mishaps with an underwater dimension, whether at sea or in a lake or river, occur regularly. Locating and recovering these aircraft can be extremely challenging, requiring a well-planned and timely response coordinated amongst many parties. Inadequate preparation or poor management of the initial investigative response has the potential to degenerate into a crisis and can threaten crucial evidence. Agencies undertaking an underwater location and recovery operation must be prepared for the onsite challenges of operations at sea such as the working environment, decisions on what to recover, issues specific to location and recovery, and the management of human remains.

Outside assistance and partnerships
Typically, safety investigation authorities will not be able to conduct an underwater location and recovery operation without outside assistance. Therefore, relationships need to be established in advance with potential partners and sources of assistance. These partners should include agencies with responsibilities for matters relating to the sea, the naval service and the diplomatic service.

Partnerships should also be established with colleagues in other safety investigation authorities. Those that have recent experience of mounting similar operations may be able to provide useful assistance. Additionally, advice should be taken from agencies such as the police, U.S. Navy and Coast Guard, but overall control of the operation should always be retained by the safety investigation authority.

Equipment and working environment
The key factor when selecting a vessel and its onboard equipment is the nature of the accident site: the sea-state conditions, probable depth of the aircraft and seabed environment. Other important factors include proximity of the nearest useful port and the availability of suitable vessels. Safety investigation authorities unfamiliar with underwater operations often underestimate the time it can take to get the necessary maritime assets into position to start work.

When considering the suitability of available vessels, take account of their capability to perform the required task in the time available as well as whether they are outrigged with specialized equipment such as acoustic devices for detecting 37.5 kHz signals and, when necessary, a hull-mounted multi-beam sonar for bathymetry of the seabed. Other considerations include the vessel’s present location and availability, transit time to the accident site and the entire charter cost, including provision for equipment and mobilization/demobilization.

Relatively small craft, for use in operations on lakes, rivers and close to shore, likely won’t be difficult to secure. For operations at sea, however, it is necessary to know where to find the appropriate type of larger vessel. If no suitable state vessels are available, it may become necessary to reach out to the chartering market and issue a call for tenders. Ancillary issues to consider may include the need for a helo deck and any auditing or certification requirements.

Mobilization of large vessels with deep-water recovery capabilities can take time. Therefore, a two-stage approach may prove advantageous — first employing a smaller vessel able to reach the location quickly and begin the task of finding the underwater locator beacons (ULB) until a recovery vessel arrives. The decision to dispatch the recovery vessel should only be made once the wreckage is located, and the delay between the discovering its location and the departure of the vessel should be kept to a minimum. If the wreckage is not found during the period in which the ULBs can be assumed to be transmitting, it will be necessary to proceed to another phase of location using sonar equipment, which will normally result in different vessel requirements.

The depth the aircraft wreckage and flight recorders are believed to be located will be the primary determinant of the recovery options. Air diving is feasible at depths up to 131 feet (40 meters), and saturation diving up to 1,640 feet (500 meters). However, for deep-water and sustained operations, the use of a remotely operated vehicle (ROV) is generally the best option. These vehicles are connected to the parent vessel by an umbilical that carries power and navigational and imagery capabilities. They come in many forms and sizes and may be equipped with one or more “manipulators” for working at the accident site.

Use of an ROV permits the whole investigation team to view and exploit in real time the images transmitted to the parent vessel. It also facilitates the mapping of the accident site. A range of ROVs can be deployed in operations up to 19,685 feet (6,000 meters), and certain specialized — and scarce — ROVs can be used below that depth.

Another type of unmanned vessel available for underwater operations is the autonomous underwater vehicle (AUV), which is a search (rather than grapple-and-recover) tool. AUVs are not tethered to a parent vessel. Rather, they are battery powered and programmed to follow a defined search program. At the conclusion of the search, the AUVs surface and upload their findings to the control center, which may be aboard a vessel or in a road vehicle parked at the lake or river side.

Operation-specific challenges
Some challenges in operations at sea derive from the length of time which the investigation team may need to be out of physical contact with the shore. There is a need to give careful thought in advance to all of the types of equipment which may be required and to the specialist personnel needed aboard.

Working vessels present particular health and safety issues for those unfamiliar with them. The investigation team should consult with the vessel’s health and safety officer to complete a risk assessment of the working environment. The planning process should include the configuration of accommodation and work spaces. The noise and movement of the vessel, the confined and less-than-perfectly-clean spaces likely available to the investigation team, and the presence of seawater and damp conditions all make for a hostile working environment for individuals and sensitive electronic equipment such as cameras and computers.

A particular problem in operations at sea is the moment when a large piece of debris is lifted out of the water. This can lead to a sudden and dangerous increase in load, with potential to damage the wreckage and lose evidence. There may be a need to counter this risk by providing additional tethering to the wreckage (to take any additional loads at key points), and the use of netting is particularly useful. Additionally, an active heave-compensated crane can help alleviate load variations on the lift line. The condition of the wreckage should be recorded before any recovery attempt is made as well as any damage sustained during the lift.

A ULB fitted to an aircraft flight recorder is triggered by immersion in water and emits an ultrasonic pulse of 10 milliseconds at 37.5 kHz and at one-second intervals. The present ICAO requirement is for ULBs (“pingers”) to transmit for at least 30 days and new Federal Aviation Administration requirements have increased to 90 days. They have a nominal audible range of 2-5 kilometers, depending on parameters such as depth, water temperature and sea conditions. It’s beneficial to begin recovery as soon as possible, using a small vessel to find the pinger(s), on the basis of a preliminary review of the loss data such as radar and the Aircraft Communications Addressing and Reporting System (ACARS). The search area may be refined later as more data becomes available.

The sonar search will begin only after the end of the pinger transmission period. The 37.5 kHz frequency is outside the audible spectrum for the human ear. Acoustic hydrophones translate the signal into the audible spectrum, a process that does not exactly reproduce the original emission, which can be polluted by the water environment and thus misprocessed. The ULB signals can be picked up using an acoustic hydrophone deployed by itself as a handheld unit, or with other units in an array.

Digitalization of the ULB signal by onboard software enables the listening for the ULB to be done by a computer rather than a human. Such an array may be deployed to good effect even in difficult sea conditions. However, in shallow waters, the amount of background noise may lead to the signal spike, experienced when the ping is detected, not being prominent and perhaps missed. With such faint signals, difficulties may also be experienced when sounds emitted by the biological environment confuse the acoustic devices. Cetacean sound emissions typically take the form of swift chirps over a wide spectrum of frequencies, which at times could be perceived as a short regular pinger signal after being sampled and processed by acoustic devices.

Towing a hydrophone array at a speed of 4 knots on a search grid of parallel tracks 1 nautical mile apart will enable 40 square miles of sea to be searched in a period of about 10 hours. Use of the vessel’s autopilot (if fitted) while following the search grid is valuable in countering the effects of strong crosswinds and crosscurrents. Strong currents may also cause wreckage and recorders to drift from their original location.

Other systems for picking up and locating ULB signals may involve the repeated “dipping” of a detector below the seasonal thermocline (which separates the noisy mixed surface layer of water from the calm, relatively quiet, deeper water below) at different locations to generate a triangulated homing point, or the deployment of acoustic listening buoys equipped with GPS and UHF radio. For searches in very shallow waters with poor visibility, such as a river or lake, grapple dragging by surface vessels and the use of metal detectors mounted on inflatable craft are options.

What to recover
The priority targets for the investigation team during the recovery phase should be flight recorders, aircraft debris/parts (including avionics components that may contain non-volatile memory), any human remains and personal effects. Wreckage observation and mapping are also important. When available, a photographic survey of the mishap site enables its original state to be recorded before it is altered by diver or ROV interventions. It is necessary to carefully select, with opinions from all investigation parties considered, the aircraft debris and parts to be recovered and to prioritize them with a view to the overall investigation. The initial analysis of the FDR and CVR may assist in this selection process.

There is a case for recovering only those parts of the aircraft judged to be relevant to the investigation, especially if the wreckage is large or fragmented. Divers or ROV operators might be given a shopping list of parts most desirable to recover, based on preliminary information gathered from recorders, sea bed images and aircraft data such as manufacturers’ drawings, parts catalogues, wiring diagrams and manuals.  

It is sometimes more straightforward to recover as much as possible, avoiding the difficulty of finding again particular items which may have been disturbed by underwater currents. The full wreckage may then be examined for its key elements in a more suitable environment. Storing wreckage on land can, however, pose a challenge, as hangar space is often scarce and in some jurisdictions a long-term storage area may not be available.

The recovery of aircraft wreckage is generally accomplished by the parts being rigged to a hoist and lifted by crane out of the water and onto the recovery vessel. In some cases, divers might attach small “parachutes” to wreckage, which are then inflated with compressed air and rise to the surface for recovery. Care is needed to avoid inflatable items being punctured by sharp metallic edges on the wreckage.

The internal components of flight recorders recovered from underwater are vulnerable to corrosion and should be kept in fresh water (deionized water) for transit and until they are opened. All wreckage recovered should be rinsed to remove salt water. Further anticorrosion application of specialized products can also help in preserving evidence.

Human remains
When recovering an aircraft underwater, there is frequently a need to deal with human remains. This poses special technical and psychological challenges beyond those associated with an accident site on land. This highlights the need to be prepared. There may be important legal reasons, such as passenger identification, for the recovery of bodies. This is an operation that should not be improvised; material preparation, ample space and good conditions are crucial. It is important to have the necessary specialized equipment available, such as refrigerated containers and body bags, and any special expertise. Medical-psychological support may also be needed to manage the psychological risks related to the recovery of human remains.

Investigators can be faced with handling large amounts of data in various formats and locations. Confidentiality issues should be considered, especially for data related to human remains. Strict procedures need to be developed and a means of secure transmission implemented between the various entities involved in the search. In most cases, a database containing, at a minimum, pictures, coordinates and descriptions of debris will be needed.

The loss of an aircraft in water may also result in fuel, oil and other noxious fluid leakage. It may be possible to contain and recover these in order to avoid ecological harm. In shallow waters, it may be feasible to surround the wreckage with special protective curtains or booms during an operation to recover the liquids. These curtains or booms may then be towed to land. Specialized assistance should be considered.

An investigation involving underwater recovery should document the operations so other investigation authorities may benefit from the lessons learned. A short report could accompany the safety investigation final report. A decision to halt an underwater recovery operation should be the prerogative of the safety investigation authority, made after careful assessment of the possible safety benefits of continuing the operation, set against the expenditure of additional resources.

Conclusion
The need to conduct an investigation into the loss of an aircraft in water is a real possibility for any agency that has a coastline or an internal body of water, or has aircraft on its register which fly over international waters. Given the number of parties that may become involved, the need to select the right equipment and expertise, the potential for spiraling costs and the challenges posed by operations at sea, any such investigation will require a well-planned and timely response.

This information provides advice on planning and preparing for such an investigation. It emphasizes the importance of establishing in advance useful partnerships and contacts, the value of checklists, the need to identify and source the necessary funding and expertise, and for the investigation authority to have a good understanding of the tools and assets required for successful search and recovery operations.

The cost of these operations can be considerable and it is important that decision-makers who control emergency funds are given realistic cost and time estimates. The challenges involved in conducting operations at sea should not be underestimated. There is often a thin line between success and failure, so anything that can be done beforehand, in preparation and planning, will increase the chances of a favorable outcome.