Retrieving The Wreckage

When Alaska Airlines Flight 261 crashed off the coast of California, technology played a critical role in the combined investigative efforts of public, private and military organizations.

by / February 12, 2001 0
Few of us realize the scope of the operations, the technological resources and the number of personnel involved in responding to a major air disaster. The task of locating and retrieving victims and wreckage -- essential in determining probable causes of the crash -- require the coordinated efforts of many civil and military agencies, hundreds of technicians and specialists and the convergence of advanced technologies. Aviation safety often depends on the timeliness and thoroughness with which these operations and subsequent investigations are carried out.

When Alaska Airlines Flight 261, en route from Puerta Vallarta to San Francisco, lost elevator control at 30,000 feet and dove into the sea off the coast of southern California last year, the crash triggered an immediate response from federal, state and local agencies, the military and several private organizations. From the initial search and rescue efforts to the final recovery of victims and aircraft remains, interagency cooperation and the application of advanced technologies such as GIS, GPS, sonar and other remote-sensing systems played a major role in expediting complex operations.

A Grim Task

The crash occurred at about 4:30 p.m. on Monday, Jan. 31. Within minutes, search and rescue helicopters from Point Mugu Naval Air Station were on scene, followed by surface craft from the Channel Islands United States Coast Guard (USCG) Station, commanded by Captain Andy Jones. Jones coordinated the search for survivors by USCG and volunteer vessels, and set up a security zone around the area of floating debris. At about 6 p.m., the smaller USCG craft was relieved by the 82-foot cutter, Point Carrew, from the Eleventh Coast Group, Long Beach.

Because the plane went down in the Channel Islands National Marine Sanctuary (CINMS), one of the many authorities the USCG alerted was the Santa Barbara headquarters of the National Oceanic and Atmospheric Associations CINMS, the agency that protects and manages the resources of the sanctuary. CINMS has an extensive GIS system with multiple data layers of the islands, information on sea conditions and bathymetric data (topography of the ocean bottom) within the sanctuary. Their staff could provide orientation maps and other supporting data.

Although no survivors were found in the immediate area of the crash site, the USCG continued to search the floating debris throughout the night. As many remains as possible had to be recovered and turned over to National Transportation Safety Board (NTSB) investigators. To assist them in tracking the floating debris field in the darkness, the USCG called on the mapping capabilities of the National Ocean Service (NOS) and requested an assessment of other potential National Oceanic and Atmospheric Association (NOAA) assets in the region that could assist with locating debris.

After receiving word that an Alaska Airlines jet had gone down in the sanctuary, NOS plotted the site coordinates and mobilized its staff. Throughout the evening, they remained in contact with the USCG, assisting in estimating the drift of the floating debris field. At the Santa Barbara office, CINMS physical scientist Ben Waltenberger forwarded the periodic coordinates of the field, along with wind and sea conditions, to the USCG and the Hazardous Materials Assessment Division (HAZMAT).

At HAZMAT headquarters in Seattle, the data was put into a modified oil-spill modeling program and produced a projected path and dispersion rate for the debris. Although the model was developed for oil spills, Chief Scientific Support Coordinator Commander Jim Morris said it has many other applications. "You just have to change a few variables to make the model do something a little different."

Coordinates of the projection were sent back to Santa Barbara and loaded into the GIS. The same evening, the USCG cutter Point Carrew had a map with a projected path of the debris field for the next 12 hours. "In emergency situations, where you have to make quick decisions, the ability to visualize data is one of the most important assets you can have," said Waltenberger.

Bring in the Subs

Within minutes of the crash, the NTSB in Washington, D.C., contacted Naval Sea Systems Command and requested mobilization of their Supervisor of Salvage (SUPSALV) Division. Since the plane was estimated to be at a depth of 700 feet, recovery operations called for side-scan sonar and ships with remotely operated vehicles (ROV) -- tethered robots capable of working for days at depths beyond the practical limit of divers.

In San Diego, the M/V Kellie Chouest, a submarine rescue support ship operated by the Navys Deep Submergence Unit, was ordered to proceed to the site with the ROV Scorpio and retrieve the planes black boxes -- the cockpit voice recorder and flight data recorder. At Port Hueneme Naval Base in Ventura, Calif., the USNS Sioux, a fleet tug operated by the Military Seal Lift Command and the research vessel M/V Independence, operated under contract by Mar Inc., were readied for side-scan mapping and ROV recovery operations.

Deep Drone, SUPSALVs primary salvage ROV system for depths to 8,000 feet, is kept in constant readiness for recovery operations in any part of the world. ROVs are controlled from the surface through an umbilical that carries control signals and power for video and lights that enable the operator to navigate the craft visually and acoustically. ROVs have an onboard sonar, a still camera and two mechanical arms (manipulators) that can work with tools, attach rigging and pick up objects ranging from the size of a teacup to objects weighing over 200 pounds.

Launching the ROVs

While the ROVs were preparing to launch, the CINMS staff in Santa Barbara shifted to developing maps of the seafloor using existing bathymetric data and geo-referenced bottom imagery captured a few years earlier by ROV cameras. "The primary purpose of bottom mapping was to look at the terrain in the area of the crash," said Waltenberger. "The Navy was about to put ROVs down and wanted to know what kind of terrain they were going to encounter."

MSD Resource Protection Coordinator Lisa Symons said the CINMS team was looking for things such as abandoned wellheads and underwater pipelines. "There is an active oil field in that area, so we were looking for anything that might hamper recovery efforts. We were also talking with SUPSALV about the natural resources they would encounter in the area. Part of our concern and part of the reason we were involved in mapping the debris field was to know where that floating debris was going to end up and how it might impact marine resources."

NOAA produced overviews of the area and imbedded the bottom imagery in 2D and 3D bathymetric maps. Users could click on different points in the area of the debris field or wreckage and actually see what the bottom looked like at those points. According to Waltenberger, the area around the crash site was flat and sandy. CINMS put up the maps in a field office at Port Hueneme and made them available to federal, state and local emergency response agencies involved in the operation. They also put maps on the Internet for the press and the general public.

The Black Box

The arrival of the NTSB investigation team early Tuesday morning coincided with the arrival of the M/V Kellie Chouest with the ROV Scorpio onboard. Since the salvage team knew the type of bottom their ROV would be working in, they began the search for the black boxes almost immediately. Using a handheld transponder, the team locked onto the characteristic pinging coming from the recorders in the tail section of the plane and pinpointed their coordinates using differential GPS, which had three- to five-meter accuracy. By Wednesday, the team had retrieved the cockpit voice recorder, and by noon the following day, the flight data recorder. Less than 72 hours after the crash, both boxes were at NTSB headquarters in Washington.

At about the same time that the Kellie Chouest was retrieving the first recorder, the side-scan sonar and the ROV Deep Drone were arriving at Port Hueneme. The sonar went aboard the Sioux, and the Deep Drone went aboard the Independence. The sonar team was tasked with producing a precision bathymetric map of the debris field, which would be used to guide the Independence and Deep Drone in recovering victims and the aircraft.

Mapping the Ocean Floor

Side-scan sonar is the principal tool for producing bottom topography and locating sunken objects (see "Sophisticated Systems Comb Ocean Floor for Clues to TWA Flight 800 Disaster," February 1997). As the torpedo-shaped sonar sensor is towed back and forth over the target area in a series of parallel, overlapping tracks, its pulses are reflected off the ocean floor and the objects on it. The echoes are received by the sensor and sent to a Data Acquisition and Processing System, which is tied into the vessels GPS-based navigation system. The output is highly detailed, georeferenced imagery in real time.

The search effort with the side scan was completed in less than a day. According to Salmon, the debris field was quite small. "The plane obviously went in at a steep angle, so we were able to quickly determine the boundaries of the primary debris field and develop a very precise map. Fortunately for us, the field was in a flat, sandy area. If you went a half mile in almost any direction, you were in rough rocky, canyons, peaks and valleys, with all kinds of problems."

At the request of the NTSB, the sonar team also ran a survey outside the primary area for isolated aircraft debris, but none was found.

Following processing and review of the sonar map by the command center in Port Hueneme, the Independence put Deep Drone down to document the debris field on videotape. The recovery team monitored the position of the drone, relative to the ship, through a transponder tracking system. Since the vessel had to stay above the drone at all times, data from the tracking system was used to keep the vessel in position through the use of thruster propellers. Images from the video were recorded along with the drones exact depth, x-y coordinates and heading.

After reviewing the tape and evaluating areas for recovery, the NTSB and SUPSALV sent the drone back for close-ups of specific targets. Retrieval of the victims was completed on Feb. 6.

"It was a slow, tedious process, because at that depth its hard to operate quickly," Salmon said. "Everything is done in slow motion."

The Final Sweep

At this point, the focus of investigator interest shifted to the tail section: specifically, the horizontal stabilizer, its control mechanism and the vertical stabilizer. Recorded conversations between air traffic control and the plane indicated the pilots had been having difficulty maintaining altitude as much as 30 minutes prior to the crash. On Boeing MD-80 aircraft, the horizontal stabilizer works on the principal of the garage door opener: A long, threaded jackscrew lifts and lowers the door as the screw rotates back and forth through a fixed nut. On the plane, the jackscrew moves the wing-like horizontal stabilizer up or down, depending on whether the pilot moves the control to climb, fly level or descend.

By Feb. 10, the Deep Drone team had recovered the essential parts of the tail section. A close-up showed stripped threads still clinging to the jackscrew. According to the Los Angeles Times, the NTSB said the jackscrew had no lubricating grease on it. Other priority parts recovered included flight control surfaces, all wing sections, flaps, rudder components and actuators. SUPSALV Operations Specialist Keith Cooper pointed out that most parts were recovered in specially constructed debris baskets by Deep Drone or slung under the vehicle in specially designed rigging.

On Feb. 22, the engines and other large parts of the plane were hauled to the surface with the aid of the fleet tug Sioux. After a subsequent video survey showed the remaining debris to be mostly fragmentary, SUPSALV hired the Sea Clipper, a commercial trawler, to retrieve the remaining pieces with a small-gauge net. The trawler completed the sweeps in two weeks. A final video survey following the last trawl on March 15 showed the area to be clear of debris.

Retrieving the victims and the wreckage from Alaska Flight 261 took the coordinated efforts of massive resources and hundreds of specialists from government agencies, military units and civilian organizations. At various times, the operation drew on combined technologies and resources from the U.S. Coast Guard, the Air Force, Navy, NOAA and private organizations. The task was completed in 48 days and provided the NTSB with enough information to identify the probable causes of the crash and prevent its recurrence in other aircraft of this design.