Speech Analysis in Accident Investigation

 

Malcolm Brenner - brennem@ntsb.gov

Human Performance Investigator

National Transportation Safety Board

 

Popular version of paper 4pSCa1

Presented Thursday afternoon, April 22, 2010

159th ASA Meeting, Baltimore, MD

 

 

The National Transportation Safety Board (NTSB) is a Federal agency that investigates major transportation accidents in the United States, determines their probable cause, and makes recommendations to prevent their recurrence. In its investigations, the Safety Board frequently obtains audio recordings and, with the support of the scientific community, has used speech analysis to develop human evidence useful to the investigation. The talk presents two examples.

 

 

Exxon Valdez

 

Shortly after midnight, on March 24, 1989, the U.S. tankship Exxon Valdez grounded on Bligh Reef near Valdez, Alaska, spilling about 258,000 barrels of crude oil and resulting in catastrophic damage to the environment.

 

One issue in the investigation was whether the master (captain) was impaired by alcohol at the time of the accident. Toxicology tests indicated the presence of alcohol in a blood sample obtained 10.5 hours after the accident, but eyewitnesses provided evidence that the master was not impaired around the time of the accident. Therefore, the Safety Board examined Coast Guard recordings for direct audio evidence.

 

Samples of the masters speech were obtained from Coast Guard recordings at five time periods relative to the accident: 33 hours before, when the vessel was inbound to Valdez (used as an exemplar of the masters normal speech); one hour before, when the master made decisions critical to the accident; immediately after; one hour after; and 9 hours after (used as an exemplar of the masters speech when awake for more than 24 hours).

 

Analysis of the masters speech revealed dramatic differences around the accident that were consistent with alcohol impairment and that were not evident in exemplar speech samples:

 

• Slowed speech. The masters speech slowed significantly before and after the accident compared to the exemplars; in fact, he was the slowest speaker on the Coast Guard tape;
• Speech errors. The master misspoke and corrected himself repeatedly before the accident;
• Misarticulation of difficult sounds. Evidence of misarticulation (slurring) was observed before the accident;
• Quality of speech. Contrary to eyewitnesses statements, the master sounded impaired before the accident with incomplete phrases, hesitations, lack of fluency, and mistakes.

 

The investigation considered alternate explanations (based on fatigue, psychological stress, drug effects, and medical problems), but alcohol impairment provided the best fit of the available evidence. Based in part on audio evidence, the Safety Board concluded that the accident reflected the failure of the master to provide a proper navigation watch because of impairment from alcohol.

 

Samples of the audio recordings will be played at this presentation.

 

 

USAir Flight 427

 

About 7:03 P.M., September 8, 1994, USAir (now USAirways) Flight 427, a Boeing 737-300 airplane, crashed while on approach to land at Pittsburgh International Airport in clear weather. All 132 persons on board were killed.

 

The cockpit voice recorder (CVR), recovered from the wreckage, provided an excellent audio record of the last 30.9 minutes of cockpit conversation. It revealed active pilot responses to the upset, including grunts and exclamations, but no direct statement of the cause of the problem.

 

The flight data recorder (FDR), also recovered from the wreckage, provided limited information about the motion of major control surfaces. However, engineering analysis of the FDR data indicated that the flightpath of the accident closely described that of a rudder hardover event (a full rudder command maintained for several seconds). Such a hardover event would imply either an unknown system failure or an unorthodox pilot input, so all investigative groups searched exhaustively for any plausible mechanism to produce a rudder hardover. The human performance group, for example, examined and discounted possibilities based on medical incapacitation, deliberate action, or panic. Audio analysis, specifically fundamental frequency and amplitude analysis of the captains statements, assisted this work by providing evidence on possible emotional reactions.

 

After several years, investigation determined that a rudder reversal sequence was conceptually possible with the Boeing 737 rudder system that might explain the accident. Under very rare circumstances, a hardware failure might cause the rudder to move temporarily opposite to the direction commanded by the pilot until it reached and maintained a full rudder command. This sequence was consistent with all available evidence but, because of its significance, a search was made for any evidence directly indicating that such a sequence occurred in the Pittsburgh accident. Audio analysis provided a possible connection. When the flying pilots grunts were overlaid on engineering reconstructions of the major flight control motions, the grunts corresponded perfectly with the motions of a rudder reversal but did not correspond to any other control input (see attached graphs).

 

Audio analysis assisted the Safety Board in its determination that the rudder surface most likely deflected in a direction opposite to that commanded by the pilots as a result of a jam of the main rudder power control unit servo valve secondary slide. As a result of the Pittsburgh accident and investigation, industry has retrofit the worldwide fleet of Boeing 737 aircraft with a modified rudder control designed to prevent any such accident in the future.

 

 

Conclusion

 

The NTSB has examined audio analysis as an investigative tool and found that it can provide useful scientific information on human reactions in accident situations. Hopefully, this talk will inspire continued cooperation with the scientific community to further examine audio responses in areas such as human fatigue.

 

 

References

 

National Transportation Safety Board. Marine accident report: grounding of the U.S. tankship EXXON VALDEZ on Bligh Reef, Prince William Sound near Valdez, Alaska, March 24, 1989. Washington, DC: National Transportation Safety Board, 1990; NTSB/MAR-90-04.

 

National Transportation Safety Board. Aircraft Accident Report: Uncontrolled descent and collision with terrain, USAir Flight 427, Boeing 737-300, N513AU, near Aliquippa, Pennsylvania, September 8, 1994. Washington, DC: National Transportation Safety Board, 1999; NTSB/AAR-99-01.

 

 

Figure 1. The timing of pilot grunting sounds overlaid on an engineering reconstruction of control surface motions during the accident sequence for wheel (top); and rudder (bottom).