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26 February 2010

Recent Aviation Safety Events and the Role of Sound

The past few days have seen three separate aviation safety related events that all have one surprising thing in common, sound. The most recent is the only one that was not connected with a fatal airplane crash, but could have a very significant effect on airline procedures in the future.

NTSB Recommends Routine Analysis of CVR Data
As reported by the USA Today and other media sources, (see pages 47 and 59), the NTSB made a recommendation (number A-10-29) which asks that the appropriate regulations be changed to allow airline operators to routinely "download and analyze all available sources of safety information, as part of their flight operational quality assurance program, to identify deviations from established norms and procedures; (2) provide appropriate protections to ensure the confidentiality of the deidentified aggregate data; and (3) ensure that this information is used for safety-related and not punitive purposes"

In plain English, the NTSB is recommending that airlines be allowed to review information from the cockpit voice recorder (CVR) to help improve safety. At, we're having a hard time understanding this recommendation. While reviewing cockpit conversations may provide crucial insights that may prevent future incidents and accidents, and such reviews have been used with very positive results in past accident investigations, doing so routinely may not be so beneficial. One big issue is logistical, and two immediately come to mind, how to choose what to review, and how to record the most useful information.

A bigger problem is technological. Currently, cockpit voice recorders in the US are required to at least the last 30 minutes of cockpit conversation. Some record up to two hours, and in 2012 all will be required to record for at least two hours. However, the average airliner may spend many more hours per day in the air. Even if recorders are reviewed at the end of every day, most recorders would likely have only the most recent flight, and only the last part of the flight. Unless the NTSB is also suggesting a radical change in voice recorder technology and procedures to include enhancing the capability to record all conversations all the time, it is hard to see how reviewing a fraction of routine cockpit recordings can be either useful.

The other two audio related aviation events involved two very different crashes of private aircraft. The first was the February 17, 2010 crash of a Cessna 310 into an East Palo Alto, CA neighborhood. Although all three on board were killed, and several houses severely damaged, no one on the ground was injured.

Fatal Crash in East Palo Alto
The city of East Palo Alto, where the crash occurred, uses an advanced audio detection system that allows police to quickly find the locations of gunshots. This ShotSpotter system recorded the sounds of the crash from two locations. According to the NTSB, these recordings will be used in their investigation, and is the first time that such a recording will be used for forensic purposes.

Recordings taken from two of the sensor locations are below. While they may be useful for the investigation, they also represent the sound of three people being killed and a neighborhood in peril. Both begin with the sound of a plane's engines, followed by the sounds of the aircraft hitting power lines, and then followed by crashing and banging sounds as parts of the aircraft hit homes and cars. In one of the recordings, you can clearly hear the screams and cries of terrified residents as the plane struck the neighborhood.

Sounds of plane crash alone

Sounds of plane crash plus screams of neighbors

Aerial Attack in Austin
The day after the crash in East Palo Alto, Joseph Stack, who apparently had serious tax issues with the IRS, allegedly flew his aircraft into a building housing several IRS offices, severely damaging the building and killing one person inside.

Less than 20 minutes before this crash, and less than an hour after allegedly setting his own house on fire, the pilot took off from a Georgetown Municipal airport, a small airport north of Austin, and had what sounded like a normal conversation with air traffic control.

Listen to ATC clearance for Joseph Stack (Source:

What Does It All Mean?
Audio happened to connect these three events, but the message here may not be about audio at all but about technology. As the two private plane crashes showed, technology is reaching into more and more areas of everyday life, and will likely play an increasingly important role in how the public experiences aviation safety events and variety and sophistication of data available to crash investigators.

In the case of the two crashes, the audio technology existed for different purposes and just happened to record part of the flight of an accident aircraft. It is no different than the way that security or traffic cameras near airports sometimes happen to catch a dramatic event like a plane crash. The decision of the NTSB may not represent a policy shift within the aviation community as much as it represents an evolution of how the aviation safety community views the ways that existing technologies can be used to enhance safety.


  1. As for airlines downloading and routinely monitoring CVRs, I, too, have a hard time understanding how this action will be helpful. I think I understand where FAA is coming from - the idea that monitoring cockpit conversations allow an airline to tell the flight crew, for example, to pay more attention to business or something similar. But, as you pointed out, the monitors are only going to catch, at best, the last two hours (more likely 30 minutes) of the plane's last flight before monitoring, which doesn't seem to do a whole lot toward improving cockpit safety.

  2. I believe it would be an added safety feature for the NTSB to suggest a radical change in voice recorder technonogy and procedures to enhance airline operators' flight operational quality assurance program. That will definately raise the bar on airline safety.