Wednesday, September 15, 2010

Mid-Air Over the Hudson - Probable Cause

The National Transportation Safety Board (NTSB) met a couple of days ago in Washington to approve the final report on the "Hudson River Mid-Air." Just over a year ago (9 Aug 2009) a transient Piper Lance and a sight-seeing helicopter collided over the river. The mishap was fatal for all nine occupants of the two aircraft.

The Board has made a synopsis of the final report available on its web site. (The report itself is in final editing and won't be posted for a couple of weeks.) The synopsis contains recommendations, conclusions and a statement of Probable Cause:

"The National Transportation Safety Board determines that the probable cause of this accident was (1) the inherent limitations of the see-and-avoid concept, which made it difficult for the airplane pilot to see the helicopter until the final seconds before the collision, and (2) the Teterboro Airport local controller’s nonpertinent telephone conversation, which distracted him from his air traffic control duties, including correcting the airplane pilot’s read back of the Newark Liberty International Airport (EWR) tower frequency and the timely transfer of communications for the accident airplane to the EWR tower. Contributing to this accident were (1) both pilots’ ineffective use of available information from their aircraft’s electronic traffic advisory system to maintain awareness of nearby aircraft, (2) inadequate Federal Aviation Administration (FAA) procedures for transfer of communication among air traffic control facilities near the Hudson River class B exclusion area; and (3) FAA regulations that did not provide adequate vertical separation for aircraft operating in the Hudson River class B exclusion area." (emphasis added)

Several of the Board's conclusions, as contained in the synopsis, relate to the inadequacy of "see-and-avoid" as a strategy for avoiding mishaps like this one (in each case, emphasis added):

  • "The airplane pilot may have believed that no other potential traffic conflicts existed because he had not received additional traffic advisories, but the pilot was still responsible for seeing and avoiding other traffic."
  • "The helicopter would not have been obscured from the airplane pilot’s view but would likely have been difficult for him to detect until the final seconds before the collision because, before that time, the helicopter would have appeared as a relatively small and stationary object against a complex background of buildings."
  • "The airplane would likely have been in the helicopter pilot’s field of view until 32 seconds before the collision, after which time the airplane was above and behind the helicopter and was outside the pilot’s field of view."
  • "The guidance in Advisory Circular (AC) 90-48C, “Pilots’ Role in Collision Avoidance,” could better assist pilots’ efforts to establish effective see-and-avoid skills if the AC were to recognize current challenges that pilots encounter in managing their see-and-avoid responsibilities, including complex, high-density airspace and the increasing presence of technology in the cockpit."
So, in sum, the airplane pilot was responsible for seeing and avoiding conflicting traffic; it was difficult (maybe impossible) for him to detect the conflicting aircraft before it was too late to avoid a collision; the helicopter pilot was completely unable to see the fixed-wing aircraft; and the FAA's existing guidance on "see-and-avoid" (AC 90-48C), is obsolete (having been issued on 18 March 1983) and not comprehensive.

None of this comes as news. I've written about this event and its aftermath in six previous posts ( 1 - 2 - 3 - 4 - 5 - 6 ). In the first of these posts, written about a week after the mishap, I linked to a 1991 report published by the Australian Transport Safety Bureau (ATSB) titled Limitations of the See and Avoid Principle. This very well written report effectively demolishes the notion that unalerted "see-and-avoid" can be an effective strategy for collision avoidance.

In that post a year ago, I wondered if "...a technological aid like the TIS system shown above, could be required for commercial operators and strongly urged for non-commercial users of the Hudson airspace." I claim no prescience, but in its recommendations, the NTSB has netted out to the same place. Here (among other recommendations) is what they are proposing (again, emphasis added):

  • "Update Advisory Circular 90-48C to reflect current-day operations, including (1) a description of the current National Airspace System and airspace classifications, (2) references to air tour operational areas as high-volume traffic environments, and (3) guidance on the use of electronic traffic advisory systems for pilots operating under the see-and-avoid concept."
  • "Develop standards for helicopter cockpit electronic traffic advisory systems that (1) address, among other flight characteristics, the capability of helicopters to hover and to fly near other aircraft at lower altitudes, slower airspeeds, and different attitudes than fixed-wing airplanes; (2) reduce nuisance alerts when nearby aircraft enter the systems’ alerting envelope; and (3) consider the different types of operations conducted by helicopters, including those in congested airspace."
  • "Once standards for helicopter electronic traffic advisory systems are developed, as requested in Safety Recommendation [4], require electronic news gathering operators, air tour operators, and other operators of helicopters used for passenger revenue flight to install this equipment on their aircraft."

"See-and-avoid" has been a cherished part of the freedom to fly as it's practiced in the US. And it can continue to be so, but the time has come to acknowledge that for high traffic areas "see-and-avoid" must be augmented with effective alerting technology.

In her opening statement for the meeting, NTSB Chairman Deborah Hersman made these observations (emphasis added):

"What I find so striking about this accident is that the airplane pilot was simply following his directions. And although the helicopter’s conspicuity was improved by anticollison lights and a high-visibility paint scheme on the blades – these enhancements didn’t help. Both aircraft were equipped with radios and traffic information systems, yet they didn’t hear or see each other, and the technology did not prevent the accident, and the air traffic system neither separated them nor alerted them that they were about to collide.


I might add that the ability to avoid future accidents like this one is dependent on a fully implemented ADS-B (automatic dependence surveillance-broadcast) program. The FAA has taken the initial steps to move towards an ADS-B-based ATC environment, but that is an expensive proposition. If we are serious about changing the paradigm and moving forward, then it’s critical that ADS-B, both in and out, be a part of this next phase of safety for collision avoidance."

The last part of this statement is quite controversial, as ADS-B (especially the "in" part) has no shortage of detractors. But some enhancing technology is needed and ADS-B is on the program.

There's a lot more that's of interest in the Board's synopsis (including some fairly severe criticism of ATC's involvement in the mishap). I urge you to read it, along with the final report when it issues.

No comments: