The US National Transportation Safety Board (NTSB) has determined that "numerous individual and institutional ATC factors" are the most likely causes of the fatal head-on collision between an Embraer Legacy 600 business jet and a Gol Transportes Aéreos 737-800SPF passenger jet at 37,000ft over Brazil's Amazon region on 29 September 2006.

All 154 passengers and crew on board Gol Flight 1907 were killed after the damaged aircraft plunged to the forest floor. The Legacy, flown by US-based air taxi operator Excelaire, was controllable and landed at a nearby military airfield with no injuries to the crew and passengers. The aircraft was being delivered from Embraer's plant in southern Brazil to the US at the time of the crash.

NTSB and representatives from several US companies, including Honeywell, ACSS and Boeing, participated in the investigation which was headed by the Brazilian Centro de Investigação e Prevenção de Acidentes Aeronáuticos (Cenipa).

"The evidence collected during this investigation strongly supports the conclusion that this accident was caused by N600XL and GLO1907 following ATC clearances which directed them to operate in opposite directions on the same airway at the same altitude resulting in a midair collision," the NTSB states in comments appended to the Cenipa final report.

"The loss of effective air traffic control (ATC) was not the result of a single error, but of a combination of numerous individual and institutional ATC factors which reflected systemic shortcomings in emphasis on positive air traffic control concepts."

NTSB also addresses the transponder issue on the Legacy Jet. "Contributing to the accident was the undetected loss of functionality of the airborne collision avoidance system technology as a result of the inadvertent inactivation of the transponder on board N600XL," the board explains. "Further contributing to the accident was inadequate communication between ATC and the N600XL flight crew."

In May 2007 NTSB asked FAA to require that collision avoidance systems include an audio warning and enhanced visual alert to inform pilots of the "loss of collision avoidance system functionality, for any reason". As currently built and certified, the units display a message only.

Following in the release of that recommendation FAA in September 2007 issued a warning to US operators of Embraer ERJ commuter jets and Legacy business jets that they could accidentally deactivate the type's transponder by placing their feet on the footrests located directly below the instrument panel, an action that would render the aircraft invisible to the collision avoidance system of an approaching aircraft.

The NTSB in its comments says that though the footrest guard is "designed to prevent unintended interactions" with the control interface for the collision avoidance system, during "in flight deck observations conducted by the NTSB and FAA, it was observed that pilots might misuse the footrests since, in certain forward seat positions, there appeared to be a very comfortable resting position that involved resting the feet on top of the footrest guards rather than inside the designated footrest areas." The report continues, "This position allowed the resting pilot to remain within easy reach of the primary flight controls but, unfortunately, located the captain's right foot in the area of the [control interface] so it could make unintended contact [with the transponder] without the captain's awareness.

"It can not be determined exactly how the accident crew commanded the transponder to standby, but the possibility of an unexpected use of the footrest guard, along with other possibilities discussed in the report, serve as important reminders of human ergonomic considerations at preventing inadvertent contacts in actual flying situations," says NTSB.

While the US team "has no substantial disagreement" with the facts gathered by Cenipa and "generally concurs" that the safety issues involved in this accident are related to ATC, operational factors, and the loss of in-flight collision avoidance technology, the NTSB report states that "interpretations, conclusions, and understandings of the relationship between certain factual items and the demonstrated risk differ in a number of respects." The Cenipa report was initially published today.

"This investigation has identified many safety issues for ATC operations, but these issues need to be further highlighted. Even though the body of the [Cenipa] report acknowledges safety deficiencies with ATC, these deficiencies are not sufficiently supported with analysis or reflected in the conclusions or cause of the accident," the NTSB states.

"These deficiencies include a lack of timely ATC action after the loss of N600XL's transponder and two-way radio communication, and features of the ATC software that may have aggravated deficiencies in altitude clearance awareness for N600XL," according to the board.

Source: Air Transport Intelligence news