The crash of GOL 1907

Sometime round about 16:00 local time on Friday 29 September, an Embraer Legacy jet on a delivery flight on airway UZ 6 in the Mato Grosso of Brasil collided with an object which took away most of a winglet and damaged the empennage. At around the same time, GOL 1907, a B737-800 (or B738 for short), also on UZ6, disappeared from radar. Parts were later found in the jungle; some pictures of the forward fuselage, taken by the military which is conducting the investigation, may be found at http://www.fab.mil.br/imprensa/Noticias/2006/10_out/0210_fotos_resgate.htm

The Legacy landed, damaged, at a local military airfield.

The purpose of this note is to say what is known, and to comment on what is being said, including rumors.

The damage to the Embraer Legacy may be seen at http://upload.wikimedia.org/wikipedia/en/2/25/Legacy2_fab.jpg

The incident (or incidents) is said to have taken place near TAROP intersection on UZ6. A scanned version of the chart showing UZ6 and TAROP may be found at http://www.atcmuseum.org/images/H4.pdf

A columnist for the New York Times, Joe Sharkey, whose column I read regularly in the International Herald Tribune, was aboard the Legacy. His story may be read at
http://www.iht.com/articles/2006/10/03/america/web.1003road.php

Some potential sources of information are the Associated Press (AP) reports, which I view at http://www.iht.com searching for “GOL 1907”, the Brazilian daily O Globo for those who read Portuguese, and the Professional Pilots Rumor Network thread starting at http://www.pprune.org/forums/showthread.php?t=246031
All of these sources must be read with some degree of caution, of course. Many interests are at play and not all contributors, correspondents or reporters are experts.

The Flight Data Recorder (FDR) and Cockpit Voice Recorder (CVR) of GOL 1907 are reported to have been recovered, but it is not clear how much of the CVR data will be recovered and usable, because part of the device which contains some memory was not recovered with the recorder itself.

The latest reports say that GOL 1907 was cleared south-east on UZ6 at 37,000 ft. This presumably means Flight Level 370, or FL 370, which is the pressure altitude of 37,000 ft (all flights at these levels fly at Flight Levels, which correspond to pressure altitudes, to make sure that everyone adheres to exactly the same altitude-measuring convention). Sharkey reported visiting the cockpit of the Legacy “minutes before” the loud bang which one presumes is a collision, and seeing the altimeter level at 37,000 ft. The crew reported that they were level at FL 370 to the military debriefers at the airbase.

I note that it has not yet been confirmed that the two aircraft did collide. I presume this will be established through reading the FDRs of both aircraft, which will show traces of the incident, and synchronising the clocks.

According to AP, Brazilian authorities are now saying that the Legacy, which had been flying at FL 370, had been cleared to descend to FL 360. Apparently the Federal Prosecutor’s Office is already investigating possible criminal offences by the Legacy crew.

One major question is why the Legacy, which is equipped with Honeywell’s TCAS 2000, apparently did not record any TCAS alerts. TCAS should have issued a Traffic Advisory about 45 seconds before point of closest approach, and a Resolution Advisory, which is advice to climb or descend, some 30 seconds before point of closest approach. The pilots are said to have reported no TCAS alerts. We must wait for the GOL FDR to be evaluated to find out what, if anything, TCAS said to the B738, which was practically a brand-new aircraft, having been delivered this year.

One rumor which was reported to have surfaced early in O Globo, before Sharkey’s article, is that the Legacy pilots wanted to climb to a higher altitude and turned the transponder off so that ATC would not perceive the violation of their clearance. Since TCAS operates through handshaking over the transponders, this would explain the lack of TCAS alerts. However, why professional pilots would do such a thing is not clear. At first, it was suggested that they might have wanted to fly higher to save fuel. However, they needed at least one refueling stop between where they were and their destination, and likely not more than one, so this does not explain why they might want to risk their lives and those of their passengers by doing such a thing. The rumor has now resurfaced that the crew may have turned the transponder off to disguise the fact that they were still at FL 370 although they had been cleared to FL 360. Why a crew would indulge in so risky an activity to save themselves 1,000 ft altitude is even less clear than the original. I can think of no reason, and I can think of lots of reasons against, namely the possibility of colliding with other traffic.

One other major questiion is whether the Legacy acknowledged the clearance to descend from FL 370 to FL 360. If not, ATC should be presuming that the aircraft continues at its last cleared altitude, on its planned route, and if they have someone else on that airway at that altitude, then they should see the conflict and resolve it, whether or not there is a communications failure with the Legacy. ATC procedures are specifically designed to cope with communicatiions failures. If the Legacy did acknowledge the clearance to descend and maintain FL 360, then the question arises why they would not do so, and I can think of no reason.

Although both aircraft were handled by one en-route center, it has been reported that they were talking to two different controllers at two physically separate locations: GOL 1907 to Manaus ATC and the Legacy to Brasilia ATC.

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