Ask the pilot

The year's most notable air travel stories (who can forget the overblown liquid-bomb terror scare). Plus: A second look at the midair collision in Brazil.

By Patrick Smith

Read more: Technology & Business, Business, P. Smith, Ask the Pilot

Dec. 22, 2006 | It's that time of year again: carolers a-caroling, chestnuts roasting, long security lines, the savory aroma of Chick-fil-A wafting through the concourse ... Doubtless you're hoping for an appropriately themed column on this weekend before Christmas -- something about the difficulties of winter flying, perhaps, or maybe a story on the Official Airline of Santa Claus.

But before moving on to the fun stuff, we need to revisit last week's column and its discussion of September's midair collision in Brazil. The article brought in a flurry of responses, a number of which, in expressing their displeasure with my analysis, seemed not to understand the basic circumstances of the accident, leading me to believe my explanations were less than clear.

If you're totally new to the topic, I'd recommend starting here. For those of you already familiar, here's a new and hopefully cogent rundown of what went wrong:

On Sept. 29, a Gol Transportes Aéreos 737 collided head-on with a U.S.-registered executive jet, an Embraer Legacy, at 37,000 feet over the Amazon jungle. The Legacy landed safely; the 737 did not. Brazilian authorities say the Legacy wasn't supposed to be at 37,000 feet. The evidence says otherwise.

In the minutes before the accident, air traffic control had lost radio (voice) communication and radar contact with the Legacy crew. Actually, radar contact had become intermittent, though controllers could no longer see the airplane's speed or height on radar, or talk to its crew. Last they knew, the Legacy was cruising at 37,000 feet, 460 nautical miles northwest of Brasília.

Temporary, short-term loss of radio and/or radar contact between pilots and air traffic control is not unusual. What precipitated disaster was not an inability to communicate, per se, but ATC's apparent assumptions about what the plane would do next, coupled with a failure to adequately monitor two converging targets.

In the event of communications loss, Federal Aviation Administration (in the U.S.) and International Civil Aviation Organization (globally) regulations mandate that pilots fly at the higher of the following two altitudes (there's a third option as well, but it does not apply in this context):

1. The altitude assigned in the last clearance received from ATC
2. The altitude ATC has advised to expect in a later clearance (clearances are sometimes issued in stages)

In both cases the correct altitude was 37,000 feet. Exactly where the Legacy was.

Prior to departing from San Jose dos Campos, a large city near São Paulo, transcripts show, the Legacy received an ATC clearance all the way to its destination (Manaus, in northern Brazil) at 37,000 feet. "Clear, 370, Manaus" were the instructions. Any pilots acknowledging this clearance would be justified in maintaining 37,000 feet all the way until their arrival, unless and until instructed otherwise.

For good measure, transcripts also record the pilots' checking in with en route controllers at 37,000 feet, and receiving no requests to climb or descend.

Brazilian authorities have repeatedly asserted that the Legacy was flying at the wrong altitude, claiming that under lost communications protocols, the pilots should have changed altitude in accordance with the values stipulated in their flight plan. At the location of impact, the flight-planned altitude was 38,000 feet. Did controllers assume the pilots had gone ahead and climbed to that height?

As I stated last time, flight plan altitudes are just that, for planning purposes. They are, for all intents and purposes, a request. The altitude at which you'll actually fly is dictated by one or more subsequent clearances from ATC. I had written that flight plan altitudes "may have meaning in the event of radio communications failure." In fact they don't; the proper reading of rule No. 2, above, has nothing to do with flight plans. It has to do with assigned clearances and direct instructions from ATC. And in this case, all clearances and instructions, from the very beginning, stipulated 37,000 feet.

One possibly important question is whether Brazilian regulations, unbeknownst to the pilots, are different from standard FAA/ICAO regulations when it comes to altitude selection after radio failure. (Researching this has proved exceptionally difficult, but there's no reason or evidence to suggest they aren't the same.) Still, at the very least, even if they had anticipated the Legacy climbing to 38,000 feet, controllers ought to have broadcast this directive into the blind. Often during radio difficulty, one of the two parties is still able to hear instructions, if not acknowledge them, perfectly well. Controllers also should have warned the Gol jet that they weren't absolutely certain what the Legacy was up to. According to the transcripts, a number of broadcasts were made into the blind, but none of them made reference to any altitude.

It sounds to me that controllers suffered a breakdown of what pilots call "situational awareness." That's a fancy way of saying they lost track of what was happening.

Last, if not least, is the transponder/TCAS issue, given a good vetting here last Friday. TCAS (or traffic alert and collision avoidance system) warnings would have, should have, warned both crews as they approached each other, independent of anything going on at air traffic control. Why this didn't happen is a perplexing mystery -- though, judging from everything else we know, it is not a crucial one, and it avoids the key questions: What was ATC thinking and doing? Why did controllers not make efforts to determine the Legacy's altitude?

Whatever the answers, as so often is the case in air crashes, we appear to have a combination of highly unlikely circumstances -- innocuous on their own, but deadly in combination -- compounding into tragedy: lost communications, erroneous expectations and malfunctioning equipment.

If you're hungry for more information, I'd recommend the January issue of Aviation Safety magazine, whose staff has put together an exhaustive dissection of the incident.

Everybody square with that? Good, so let's move on to things less technical and more seasonal ...

This being Ask the Pilot's final installment for 2006, we'll start with a brief look back at the most notable air travel stories of the past 12 months. What were they?

Next page: Ask the Pilot's first annual Hook, Line and Sinker Award goes to ...