Ask the Pilot

Tough questions after a deadly catastrophe in Brazil. When is a runway "too short"? And, are some of our busiest airports unsafe?

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

July 27, 2007 | This week, no self-indulgent travelogues or embarrassing confessionals. There's nothing like an air disaster to give this column a good kick in the ass.

"Brazil's worst-ever crash." Seems like only yesterday I was typing those words. It was last September, to be precise, after a Gol airlines 737 collided with a U.S.-registered business jet over the Amazon jungle, killing 155 people. The accident brought heavy scrutiny upon Brazil's civil aviation infrastructure. Air traffic in the country has grown tremendously over the past 10 years, putting intense pressure on its air traffic control network, airports and terminals. The last thing Brazil needed was another, even deadlier crash.

Unfortunately, on July 17, an Airbus A320 operated by TAM Linhas Aereas crashed on landing at São Paulo's Congonhas Airport. The twin-engine jet skidded off the rain-slicked runway, slammed into a row of buildings (including a Shell gas station and one of TAM's cargo offices) and exploded. All 187 passengers and crewmembers perished, as did at least four people on the ground. As of this writing the exact death toll is unconfirmed and may climb. Already the accident is not only Brazil's deadliest ever, but the deadliest ever in all of South America.

Hemmed in by dense neighborhoods, Congonhas is the downtown airport for São Paulo, used primarily for domestic flights. Although physically much smaller than nearby Guarulhos International (GRU), Congonhas is the busiest airport on the continent, handling 18 million passengers annually, and is somewhat notorious for the shortness of its main runway. In the wake of the disaster, much has been made of the strip's relatively meager 6,365-foot length. Almost every news report has referenced this as a probable factor in the tragedy, or even its direct cause. Prior to last Tuesday, numerous crews had complained to authorities about unusually slippery conditions when landing in rainy weather. Only a day earlier, an ATR-42 turboprop belonging to a local regional carrier had slid off the same runway and into the muddy grass alongside. Earlier this year, in a ruling that was overthrown on appeal, residents and politicians had successfully lobbied for the banning of certain-sized jetliners from Congonhas altogether. They may yet get their wish.

Although it is highly likely that the stubby Congonhas runway played a role in the tragedy, a ban may or may not be a useful idea. The relationship between planes and runways is a tad more nuanced than the simplistic accounts you've been reading in the papers and watching on CNN.

"Why would pilots be dumb enough to land on a runway that's too small?" asked an e-mailer who'd been following the story on television. "Are there not specific measurements and rules they have to follow?" This scenario evokes the image of a hapless crew, looking down at an airport from several thousand feet and sizing up the available pavement. "That looks about right," says the captain, and down comes the landing gear. Moments later his judgment is proved wrong as the jet goes barreling through a fence and headlong into a gas station.

That's not how it happens. We hear a lot about pilots needing to possess expert judgment and seat-of-the-pants skill. While maybe that's true, there is almost nothing subjective about choosing where to land on or take off from.

Let's take landings first. Per regulations, no crew is allowed to land on a runway that does not guarantee stopping distance under existing conditions. The plane will not be dispatched to an airport in the first place unless it has room enough to touch down and come to a complete stop within the first 60 percent of the intended runway's total length. The exact stipulations vary country to country, rulebook to rulebook, but that's a fair gauge.

For takeoff, two things need to be assured before any commercial jet begins to roll. First, the plane must be able to climb away safely following an engine failure at the worst possible moment -- that is, at the "takeoff decision speed," or "V-1." Second and no less important, the jet must be able to come to a safe and full stop if the takeoff is aborted at any time prior to that speed. (Think of V-1 as a fulcrum. If there's a problem after V-1, the crew knows the plane will fly and clear obstructions. If there's trouble before V-1, the crew knows it has room to stop. This is called a "balanced field.")

So, either you have the numbers or you don't. But what makes it difficult to dismiss any given runway as "too small" is that the numbers will vary flight to flight, dependent on a host of variables: weight, flap/slat settings, wind velocity, elevation, temperature, barometric pressure and surface conditions. Weight alone can vary by hundreds of thousands of pounds, depending on the fuel, cargo and passenger load. Is the pavement slippery? Winds less than favorable? Autobrakes inoperative or an anti-skid system on the fritz? All of that is factored in. There are charts for these things, and information is sent to the crew via radio or cockpit data link.

Thus there is no consistent minimum length that a plane requires. It depends. Granted, a Boeing 747 at any weight will be too heavy for a lot of airports, but there are plenty of occasions when a large plane at a low gross weight requires less pavement than a smaller plane at a heavy weight. (The Airbus A320, with typical seating for about 150 people, is relatively small.) The accident runway at Congonhas is only slightly shorter than the main runway at Washington-National, on which planes as large as the 757 land and depart daily by the dozen. At New York's LaGuardia, the strips top out slightly longer, at just 7,000 feet; widebodies like the 767, DC-10, and L-1011 have used LGA routinely over the years.

(Consider for a moment the much-hyped Airbus A380, the largest commercial airliner ever conceived, still in trial runs but expected to enter service next year. We're constantly reminded that the plane will be "too big" for many airports. What they are talking about, mostly, is the room required for maneuvering on taxiways and aprons, not runway length. Thanks to its powerful engines and high-lift wing, the A380 will require no more runway than the typical 747, A340, or other widebody.)

It should go without saying, however, that if given the option, a pilot will always prefer a longer runway to a shorter one. While the letter of the law is one thing, the problem with short runways is that they leave less margin for error. And with that, let's go back to São Paulo.

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