United Airlines' Tech Meltdown -- It Could Happen Again

June 21, 2007 — -- What happened to United Airlines on Wednesday has happened before to other large carriers, and it will happen again — the unexpected shutdown of a computer system that has become so integral to the airline's operations that planes simply can't fly without it.

In Wednesday's case, it was United's turn to experience a sudden inability to compute the weight and balance figures (and other performance data) for each outbound flight, and that inability struck at the heart of the airline's schedule.

Each airline designs its own system for tracking and figuring the amount of weight on an outbound flight. Decades back, all airlines calculated such items with pencil and paper, but today computers do the work. For most big airlines, the computer systems are centralized — United's is in the Chicago area; American's is near Tulsa, Okla., etc.

But the problem is that once the Federal Aviation Administration approves the way that computer system is used by an airline for such critical functions as weight and balance, the system has to work to run the airline. If it stops working, no flights can fly until it either comes back on line or the airline goes to some approved backup system for manual calculation. In the meantime, as little as a half-hour delay can throw the system into chaos.

Like Clockwork … Most of the Time

Today's big airlines and airports both operate like an intricate Swiss watch: As long as everything functions reasonably close to the way it's been designed, the whole mechanism can self-correct for minor deviations. But throw sand in the delicate gears — or a multihour delay in launching hundreds of flights — and the mechanism begins to break down rapidly.

Perhaps the most basic problem is just gate space. In a major so-called hub airport, such as Chicago's O'Hare, arriving flights are scheduled into gates mere minutes after departing flights are scheduled to push back. Obviously, when the outbound aircraft doesn't leave the gate, an inbound flight is orphaned.

When the airport is too congested to have spare gates sitting open to receive that newly arrived flight, planes have nowhere to go and must wait on the tarmac. Even the pilots are essentially powerless to solve the problem, because we're a long way from the days when airliners had their own built-in stairs and passengers could be deplaned without a gate or outside assistance.

When hundreds of flights are still at their gates up to an hour after scheduled departure, the number of inbound orphaned flights starts piling up to serious numbers at places like O'Hare. This, by the way, is why airlines prefer to push outbound jetliners off their gates on schedule even if they know, as United did Wednesday, that there's a major problem in progress that's going to delay takeoff.

In addition, with flights from distant outlying cities delayed in heading to a major hub such as Chicago, Denver, Atlanta or Cincinnati, the entire connection sequence starts to melt down. As United grappled with the problem of bringing its weight and balance functions back online Wednesday, every minute that passed with planes still at the gate compounded the problem. If the shutdown had lasted for much longer than two hours, the results across United's system could have resembled the average East Coast winter shutdown and taken days to repair.

Weighing the Center of Gravity

Weight and balance, by the way, is a catchphrase for aviation, but it's a vital concern. Aircraft fly only because the engines have the ability to produce enough power to lift the weight of the aircraft off the ground. There are, however, limits to how much weight a plane can carry without starting to damage the structural integrity, called maximum gross weight. In addition, if too much weight is too far forward or aft in an airplane, the flight controls may no longer be able to keep it under control in the air.

Calculating the precise weight and what's called the CG, or center of gravity, is what the weight and balance function of the airlines' computers do with great precision. The computers also take into account the amount and weight of the fuel carried, and they carry out a set number of internal checks to make sure every single flight is loaded safely and ready for flight.

Can that be done by hand? Yes, but only with a backup plan the FAA has previously approved, and even then the delays are going to be serious because the manual process takes more time.

High-speed, complex computer systems are a fact of life in aviation, and the normal benefits are great. But as with any mechanism we become dependent on, when it isn't there and our backup systems aren't sufficiently practiced, approved and instantly useable, the whole operation grinds to a halt.