Adjusting Weight on Planes

Q: We recently caught a flight aboard a Boeing 767. The plane was fairly empty, and most of the two dozen passengers were seated near the front. Just before takeoff the flight attendant announced that “for balance purposes” seven people from the front of the plane would be required to move to the back. Now, maybe it’s just me, but it’s quite a confidence buster to learn that the safety of an aircraft the size of a 767 might hinge on the seating position of seven people.

You’re getting the wrong idea. The juggling was necessary to fine tune the aircraft’s center-of-gravity (CG) as mandated by the preflight weight-and-balance scheme. Had those passengers not been relocated, rest assured the plane would have taken off and landed just fine, albeit officially out of tolerance. A plane’s center of gravity will shift as it consumes fuel, so both a takeoff and landing CG are predetermined and checked against the limits.

The weight and placement of passengers, fuel, luggage and freight, are worked out prior to flight. Occasionally — at smaller regional carriers, for one — the pilots themselves are responsible for the number crunching, but generally the grunt work is taken care of by the folks backstage, who run the data through a computer. Usually the task is complete before leaving the gate, but sometimes, especially if last minute changes were made, it’ll be sent to the crew remotely by radio or data-link after pushing back.

Obviously the placement of seven people on a small commuter plane is more critical than on a widebody Boeing, but only an extreme out-of-balance condition will render any aircraft unflyable. Improper CG computation is more likely to increase fuel consumption than cause an accident. Off the top of my head I can’t recall any large airliner having crashed from weight or CG-related trouble. Initial speculation blamed the 2003 crash of a US Airways Express turboprop on a balance problem, but the investigation soon turned to mechanical failure.

Remember that even a full complement of passengers entails only a fraction of a widebody plane’s sum heft. With tanks filled and every seat taken, a 767 tips the scales at about 400,000 pounds, but the portion represented by passengers and their luggage weighs but a tenth of that.

This article is part of a collection that originally appeared on Salon.com. Patrick Smith, 38, is an erstwhile airline pilot, retired punk rocker and air travel columnist. His book, Ask the Pilot (Riverhead) was voted “Best Travel Book of 2004″ by Amazon.com. Patrick has traveled to more than 55 countries and always asks for a window seat. He lives near Boston.

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