Ask the pilot

The science of weighing airplanes. And, how many things can go wrong with a jet before it's not allowed to fly?


Patrick Smith
November 22, 2002 5:30PM (UTC)

A reader from India informs us that the Emperor Ashoka, the Air India 747 spoken of earlier, complete with its Moghul arches, was destroyed in 1978 when it plunged into the sea off Bombay. I've read about this accident many times -- caused when a very hung-over captain ignored a very malfunctioning instrument and flopped his jet into the ocean -- but never realized it was the same 747 from the picture I own.

A reader in Japan explains that many of the old Pan Am Clipper titles, the nautically oriented ones in particular, are taken from the names of ships and racing sloops. This is interesting, if not surprising, but I've yet to decipher the origins of "Neptune's Car" or "Young Brander." Anybody?

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And, um, an outraged reader from Michigan calls me "a bonehead" for suggesting the width of airliner seats is reasonable, and he demands the bankruptcy of each and every airline for wedging him into such a tiny space and expecting him to eat such crappy food. A similarly appalled e-mailer, commenting on my generally beholden observations, says, mockingly, "Oh yes, sure, I should feel so goddamn lucky to fly."

Actually, yes, you should.

The Pilot, apparently, is shilling again for the "corporate bullshit airlines," taken to task for daring to ask the public to, gulp, appreciate the chance to voyage across oceans and continents with all the circumstance of motoring your Subaru to Trader Joe's.

I hate to say it, but the only reason you're served an inflight meal at all, never mind one cooked to order, is that to a large extent the tradition happens to be a lingering ritual from earlier days. Am I suggesting the elimination of galley carts and plastic trays? No, and I fully concede that nobody deserves the indignities of prolonged physical discomfort or rudely administered customer service. But service concepts should be moved away from anachronistic pretensions of pampering and toward something more modern and effective. For now the airlines, which are not the most imaginative animals, have yet to settle on alternative means of distracting or entertaining passengers (at least those in coach).

On a plane in the 1930s or 1940s, of course, you'd have had a big fat reclining chair, a sleeping berth, five-course meals served by a tuxedoed steward, and maybe an onboard lounge where you could sit and read the New York Times. But in 1939 aboard Pan Am's Dixie Clipper, it cost $375 to fly each way between New York and France. This fact seems lost on the Great Unwashed who, in 2002, can lug aboard their backpacks and flip-flops and traverse the Atlantic in a quarter-billion-dollar jetliner for $249 in six hours.

If Mr. Michigan really wants to, he can revisit the glamorous indulgences of commercial aviation by buying himself a first- or business-class ticket, at a fraction of the cost of 50 years ago.

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And he'll get there four times as fast.

You previously wrote that the weight of an aircraft affects its performance and must be calculated prior to takeoff. How is this accomplished with any degree of accuracy?

Passengers are not required to divulge the quantitative specs of their waistlines, obviously, and instead the airlines use standard weights both for people and luggage. An example is 180 pounds per person (including carry-on) and 25 pounds per checked bag. Sometimes this is adjusted slightly higher during winter to account for heavier clothing (please don't ask me about trans-climate routes). Thus, passenger and luggage tallies are approximations. The numbers are derived empirically and are surprisingly accurate.

They are added to something called the BOW, or basic operating weight, of the aircraft, which includes the ship itself, replete with all furnishings, supplies and crew. Compounded with fuel and cargo, the result is the total gross "ramp" weight. Fuel used for taxiing, which can be several thousand pounds, is subtracted to reveal the takeoff weight.

Both weight and its distribution are very important. Every flight's center of gravity, which changes as fuel is consumed, is calculated (and kept track of), in part to determine the control surface settings for takeoff. Pilots are trained in the particulars of weight and balance, but in practice the grunt work is usually taken care of electronically, presented to the crew in dot-matrix splendor with the rest of the preflight paperwork.

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It might surprise you to hear that in the case of a Boeing 747, three hundred passengers and their suitcases, about 60,000 pounds en masse, would make up less than 10 percent of the airplane's total bulk.

So how much do these things weigh?

A 747's maximum certified takeoff weight is in excess of 800,000 pounds, and the new Airbus A380 will break the million mark. A fully packed 757 might be 255,000 pounds, while a 50-passenger regional turboprop will top out around 50,000.

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There are weight limits for the different operational regimes, including ones for sitting at the gate, taxiing, taking off and landing. But the constraining factor for a specific takeoff or landing, remember, is not necessarily the structural restriction of the plane. Runway length, temperature, wind, barometric pressure, etc., can all influence payload.

As most people know, smaller airports with smaller runways are generally served by smaller planes. But this is more a function of practicality than size, strictly speaking. While you'll never see a 747 at La Guardia, that's not to imply one couldn't fly there. Rather, its payload would be so restricted by, to put it one way, the proximity of Flushing Bay as to render it economically unfeasible.

Because fuel loads are such a large percentage of overall weight, pilots rarely think of fuel in terms of gallons but almost always as pounds. (Some quick metrics, just so you know: It's about 6.7 pounds to the gallon. One kilo is 2.2 pounds and a gallon equates to 3.78 liters.) Everything from initial fueling to enroute burn is measured by weight, not volume. A fuel load of, say, 200,000 pounds may be a third or more of a wide-body airplane's sum heft.

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Before boarding, we were told our flight was weight restricted because of a malfunctioning system. Is it the crew's decision to take off when something important is not working?

Airplanes can depart with various inoperative components -- usually noncritical equipment carried in duplicate or triplicate -- depending on guidelines laid out in something called the MEL (minimum equipment list) or CDL (configuration deviation list). Any component listed in either of those books is "deferrable," as we call it, so long as the outlined conditions are met. These conditions can be quite restrictive and complicated, depending on what's broken. Many things, of course, are not deferrable at all, and any malfunctioning item must be repaired within a set number of days or flight hours. Before any flight is dispatched with a deferral, it must be documented and coordinated between the crew and maintenance personnel.

Above and beyond the deferral process, no respectable airline will pressure a crew to operate any flight. The final call, if you will, is the captain's, regardless of what the MEL or CDL allows.

What are pilots looking for when they walk around the plane prior to departure? Watching this procedure from the terminal, it doesn't seem a very in-depth inspection.

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Money, usually. Loose change, jewelry, that kind of thing. Or reading graffiti that the workers sometimes leave on the grimier panels ("Vote No" is a common one around contract time).

Actually, the walk-around inspection is a supplemental, for-the-record sort of thing done in addition to the more serious checks performed at various intervals by the maintenance staff. It's essentially a superficial perusal and not a whole lot different from checking your oil, tires and wipers before a road trip.

Much of the more technical preflight routine takes place out of view in the cockpit, where systems are put through tests before departure. Mechanics and pilots each have their own procedures to run through, before and after every flight.

Do you have questions for Salon's aviation expert? Send them to AskThePilot and look for answers in a future column.

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Patrick Smith

Patrick Smith is an airline pilot.

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