You mentioned the fuel efficiency of airplanes vs. cars. How much fuel does it take to fly me across the country?
Traveling between New York and San Francisco, a medium-sized jetliner like a Boeing 767, which is a common model for this route, will consume roughly 7,000 gallons of jet fuel (not even a third of what its tanks can typically carry, by the way).
That's equivalent to a little less than half a mile per gallon. But on a full flight with 200 passengers, it's 32 gallons per person, or nearly 80 miles per gallon, per person, which sounds much more impressive. If you're the type who likes calculators, further crunching reveals 0.014 gallons for each, as they're termed in the trade, seat mile.
To get a sense of industry-wide economy, you'd have to cipher averages of per-flight occupancy (in 2001, flights were operating at about 70 percent capacity), per-hour fuel burn, and flight distance. And in deference to critics, if jetting across the continent weren't such a practical endeavor, only a fraction of today's passengers would actually be doing it. Still, overall efficiency is far and away better than a 16 mpg SUV carrying one or two people.
As for emissions, like I said, commercial aviation accounts only for about 5 percent of worldwide fossil fuel use. Not fully understood, however, are the impacts of chemically laden contrails, and whether jet exhaust, injected directly into the upper troposphere, is uniquely harmful to the atmosphere.
How is airplane fuel different from other kinds of fuel?
Jets and turboprops run on jet fuel, which is kerosene. It is vaporized and fed into the combustion chambers of jet and turboprop engines. Televised fireballs notwithstanding, it's surprisingly stable and less combustible than other fuels, at least until vaporization. You can hold a lit match above a pool of jet fuel and it will not ignite. (Neither Patrick Smith nor Salon Media Group shall be responsible for injuries or damage caused in connection with this statement.)
Piston-powered aircraft -- like privately-owned Pipers and Cessnas -- burn a high-octane fuel commonly called "avgas," analogous to gasoline. Some small planes can be certified to run on ordinary automobile gas.
I've been on flights where we've had to circle for an hour before landing. How much fuel is on board for these situations? Do airlines cheat to save money?
Fuel loads are ascertained beforehand, according to regulation and subject to weather, traffic and other variables. The regulations are intricate, differing between domestic and overseas operations, but the old domestic rule is a good indicator of how conservatively things work: there must always be enough fuel to carry a plane to its intended destination, then to its designated alternate airport(s), and then for at least another 45 minutes. Sometimes, two or more alternate airports must be filed in a flight plan (another batch of rules), upping the total accordingly. When delays or holding patterns are expected, even more is added. The fuel portion of the preflight paperwork can be lengthy.
Although the dispatchers and planners devise the figures, pilots have the final say and can request extra. Carrying surplus fuel costs money, but it's not nearly as expensive as the hassles of diversions. Or accidents. A scan of my memory banks comes up with exactly two cases of fuel depletion crashes in the modern aviation era -- a United DC-8 near Portland, Ore., in 1978, and an Avianca 707 near Kennedy airport in 1990.
I know that planes sometimes jettison fuel. Is this to prevent an explosion during a crash?
With larger planes, the maximum allowable weight for takeoff is greater than the one for landing. This makes sense, if you think about it, as touching down puts higher stresses on the airframe than taking off. During flight, fuel is burned away -- to the tune of hundreds of thousands of pounds on longer routes -- reducing weight to within landing limits. (All of this is planned out before departure, as maximum weights for a given flight are contingent on many things, not simply the book restriction of the plane itself.)
Now, let's say something happens soon after takeoff and a plane must return to the airport. In many cases it's too heavy. Rather than tossing passengers or cargo overboard, it will jettison fuel through plumbing in its wings. I once had to dispose of more than 100,000 pounds this way over northern Maine, a procedure that took many minutes and afforded me a lavish overnight at the Bangor airport Hilton.
Unless there's a serious emergency, "dumping" takes place at high enough altitudes where the kerosene dissipates long before reaching the ground, and no, the engine exhaust will not ignite the stream.
In certain situations a crew may elect to land even when exceeding max landing weight. There is the risk of damage to the plane, of course, but the risk of delay could be greater.
- - - - - - - - - - - -
Two short poems about airplanes
"Tarmac"
At the far end of Heathrow Airport is the row of hangars
where they load up the cargo planes. You walk across the asphalt and hide
from the rain under the wing of a Virgin Atlantic 747, orange and white,
and below the windscreen is a hideous emblem of a woman with a Union Jack
trailing from her hand. Through the mist are the lights of Terminal Four,
where the Concorde docks and where you can walk from airplane to hotel room,
or onto the subway, without even stepping outside. You remember
a thirty-dollar breakfast and the rows and rows of pilgrims headed to Mecca,
in the spring of 1992. Now a British Airways jet turns onto the runway,
Your adrenaline surges as its engines rev, and then it's gone in a blast of heat
and a vortex of fog around the wingtips. Up on the fin is a small crest,
and if you bother to read the small print it says "To Fly, To Serve" and you think,
hey that's a noble enough ambition.
Seven hours away the twin blue towers of the Whitestone Bridge, camouflaged
against the noon sky, poke their heads above a checkerboard runway barrier.
Sitting at the gate, it strikes you, like New York fist in the nose, that it was seven
years ago this very afternoon, when you made your first trip to Manhattan,
21st Street, and how you were at least as excited
as Neil Armstrong must have been, skipping across the surface of the moon.
It was a warm day -- men and women in shorts in Gramercy Park, piles
of old snow still in the gutter. What do they mean anymore, these fragments
of a city? The cab driver who knew the capital of every country in the world.
The taste of lemon on your girlfriend's breath. None of it much like
the romantic gray Gotham of the movies, and today the other fifteen people
on the two o'clock shuttle don't seem too concerned with history, yours or theirs.
The order of the day, in this so-different life, is only to get home. Outside, seagulls
are circling like buzzards, gleaming yellow taxis slam shut their doors.
- - - - - - - - - - - -
"Muir Woods, Thinking Of Earthquakes"
The approach to the airport is from the west, snowcaps buttered
yellow at sunrise, and then the land flattens out.
Next come the lights of the lower East Bay, and the long skinny span
of the San Mateo. The skyline like a pincushion between the hills.
But I'm looking northward, below the fog as if peeking
beneath the corner of a worn carpet. Follow the coast.
Any of a hundred of those quiet seaside towns,
could have been the death of me. Funny, the blurred recollections
after five quick years and a hundred unanswered letters. Ready I was,
to fell an acre of redwoods with the side of my bare hand,
if that's what it took. To stand with crazy Californians
scrambling in disaster drills, our houses toppling
from mud-soaked hillsides, if that's what you asked of me.
Memories and their aftershocks -- these clear new earthly fears:
fault lines, fires, the cold Pacific tide against my feet.
Before you say anything, it was never my intention to give up the day job. They took it from me, and part of my revenge is subjecting would-be passengers to samples of free verse. And if we can play the rock lyrics game, why not poems too? Except I'd probably win; there are dozens more where these came from. Also, yes, you're right, they're not wholly about airplanes at all. They're also about women. Two themes not so distantly related -- think dogged pursuit and infatuation; nothing to show for it. Maybe it was the cockpit checklists that inspired me, free verse masterpieces that they are ...
Stabilizer trim override, normal
APU generator switch, off
Isolation valve, closed
Autobrakes ... maximum!
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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