Ask the pilot

I've heard that some airlines are now carrying less oxygen in order to save weight and burn less fuel. According to some articles, this partly explains the jet lag/fatigue symptoms after long flights, and that the wise passenger can even ask to have the oxygen flow increased.

British Airways says it will save $5 million annually, along with 6,000 tons of fuel, by reducing the amount of supplemental oxygen carried in the underfloor tanks of its long-haul 747s. The 747 was designed to transport in excess of 500 passengers. Those at BA, and many other operators, hold considerably fewer. As pointed out by a BA spokesman in the most recent issue of Air Transport World, there is little point in lugging around "way over what would be required in any conceivable emergency." Even with the reductions, amounts will remain well within regulatory minimums.

And again, these are emergency tanks. They are small, and used only if there's a loss of cabin pressure. In normal operations, passengers do not get oxygen from a tank. The air you breathe exists as it does from pressurizing the cabin. That's what pressurization is for. The atmosphere at higher altitudes is very thin; pressurization squeezes it back together, replicating conditions near sea level. The whole point is to not need a tank.

That brings us to an enduring variation of this myth, which maintains that crews intentionally dial down the cabin pressure as a way to save fuel and, in the process, subject passengers to an oxygen-scarce environment that helps keep them woozy and docile. Not only is this patently untrue, it would have quite the opposite effect. Although the symptoms of hypoxia can, at first, make a person feel dizzy and relaxed, they also induce confusion, nausea and migraine-strength headaches -- like the one I had in Cuzco, Peru, for two days straight. I wouldn't wish that on my worst enemy, let alone a planeload of passengers.

And as I've reminded readers before, the pilots are breathing the same air as everybody else. An aircraft fuselage does not contain separate compartments with different pressure values in each. The entire vessel is pressurized equally from the forward pressure bulkhead to the aft pressure bulkhead. This normally includes the cabin, cockpit and lower-deck cargo holds (but, alas, as we've seen, not the landing-gear wells).

In practice there is very little difference, flight to flight or plane to plane, in the density of available oxygen. Crews set up the pressurization system (there's a main and a backup) prior to departure, dialing in the intended cruising altitude and elevation of the destination airport. The rest happens automatically, holding the cabin to the equivalent of about 8,000 feet above the ground. In other words, you're breathing more or less as you would in Mexico City -- minus the pollution. Pressurizing all the way to sea level is unnecessary, and would put undue stress on the airframe.

A co-worker insists that while flying to New York recently, the pilot stated that, thanks to unusually high tailwinds, we were traveling at more than 750 miles per hour. How is it possible for a subsonic commercial plane to be flying faster than the speed of sound?

Well, it is and it isn't. It's fully plausible that the plane in question was traveling at 750 knots, sure, but that's groundspeed, not airspeed. Velocity over the ground and velocity through the air can be, and usually are, different things.

Say a plane is flying in still, windless air at 300 knots. Kick in a 200-knot tailwind, and relative to the ground you're now at 500 knots. However, the plane's "true" speed, relative to the air itself, remains where it was at 300. The reverse is true too. Imagine that same plane fighting a 200-knot headwind. Those 300 knots of airspeed equal only a hundred over the surface.

Remember that a plane pays no mind to how fast, or in what direction, it moves across the ground. It is the flow of air under and around the wings that keeps the damn thing from dropping out of the sky, and so it cares only how fast it is moving through the air. This is why pilots prefer taking off and landing into the wind, reaping the benefit of free airspeed. If a plane is landing into a 20-knot headwind, and its target approach speed is 120 knots, then 20 of those knots are already taken care of. And as discussed here before, the difference between airspeed and groundspeed illustrates how a plane can, under certain conditions, fly backward. Point the nose into a strong wind while slowing the machine to as low an airspeed as possible without stalling; look down, and sure enough you're drifting backward like a kite.

It's likely that the instance of the "supersonic" flight mentioned by the question writer took place earlier this month, on the weekend of Feb. 2. For most of this winter, the jet stream had been staying north, over Canada, resulting in abnormally mild temperatures for much of the United States. But over a three-day period, the jet stream moved south, settling over the Eastern seaboard, bringing with it colder temperatures and some extremely powerful upper-level winds. I have heard from several pilots who, over the course of that weekend, experienced their fastest-ever groundspeeds. (Yes, some pilots do keep track of those things, often with a notation in their logbook or some such.)

"We had the wind right on our tail," says Capt. James Haney, a UPS pilot who was flying from Louisville, Ky., into Richmond, Va., early in the morning on Saturday, Feb. 3. "This was only the second time I've ever experienced a groundspeed over 700 knots. Air traffic control told us they had never seen anything like it -- half a dozen airplanes on their screen, all moving at 690 or higher!"

In the opposite direction, of course, planes were crawling along at an agonizingly slow pace. I traveled from Boston to Atlanta on Feb. 4, and what is normally a two-hour trip took well over three hours.

Another sign of climate change? I don't know, but over the past few years I've encountered a startling number of superlatives from pilots (and passengers) relating to weather: strongest-ever winds, largest-ever storms, worst-ever turbulence and so forth.

I have an opportunity to fly between Mexico and Havana on Cubana, the Cuban national airline. Is it safe to fly this carrier, particularly in light of its ancient Russian jets?

Is it just me, or is there something appalling about the fact that human beings are not allowed to fly between Cuba and the United States? (Mexico, Canada, the Bahamas and the Dominican Republic are the most common jumping-off points for those who skirt the rules.) But for now I'll forgo any editorializing, and instead correct a negative stereotype pertaining to former Soviet aircraft types.

For all intents and purposes, most of the various Antonovs, Tupolevs, Yakovlevs and Ilyushins -- the staples of Soviet commercial aviation, still widely used around the globe -- are economically obsolete. They're fuel-thirsty, are difficult to maintain and cannot be outfitted with the sorts of passenger accouterments demanded by leading airlines. But they are not dangerous, and while the designs themselves are old, some of these planes had very long production runs, meaning they are often surprisingly young. Putting aside the fact that age, by itself, has very little to do with how safe an airplane is, you'll be surprised to learn that the oldest of Cubana's jets, a Yakovlev Yak-42, was built in 1987, making it newer than many of the aircraft belonging to American, Delta, Northwest and the other major carriers. Additionally, Cubana is known to lease late-model Airbuses for use on certain routes.

True story from a few years ago, when I flew cargo planes: Taxiing at Mexico City one morning, we passed a Cubana IL-62 docked at the gate. Our captain, using a gruff, you-couldn't-pay-me-enough cadence, remarked, "Man, look at that old thing." I noted the registration and, later, checked it against my books. The Cubana jet was vintage 1990, more than 20 years younger than our freighter.

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