Ask the pilot

Do pilots jettison fuel before landing? Plus: JetBlue's troublesome nonstops, and the scoop on in-flight "shortcuts."


Patrick Smith
December 14, 2007 4:19PM (UTC)

I travel frequently, and the following is a common occurrence: Departure is delayed by an hour or more, but once we're in the air, the pilot informs us that he will "try to make up some time," perhaps with the help of "shortcuts." What is going on here? And if it's possible to fly faster, why not do it all the time?

Basically there are two ways to make up time: increase speed or ask for a shorter routing. With respect to the latter, planes are seldom cleared in a straight line from departure point to destination; assigned routes tend to turn, bend and twist -- a product of crowded skies and the convoluted (i.e., outdated) architecture of our airway system. If a flight is late, the crew will sometimes request to cut corners or bypass certain fixes, shortening the distance. Permission depends mostly on traffic, and requests are frequently denied. With so many planes in the air, all of them in the same proverbial boat, pilots have to be judicious when seeking such favors.

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The same holds true for speeding up. Traffic constraints often require that flights adhere to their originally planned speeds. (As a rule, crews do not select their own cruising speeds. An optimum speed, usually determined by computer as part of the preflight number-crunching, seeks to strike a balance between economy and time.) There can also be complications, particularly on long-haul flights, pertaining to fuel. Going faster burns more gas, and this isn't always feasible if regulatory reserve parameters are tight. On transoceanic flights, planes are not visible to radar over long stretches; after initial clearance, exact adherence to routing and speed is critical for traffic separation.

In a recent column you wrote that "planes do not jettison fuel except during emergencies." I beg to differ. Commonly during landings I see a long trail of what appears to be fuel spitting from the wingtip, close to where the fuel jettison nozzles are located. This is done, I assume, to lighten the aircraft for landing. I understand airlines' not wanting to admit they are spraying gas over people's neighborhoods, but I see it all the time.

Or so you think. As I've stated before, airlines would sooner toss bags of hundred-dollar bills over the side than spit away fuel for no good reason. What you are seeing is moisture -- the condensed cores of the wingtip vortexes. At or near the wingtips, higher-pressure air beneath is drawn toward the lower-pressure air on top, resulting in a rotational flow that can trail behind an aircraft for miles. When moisture levels are high, such as when passing through mist, clouds or conditions of high humidity, the hearts of these vortexes become visible, shooting from the wings as strands of instantly condensed vapor. They are most pronounced when the wing is working hardest to produce lift -- i.e., when flying slowly during approach or departure. Here's a spectacular image of this phenomenon, showing both the condensed cores and the outer spin.

Not only can you sometimes see the vortexes, but from the ground you can sometimes hear them. Stake out a position below a runway approach path, and several seconds after a plane passes overhead, listen for the eerie, whiplike noise of the vortices as they float downward.

They don't always emanate from the outermost tip. One common spot is slightly inboard, near the trailing edge of the outboard wing flap, as seen here. Moisture will condense in other areas too, such as around the engine attachment pylons. You'll witness what appears to be a stream of gray smoke pouring from the top of an engine during takeoff. This is actually water vapor caused by invisible currents around the pylon. Other times, the area just above the surface of the wing will suddenly flash into a white puff of localized cloud. Again, this is condensation. That "cloud" appears as shadow in this photograph of a British Airways 747.

Some commercial aircraft, particularly larger ones, do have the capability to jettison fuel. The maximum weight for takeoff is usually greater than the one for landing. This is true for a few reasons, the obvious one being that touching down puts more stress on an airframe than taking off. Occasionally, something happens soon after departure and a plane must return to the airport. Rather than tossing passengers overboard, it will eject fuel through plumbing in its wings. As described in this 2005 column, I once had to dispose of more than a hundred thousand pounds this way over northern Maine, a procedure that took many minutes and afforded me a lavish night's stay at the Bangor airport Hilton. Unless the trouble is urgent, dumping takes place at high altitudes, allowing the kerosene to dissipate well before reaching the ground. And no, engine exhaust will not set the discharge on fire.

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I've flown JetBlue several times from Burbank, Calif., to New York. Out of six flights, all booked as nonstop service, we've made unscheduled refueling stops three times. The stops were announced just after or before takeoff. They say it's due to winds but I'm not really buying it. I haven't had this experience on other carriers.

JetBlue is somewhat notorious, if that's the right word, for turning its coast-to-coast nonstops into coast-to-coast one-stops at the last minute. The airline uses Airbus A320s for these routes -- a plane that was designed for generally shorter-haul services. Its performance on these five-plus hour trips is marginal, weather and weight depending.

Remember that commercial aircraft do not have a fixed range. You'll often see a plane's range stated as a particular number of miles, but this is a typical or average value, not a hard number. It's best to think of range in terms of time, not miles, endurance varying with winds, payload and other factors. Most of the time, JetBlue's A320s can make a transcontinental trip without any trouble. Occasionally, however -- particularly if headwinds are strong -- this isn't the case. With a full load of freight, baggage and passengers, a plane may not be able to carry enough fuel to make the full journey without exceeding its maximum takeoff weight. Thus it comes down to a choice between making an unplanned stop or bumping revenue. In most cases, the airline opts for the stopover. Even a full complement of fuel might not be enough if headwinds are sufficiently powerful.

Poor weather at or around the destination is another issue. Regulations say that a domestic flight has to have enough fuel to reach its destination, then proceed to one or more alternate airports, if necessary, plus a 45-minute buffer. Because those designated alternates need to meet specific ceiling and visibility criteria at the expected time of arrival, they can sometimes be a long way from the intended destination, pushing the required fuel beyond what the flight can actually carry.

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At Burbank, the airport itself presents another problem. Neither of the two runways is particularly long, and there is a close-in mountain range. When prevailing winds require the use of the shorter strip, the maximum allowable takeoff weight can be quite low.

Fuel burn is calculated prior to departure, so usually the crew knows well ahead of time if an unscheduled stop is needed. Once in a while though, that decision is made en route. Winds can turn out stronger than forecast, or the destination and/or alternate weather might change. In no circumstances are you "running out of fuel." The diversion is made to keep within the rules and ensure that this doesn't happen.

If JetBlue had the resources, it'd behoove the airline to invest in a more suitable plane for these services. Until then, look on the bright side: Its fares are low and you can watch TV during the delay.

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I travel to La Paz, Bolivia, several times a year. The airport there is the highest commercial airport in the world, at 13,300 feet. Travel guidebooks, along with fellow passengers, love to talk up the weirdness of taking off or landing in La Paz. They claim that operations can only take place early in the morning or late at night, and only narrow-body planes are allowed. How much of this is true, and why?

In fact the world's highest commercial airport is Qamdo Bangda, in Tibet, at 14,219 feet (4,334 meters). But at 13,325 feet La Paz is still very high. The air up there is thin, which in turn means the engines develop less power, and the plane needs to be moving very fast (i.e., at an unusually high groundspeed) before the wing generates the same amount of lift it would generate at sea level. The result: unusually long, high-speed takeoff and landing rolls, and shallow climbs.

The La Paz airport, appropriately named El Alto -- "the Top" -- has a very long, 13,000-foot runway (nearly matching its height above sea level), but the peaks of the surrounding Andes mean that aircraft can still be weight-restricted when taking off due to obstacle clearance requirements -- and possibly on landing too, due to complications should the plane have to execute a missed approach or go-around. The maximum allowable takeoff weight will vary flight to flight, depending on exact conditions. The same is true, though to a lesser extreme, at other high-altitude airports such as Mexico City and Denver.

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But there's no reason a plane cannot arrive or depart at La Paz in midday. It's cooler in the evening and morning (cooler air, like that at lower altitudes, is more dense), meaning that performance is better, but it depends on the present temperature, winds and weight. And there's no reason it can't be a wide-bodied plane. Wide bodies are heavier, obviously, but they generate proportionately more lift and power. In many circumstances, a half loaded 777 is going to require less runway than a fully loaded, but much smaller 737.

Crews flying into La Paz need to don oxygen masks during arrival and departure, and are usually required to undergo airport-specific instruction prior to their first trip there.

Meanwhile, be wary of what the guidebooks and your seatmates have to say. Lonely Planet might publish the world's best travel guides, but I have noticed mistakes in references to airlines and flying. Its readers aren't immune either, and you might remember, a few years back, my takedown of a Lonely Planet "Thorn Tree" discussion on airline safety records.

The Bolivian national carrier, Lloyd Aereo Boliviano (LAB), was one of the world's oldest airlines, founded in 1925. Last spring, it suspended operations because of financial problems.

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GO-AROUNDS

Re: Weather delays and that blasted iPhone commercial

Some of the points you raise in your iPhone column are disingenuous. Don't insult our intelligence by sweeping the airlines' continually poor treatment of its customers under a rug of technical and systematic deficiencies and complexities. The airline business is still at least partially a customer service business: It's people dealing with people. No matter how complicated the workings of the airline industry are under the hood, it comes down to basic customer service and human decency. The airlines, by and large, don't understand this, and the apocryphal snarky pilot in your story is as emblematic of that fact as the doofus with the iPhone is of ignorant customers.

-- Jason Botwick

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Re: Hotel room clutter

Another perspective on the hotel brochure flock: Some years ago I worked as a hotel housekeeper. I've had some tough jobs in my life, but housekeeping was the toughest. Changing linens, vacuuming and cleaning a bathroom in 10 minutes is a challenge in itself -- but then you have to do it again and again and again. And the infuriating cherry on the top is Arranging the Brochures. At one top chain hotel there were 15 of them, and they were all supposed to be arranged with the kind of precision usually associated with microsurgery. This was maddening, especially as the chief housekeeper followed me around and ostentatiously rearranged them every time while making sighing noises. So next time you're about to fling the things into a corner, consider staying your hand on behalf of the housekeeper who's going to have to scoop them all up again.

-- Gavrielle Perry


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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