Ask the pilot
Do pilots jettison fuel before landing? Plus: JetBlue's troublesome nonstops, and the scoop on in-flight "shortcuts."
Editor's note: This story has been updated since it was originally published.
By Patrick Smith
Read more: Technology & Business, Flying, Airlines, Business, P. Smith, Ask the Pilot, Food and Travel
Dec. 14, 2007 | 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to destination; assigned routes tend to turn, bend and twist -- a product of crowded skies and the convoluted (i.e., outdated) architecture of our airway system.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.
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.
Next page: Can high winds be the cause of unscheduled fueling stops?