Q: Partway through an America West flight from Las Vegas to New York-Kennedy, the pilot announced that we’d experienced stronger than expected headwinds and would need to land in Columbus, Ohio, to take on extra fuel. How often does this happen and why? Did America West roll the dice before we took off, hoping we’d have enough gas?
Determining required fuel is a serious and somewhat scientific undertaking. Crews do not ballpark the load with a cursory glance at a gauge, as you might in a car before a long road trip. The regulations get knotty, particularly on international routes, but a good place to start is the standard US domestic rule: You cannot depart without enough fuel to reach your intended destination, then proceed to the most distant of any requisite alternate airport (designated in accordance with forecast ceiling and visibility minima), plus maintain a 45-minute cushion on top of that.
The numbers are wrangled backstage, so to speak, by an airline’s dispatch/flight planning staff. Intended routing and cruise altitude(s) are balanced against wind and weather conditions to formulate a minimum legal carriage. Air traffic considerations may further increase the total. The ascertained sum is presented to the captain, who has final word, as part of the preflight paperwork package. If in the captain’s judgment the situation so warrants, he (or she) can request extra.
Once aloft, quantity on longer flights is kept track of progressively over a series of waypoints. Hitting a certain fix, the crew compares actual remaining fuel with a predicted value shown in the fuel synopsis portion of the flight plan. Flash back for a minute to that old freighter I once flew to Europe. Approaching a waypoint over the North Atlantic, we’d run a fuel score. I’d total up what remained in the jet’s eight tanks, and compare that amount to what was anticipated on the sheet. If, for example, at 40 degrees west longitude, roughly midway across the pond, the paperwork called for 75,400 pounds, and I counted 76,200 pounds, I’d tell the captain we were “ahead 800” (or 120 gallons if you prefer).
That’s pretty old fashioned, but a good illustration. On modern aircraft there’s no need to have a hack like me staring at eight different dials with a calculator, but the basic procedure is no different.
Usually, the estimated numbers are accurate and reliable. With sophisticated software and thousands of daily flights, carriers have cutting-edge prognosticating tools and an immense bank of empirical data to work with. Still, every so often, whether due to shifting upper-level winds or a surprise ATC re-route, you drop below target values. Back on our freighter, it wasn’t unusual for the captain to hear, “We’re down 1,500.” Even so, it’s seldom a big deal unless you begin to fall substantially behind. (Keep in mind that even over the ocean planes stay within prescribed distances to diversion airports.)
When loads are heavy or other factors make takeoff weight an issue, a flight might set out with exactly the minimum legal fuel. Though all mandated buffers are accounted for, there isn’t much wiggle room for unforeseen problems. Should the cards include drastically changed winds or holding patterns, a diversion may be in order. This rarely happens, but as the emailer can attest, it’s not unheard of. You aren’t making a pit stop because you’re “running out of fuel,” exactly (reference the British Airways stories from a few weeks ago). More specifically, you’re unable to maintain those regulatory safety margins.
Critical to all of this is the fine print of how and when to designate a so-called alternate airport. Take the emailer’s example of Las Vegas to JFK. If the weather in New York is forecast below certain parameters, a diversion point, or “alternate” must be filed as well. Think of it as a backup destination, and here too the weather is obliged to meet specific ceiling and visibility criteria. In some cases two alternates need to be filed, with fuel enought to reach both. Occasionally, such as when entire region is blanketed by heavy fog, hunting down a permissible alternate can take you hundreds of miles away. The closest option to New York might be Pittsburgh (that’s fairly extreme, but not unprecedented). Now, not only do you need enough in the tanks to go LAS-JFK, but enough to then backtrack to PIT. Add still more for the 45-minute rule, and yet more for any anticipated holds or delays — what an airline calls provisionary or contingency fuel. Down at the gate in Vegas, you might hear chatter of the B-word (bump) if the flight is already heavily laden with passengers and cargo.
One way of working the system is to request a change of alternates while en route, if possible. Should Hartford turn bright and sunny, that can free up several thousand pounds on reserve for the much longer ride to Pittsburgh — useful if ATC springs an hour-long holding pattern.
With all of these safeguards, you’d figure it a virtual impossibility for a plane to succumb to fuel depletion. If by virtual impossibility you mean four times, that’s an accurate assessment. The most notorious and widely known incidents in which otherwise operable jetliners became hundred-ton gliders are those of a United Airlines DC-8 near Portland, Oregon, in 1978; an Air Canada 767 five years later; the tragedy of Avianca flight 52 near JFK airport in 1990, and the strange story of Air Transat flight 236 in 2001. Air Canada and Air Transat landed safely. United and Avianca did not.
Dissecting the how and why of these events would involve many pages. Suffice to say the circumstances were complicated and, in at least two of the examples, involved some hideously unusual decision making. I choose to think the rarity of such occurrences — a mere four flights out of many millions — is the most noteworthy thing about them.
This Q&A is part of a collection that originally appeared on Salon.com. Patrick Smith, 38, is an erstwhile airline pilot, retired punk rocker and air travel columnist. His book, Ask the Pilot (Riverhead) was voted “Best Travel Book of 2004” by Amazon.com. Patrick has traveled to more than 55 countries and always asks for a window seat. He lives near Boston.