Ask the pilot

Can commercial jets fly upside down? Has terrorism forced a change in transoceanic flight paths? And other probing questions for our expert.

By Patrick Smith

Read more: Technology & Business, Flying, Airlines, Business, P. Smith, Ask the Pilot

Photo by Patrick Smith

An Antonov AN-124 cargo jet as seen from a 767 over the North Atlantic.

Dec. 12, 2008 | Nothing philosophical this week. We'll keep it real, so to speak, with some red-meat questions and answers from reader-submitted e-mails. A few of these concerns have been dealt with in this column before, but judging from how often I'm asked about them, it's probably time to review.

In your description a week ago of a Colombian jetliner that ran out of fuel over Long Island, N.Y., you write that the plane, all four of its engines having failed, "glided to a crash landing into a wooded hillside." I understand your desire to avoid alarmist language, but was that not a little soft? "Glided"? I suspect that "plunged" or "fell" might have been more accurate.

"Glided" was a perfectly apt term. Many passengers presume that only a glider can glide, and that a large plane, sans power, will exit from the sky like a stone. But this is not at all true. Granted, the complete ceasing of all engines brings on a host of ancillary problems (electrical, pneumatic, hydraulic), to put it mildly, but a jet will not -- repeat, not -- "fall," "drop," "plummet," "spin," "plunge," "nose-dive" or otherwise come flailing out of the sky like a pheasant full of buckshot.

Though some can do so more efficiently than others, any airplane, no matter how large or heavy, is able to glide. By "efficiency" we mean the number of feet that a plane moves forward in relation to the number of feet that it drops vertically -- the "glide ratio." The glide ratio of a large passenger jet often exceeds that of, say, a small Cessna. A jet needs to perform this glide at a higher speed, but the result is the same: a surprisingly long distance traveled. Consider the strange fuel-depletion incident involving Air Canada Flight 143 in 1983. During its glide to a perilous but fatality-free landing, the Boeing 767 was moving at a speed of around 240 miles per hour, losing approximately 1,000 vertical feet for every 10,000 feet it covered horizontally. In another bizarre incident, an even larger plane, an Air Transat (also Canadian) Airbus A330, glided for more than 80 miles after the loss of both engines over the Atlantic in 2001.

As for what such a glide feels like, most of you already know. Even in normal operations, it is common for passenger jets to descend at what a pilot would call "idle thrust." That is, the engines are run back to a zero-power condition. They are still operating, but in a way that produces little or no thrust. The descent rate and the accompanying sensations on those Colombian or Canadian jets would have been no different from what you've felt many times.

Midway across the ocean between England and New York, I watched as another 747 approached us and flew surprisingly close alongside for several minutes. It was just to the left and slightly beneath us, so close that you could see people in the other plane. Then we separated. Any ideas?

Planes on the transoceanic "track" systems commonly encounter one another more or less as you describe. However, any two planes along the same track (that is, a string of latitude and longitude points forming a sort of high-altitude highway) must be separated by at least 1,000 feet vertically, or several miles horizontally. I suspect the plane that approached you was flying the same track, slightly offset (as is often the case), 1,000 feet beneath you.

This photograph shows an Antonov AN-124 cargo jet as seen from a 767 over the North Atlantic, approaching the coast of Labrador. It was taken by me out the cockpit window. Believe it or not, the Antonov is 1,000 feet below us.

Not to belittle your powers of observation, but distances aloft can be very hard to judge, and passengers have an extremely common habit of underestimating the separation from other aircraft. Even during the most harrowing near miss it would be all but impossible to see the occupants of another jetliner. Even if an airplane is parked at the gate, a few feet away and stationary, it can be very difficult.

The distance from New York to Copenhagen, Denmark, is about 3,400 nautical miles. Continental Airlines uses a Boeing 757-200 on this route, and according to online sources this plane has a maximum mileage of just over 3,900. That seems to be pushing the limit. What about delays, storms, etc.?

First off, range is more accurately a measure of time, not distance, and the mileage values you see in books or on Web sites are a rough gauge only, predicated on specific conditions. In real life, duration is never a hard and fast value. It differs from flight to flight depending mainly on weight and winds. Even so, the values you cite leave 500 miles of slack, which is quite a bit.

As I've explained before, every flight, by legal stipulation, must have adequate fuel to account for delays. The regulations differ, depending on whether they're for domestic or international flights, but basically they work like this: A plane needs enough fuel to reach its destination, plus enough to reach any designated alternate airport (a minimum of one, with acceptable forecast weather, is required on any international route), plus an added buffer of at least 45 minutes. While en route, when crossing a fix or waypoint, the crew will match existing fuel levels with those calculated on the flight plan to see if they are running ahead or behind. Occasionally, such as when headwinds are higher than forecast, the burn will run higher than expected, but only in rare cases does this entail a diversion.

The Boeing 757 was not designed with ocean crossings in mind, necessarily. But that doesn't mean it is risky to fly them. In recent years, with hub-and-spoke networks increasingly bypassed by point-to-point nonstops, more and more planes that were conceived for short to medium haul are being put into service on longer pairings. We've seen 737s flying to Hawaii from California, and even between the U.S. and Britain. Domestically, JetBlue has been using the little A320 on coast-to-coast nonstops for several years now -- though once in a while, particularly in the winter when westerly winds are stronger, those aforementioned rules require that a nonstop become a one-stop.

Next page: Have transoceanic flight paths changed because of terrorism fears?