do planes ever fly parallel to the meridian?

As I understand it, commercial flights that travel from continent to continent fly parallel to the equator. Meaning, a plane traveling to Moscow from Los Angeles would travel over the Pacific, over Asia, and into Moscow.

But, why couldn’t that plane fly northward? That is, over Canada, over the North Pole, and then into Moscow.

My first assumption would be because of weather conditions (turbulence, visibility rating, etc.). But is that all? Or, maybe not enough possible landing sites in case there was an emergency in the air? Or…?

I’m no expert, and perhaps someone will be along soon with more information. But equator-parallel routes are rare.

All other things being equal, a great circle route is preferred. Non-great-circle routes are chosen because of politics and technology (available along the route), and perhaps other reasons.

You might find this interesting: First Transpolar Flight Routes Set to Open.

Doesn’t anyone remember the Arlo Guthrie song “Coming in to Los Angeles”? The opening lines go something like this:

Coming in from London from over the pole
Flying in a big airliner

Anyway, the point is that most flights don’t parallel the equator. If fact this would be an impossible way to get to any desination not on the same line of latitude as the departure airport. If you parallel the equator your latitude never changes. I suspect what you meant to say was that the angle with respect to the equator never changes. This would make the true course of the aircraft constant from the departure airport to the destination airport.

But as you and JonF pointed out, these are not the shortest routes. In fact, althouth I’m not sure if I can prove this, on a long flight that traverses many meridians the constant angle route may be several times longer than the great circle route. I believe great circle, or near great circle, routes are by far the most common routes traveled by airlines.

Politics may be the only reason that great circle routes are not used. There needn’t be any land naviagtional aids along the route in order for a airline to navigate over it. In the past the INS (inertial naviagtion system) was sufficient. It is now being replaced by GPS. Landing sites can be a problem. ICAO (international civil aviation organization) and the FAA have some rules about this. Airlines must be within a distance from an airport that allows them to fly there on one less than all engines in a certain amount of time at all times.

For twin engine airliners this used to be 90 minutes. So at all times a twin engine airliner had to be within 90 minutes flying time on one engine from an airport it could land at. This eliminated many routes for twin engine international airliners. Now new regulations are being granted on a per model basis to extend that time. For four engine airliners, I’m not sure what three-engine-to-an-airport time is, but it is long enough to allow nearly all routes for these aircraft.

Visiblity is never a problem, airlines can fly through zero visiblity. Some can even land in zero visibility. Turbulance is more of an altitude problem, not a latitude problem. I don’t think the 35000 feet above the north pole has any worse turbulence than anywhere else 35000 feet up.

JonF I think that article title is misleading. Airlines have always used routes that traverse high latitudes. Now more of them are open due to Russia opening its airspace. I seem to remember that quite a few years ago a western airliner was shot down when it was flying over the pole and wandered off course into Russian airspace.

To answer the OP, yes airlines can use meridians. If two airports are on the same line of longitude then the shortest route would follow that line. This is because all lines of logitude are great circle arcs. Only one line of latitude (the equator) is a great circle.

Actually polar routes are fairly common. There are still some political airspace restrictions, but commercial airliners can and do use Great Circle routes.

A Korean Boeing 747 was shot down off of Sakhalin Island in the 80s after it wandered into Soviet airspace. It had overflown a ver sensitive Naval installation. Compounding the problem was that there was an Air Force jet in the area doing surveilance. The AF jet is basically a Boeing passenger jet (a 707, I think) that has been equipped for military use. The Soviets mistook the Korean airliner for what they thought was a U.S. spy aircraft. The reason the Korean airliner was off course was because one of the pilots entered the wrong number (I think he transposed a couple of numbers) on the navigational equipment.

The great-circle route between L.A. and Moscow crosses Canada and northern Greenland. The distance is around 8000 miles. To find the great-circle route between any two places, take a globe and stretch a string between the two points. It turns out that Anchorage, Alaska, is close to the halfway point between New York and Tokyo, which is why airliners used to stop there for refueling. (I suspect they still do in many cases, but there are now airliners that can make the trip nonstop with fuel to spare.)

Quoth Dr. Lao:

I’m pretty sure that the limit of the ratio is pi/2, if you’re using the shortest possible constant-angle path. Of course, you could fly around the world a few times before landing, but that’d be silly.

Also, another reason that routes might deviate from great circles is to take advantage of or avoid the jet stream, or other air currents.

747’s can make the trip non-stop, usually. I read something once about 747 captain. On the New York to Tokyo flight in the winter time they would file a flight plan with Anchorage as their destination. The upper altitude winds tend to be most severe in the winter. If they had bad head winds they would land at Anchorage to refuel. The problem was that although they didn’t have enough fuel to make it to Tokyo they had too much to land at Anchorage. That is to say, they were above their minimum landing weight. As a result they had to dump fuel in the ocean in order to land and refuel. The passengers thought they were crazy, but there wasn’t any other airports further along the route they could land at.

Replying to the tangent. Johnny L.A. is referring to the 1983 shootdown of KAL Flight 007 in 1983; less well known is the Russian shootdown of another KAL flight that was 1000 miles off course and crash landed on a frozen lake near Murmansk in 1978. Considering that it was a Paris - Seoul-Kimpo flight, and attempting to drag this reply somewhere near on-topic, I’ll venture a WAG that their planned course was near trans-polar.