Why did my flight pass over so many major cities?

I flew from SJ to Baltimore and back this past week. I could not help but notice that it flew pretty much directly over several major cities along the way.

I recognized Denver, KC, St. Louis, and Louisville. There were probably some others I missed. This plane flew pretty much DIRECTLY over these cities, so much so that I could recognize buildings, monuments, Churchille Downs, etc. There isn’t much out there in the middle of the country, so this was obviously by design, but why?

My guess is that the pilot uses these cities as landmarks on his route to make sure he knows where he’s going. But I could be mistaken, as I know nothing about aviation besides what I’ve learned from fooling around with flight simulators.

Until someone like pilot141 shows up, I’ll have a stab at it.

All “scheduled airline” flights fly on instrument flight plans. That’s flying using navigational radio beacons or (I think) some GPS these days. The airliners fly from point to point on a system of beacons - which may or may not line up with ground-based visual landmarks.

Most likely, your particular airline flight was assigned a route using beacons based in or very near large cities

This system means airlines seldom really fly a straight line from city to city - their path may zig and zag quite a bit. A “direct” flight is direct only in the sense it doesn’t stop anywhere else before its final destination.

but surely as a precaution shlouldn’t they avoid flying over built up areas?

Sure plane crashes are rare but what advantage is there in deliberatley tempting fate?

They may be flying from airport to airport, to stay in parallel flight paths with the planes that are doing shorter hops.

When was the last time that you heard of a plane just falling out of the sky mid-flight?

It never happens.

Hmmm. Lets see. A Swiss Air DC10 fell outta the sky around Peggy’s Cove Nova Scotia. TWA 747 fell outa the sky off the east coast a few years back. That MD80 fell outta the sky around LA a few years ago. That Airbus 300 fell outta the sky around LaGuardia shortly after takeoff just after the 9/11 attacks… 737’s have fallen out of the sky a few times due to rudder control problems. Those are the only ones that come to mind after about 20 seconds of thought.

Nah, never happens.

As a side question:

Do commercial airlines use high altitude airways most of the time still (VOR-VOR?) or GPS “direct” navigation when possible? (IE - not over restricted airspace)

To understand this, you have to remember that the national airspace system and modern jetliners did not just appear one day in their present form. It has been an evolutionary process.

In the old air mail days, pilots navigated visually - following rivers, railroad tracks and flying over known cities and towns. This worked fine during the daytime, but was useless at night. When the demand forced air mail flights to fly at night a series of bonfires was lit along the route of flight. There needed to be someone there to light the fire, so the flight paths tended to “wander” along the ground to coincide with places that had at least some people on the ground.

Even bonfires and railroad tracks couldn’t help pilots fly in clouds, though, so something else was needed. Eventually radio navigation evolved. A radio transmitter was placed on the ground and the pilot tuned it up. Initially this was a very crude system, as you had to listen to the volume level increase or decrease to tell if you were going toward or away from a station.

These radio transmitters were also placed near cities and minor population centers. Not only did it help make maintenance easier, but often the town was the destination for the aircraft, and you now had a way for the pilot to navigate directly to your airport.

As the radio beacons got more sophisticated so did the aircraft. Now the beacons could drive a pointer in the cockpit, making navigation much easier. Easier navigation meant more reliable transportation, and air traffic increased. After a series of mid-air collisions the government realized that a system was needed to separate aircraft not just around airports but also all across the country.

They came up with the national airspace system. Airways, often called “highways in the sky” were plotted out. These airways provide a way to fly between two radio beacons. For example, fly outbound from the Los Angeles beacon on the 080 radial (an imaginary line that extends outward at a 080 heading from the beacon) until you intercept the 260 radial inbound from the Needles beacon. This allows you to know your position the entire way, and you are able to stay on course. It also allows for airways to be constructed from beacons that are very far away - you don’t have to pick up the Needles beacon right after takeoff, but only after flying outbound from the Los Angeles beacon for quite a while.

By having aircraft stay along defined paths in the sky, many more aircraft could fly safely in the same airspace. The route system became a valuable tool for Air Traffic Control - they could route aircraft in ways to make the system more efficient. For example, a “corridor” is developed for all aircraft coming in to New York from the south. A different one exists coming from the west, and still more exist for aircraft departing New York.

As traffic increased the beacons got better, becoming VORs (Very high frequency Omni-directional Radio) that included DME (Distance Measuring Equipment). More and more were activated around the country, but the practice of routing aircraft over these beacons continued.

Today we have the technology to go direct to anywhere (inertial reference systems and GPS are the two most prevalant technologies). However, the same limitations concerning traffic flow remain - you can’t have 300 airplanes converge on JFK at once. The FAA is working on what it calls “free flight” which will allow pilots to spend most of the flight going direct to their destination, only jumping into an arrival corridor near the end. This requires them to completely re-work how they accomplish en-route separation, so “free flight” has been the perpetual “next year’s” technology.

So it comes down to the national airspace system being an evolutionary technology rather than a revolutionary one.

You could just as easily say: “as a precaution, they should surely stay near built up areas, where there would be a nearby airport to head for if an emergency happened.”

Time lapse photography of airplanes following a corridor from LaGuardia out to the Atlantic.

1978, a DC-10 lost an engine (as in “departed the airframe”, not just merely quit) shortly after takeoff and crashed into a residential neighborhood of Chicago. Yeah, it happens.

But mostly, the Bad Things happen just after take-off or landing, not in the middle of cruise flight. So you might ask if it would make more sense to land and takeoff in the middle of nowhere rather than in or near population centers. If something went fatally wrong at 30,000 feet directly over a city center, momentum alone would most likely carry debris away from the buildings directly below.

But, really, airline accidents are so statitiscally rare that the risk becomes acceptable to society - far more people die each year on the road in cars, or slipping on their bathroom floors at home. Forcing airlines to never fly over densely inhabited areas would seriously undermine their utility. Other potentially much worse hazards can be found in the railroad cars hauling highly toxic chemicals through urban centers, as just one example.

I hate to tell you this, but most major airports are in the middle of built up areas. And most bad stuff is likely to happen on takeoff or landing…

In addition to what pilot141 said, it should be noted that flight over congested areas is not prohibited. Although there are minimum altitude restrictions, they’re not terribly high. Federal Aviation Regulation 91.1199© states:

*Except when necessary for takeoff or landing, no person may operate an aircraft below the following altitudes:

…Over any congested area of a city, town, or settlement, or over any open air assembly of persons, an altitude of 1,000 feet above the highest obstacle within a horizontal radius of 2,000 feet of the aircraft.*

There is also another reg that states you need to operate your aircraft in a manner that allows for a safe emergency landing. Thus, even if one reg says it is legal for you to fly 1,000 feet above a skyscraper, if your aircraft is such that if the engine fails while you are above that skyscraper you can’t glide to an open space suitable for a safe landing, then 1000 feet above the obstacles is NOT the minimum legal altitude because of this other reg.

An illustration of this would be when I approach my home airport in a Cessna 150, 172, or Piper Warrior and fly over surrounding subdivisions at 1500 feet above the ground. The characteristics of those airplanes AND the surronding terrain are such that if the engine fails anywhere along my flight path I should be able to reach either an open field or the actual airport and land with little or no damage. However, when I’m in a Piper Arrow I approach the home airport at a much higher altitude when flying over the built up areas because that airplane glides like a brick if the power is off and I need a lot more altitude to start with if I’m going to reach a safe landing spot. If I were to fly into my homebase at the “legal minimum” in an Arrow, run into trouble, and cause a mess on the ground I could, in theory, still be heavily penalized for flying in a “hazardous manner”* and possibly been found to be operating at too low an altitude for safety even if I was more than 1000 feet above the ground/highest obstacle.

Now, the flight characteristics of, say, a 747 are actually pretty good - google on “Gimli Glider” for the story of a successful landing of an airliner with no engine power whatsoever. So, if you’re over a city at 30,000 feet and the engine quits it will (assuming the steering is working properly) take about a half an hour to reach the ground at best glide speed, which should put you considerable distance away from that built-up urban center (I figured about 50-80 miles, but honestly, that’s a WAG since I’m not that up on airliner glide ratios and best glide speeds)

If, hypothetically, an airliner had an engine failure over the Chicago Loop at 30,000 feet (as an example - I live and fly around Chicago so I know the airspace) there are several airports it could glide to within that radius I mentioned that could easily accomodate an airliner. O’Hare and Midway, for starters. There are others that could at least accomodate a landing, even if it couldn’t take off from there again or might damage the runway (because airliners are really heavy objects). There was a recent thread speculating about such things, actually.

Most, if not all, large cities are likewise equipped with airports that could serve a similar functions. As t-bonham@scc.net pointed out, you could even use this as an argument for requiring airliners to fly over city centers - because city centers are where airports that can handle an airliner in distress are usually located, and where there are maximum emergency services available.

  • “Hazardous manner” is actually a term used in the regs and is quite a catch-all. It has caught a lot of pilots, even when no harm has been done, and frequently appears when pilots are penalized for breaking one or multiple regs.

Most sources suggest a best glide ratio of around 18 or so for a typical modern airliner. This gives a glide range of just over 100 statute miles from an altitude of 30,000’.

And that equals exactly 90 nautical miles.

Nice WAG - are you that wasn’t a SWAG? (Scientific WAG?):wink: