When I am driving home from work at night, I pass by O’Hare. If it is a clear night, I’ll see the planes lined up in a string for the approach. I was wondering how far apart the planes are from each other? If I can see five in a string, how far away is the fifth one? How about the eight one?
I realize that the distances are not fixed; the planes are all converging on a single location, the airport. I also assume that there are a ton of factors that affect the range. I am guessing these would include the airport, the visibility, the weather, maybe the types of planes, possibly how busy the schedule is. So feel free to specify a range and make any assumptions. I’ll provide some as a staring point: consider a major airport like O’Hare, full-size passenger jets, visibility is very good, weather is “normal”, it is the busy part of an average day (i.e. not Thanksgiving Eve).
I work at Baltimore Washington Intl… I’ll take my stop watch to work tomorrow and get some times during the morning rush. If I had to make an educated guess, I’d say there’s no more than one minute between landings at peek times, maybe ninety seconds. That’s assuming each jet clears the runway swiftly after landing. On runways that intersect, that time can be cut in half.
I will get it started. For commercial airliners, Air Traffic Controllers are the ones tracking all the planes, developing an overall strategy, and ensuring proper separation between the planes. These are rigidly spelled out in the FAR manual (Federal Aviation Regulations) that is published every year. I have a copy here but it is rather thick and involved. It is similar to a traffic rulebook you might see at your local DMV but it covers all of aviation in the U.S. I am just a perpetual student pilot in small aircraft so I only have a general idea of the exact spacing allowed.
c. When radar is employed in the separation of aircraft at the same altitude, a minimum of 3 miles separation is provided between aircraft operating within 40 miles of the radar antenna site, and 5 miles between aircraft operating beyond 40 miles from the antenna site. These minima may be increased or decreased in certain specific situations.
Excerpted from the FAA’s Air Traffic Control Handbook (there’s a whole list of seperation requirements, I’ve cut it back to just terminal stuff). The guts of it are underlined:
5-5-4. MINIMA
WAKE TURBULENCE APPLICATION
e. Separate aircraft operating directly behind, or directly behind and less than 1,000 feet below, or following an aircraft conducting an instrument approach by:
NOTE-
Consider parallel runways less than 2,500 feet apart as a single runway because of the possible effects of wake turbulence.
Heavy behind heavy- 4 miles.
Large/heavy behind B757- 4 miles.
Small behind B757- 5 miles.
Small/large behind heavy - 5 miles.
WAKE TURBULENCE APPLICATION
f. TERMINAL. In addition to subpara e, separate an aircraft landing behind another aircraft on the same runway, or one making a touch-and-go, stop-and-go, or low approach by ensuring the following minima will exist at the time the preceding aircraft is over the landing threshold:
NOTE-
Consider parallel runways less than 2,500 feet apart as a single runway because of the possible effects of wake turbulence.
Small behind large- 4 miles.
Small behind B757- 5 miles.
Small behind heavy- 6 miles.
g. TERMINAL. 2.5 nautical miles (NM) separation is authorized between aircraft established on the final approach course within 10 NM of the landing runway when operating in single sensor slant range mode and aircraft remains within 40 miles of the antenna and:
The leading aircraft’s weight class is the same or less than the trailing aircraft;
Heavy aircraft and the Boeing 757 are permitted to participate in the separation reduction as the trailing aircraft only;
An average runway occupancy time of 50 seconds or less is documented;
CTRDs are operational and used for quick glance references;
REFERENCE-
FAAO 7110.65, Use of Tower Radar Displays, Para 3-1-9.
Turnoff points are visible from the control tower.
Browsing through here, I found the minimum radar separation to be 3 miles. There’s more to it than that as greater separation may be required between a small aircraft following a large aircraft but I had difficulty finding specific numbers.
A B757 is unusual in that its weight fits into the “medium” category, but it has powerful wake turbulence requiring it to be treated similar to a “heavy” aircraft. So it tends to gets its own special mention when considering wake turbulence.
Wake turbulence often corresponds pretty well with aircraft size. It is easy to guess that wing vortexes generated by a 747 may cause problems for a Cessna even a long distance behind it.
However, wake turbulence is an aerodynamic property of a given aircraft and this plays a big role. The 757 has a design that makes it particularly bad at causing devastating wake turbulence for other planes and has caused accidents because of it.
"In the past year, there have been at least four accidents/incidents involving aircraft flying behind Boeing 757s. Although the investigations are not all complete, there is considerable evidence that wake turbulence from the Boeings was the cause.
What’s different about the 757? It’s not a “heavy” � an aircraft with a gross takeoff weight of more than 300,000 pounds. Heavies are accorded more respect and space by air traffic control and most pilots because the wing-tip vortices they produce are known to be big and powerful. It now appears that something about the aerodynamics of the 757’s very efficient wing allows it to produce “vortex tangential velocities greater than any ever recorded, including those of the Boeing 747 and Lockheed C-5A.” This is according to a study done by the National Oceanic and Atmospheric Administration back in 1990."
On a good day 90 seconds seems to happen at O’Hare when they get a conga line of big passenger jets. Less than ideal conditions would require greater separation, as would the appearance of a significantly smaller or larger airplane than other in line.
Planes can land on parallel runways at the same time. I was next to one flying into Atlanta just a couple of weeks ago. There are also procedures for Parallel landings, but I can’t find them right now.
Then use the GE ruler to check separation. At the moment, the surface separation of flights on the westbound approach over Lake Michigan is 4-5 miles. This isn’t official, of course, but it is close to the official rules quoted above.
At the “Oshkosh” Fly in, we do 3 at a time, one shot, one middle and one long about 30 seconds apart without the pilots transmitting, just listening.
*::: OK, that is a special case ::: *
Small aircraft can land very quickly at big airports as long as there is 3000’ separation on the runway, takes a good controller and pilots that are paying attention.
The pipeline patrol outfit I flew for were allowed to make formation takeoffs, the only civil aircraft allowed to do that there in an non-airshow condition. They did not let us do it except early in the mornings when there were not a lot of planes waiting to take off. The dummy corporate jocks never did understand that if we three went all at once, they got off faster.
Of course we were not carrying paying passengers either.
I’ve worked at MIA before and they have parallel runways and I don’t recall seeing simultaneous landings. They’ll take off on one and land on the other frequently, but not landings. That would require them to make the approach side by side. Of course their parallels are very close to each other, and I’m sure all are not.
Pearson Airport in Toronto has three parallel east-west runways, two side-by-side at the south side of the airport property, and one a couple of kilometres away along the north side of the airport property. (There are also two parallel north-south runways.)
I work directly under the flight path for the northern runway. Directly, as in, “plane shadows cross the building and my desk if the sun angle is right”. When I’m going home, I often see a plane go over about once every minute or so, and off to the south, I can see a similar stream of planes for the southern runways.