Intersecting runways?

This thread got me curious, poking around with Google Maps*. I was checking out one of the aiports I use most frequently (Philly.) I noticed that there are two runways (17 and 27R, not that I have the slightest idea of what that means really - they’re both to the right of the airport) intersect. So, apparently, do the two runways at the Albuquerque airport. This seems slightly counterintuitive to this non-pilot, for two reasons. For one, I’d think that this reduces any chance of using both runways at the same time, which seems inefficient. Secondly, and I’m fairly certain I’m wrong about this but I don’t understand why, isn’t that more dangerous? I assume that intersecting runways = greater chance of colliding planes, which is something best avoided.

So…what’s the dope? Why have them intersect each other?

Bonus question: scroll a little bit southeast of the ABQ airport. There’s a couple clearly-defined circles, some with airplanes in them, and two of them seem to have heart-shaped roads going into them. I assume that they’re connected to/part of the military base (Air Force, I think) right next to the airport, but what are they?

Non-pilot here (so correct me if I’m wrong), but usually they don’t even want to use both runways simultaneously. They use just the one (or two if there is another parallel one), and which one it is depends on that day’s wind direction.

I am sure others with more knowledge will be along to answer this shortly…

From my understanding planes take-off and land into the wind. At certain airports the winds blows in a fairly constant direction so the runways all go in the same direction. However, at some airports the wind shifts which allows the aircraft to choose the runway that maximizes their lift.

This is a good thing to know.
Runways are named according to the first two digits of their compass heading. That is runway 17 runs 170 degrees from start to end. 27R runs at 270 degrees and the R at the end stands for “Right”. There could possibly be a 27L running parallel next to it.

You may have figured out that runways may have two names if they are used in both directions. Runway 27 can become runway 9 (for 90 degrees) if traffic lands in the other direction.

Only some of the runways at a large airport are open at any given time. The biggest reasons for this include wind direction and traffic density.

There are two sets of Air Traffic Controllers that ensure that traffic lands, departs, and moves around the airport on the correct runways and taxiways. Intersecting runways would not usually be in use at the same time especially for larger planes.

I’m assuming that commerical runways intersect for the same reason that private runways intersect: To take advantage of prevailing winds.

Planes have an advantage taking off into the wind, since it means the air flowing over the wings is faster than the speed applied by the engines alone. Thus they can get the same lift faster and with less fuel. Same thing happens in reverse when landing, the wind blowing across the wings means the plane can be going slower with reference to the ground without stalling and falling out of the sky. A strong crosswind provides no real advantage and can be outright dangerous since it tries to push the plane of the runway.

By building runways at near right angles you can take advantage of the wind no matter what direction it’s coming from while at the same time you never need to closed down because of crosswinds. A single strip will only be able to take advantage of the wind if it’s coming from two directly oposite directions, while if there is a strong crosswind landing becomes more difficult and can even require closing the strip.

Yes, they won’t use both runways at the same time. The runway in use is the one that allows aircraft to land and take off as close to the wind direction as possible.

The numbers give the headings of the runway, to the nearest ten degrees. If you’re landing on Runway 17, you’ll have a heading of 170° (ten degrees east of south). Incidentally, each runway will have two numbers, depending on which way you’re approaching it - so, the same physical piece of tarmac as Runway 17 is Runway 35 if planes are landing in the south-north direction rather than north-south.

The “R” and “L” mean “Right” and “Left” - this is used if you have two parallel runways.

I just wanted to amplify on this. Runway 9 ir 90 degree clockwise (actually, anywhere between 85 and 95) from magnetic north, which means facing east, but not geographic east unless you at a location where magnetic north happens to be true north. And the same runway in the opposite direction is runway 27, since that is 270 clockwise from north.

Another point worth mentioning is that magnetic north wanders and occasionally pilots will get an advisory that runway 9 is now runway 8 or 10. Then there are places in northern Canada where the system is useless since they are so near the magnetic pole that the direction is very unstable. I don’t know what they do, but I guess there are no population centers large enough for them to have a controlled airport.

The relevant Transport Canada document is here in PDF. It says:

The designated “area of compass unreliability” is the Northern Domestic Airspace, whose boundaries are set out on page 142 of the Designated Airspace Handbook. The Northern Domestic Airspace carefully excludes Inuvik, Yellowknife, Churchill, and Iqaluit, which are the biggest airports in Canada’s arctic. As far as I can tell, there are no airports in the Northern Domestic Airspace which have control towers, but the existence of an FSS frequency at Rankin Inlet suggests a control zone. I don’t have my Canada Flight Supplement on hand to check that, though.

A triangular or hexagonal arrangement was common for WW2-era bases. London Heathrow still has the remnants of a hexagon arrangement. Also scroll up a mile or so (Google Maps is playing up at the moment so I can’t link) and you’ll find RAF Northolt - a good example of three intersecting runways, and very much an active military base.

Contrast the runway alignment at O’Hare International in Chicago, where there are three sets of parallel runways (E-W, NW-SE, and NE-SW), as well as one N-S runway. See this diagram. At any given time, there are usually four runways in use at that airport; two landing and two takeoff. The most common in my experience are the NW-SE and W-E pairs.

Note that this requires a highly co-ordinated plan for execution. In such cases, pilots are told when they are allowed to cross the other active runways; they may also be given requests to “land and hold short” of a particular point on their runway, which allows the simultaneous use of a runway that intersects towards the end of the landing runway for other landings or for take-offs.

If you can watch what is going on during busy hours at O’Hare and make sense of it, you are doing well. I prefer LAX; they never use intersecting runways.

Also, intersecting runways can be used at the same time-- often at a big airport the runway is long enogh for small plane to land before the intersectiion. This is called land and hold short.


To amplify, LAHSO (land and hold short operations) allow airports to use intersecting runways to increase traffic capacity and facilitate air traffic using runways that are convenient for traffic arrival/departure. A pilot cannot be forced to accept a LAHSO clearance, so if he or she wants to use the entire length of a runway to land, and that runway intersects with another active runway, the controller will have to find a way to accomodate this (pilot must of course let the controller know, preferably early).

Right - indeed, there would essentially always be a parallel runway in this case - otherwise, the “R” or “L” would be confusing (could induce someone to land on a parallel taxiway, for instance).

No one has to be landing for the strip of pavement to have two numbers: The whole runway is called 09-27; when landing to the east, you’d be “cleared to land on Runway 9”. IOW, the runway hasen’t changed names; instead, the Tower has designated a new landing direction.

To be pedantic, they are open most all the time (except when under repair, buried in snow, etc.). Only some of them will be in active use.

As a side note, LAX has four parallel runways. 2 to the south of the terminal, and 2 to the north.

Even though they are parallel, one set is called 25L and 25R, and the other is 24L and 24R. Of course, with their corresponding reverse designations from the other direction.

Also to chime in as a private pilot…at Paine Field in Everett WA there are two parellels, plus a crossing diagonal that crosses one of the parallels. The intersecting runways are often both open simultaneously, with ATC managing the traffic. As a pilot I could request one or the other, but was frequently overridden and asked to use a different runway based on the other traffic in the air.

I would frequently squeeze out on the diagonal in between the heavy jets from Boeing using the main runway.

Wow. Lots of extraneous information in this thread, and a few blatant falsehoods.

Airports most definitely will use intersecting runways simulateously at an airport. It’s not uncommon to have one runway used only for departures and another used only for arrivals. That is what local controllers (“Tower”) are for: To sequence landing and departing planes and make sure that none are occupying the same part of runway at the same time. There are strict rules the controllers must follow for sequencing and maintaining separation on intersecting, parallel, and same runways. To list all of them would be time-consuming, but basically, arrivals have priority over departures, and controllers must wait until a possible conflict has cleared.

There was a near-miss at Boston Logan airport in June that arose from exactly this scenario - two commercial aircraft (1 United 737, 1 Aer Lingus Airbus) cleared to depart near-simultaneously from intersecting runways 15R and 09. According to reports, they were cleared by two different controllers using different frequencies.

An informative report of the incident may be found here:

So yes, it is potentially a very dangerous situation.

Dad abd I used to fly to Las Vegas occasionally ‘for lunch’. (Really to get a couple of hours of stick time each. :wink: ) Light aircraft would take off and land on the shorter runway (IIRC, to the South) while heavies would use the longer intersecting runway.

Just to add to the (already voluminous) information…the tower guys pretty much have complete control over what runways are in use, up to and including using runways with a tailwind. At a large airport like O’Hare or DFW they often continue to use runways with small tailwinds rather than “turn the airport around”.

When an airport gets “turned around” (ie switch from north flow to south flow) it can cause serious delays, especially if it’s during a busy departure/arrival period.

On the other hand, airports that are not busy can change active runways rapidly. In a previous life I was an instructor and we sometimes needed to fly specific approaches for training. Often these approaches went against the prevailing winds or were to intersecting runways. We could almost always get clearance to shoot the approach, but sometimes it took some creative sequencing by the approach controller.

In my present job we’re concerned about being on time. We also go into some smaller airports served by only one or two parallel runways. Last week we were on approach into airport X, which was landing to the south. We were coming from the south, and we park on the north side of the airport. If we could land north we would not only save about 5 minutes of flying time but also about two miles of taxiing after landing. (For you pilot types out there: Rwy 17/35, ATIS calls winds 180/5, landing runway 17. We park at the very NW ramp, so an approach to 35 saves us tons of time).

We ask approach control if we can get Rwy 35. They say yes, then no, then yes again. A Delta guy took off on Rwy 17 and turned on his departure route, followed by us landing on Rwy 35 and clearing the runway. Shortly after we cleared a Southwest 737 coming in from the north landed on Rwy 17. Three airport movements with an opposite-direction landing in the middle. All safely done, but only possible when the airspace is not busy.

Sooo…not exactly “intersecting” runways, but the concept is the same.

OK, another post to hopefully address the actual OP.

As others have mentioned, several considerations go into building an airport. In the hectic days of WWII they didn’t have time to figure out what the prevailing winds were over the course of a year at New Army Air Corps Base of the Week, so they often built the triangle-design airport. This is three runways each 120 degrees off from the other forming an equilateral triangle. The advantage to this design is that you can always have a runway without a direct (ie 90 degrees off) crosswind. At worst if one runway had a 90-deg crosswind another would only have a 30-deg crosswind, so you’d land and take off on the one with the 30-deg crosswind. This would be the only active runway and no departures/arrivals would take place on the intersecting runways.

As airports grew and became busier everyone realized it made good sense to line the runways up with the prevailing winds. Sometimes this can lead to what seems like uncanny results: look at the airports in the Dallas/Fort Worth, TX area. North/south runways are the way to go, period. DFW, Alliance, Carswell, Meacham, even Love are all aligned N/S.

Other areas (ABQ for example) are tougher. Sometimes the wind blows from one direction, sometimes another. The trick is to find the combination of runways that will allow the greatest flexibility of operations without paving the entire city. What ABQ did is essentially what was done during WWII: throw down runways 120 degrees off and hope for the best. But ABQ is at a high altitude, and aircraft require much longer runways for takeoff at high altitude airports. Rather than lay down a 2 mile x 2 mile x 2 mile triangle, ABQ went with the intersecting runway option. (Looking at a Google Earth image of ABQ really helps you visualize this).

They saved immensely on space but now must be effective controlling the runways. In today’s environment of reliable communication this is not a problem.

The final restriction on laying runways is available space. La Guardia airport’s runways might have some basic knowledge of prevailing winds, but it is a rudimentary knowledge at best. Some runways are layed down because there is only one place they CAN be layed down, winds be damned. La Guardia has two runways so that they can run departures off of one and landings on another. They intersect because LaGuardia is VERY space limited. There is no option but to sequence departures between arrivals. Yes, it makes the airport busy. But the reason the airport is busy is because it’s convenient for millions of travelers. Location, location, location!