In our Ask a Pilot series, pilot Spencer Marker answers one of your aviation-related questions each week. See past installments here and submit your own to Whitney@johnnyjet.com.
The question
On the last flight I took, another airplane passed overhead and it seemed really close. How close together can airplanes be when they fly?
—Robert B.
The answer
Hey Robert. Thanks for posing such a great question for this week’s Ask a Pilot. Whether it’s at cruise altitude or landing at airports with parallel runways, I receive lots of questions from passengers like this.
Our friends at the FAA call this aircraft separation. They’ve set up rules regarding separation that pilots and Air Traffic Controllers must abide by. Let’s have a look at some of these rules established by the FAA and how close together airplanes are during certain phases of flight.
Cruise altitude
While flying at cruise altitude, the FAA dictates that from the ground to 29,000 feet, no airplane must come within 1,000 feet vertically of one another. This means that while flying in an airliner, the aircraft you see pass over you is a minimum of 1,000 feet above.
From 29,000 feet to 41,000 feet, the rules change slightly. At these altitudes, the FAA requires airplanes to be separated by 2,000 feet. This rule was introduced in 1958 and was founded in the inaccuracies in altimeters (the instruments pilots use to read altitude) at higher altitudes.
With the advent of more advanced altimeters and computerized flight instruments, the FAA reduced the spacing to 1,000 feet with the introduction of RVSM, or reduced vertical separation minima. This change acknowledged advances in technology while also allowing for more air traffic in a volume of airspace.
Airplanes are separated laterally at cruise as well. The distance does vary, but generally speaking, airplanes at high altitudes are spaced a minimum of 10 nautical miles horizontally. During approach and landing segments, the spacing required is reduced.
Takeoff spacing
When an airplane is departing, Air Traffic Controllers can place aircraft much closer together than they do at cruise altitude. This allows controllers to maximize their rate of departures for efficiency.
Airports like Atlanta and San Francisco will allow simultaneous takeoffs on their parallel runways. After liftoff, the aircraft turn opposite directions, thus creating safe separation.
Controllers will also space out aircraft departing on the same runway. This is due primarily to a phenomenon known as wake turbulence. This spiral vortex that originates on a plane’s wing tip can be quite strong. Its intensity is directly related to the weight of the airplane, so an A380 will create a much stronger wake vortex than a Boeing 737 will. If a smaller jet gets caught in this vortex, it can induce a sudden roll. This sudden maneuver can be quite startling to passengers.
To mitigate this, ATC will space out airplanes based on their wake turbulence categories. Heavier airplanes require more distance before a subsequent smaller plane can takeoff.
Landing
Separating landing airplanes presents a unique challenge for Air Traffic Controllers. Not only do you have to pay heed to wake turbulence from different types of aircraft, but you also are responsible for guiding the airplanes to their respective approaches when the weather deteriorates.
When an airport has parallel runways, like New York’s JFK and SoCal’s LAX do, the runways are required to be built a minimum of 4,300 feet apart to allow two airplanes to approach simultaneously on each runway. Certain airports have approval to land airplanes simultaneously with as little as 3,000 feet separating them, so long as the airport has an enhanced radar system.
SFO
So, right about now, those of you that fly into SFO regularly are thinking, “SFO has runways that are way closer together than that! How does that work?” This brings up a good point about one of the most delay-prone airports in the country.
San Francisco International Airport was constructed into the San Francisco Bay and it has four runways, two pairs of parallels. The parallel runways most often used (28 Left and 28 Right) have a mere 750 feet separating them. Yet SFO will still land an airplane on each runway simultaneously. How is this possible?
Well, during good weather, ATC relies on a pilot’s ability to see the airport and the airplane they will be flying adjacent to during their approach. When a pilot tells ATC that they see the other airplane and the airport is in sight, the controller will clear the pilot for a visual approach. This means that it is now the pilot’s responsibility to maintain a safe distance from the other jet and fly their airplane to the runway visually. The 750 feet separating two jets feels very close to most passengers and makes for some spectacular photos for avgeeks on the ground!
When the famous San Francisco fog rolls in (as it does frequently), the arrival rate at the airport decreases significantly and the delays start piling up. This is because pilots must fly instrument approaches to the airport rather than visual approaches. And while two runways can still be used, one approach path must be offset (not lined up with the runway) to provide adequate spacing in the clouds. Additionally, controllers will stagger aircraft on the approach and as a result, the arrival rate decreases.
Constructing a runway further out into the bay to alleviate this kind of congestion has been discussed but is met with fierce opposition from environmentalists in the Bay Area. Check out a great video from pilotsEYE.tv and see what it’s like to land an A380 in SFO.
To sum up
Thanks for the great question, Robert! The space between two airplanes often seems like less than it actually is. The FAA has well-established rules to dictate the space that must exist between two airplanes at all times. And as a pilot who has frequented the SFO airport, I’m glad you gave me the opportunity to discuss what makes this airport’s operation so unique.
Thanks again for your question! And if anyone have a burning aviation question or something you would like cleared up, drop us a line at Whitney@johnnyjet.com to get your question featured in an upcoming Ask a Pilot column.
Tailwinds,
—Spencer
Thanks for this post, I was actually watching a live map of all the places over our country flying around and there seemed to be like, A BILLION, PLANES ALL FLYING IN THE SAME FREAKING AREA. So, that’s answered.
Hi, Spencer!
Having worked with the FAA for many years, I’d like to mention the FAA has a system called traffic management that sets up departures well in advance of flight time (at least 12 hours) so that there will not be too many aircraft in any one place for the controllers to keep separate. That greatly reduces delays en route, since most delays are taken on the ground. (The airlines negotiate with the FAA in advance for which flights will delay — often based on how full the flight is, but also based on where the airline plans to use the plane later).
That means that by the time your flight takes off, a moving space for your airplane has been reserved all the way to airport where your aircraft will land. En route, controllers may adjust this plan because of minor changes that creep in, but for the most part (as you know) things go smoothly and there are no long holding patterns (like there used to be in the 1970’s and before).
One of the biggest flies in the ointment for this scheme is weather. Especially in summer, convective events (thunder storms, for example) may crop up and block traffic routes faster than the forecasters and computers can adapt. In those cases, there may be additional delays, and they are engineered into the traffic management on the fly (so to speak), minimizing airborne delays even when the system is stressed.
So the FAA is carefully planning your flight long before takeoff so the controllers and pilots will have a clear path for most flights once they take off.