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Why Is My Flight Delayed At SFO? Blame It On Flow Control

SFO is one of the most unique airports in America both because of its local weather and its design. In good weather, the airport has some of the most unique simultaneous approaches and departures.  In less than ideal weather, delays are unfortunately very common.  What makes SFO so unique?  Let’s explore.

In good weather, planes fly simultaneous approaches and departures

The Bay Area is blessed with a very temperate climate that makes for comfortable weather year-round.  Yet the weather can also create chaos and do so quickly.

Have you ever looked out the window while landing at San Francisco International in good weather and noticed a plane flying seemingly in a close formation with you?  There’s a reason for that.  San Francisco International Airport has two sets of parallel runways.  On most days, traffic lands on runways 28L and 28R.  These two runways are only 750 feet apart.  The non-standard separation of the runways combined with the high volume of traffic at SFO along with noise abatement procedures has forced some creative and complex approaches into the airport.

During visual conditions, controllers are able to stagger the aircraft by assigning precise speeds at each waypoint of the visual approach augmented by navigational aids.  This type of approach results in a relatively high flow of traffic  of around 60 aircraft per hour(that keeps airline schedules running smoothly) and unique photos for Avgeeks.

A veces tengo suerte y veo algunas maravillas.

A video posted by Hugo Morales (@gonzomau) on

When the low ceilings (or fog) roll in, get ready for delays

During less than visual conditions, the aircraft are spaced further apart laterally but still closer than standard separation at most airports.  The airport utilizes what is known as a PRM approach or Precision Runway Monitoring combined with SOIA or Simultaneous Offset Instrument Approach.  A PRM approach is an approach that utilizes an additional radar controller to ensure separation.  It is utilized at other major airports across the country to keep planes safe while flying closer together than standard on approaches during instrument conditions.  SOIA is also employed at SFO to keep aircraft spaced further than 750 feet laterally until they are visual.

One aircraft on 28L flies the ILS (precision approach) and the aircraft on 28R flies an LDA or GPS which is slightly offset to the runway.  (See approach plate here with myriad of notes).  This unique approach does slow down the traffic flow to 38 aircraft per hour but keeps aircraft safely flowing into SFO during less than ideal weather conditions.  During really poor weather (like fog or low ceilings), SFO is restricted even further to just 30 aircraft per hour because arriving aircraft must approach the field in a single file line as they utilize the ILS (up to CAT-III) to land on 28R.  Check out this guide if you are interested in learning more about SFO’s weather operations plan.

The next time you fly into SFO, thank the pilots for flawlessly executing a very complicated approach procedure to get you there safely.

Read more about SOIA at SFO here:
Read more about SOIA at SFO here:

Written by Avgeekery

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