The FAA as stated in Order JO 7110.65X defines winds as calm when the wind velocity is less than three (3) knots.  The calm wind threshold was established to ensure the safety of the flight.   When wind velocity exceeds this threshold air traffic control must change the flow direction to ensure airplanes take off and land into the prevailing wind. 

When calm wind conditions exist the FAA air traffic controllers have the option to operate Norman Y. Mineta San Jose International Airport (SJC) in either North or South flow. The preferred flow direction at SJC is a North Flow with the airport operating in this configuration 83% of the time in 2023 according to the airport's 2023 Annual Noise Report

During times of inclement weather, or during frontal passages, and on some mornings, the wind direction shifts and blows out of the south exceeding the three (3) knot calm wind threshold, requiring the airport to operate in a South Flow configuration. 

Generalize Runway 12R Approach Pattern (South Flow)

When SJC operates in a south flow arriving aircraft overfly the community of Sunnyvale on the downwind leg of their race track type approach to line up with the runway and land.  At any time the current wind speed/direction and SJC flow can be viewed on the homepage of the Noise Lab website or on the City's Casper Flight Tracker both of which display the current weather conditions as measured at SJC Airport.

Arriving aircraft in South Flow based on their performance, and onboard navigation equipment are assigned one of two primary approaches by the FAA to land at SJC.  The first is the Instrument Landing System/Localizer (ILS/LOC) approach to either Runway 12L or 12R.  The second is the Required Navigation Performance (RNP) Z Approach to either Runway 12L or 12R.  The majority of landing aircraft that fly over Sunnyvale in South Flow are landing on Runway 12R which is closest to the City.

ILS/LOC Runway 12R Approach

The ILS/LOC Runway 12R Approach is a straight-in approach to Runway 12R, which requires arriving aircraft to fly parallel to the runway at SJC on the downwind leg of the approach before turning back south towards the runway to land.  Airplanes assigned this approach over fly Sunnyvale from south to north with the flight tracks being dispersed laterally over a large area of the city.  This large lateral dispersion of flight tracks is caused because the planes are being "vectored" or instructed to fly a specific compass heading by ATC. However, due to wind drift and the individual aircraft's current track, speed, and turn rate when aligning onto the assigned heading to enter the downwind leg the dispersion of flight tracks over the city can be significant.  The flight track graphic below captured from Sunnyvale's Casper Noise system shows twenty-three (23) individual flight tracks with a high degree of lateral dispersion.

ILS/LOC Runway 12R Approaches to SJC Airport

Generally, speaking aircraft that are being radar vectored onto the downwind for the ILS/LOC approach by ATC are instructed to descend in a series of stair steps called a "step down" approach.  When the airplane reaches the newly assigned altitude it must level off until it is cleared to a lower altitude.  This maneuver requires the pilots to increase engine power to maintain the altitude which increases noise on the ground.

Typical Step-Down ILS/LOC Approach to Runway 12R at SJC

The high variability in the vertical flight profile from flight to flight is also based on when the aircraft is instructed by ATC to turn back towards the airport onto's its final approach to line up with the runway.   For Visual Meteorological Conditions (VMC) to exist in Class C Airspace, such as SJC, there must be at least 3 statute miles visibility and 2,000 feet horizontal, and 500 feet vertical separation underneath the clouds. If these conditions exist a pilot flying under an Instrument Flight Rules (IFR) flight plan may be cleared by ATC to fly a Visual Approach to the runway and if traffic volumes permit will not fly the entire length of the ILS approach from the initial approach fix at ARTAQ in.  Instead, the airplane will most likely join the final approach course somewhere between ARTAQ and SUNNE.

Vertical Flight Profiles of ILS/LOC Runway 12R Approaches to SJC

When the cloud ceiling and or visibility drops below the VMC threshold, Instrument Meteorological Conditions (IMC) govern all arriving and departing aircraft at SJC.  Under IMC conditions landing aircraft in south flow assigned the ILS/GPS approach must fly the entire approach from ARTAQ in towards the runway.  In addition, greater aircraft separation standards between landing aircraft are required by ATC to ensure aircraft are kept at a safe distance from each other as pilots are no longer able to maintain visual separation from each other.

RNP Z Runway 12R Approach

The RNP Z Runway 12R Approach at SJC Airport is an all weather RNP AR Approach with a minimum performance level of 0.3.  This means that in order to be authorized to fly the approach the aircraft is required to have both aircraft and operational approval for RNP 0.3  and the navigation system must be able to continually calculate its position within a circle with a radius no larger than three (3) tenths of a nautical mile (approximately 1,823 feet) left or right of the track centerline. 

This approach also contains two Radius to Fix (RF) legs between ZORSA->PUAKO and PUAKO->FOXAG that require a much higher degree of accuracy in navigation in order for the aircraft to maintain the track keeping precision required during the fixed radius turn.

RNAV (RNP) Z Runway 12R Approaches to SJC Airport

Unlike the ILS/LOC approach which uses altitude step-downs and has a high degree of lateral and vertical dispersion and can be hand flown by the pilot, the RNP approach to Runway 12R due to the high level of precision required is flown by the aircraft's onboard autopilot using the track position information from the Flight Management System (FMS).  The autopilot manages the aircraft's speed and pitch angle to ensure it stays within a very narrow prescribed descent window.  While at the same time the autopilot constantly corrects for lateral wind drift to keep the airplane within the narrow 0.3 nautical miles lateral boundary of the approach.  The result, as shown in the above flight track graphic taken from Sunnyvale's Casper Noise system, shows forty-four (44) individual flight tracks stacked in a very narrow, highly concentrated flight path with very little lateral dispersion.

The vertical dispersion of airplanes flying the RNP Z Runway 12R approach is very small, due to the vertical speed management performed by the aircraft's autopilot.  There are prescribed minimum altitudes coded into the approach procedure for various segments of the approach as shown in the graphic above.  The autopilot uses this information and continually calculates based on the aircraft's current speed, altitude, and descent rate what adjustments to make to the airplane's vertical path to ensure it stays within the allowed limits of the approach procedure.

Vertical Flight Profiles of RNP Z Runway 12R Approaches to SJC Airport