In 1994 City of Tempe and City of Phoenix came to an agreement on noise mitigation flight procedures for all jets and large commercial turbo propeller aircraft that depart from and arrive at Phoenix Sky Harbor International Airport over Tempe. In the Intergovernmental Agreement (PDF). City of Tempe agreed not to oppose the construction of a new third runway in return for noise mitigation flight procedures and the installation of a Noise and Flight Track Monitoring System at the airport.
With the agreement, the City of Tempe received a binding commitment from the City of Phoenix and assurances from the Federal Aviation Administration (FAA) that commercial aircraft operations close to the airport would be restricted to the airspace over the Salt River riverbed in Tempe. The FAA made assurances to Tempe in a Record of Decision Amendment (PDF).
The riverbed designated flyway is designed to limit the noise impact of arriving and departing aircraft on the adjacent Tempe neighborhoods. The agreement has three mitigation measures:
The "4-DME" procedure, called the "One-DME" procedure prior to the move of a navigational aid (VORTAC) located in Tempe.
The "side-step" procedure.
1. The 4-DME procedure:
When jets and large turbo propeller aircraft* depart to the east from any runway, they are directed to follow specific headings outbound over the Salt River for a distance of 4 nautical miles, east of a navigational aid (VORTAC), before continuing on the designated departure route.
*The 4-DME procedures was not implemented for large turboprop aircraft.
2. The side-step procedure:
When jets and large turbo propeller aircraft bound for the south runway are approaching from the east, they are to be on a stabilized approach path to the center runway, and have clearance by Air Traffic Control to change over to the south runway (25L) approach when alongside the Sun Devil Stadium at Mill Avenue. The maneuver to change approach course and line the aircraft up with the center line of an adjacent parallel runway is called to "side-step".
The FAA implemented an informal side-step procedure when the new south runway was opened up for commercial air traffic, but the practice and the attempt to formalize the procedure was suspended by the FAA on March 27, 2002.
To the extent possible over the year, to disperse the noise impact of departing eastbound and westbound jets and large turbo propeller aircraft evenly between Phoenix and Tempe during day and nighttime hours.
Oversight and Supervision:
The IGA includes provisions for the City of Phoenix to develop, install, and operate a noise and flight track monitoring system. This system was installed for the Phoenix Sky Harbor International Airport in the late 90's and makes it possible to monitor how well airlines keep to the agreed procedures and avoid flying over noise sensitive areas in Tempe on the north and south sides of the riverbed just east of the Phoenix Sky Harbor International Airport. The City of Phoenix is responsible for giving written notifications to owners or operators of aircraft that do not comply with the agreed upon flight procedures.