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Separation Standards

Introduction:

  • Separation standards are the processes, procedures and rules in place by Air Traffic Control to improve safety in the National Airspace System (NAS)

Instrument Flight Rules Separation Standards:

  • ATC effects separation of aircraft vertically by assigning different altitudes; longitudinally by providing an interval expressed in time or distance between aircraft on the same, converging, or crossing courses, and laterally by assigning different flight paths
  • Separation will be provided between all aircraft operating on IFR flight plans except during that part of the flight (outside Class B airspace or a TRSA) being conducted on a VFR−on−top/VFR conditions clearance
    • Under these conditions ATC may issue traffic advisories, but it is the sole responsibility of the pilot to be vigilant so as to see and avoid other aircraft
  • When radar is employed in the separation of aircraft at the same altitude, a minimum of 3 miles separation is provided between aircraft operating within 40 miles of the radar antenna site, and 5 miles between aircraft operating beyond 40 miles from the antenna site
    • These minima may be increased or decreased in certain situations
    • Separation standards are increased, for example, in the terminal environment when CENRAP is being utilized

Speed Adjustments:

  • ATC will issue speed adjustments to pilots of radar-controlled aircraft to achieve or maintain required or desire spacing
  • ATC will express all speed adjustments in terms of knots based on indicated airspeed (IAS) in 10 knot increments except that at or above FL 240 speeds may be expressed in terms of Mach numbers in 0.01 increments
    • The use of Mach numbers is restricted to turbojet aircraft with Mach meters
    • Pilots complying with speed adjustments are expected to maintain within ± 10 knots/0.02 Mach number or 5% of the specified speed in accordance with reporting requirements
  • When ATC assigns speed adjustments, it will be in accordance with the following recommended minimums:
    • To aircraft operating between FL 280 and 10,000 feet, a speed not less than 250 knots or the equivalent Mach number
      • On a standard day the Mach numbers equivalent to 250 knots CAS (subject to minor variations) are:
        • FL 240-0.6
        • FL 250-0.61
        • FL 260-0.62
        • FL 270-0.64
        • FL 280-0.65
        • FL 290-0.66
      • When an operational advantage will be realized, speeds lower than the recommended minima may be applied
    • To arriving turbojet aircraft operating below 10,000 feet:
      • A speed not less than 210 knots, except;
      • Within 20 flying miles of the airport of intended landing, a speed not less than 170 knots
    • To arriving reciprocating engine or turboprop aircraft within 20 flying miles of the runway threshold of the airport of intended landing, a speed not less than 150 knots
    • To departing aircraft:
      • Turbojet aircraft, a speed not less than 230 knots
      • Reciprocating engine aircraft, a speed not less than 150 knots
  • When ATC combines a speed adjustment with a descent clearance, the sequence of delivery, with the word "then" between, indicates the expected order of execution
    • ATC: "Descend and maintain [altitude]; then, reduce speed to [speed]"
    • Example: Reduce speed to (speed); then, descend and maintain (altitude)
  • The maximum speeds below 10,000 feet as established in 14 CFR Section 91.117 still apply
    • If there is any doubt concerning the manner in which such a clearance is to be executed, request clarification from ATC
  • If ATC determines (before an approach clearance is issued) that it is no longer necessary to apply speed adjustment procedures, they will:
    • Advise the pilot to “resume normal speed.” Normal speed is used to terminate ATC assigned speed adjustments on segments where no published speed restrictions apply. It does not cancel published restrictions on upcoming procedures. This does not relieve the pilot of those speed restrictions which are applicable to 14 CFR Section 91.117
      • Example: (An aircraft is flying a SID with no published speed restrictions. ATC issues a speed adjustment and instructs the aircraft where the adjustment ends): “Maintain two two zero knots until BALTR then resume normal speed”
      • The ATC assigned speed assignment of two two zero knots would apply until BALTR. The aircraft would then resume a normal operating speed while remaining in compliance with 14 CFR Section 91.117
    • Instruct pilots to “comply with speed restrictions” when the aircraft is joining or resuming a charted procedure or route with published speed restrictions
      • Example: (ATC vectors an aircraft off of a SID to rejoin the procedure at a subsequent waypoint. When instructing the aircraft to resume the procedure, ATC also wants the aircraft to comply with the published procedure speed restrictions): “Resume the SALTY ONE departure. Comply with speed restrictions"
      • Caution: The phraseology “comply with restrictions” requires compliance with all altitude and/or speed restrictions depicted on the procedure
    • Instruct the pilot to “resume published speed.” Resume published speed is issued to terminate a speed adjustment where speed restrictions are published on a charted procedure
      • When instructed to “comply with speed restrictions” or to “resume published speed,” ATC anticipates pilots will begin adjusting speed the minimum distance necessary prior to a published speed restriction so as to cross the waypoint/fix at the published speed. Once at the published speed, ATC expects pilots will maintain the published speed until additional adjustment is required to comply with further published or ATC assigned speed restrictions or as required to ensure compliance with 14 CFR Section 91.117
      • EXAMPLE: (An aircraft is flying a SID/STAR with published speed restrictions. ATC issues a speed adjustment and instructs the aircraft where the adjustment ends): “Maintain two two zero knots until BALTR then resume published speed"
      • NOTE: The ATC assigned speed assignment of two two zero knots would apply until BALTR. The aircraft would then comply with the published speed restrictions
    • Advise the pilot to “delete speed restrictions” when either ATC assigned or published speed restrictions on a charted procedure are no longer required
      • EXAMPLE: (An aircraft is flying a SID with published speed restrictions designed to prevent aircraft overtake on departure. ATC determines there is no conflicting traffic and deletes the speed restriction): “Delete speed restrictions"
      • NOTE: When deleting published restrictions, ATC must ensure obstacle clearance until aircraft are established on a route where no published restrictions apply. This does not relieve the pilot of those speed restrictions which are applicable to 14 CFR Section 91.117
  • Approach clearances supersede any prior speed adjustment assignments, and pilots are expected to make their own speed adjustments as necessary to complete the approach. However, under certain circumstances, it may be necessary for ATC to issue further speed adjustments after approach clearance is issued to maintain separation between successive arrivals. Under such circumstances, previously issued speed adjustments will be restated if that speed is to be maintained or additional speed adjustments are requested. Speed adjustments should not be assigned inside the final approach fix on final or a point 5 miles from the runway, whichever is closer to the runway
  • The pilots retain the prerogative of rejecting the application of speed adjustment by ATC if the minimum safe airspeed for any particular operation is greater than the speed adjustment
    • NOTE: In such cases, pilots are expected to advise ATC of the speed that will be used
  • Pilots are reminded that they are responsible for rejecting the application of speed adjustment by ATC if, in their opinion, it will cause them to exceed the maximum indicated airspeed prescribed by 14 CFR Section 91.117(a), (c) and (d). IN SUCH CASES, THE PILOT IS EXPECTED TO SO INFORM ATC. Pilots operating at or above 10,000 feet MSL who are issued speed adjustments which exceed 250 knots IAS and are subsequently cleared below 10,000 feet MSL are expected to comply with 14 CFR Section 91.117(a)
  • Speed restrictions of 250 knots do not apply to U.S. registered aircraft operating beyond 12 nautical miles from the coastline within the U.S. Flight Information Region, in Class E airspace below 10,000 feet MSL. However, in airspace underlying a Class B airspace area designated for an airport, or in a VFR corridor designated through such as a Class B airspace area, pilots are expected to comply with the 200 knot speed limit specified in 14 CFR Section 91.117(c)
  • For operations in a Class C and Class D surface area, ATC is authorized to request or approve a speed greater than the maximum indicated airspeeds prescribed for operation within that airspace (14 CFR Section 91.117(b))
    • NOTE: Pilots are expected to comply with the maximum speed of 200 knots when operating beneath Class B airspace or in a Class B VFR corridor (14 CFR Section 91.117(c) and (d))
  • When in communications with the ARTCC or approach control facility, pilots should, as a good operating practice, state any ATC assigned speed restriction on initial radio contact associated with an ATC communications frequency change
  • Pilot:

    • Advises ATC any time cruising airspeed varies plus or minus 5 percent or 10 knots, whichever is greater, from that given in the flight plan
    • Complies with speed adjustments from ATC unless:
      • The minimum or maximum safe airspeed for any particular operation is greater or less than the requested airspeed. In such cases, advises ATC
        • It is the pilot's responsibility and prerogative to refuse speed adjustments considered excessive or contrary to the aircraft's operating specifications
      • Operating at or above 10,000 feet MSL on an ATC assigned SPEED ADJUSTMENT of more than 250 knots IAS and subsequent clearance is received for descent below 10,000 feet MSL. In such cases, pilots are expected to comply with 14 CFR Section 91.117(a)
    • When complying with speed adjustment assignments, maintains an indicated airspeed within plus or minus 10 knots or 0.02 Mach number of the specified speed
  • Controller:

    • Assigns speed adjustments to aircraft when necessary but not as a substitute for good vectoring technique
    • Adheres to the restrictions published in the FAAO JO 7110.65, Air Traffic Control, as to when speed adjustment procedures may be applied
    • Avoids speed adjustments requiring alternate decreases and increases
    • Assigns speed adjustments to a specified IAS (KNOTS)/Mach number or to increase or decrease speed using increments of 10 knots or multiples thereof
    • Terminates ATC-assigned speed adjustments when no longer required by issuing further instructions to pilots in the following manner:
      • Advises pilots to "resume normal speed" when the aircraft is on a heading, random routing, charted procedure, or route without published speed restrictions
      • Instructs pilots to "comply with speed restrictions" when the aircraft is joining or resuming a charted procedure or route with published speed restrictions
        • The phraseology "comply with restrictions" requires compliance with all altitude and/or speed restrictions depicted on the procedure
      • Instructs pilots to "resume published speed" when aircraft are cleared via a charted instrument flight procedure that contains published speed restrictions
      • Advises aircraft to "delete speed restrictions" when ATC assigned or published speed restrictions on a charted procedure are no longer required
      • Clears pilots for approach without restating previously issued speed adjustments
    • Gives due consideration to aircraft capabilities to reduce speed while descending
    • Does not assign speed adjustments to aircraft at or above FL 390 without pilot consent

Runway Separation:

  • Tower controllers establish the sequence of arriving and departing aircraft by requiring them to adjust flight or ground operation as necessary to achieve proper spacing. They may “HOLD” an aircraft short of the runway to achieve spacing between it and an arriving aircraft; the controller may instruct a pilot to “EXTEND DOWNWIND” in order to establish spacing from an arriving or departing aircraft. At times a clearance may include the word “IMMEDIATE"
    • For example: “CLEARED FOR IMMEDIATE TAKEOFF.” In such cases “IMMEDIATE” is used for purposes of air traffic separation. It is up to the pilot to refuse the clearance if, in the pilot's opinion, compliance would adversely affect the operation

Gate Holding Due to Departure Delays:

  • Pilots should contact ground control or clearance delivery prior to starting engines as gate hold procedures will be in effect whenever departure delays exceed or are anticipated to exceed 15 minutes
    • The sequence for departure will be maintained in accordance with initial call up unless modified by flow control restrictions
    • Pilots should monitor the ground control or clearance delivery frequency for engine startup advisories or new proposed start time if the delay changes
  • The tower controller will consider that pilots of turbine-powered aircraft are ready for takeoff when they reach the runway or warm-up block unless advised otherwise

Runway:

  • Tower controllers establish the sequence of arriving and departing aircraft by requiring them to adjust flight or ground operation as necessary to achieve proper spacing
  • The may "hold" an aircraft short of the runway to achieve spacing between it and an arriving aircraft; the controller may instruct a pilot to "extend downwind" in order to establish spacing from an arriving or departing aircraft
  • At times a clearance may include the word "immediate"
    • For example: "cleared for immediate takeoff"
    • In such cases "immediate is used for the purposes of air traffic separation
  • It is up to the pilot to refuse the clearance if, in the pilots opinion, compliance would adversely affect operation

Visual Separation:

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  • Visual separation is a means employed by ATC to separate aircraft in terminal areas and en route airspace in the NAS. There are two methods employed to effect this separation:
    • The tower controller sees the aircraft involved and issues instructions, as necessary, to ensure that the aircraft avoid each other
    • A pilot sees the other aircraft involved and upon instructions from the controller provides separation by maneuvering the aircraft to avoid it. When pilots accept responsibility to maintain visual separation, they must maintain constant visual surveillance and not pass the other aircraft until it is no longer a factor
    • Traffic is no longer a factor when during approach phase the other aircraft is in the landing phase of flight or executes a missed approach; and during departure or en route, when the other aircraft turns away or is on a diverging course
  • A pilot’s acceptance of instructions to follow another aircraft or provide visual separation from it is an acknowledgment that the pilot will maneuver the aircraft as necessary to avoid the other aircraft or to maintain in−trail separation. In operations conducted behind heavy aircraft, or a small aircraft behind a B757 or other large aircraft, it is also an acknowledgment that the pilot accepts the responsibility for wake turbulence separation. Visual separation is prohibited behind super aircraft
    • When a pilot has been told to follow another aircraft or to provide visual separation from it, the pilot should promptly notify the controller if visual contact with the other aircraft is lost or cannot be maintained or if the pilot cannot accept the responsibility for the separation for any reason
  • Scanning the sky for other aircraft is a key factor in collision avoidance. Pilots and copilots (or the right seat passenger) should continuously scan to cover all areas of the sky visible from the cockpit. Pilots must develop an effective scanning technique which maximizes one’s visual capabilities. Spotting a potential collision threat increases directly as more time is spent looking outside the aircraft. One must use timesharing techniques to effectively scan the surrounding airspace while monitoring instruments as well
  • Since the eye can focus only on a narrow viewing area, effective scanning is accomplished with a series of short, regularly spaced eye movements that bring successive areas of the sky into the central visual field. Each movement should not exceed ten degrees, and each area should be observed for at least one second to enable collision detection. Although many pilots seem to prefer the method of horizontal back−and−forth scanning every pilot should develop a scanning pattern that is not only comfortable but assures optimum effectiveness. Pilots should remember, however, that they have a regulatory responsibility (14 CFR Section 91.113(a)) to see and avoid other aircraft when weather conditions permit

Pilot Responsibilities:

  • A pilot's acceptance of instructions to follow another aircraft or provide visual separation from it is an acknowledgment that the pilot will maneuver the aircraft as necessary to avoid the other aircraft or to maintain in-trail separation
    • Pilots are responsible to maintain visual separation until flight paths (altitudes and/or courses) diverge
    • In operations conducted behind heavy jet aircraft, it is also an acknowledgment that the pilot accepts the responsibility for wake turbulence separation

NOTE:
When a pilot has been told to follow another aircraft or to provide visual separation from it, the pilot should promptly notify the controller if visual contact with the other aircraft is lost or cannot be maintained or if the pilot cannot accept the responsibility for the separation for any reason

  • Pilots and copilots (or the right seat passenger) should continuously scan to cover all areas of the sky visible from the cockpit
    • Pilots must develop an effective scanning technique which maximizes one's visual capabilities
    • Spotting a potential collision threat increases directly as more time is spent looking outside the aircraft
    • One must use timesharing techniques to effectively scan the surrounding airspace while monitoring instruments as well
  • Since the eye can focus only on a narrow viewing area, effective scanning is accomplished with a series of short, regularly spaced eye movements that bring successive areas of the sky into the central visual field. Each movement should not exceed ten degrees, and each area should be observed for at least one second to enable collision detection. Although many pilots seem to prefer the method of horizontal back-and-forth scanning every pilot should develop a scanning pattern that is not only comfortable but assures optimum effectiveness. Pilots should remember, however, that they have a regulatory responsibility (14 CFR Section 91.113(a)) to see and avoid other aircraft when weather conditions permit

Controller Responsibilities:

  • With regard to visual separation:
    • Within the terminal area when a controller has both aircraft in sight or by instructing a pilot who sees the other aircraft to maintain visual separation from it
    • Pilots are responsible to maintain visual separation until flight paths (altitudes and/or courses) diverge
    • Within en route airspace when aircraft are on opposite courses and one pilot reports having seen the other aircraft and that the aircraft have passed each other

Conclusion:

  • When weather conditions permit, during the time an IFR flight is operating, it is the direct responsibility of the pilot to avoid other aircraft since VFR flights may be operating in the same area without the knowledge of ATC
    • Traffic clearances provide standard separation only between IFR flights

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