Appendix E E-2
• Inspectors and air carriers can access FAA Order 8900.1, Flight Standards Information Management System, on DRS. Operators and the public can find this order on the FAA’s website at https://www.faa.gov/regulations_policies/orders_notic es and DRS.
• You can find a current list of Technical Standard Orders (TSO) on DRS.
E.2.1 FAA Guidance Material.
1. AC 20-165, Airworthiness Approval of Automatic Dependent Surveillance - Broadcast OUT Systems.
2. AC 20-172, Airworthiness Approval for ADS-B In Systems and Applications.
3. FAA Order 8900.1, Volume 3, Chapter 18, Section 3, Operations Specification (OpSpec)/Management Specification (MSpec)/Letter of Authorization (LOA) A354, Automatic Dependent Surveillance-Broadcast (ADS-B) In-Trail Procedure (ITP).
4. Aeronautical Information Manual (AIM), Paragraph 4-5-7, Automatic Dependent Surveillance-Broadcast (ADS-B) Services, through Paragraph 4-5-10, Automatic Dependent Surveillance-Rebroadcast (ADS-R).
5. Surveillance and Broadcast Services Description Document (SRT-047).
E.2.2 FAA TSOs (edition listed below or later revision).
1. TSO-C129a, Airborne Supplemental Navigation Equipment Using the Global Positioning System (GPS).
2. TSO-C145c, Airborne Navigation Sensors Using the Global Positioning System Augmented by the Satellite Based Augmentation System (SBAS).
3. TSO-C146c, Stand-Alone Airborne Navigation Equipment Using the Global Positioning System Augmented by the Satellite Based Augmentation System (SBAS).
4. TSO-C154c, Universal Access Transceiver (UAT) Automatic Dependent Surveillance-Broadcast (ADS-B) Equipment Operating on Frequency of 978 MHz.
5. TSO-C157a, Aircraft Flight Information Services-Broadcast (FIS-B) Data Link Systems and Equipment.
6. TSO-C166b, Extended Squitter Automatic Dependent Surveillance - Broadcast (ADS-B) and Traffic Information Service - Broadcast (TIS-B) Equipment Operating on the Radio Frequency of 1090 Megahertz (MHz).
7. TSO-C195b, Avionics Supporting Automatic Dependent Surveillance – Broadcast (ADS-B) Aircraft Surveillance Applications (ASA).
8. TSO-C196a, Airborne Supplemental Navigation Sensors for Global Positioning System Equipment Using Aircraft-Based Augmentation.
E.2.3 RTCA, Inc. Documents. You can find the most recent editions of RTCA documents available for purchase at https://my.rtca.org/nc__store.
Appendix E E-3
1. RTCA DO-260B, Minimum Operational Performance Standards for 1090 MHz Extended Squitter Automatic Dependent Surveillance - Broadcast (ADS-B) and Traffic Information Services - Broadcast (TIS-B).
2. RTCA DO-282, Minimum Operational Performance Standards for Universal Access Transceiver (UAT) Automatic Dependent Surveillance - Broadcast (ADS-B).
3. RTCA DO-312 Supplement, Safety, Performance and Interoperability Requirements Document for the In Trail Procedure in Oceanic Airspace (ATSA-ITP) Application.
4. RTCA DO-317B, Minimum Operational Performance Standards (MOPS) for Aircraft Surveillance Applications (ASA) System.
5. RTCA DO-328B, Safety, Performance and Interoperability Requirements Documents for Airborne Space – Flight Deck Interval Management (ASPA-FIM).
6. RTCA DO-354, Safety and Performance Requirements Document for CDTI Assisted Visual Separation (CAVS).
7. RTCA DO-361A, Minimum Operational Performance Standards (MOPS) for Flight-Deck Interval Management (FIM).
E.3 European Union Aviation Safety Agency (EASA) Publications (current editions).
1. EASA Acceptable Means of Compliance (AMC) 20-24, Certification Considerations for the Enhanced ATS in Non-Radar Areas Using ADS-B Surveillance (ADS-B-NRA) Application via 1090 MHz Extended Squitter.
2. EASA Certification Specifications and Acceptable Means of Compliance for Airborne Communications, Navigation and Surveillance (CS-ACNS).
Appendix F F-1 APPENDIX F. INTERVAL MANAGEMENT
CONTENTS Paragraph Page
F.1 PurposeF-3
F.2 BackgroundF-3
F.3 IM DescriptionF-3
F.3.1 IM Description—Ground ComponentF-3
F.3.2 IM Description—Airborne ComponentF-3
F.3.3 Notional PhraseologyF-4
F.3.4 Early FIM AvionicsF-4
F.4 IM TerminologyF-4
F.4.1 Assigned Spacing Goal (ASG)F-4
F.4.2 Cross Point (CP)F-4
F.4.3 Designated TrafficF-4
F.4.4 Flight-Deck Interval Management (FIM) AvionicsF-4
F.4.5 Flight Identification (FLT ID)F-5
F.4.6 Interval Management (IM) AircraftF-5
F.4.7 Interval Management (IM) Clearance TypeF-5
F.4.8 Interval Management (IM) Path MonitoringF-5
F.4.9 Interval Management (IM) SpeedF-5
F.4.10 Interval Management (IM) Speed Compliance MonitoringF-5
F.4.11 Planned Cancellation Point (PCP)F-5
F.4.12 Selected TrafficF-5
F.5 IM Clearance Types DescribedF-5
F.5.1 Clearance Type FormatF-6
F.5.2 Maintain Current SpacingF-6
F.5.3 Capture Then MaintainF-7
F.5.4 Achieve-by Then Maintain (“Cross”)F-7
F.5.5 Final Approach SpacingF-8
F.5.6 IM SuspendF-8
F.5.7 SafeRoute+ DifferencesF-8
F.5.7.1 Maintain Current Spacing and Capture Then MaintainF-8
Appendix F F-2
F.5.7.2 Achieve-by Then MaintainF-9
F.5.7.3 No IM SuspendF-9
F.6 IM ProcedureF-9
F.6.1 ATC Identifies IM PairF-9
F.6.2 ATC Checks FeasibilityF-9
F.6.3 ATC Issues IM ClearanceF-9
F.6.4 IM Flightcrew Reads Back ClearanceF-10
F.6.5 IM Flightcrew Verifies Lead AircraftF-10
F.6.5.1 FLT ID VerificationF-10
F.6.6 FIM Avionics Internal CheckF-10
F.6.7 IM Flightcrew Evaluates IM SpeedF-10
F.6.7.1 Rejected ClearanceF-10
F.6.7.2 Accepted ClearanceF-10
F.6.8 IM Flightcrew Flies IM SpeedsF-10
F.6.8.1 Compliance With IM SpeedsF-10
F.6.8.2 Mandatory Published SpeedsF-11
F.6.8.3 Compliance With Operational or Regulatory Speed RestrictionsF-11
F.6.9 IM CancellationF-11
F.7 Off-Nominal SituationsF-11
F.8 IM Flight PlanningF-12
F.8.1 IM Outside the U.S. NASF-12
F.8.2 FP CodesF-12
F.9 Authorization to Conduct IMF-12
F.9.1 Initial Request for AuthorizationF-12
F.9.2 Documentation to Submit With Formal RequestF-12
F.9.2.1 Request LetterF-12
F.9.2.2 Airplane Flight Manual (AFM) Compliance DocumentationF-13
F.9.2.3 Operation Manuals and ChecklistsF-13
F.9.2.4 Maintenance DocumentationF-13
F.9.2.5 Revision of Minimum Equipment List (MEL)F-13
F.9.2.6 Pilot TrainingF-13
F.9.2.7 Dispatch/Flight Follower TrainingF-14
Appendix F F-3 F.1 Purpose. This appendix describes Interval Management (IM) and provides guidance to operators seeking FAA authorization to conduct IM operations.
F.2 Background. The busiest airports operate most efficiently when air traffic control (ATC) delivers the arriving aircraft in a well-spaced sequence within an appropriate interval.
The benefits of this spacing may include reduced fuel burn, reduced emissions, reduced noise, reduced ATC transmissions, and, as applied to the National Airspace System (NAS), greater throughput and reduced delays. IM is a set of procedures and capabilities designed to provide precise aircraft-to-aircraft spacing, and is suitable for use en route, on a terminal arrival, and final approach. It is an ADS-B In application that takes advantage of NAS-wide presence of ADS-B Out-equipped aircraft.
F.3 IM Description. IM has a ground component and an airborne component. On the ground, ATC uses automation support tools to identify candidate pairs of IM aircraft and develop an IM clearance. The flight deck component, called Flight-deck Interval Management (FIM) avionics, consists of equipment that provides IM speeds to be flown by the flightcrew based on the information provided in the ATC clearance. The purpose of the IM clearance is to establish and maintain a precise spacing between a lead ADS-B Out aircraft and a trail IM aircraft. The IM speeds provided by the FIM avionics are expected to be flown as if each one was an individual ATC speed instruction for the duration of the IM procedure. The flightcrew complies with the ATC IM clearance by flying the IM speeds. ATC continues to retain responsibility for safe separation between the lead ADS-B Out aircraft and the trail IM aircraft, as well as all other aircraft.
Note: Some may describe the IM aircraft as “following” a lead aircraft. The IM aircraft does not actually follow the lead aircraft but rather spaces from it in time or distance. At all times, each aircraft is navigating independently, as in non-IM operations. There are IM scenarios where the lead aircraft and IM aircraft are on different navigation routes and can be at different altitudes.
F.3.1 IM Description—Ground Component. ATC initiates an IM operation when it is determined to be operationally beneficial. ATC, in conjunction with the IM automation support tool, identifies a candidate pair of aircraft consisting of a lead ADS-B Out aircraft and a FIM-equipped IM aircraft. The tool also calculates a desired spacing, called the assigned spacing goal (ASG). Based on when ATC needs the ASG, the controller can choose one of the IM clearance types (covered in paragraph F.5).
F.3.2 IM Description—Airborne Component. IM is just one application that can be installed in approved ADS-B In avionics as described in Chapter 2, Overview and System Description, paragraph 2.4.1, and meets the performance requirements of Technical Standard Order (TSO)-C195c, Avionics Supporting Automatic Dependent Surveillance – Broadcast (ADS-B) Aircraft Surveillance Applications (ASA). As with other ADS-B In applications, FIM avionics consist of the Cockpit Display of Traffic Information (CDTI) and application-specific software. The CDTI consists of an input device for the IM clearance elements, a traffic display, and a means to display the IM speeds and provide FIM-specific indications and alerting. A speed algorithm calculates the IM speed: the indicated airspeed (IAS) or Mach number the flightcrew flies to
Appendix F F-4 achieve or maintain the ASG. Calculation of the IM speed is based on a number of variables, including the along-path distance between the two aircraft, their groundspeeds (GS), and the forecast winds and temperatures entered into the FIM avionics or other flight deck systems. The algorithm will limit calculated IM speeds to prevent overly large speed changes and exceeding certain aircraft, regulatory, or procedural limitations.
Precise spacing requires that the algorithm use a defined horizontal and vertical path for both aircraft. Therefore, either both aircraft must be on the same horizontal, or near parallel route, or the lead aircraft’s navigation clearance1 must be provided to the FIM avionics. The FIM avionics monitor for horizontal path conformance of both the lead and IM aircraft and IM speed compliance. The avionics will provide appropriate indications to the flightcrew if any path conformance or speed compliance issues arise.
1 The lead aircraft’s navigation clearance may be provided as a named route or a sequence of waypoints. “Same route” may be used to indicate when the lead aircraft will be on the same route as the IM aircraft.
F.3.3 Not ional Phraseology. IM has been developed through years of extensive study, simulation, and limited operational trials. However, at the time of this writing, the situations in which ATC may use IM, and the actual phraseology, are still evolving.
Therefore, we use notional phraseology in this appendix to explain the IM procedure. We will add approved phraseology to subsequent revisions of this guidance.
F.3.4 Early FIM Avionics. The early versions of FIM avionics may have only a subset of the full IM capabilities described in the avionics standards. We will identify any differences in this AC. As later versions of FIM avionics are produced that meet the full set of capabilities described in the avionics standards, we will revise this AC to reflect the added functionality. Additionally, equipment terminology may evolve as new FIM avionics designs are fielded. We will use terminology from the avionics standards in this AC.
F.4 IM Terminology. We use the following terminology in this appendix.
F.4.1 Assigned Spacing Goal (ASG). The time or distance interval between the IM aircraft and lead aircraft assigned by ATC in the IM clearance. The ASG includes both a value and unit of dimension as time or distance.
F.4.2 Cross Point (CP). A point on the IM aircraft’s planned flightpath where the ASG is intended to be met. Also referred to as the “Achieve-by” point in industry standards.
F.4.3 Designated Traffic. Traffic designated in the CDTI as the lead aircraft for a specific ADS-B In application. The FIM avionics will make designated traffic distinguishable from all other traffic on the traffic display.
F.4.4 Flight-Deck Interval Management (FIM) Avionics. FIM avionics refer to the avionics that provide the IM capabilities defined in this AC. The FIM avionics act as the flightcrew interface and provide features like the IM clearance input interface and the display of IM speeds and any indications or alerts.
Appendix F F-5 F.4.5 Flight Identification (FLT ID). The flight identification, or FLT ID, can be the telephonic “call sign” (e.g., “Brickyard 3691”), the International Civil Aviation Organization (ICAO) three-letter designator (3LD) combined with a flight number (e.g., “RPA3691”), or the registration number of the aircraft (e.g., “N12345”). The FLT ID of the lead aircraft is given by ATC in the IM clearance.
F.4.6 Interval Management (IM) Aircraft. An aircraft that is equipped with FIM avionics, flown by an IM-qualified flightcrew.
F.4.7 Interval Management (IM) Clearance Type. The basic action that ATC instructs IM aircraft flightcrew to perform as part of the IM operations. The IM clearance types are Achieve-by Then Maintain, Maintain Current Spacing, Capture Then Maintain, Final Approach Spacing, and IM Turn.
F.4.8 Interval Management (IM) Path Monitoring. An internal FIM avionics function that leads to the removal of IM information when either the IM aircraft or the lead aircraft vary from their expected route by a predetermined amount.
F.4.9 Interval Management (IM) Speed. The speed in IAS or Mach calculated by the FIM avionics that the flightcrew flies to conform to the ATC IM clearance and to achieve or maintain the ASG.
F.4.10 Interval Management (IM) Speed Compliance Monitoring. An internal FIM function designed to ensure precise flightcrew adherence to IM speed changes. The speed monitoring may include visual and aural alerting that notifies the flightcrew when the aircraft speed varies from the IM speed by a predetermined amount, or that the flightcrew has not responded to an IM speed change for a predetermined amount of time.
F.4.11 Planned Cancellation Point (PCP). A point on the IM aircraft’s planned flightpath where the IM operations end normally. Also referred to as the “Planned Termination Point” in industry standards.
F.4.12 Selected Traffic. Traffic uniquely chosen by the flightcrew via the CDTI to obtain additional information (e.g., FLT ID and GS). The FIM avionics will make selected traffic distinguishable from all other traffic on the traffic display. Selected traffic is not participating in an IM operation (but designated traffic is).
F.5 IM Clearance Types Described. The FIM avionics standard contains five clearance types: Maintain Current Spacing, Capture Then Maintain, Achieve-by Then Maintain, Final Approach Spacing, and IM Turn. These clearance types describe the functions the FIM avionics perform; however, the terms used over the voice frequency and that are shown on the flightcrew interface may change to align with approved phraseology. ATC does not currently plan to implement the IM Turn clearance type in the NAS, so we do not address it here. We will add information related to IM Turn to a future revision of this appendix should that change.
Appendix F F-6 F.5.1 Clearance Type Format. Each clearance type has required clearance elements and optional clearance elements for ATC to provide. For all clearance types, ATC will provide the FLT ID of the lead aircraft along with the clearance type. For all clearance types, there is an ASG; ATC will state the ASG (in units of time or distance), or it will be implied (i.e., current spacing). ATC will provide the IM aircraft the planned route for the lead aircraft and the CP for the Achieve-by Then Maintain clearance type. The PCP is optional for all clearance types, but the FIM avionics will default the PCP in close proximity to the destination airport if a PCP is not entered.
F.5.1.1 See Table F-1 for a summary of the acceptable clearance element formats.
The ASG will be expressed in time (seconds) or distance. ATC will provide the CP and PCP as navigational fixes. The lead aircraft planned route will be given as a Standard Terminal Arrival Route (STAR), airway or approach procedure, or as a combination of those procedure types.
Table F-1. Clearance Element Formats Clearance Element Format Examples Assigned spacing goal (ASG) Time or distance 90 seconds, 10 nautical miles (NM) Lead aircraft planned route Standard Terminal Arrival Route (STAR), airway, approach, waypoint, or sequence of waypoints EAGUL6, J186, ILS 36R Cross Point (CP) Navigation fix SLIDR, PINNG, 10 NM EAST OF SLIDR Planned Cancellation Point (PCP) Navigation fix EAGUL, DRRVR F.5.2 Maintain Current Spacing. ATC can issue a Maintain Current Spacing clearance expressed in either time or distance. For example, while an IM aircraft is transiting an en route sector, the controller seeing that the current spacing is suitable for delivery to the next sector gives the following: “TRAIL 456 Maintain current time behind LED123,” or “TRAIL 456 Maintain current distance behind LED123.”
Appendix F F-7 Figure F-1. Maintain Current Spacing F.5.3 Capture Then Maintain. ATC can issue a Capture Then Maintain clearance when the IM aircraft and the lead aircraft are on similar routes. The FIM avionics will provide IM speeds to move the IM aircraft to the ASG. A sample IM clearance could be:
“TRAIL 456 Capture then maintain 10NM behind LED123.”
Figure F-2. Capture Then Maintain F.5.4 Achieve-by Then Maintain (“Cross”). When ATC needs the ASG achieved by the time the IM aircraft reaches a particular fix, the controller may issue an Achieve-by Then Maintain clearance. The lead aircraft and IM aircraft may be on different paths at the time of the clearance, but will cross a common fix. The clearance will contain the lead aircraft’s planned route and the identification of the fix to cross, and optionally the PCP.
The ASG can be expressed in time or distance. An example could be: “TRAIL 456 Cross HONEE 90 seconds behind LED123 on the BRAVO Transition. Cancel at LASFX.” The IM flightcrew will fly the IM speeds to achieve the ASG by the time they cross the CP.
They will maintain the ASG until the PCP, or until the controller cancels the IM operation.
Appendix F F-8 Figure F-3. Achieve-by Then Maintain (Cross) F.5.5 Fi nal Approach Spacing. ATC may issue a Final Approach Spacing clearance when one or both of the lead aircraft and the IM aircraft will be on the same final approach course.
One aircraft can be on an intercept to final if the other aircraft is already established on the final approach course. The controller will include the lead Aircraft Identification (ACID) and the ASG.
Figure F-4. Final Approach Spacing F.5.6 IM Suspend. ATC may have a need to interrupt an active IM operation, such as to resolve a developing traffic conflict. IM Suspend is not strictly a clearance type. It is functionality in the FIM avionics that temporarily removes IM speeds, indications, and alerts, but retains the basic IM clearance information.
F.5.7 SafeRoute+ Differences. SafeRoute+ is the Aviation Communication & Surveillance Systems’ (ACSS) commercial name for the only FIM avionics in production at the time of this writing (2025). It contains a subset of the standard IM functions and has some functionality that differs from the industry standard. The SafeRoute+ avionics only accommodate two of the clearance types (i.e., Capture Then Maintain and Achieve-by Then Maintain), and they differ slightly from the standards invoked in TSO-C195c.
F.5.7.1 Maintain Current Spacing and Capture Then Maintain. In the absence of an ASG entry by the flightcrew, the SafeRoute+ FIM avionics calculate IM speeds to maintain the current spacing. A time or distance input must still be made by the flightcrew.
Appendix F F-9 F.5.7.2 Achieve-by Then Maintain. A SafeRoute+ Achieve-by Then Maintain clearance differs from the standards in two ways. It does not accept the lead aircraft’s planned route; therefore, the lead aircraft and IM aircraft must be flying direct to a common merge point.
F.5.7.3 No IM Suspend. SafeRoute+ does not include a “suspend” function. Instead, the pilot uses the “terminate” (also known as “cancel”) function.
Table F-2. Interval Management Clearance Types Summarized
Clearance Type Maintain Capture Then Maintain Cross Final Approach Spacing Notional Phraseology “TRAIL 456 Maintain distance behind LED123” “TRAIL 456 Maintain 90 seconds behind LED123” (or “Maintain 10 miles”) “TRAIL 456 Cross HONEE 90 seconds behind LED123 on the BRAVO transition. Cancel at LASFX.”
“TRAIL 456 Cross GOSER 90 seconds behind LED123” Clearance Elements Lead A/C FLT ID Lead A/C FLT ID Lead A/C FLT ID Lead A/C FLT ID ASG (implied) ASG ASG ASG CP Lead aircraft planned route if different than the IM aircraft planned route Lead aircraft planned route if different than the IM aircraft planned route PCP (optional) PCP (optional) PCP PCP defaulted Notes Underline indicates clearance type cue.
Not in SafeRoute+.
Underline indicates clearance type cue.
Lead aircraft is on different route— “BRAVO” transition.
Not in SafeRoute+.
F.6 IM Procedure. ATC may initiate an IM operation when operationally beneficial. The IM operations will proceed as follows:
F.6.1 ATC Identifies IM Pair. Using automation support tools, ATC identifies the potential IM lead aircraft and IM aircraft, ASG, CP (when applicable), and PCP.
F.6.2 ATC Checks Feasibility. ATC, supported by automation, determines the feasibility of the IM operation. Some of the factors used in this determination are current spacing, remaining distance, conflicting traffic, and weather.
F.6.3 ATC Issues IM Clearance. ATC issues the IM clearance to the IM aircraft flightcrew using one of the four clearance types.
Appendix F F-10 F.6.4 IM Flightcrew Reads Back Clearance. The IM aircraft flightcrew reads back the IM clearance and enters the IM clearance information into the FIM avionics.
F.6.5 IM Flightcrew Verifies Lead Aircraft. The flightcrew verifies that they have selected the correct lead aircraft and that they have entered the IM clearance elements correctly.
F.6.5.1 FLT ID Verification. Verifying the correct lead aircraft is an essential step.
It can be complicated by two factors. The first factor is that the telephonic or spoken call sign of the lead aircraft might not easily translate into the FLT ID displayed by the FIM avionics. For example, “Brickyard” does not readily translate to “RPA.” The second factor is that the lead aircraft or nearby nonparticipating aircraft might have erroneously programmed their FLT ID in their ADS-B Out system. The flightcrew should not accept the IM clearance if there is any doubt that they have correctly identified the lead aircraft as that given in the IM clearance.
F.6.6 FIM Avionics Internal Check. The FIM avionics perform an internal check to ensure all performance criteria are met. If the performance criteria are met, the FIM avionics display the first IM speed.
F.6.7 IM Flightcrew Evaluates IM Speed. The flightcrew evaluates the IM speed to determine whether it is acceptable given operational considerations, such as ride conditions and own aircraft limitations.
F.6.7.1 Rejected Clearance. If the flightcrew determines that the IM speed is unacceptable, they will immediately notify ATC that they are unable to accept the IM clearance.
F.6.7.2 Accepted Clearance. If the flightcrew determines that the IM speed is acceptable, they begin complying with the IM speeds with no further instruction by ATC (i.e., further read-back or reporting the implementation of each IM speed). ATC does not require any further acknowledgment by the flightcrew that they have accepted the IM clearance. This is similar to acknowledging a clearance to descend or climb; ATC expects the flightcrew to promptly comply or notify ATC that they are unable to comply.
F.6.8 IM Flightcrew Flies IM Speeds. The FIM avionics calculate IM speeds to maintain or achieve the ASG. In the case of an Achieve-by Then Maintain (also known as “Cross”) clearance, the IM speeds are flown to achieve the ASG by the time the IM aircraft reaches the CP. In the case of a Capture Then Maintain clearance, the IM speeds are flown to achieve the ASG quickly and then maintain that ASG until reaching the PCP.
F.6.8.1 Compliance With IM Speeds. It is important to emphasize that flightcrews should resist the temptation to “out-guess” the FIM algorithm by flying speeds other than the IM speed. Flying the IM speeds is how the flightcrew complies with the ATC-issued IM clearance. The FIM algorithm is designed to account for speed changes that result from the lead aircraft trajectory as well as actual winds encountered along the IM aircraft’s route, which inevitably will differ