Free Flight with Airborne Separation Assurance


OPERATIONAL CONCEPT: Our "Rules of the Road"

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"Free Flight" is an evolving concept. Nonetheless, before we could carry out an exploratory study into the feasibility of Free Flight in en-route airspace, we had to adopt some basic definition of the operational scenario in which aircraft would operate. This definition had to specify how aircraft would behave, including

The cornerstone of our ATM concept was the set of "Rules of the Air," or Extended Flight Rules (EFRs), that could be used to conduct Free Flight simulations. EFRs were intended to dictate how aircraft should self-separate, under conditions of minimal (or no) ground intervention. They had to do so both comprehensively (i.e., for all possible traffic encounters) and unambiguously (i.e., each party had to have a clear understanding of the responsibilities of all aircraft). Further, to expedite pseudopilot training, it was decided that the number of extended flight rules had to be kept to an absolute minimum.

EFRs were based upon reviews of the following sources:

These EFRs had to make some simplifying assumptions, for the purposes of experimental design. For instance, these EFRs disregard such factors as: Model-of-aircraft differences in manoeuvrability; Gross mismatches in aircraft equipage; and flight priority differences.


Rule 1: Altitude for Direction-of-Flight (AFDOF)

Aircraft are required to fly at alternating flight levels determined by their heading, and separated by a minimum of 1,000 feet (i.e., 10 FLs). On headings of 360 (i.e., due north) to 179 degrees, aircraft must fly at odd numbered Flight Levels (FLs). On headings of 180 (i.e., due south) to 359, aircraft must fly at even numbered FLs. For example, eastbound aircraft may operate at FLs 170, 190, 210, 230, etc. Westbound aircraft may operate at FLs of 160, 180, 200, 220, etc. These rules apply to cruise, not transitional phases of flight. This prevents the need for aircraft to ascend / descend in a "corkscrew" fashion.


Altitude for Direction-of-Flight EFR


Rule 2: Phase-of-Flight Priority (PFP)

Differences in phase-of-flight (e.g., climb, cruise, descent) are associated with differences in aircraft manoeuvrability. It was decided that this should be reflected in the determination of which aircraft should bear the greater responsibility for evasive manoeuvres . Duong, Hoffman, Floc’hic and Nicolaon (1996) developed a matrix of priorities for aircraft in encounter situations, based on both the phase (i.e., climb, cruise, descent) of each, as well as the sub-phase (e.g., initial, intermediate, final sub-phases of climb). It was decided that such a fine categorisation would have been impractical to implement with experimental pseudopilots. Therefore differences in sub-phase were disregarded. As a result, the following matrix of Phase-of-Flight Priority (PFP) can be used to resolve only encounters in which aircraft differ in phase-of-flight.

Matrix of Phase-of-Flight Priority (PFP)


Rule 3. Extended VFR Overtaking Rules (EVOR)

ICAO sets forth standard rules for overtaking in VFR conditions. These rules were designed for use in conditions in which electronic surveillance data are not available, and they have the advantage of being straightforward and reliable. The basic requirement of VFR states that any overtaking aircraft do so on the starboard (right) side of other craft, so that the other craft remains to the port (left) side of ownship during the overtake. The presumed rationale for this requirement stems from the enhanced useful field of view that this configuration affords the left seat pilot.

Clearly, VFR overtaking rules can be very inefficient and ambiguous under certain circumstances. For instance, VFR state that if a faster aircraft wishes to overtake a slower aircraft (on roughly the same heading), though is already established to the port side of the slower aircraft, it must alter its heading so as to pass to the starboard side of the other craft. However, VFR do not address at what level of lateral separation this overtaking rule no longer need apply.

To correct some of the ambiguities and inefficiencies of VFR overtaking rules, the ATLAS project suggested a set of relative separation rules that were designed for autonomous self-separating flights, operating at the same altitude. These rules allow for different evasive manoeuvres, depending on the angle of closure between two aircraft. As shown in figure 3, this set of rules defines the following three lateral segments of airspace around an aircraft: · Port Forward · Starboard Forward · Rear

Lateral airspace segments, for Extended VFR Overtaking Rules (EVORs)


These airspace segments permit four EVORs to be specified, as follows.

Rule 1.
If both aircraft are both in the P region of the other, both turn right.

Rule 2.
If both aircraft are in the S region of the other, both turn left. Figure 4b.

Rule 3.
If aircraft A is in the P region of aircraft B’s airspace, and aircraft B is in the S region of aircraft B, then A must turn in the direction that achieves separation with minimal deviation.

Rule 4.
If aircraft A is in the R region of aircraft B, and aircraft B is in the S or P region of aircraft A (i.e., A is in-trail behind B), then aircraft A will turn in either direction to achieve separation with minimal deviation.

These four rules apply to situations in which vertical separation is not maintained. This might refer to two aircraft at cruise. It might also, though, refer to transitioning (ascending / descending) aircraft. For instance, EVOR Rule 2 would dictate that aircraft travelling on reciprocal headings, though in identical climbs, should achieve lateral separation. Second, in cases of identical heading (i.e., precisely head-on or in-trail), in which no preferred evasive path exists, VFR rules should still apply—that is, aircraft should turn right.

Rule Priority

In some cases, it might not be clear which rule to apply to a local conflict. For instance, if two aircraft are approaching at a slightly oblique angle and at the same flight level, it would not be clear whether to apply AFDOF (such aircraft should have already been separated by 1,000 feet) or EVOR (and allow the aircraft to pass to the starboard side of one another). In such cases, separation assurance is left to the discretion of the user (in this case, the pseudopilot), who is expected to consider the time available for establishing minimum safe separation. The rules 1, 2, and 3 will generally correspond, respectively, to more tactical timeframes. That is, if there is insuffient time to loss-of-impact to apply AFDOF, then the pseudopilot would be expected to separate aircraft using EVOR.

Summary

It is believed that these three broad Extended Flight Rules (EFRs)—AFDOF, PFP, and EVOR-- are sufficient to achieve self separation in localised encounters for most potential scenarios. AFDOF and EVOR together should apply to all possible cruise encounters. PFP rules will apply in most mixed-phase encounters (e.g., when one aircraft is ascending, and the other is at cruise). Certain same-phase encounters (e.g., both ascending through identical flight levels, though on reciprocal headings), cannot be addressed by the current EVOR.

The purpose of these rules was to facilitate real-time human-in-the-loop simulations of free flight scenarios. These rules do not purport to cover all conceivable conflict situations, nor always provide the most optimal solution to a given situation. Instead, they were seen as a minimal set of guidelines that could guide the self separation behaviour of simulated autonomous aircraft.

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