Longitudinal Separation After a Level Change
You are the area controller at a procedural (non-radar) sector. Aircraft A, a B763 (wake category H) at FL350, reports over waypoint ALFA at 10:00, estimating BRAVO at 10:24, and next reporting point CHARLIE. Aircraft B, a B738 (category M), is at FL330 on the same track, 12 minutes behind A, and requests FL350. The sector has no radar coverage; the only navigation aid is an NDB at BRAVO. The route segment ALFA–BRAVO is 240 NM. The aircraft are on the same airway with no lateral offset. Minimum longitudinal separation on the airway is 15 minutes.
1. Can you clear B to FL350 now? Work through the longitudinal and vertical separation logic.
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No. Clearing B to FL350 would put both aircraft at the same level on the same track, and the only separation available is longitudinal. The 15-minute minimum is met at the moment of the request (12 minutes is not 15), so even before the climb the standard is not satisfied if B is to be at the same level.
More importantly, the climb changes the geometry. Once B is cleared to FL350, B is no longer 1000 ft below A; B is level with A and closing. The 12-minute gap must be converted to a distance and compared with the time needed for the climb. At a closing speed of, say, 20 kt (if B is faster), 12 minutes corresponds to roughly 4 NM of closure - negligible - but the 15-minute standard is a time standard, not a distance standard, and it is not met.
Correct action: hold B at FL330 or issue a crossing restriction. If B is cleared to FL350, the controller must first establish 15 minutes' longitudinal separation at the same level, which means waiting until B is at least 15 minutes behind A at FL350, or applying a Mach number technique with a documented speed control and a positive check at the next reporting point.
2. A now reports over BRAVO at 10:26, 2 minutes later than estimate. B reports over ALFA at 10:12. Reassess the separation and decide whether B may climb.
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Recompute from actual reports, not estimates. A passed BRAVO at 10:26; B passed ALFA at 10:12. The segment ALFA–BRAVO is 240 NM. B's time over ALFA is 10:12, and A's time over ALFA was 10:00, so the actual time interval at ALFA was 12 minutes - still short of 15.
Now project forward. A's ground speed over ALFA–BRAVO was 240 NM in 26 minutes, about 554 kt. B has not yet reported BRAVO. The interval at BRAVO will be the interval at ALFA plus any change in the gap. If B maintains the same speed as A, the interval stays 12 minutes; if B is faster, it shrinks. The controller must not assume the gap will grow.
Decision: B may not climb to FL350 on this evidence. The controller should either keep B at FL330 (vertical separation of 1000 ft is satisfied because A is at FL350), or apply a speed control to B to build the interval to 15 minutes, or wait for B's BRAVO report and recompute. If a Mach technique is used, the controller must assign a Mach number to B that is equal to or less than A's and verify it at the next reporting point.