Narrative:

Small area of thunderstorms about 20 miles southeast of ovr. Aircraft X; flight of six; departed oma; and appeared to be on course direct irk. Aircraft Y departed oma a few minutes later; on course direct jalap. Both routes put aircraft in close proximity to thunderstorm. Aircraft X flight was about 7 or so miles in trail; non standard; with the trailing aircraft squawking 4000; but no mode C; and was significantly slower than aircraft Y. The aircraft between the lead and trail aircraft were primary only. Controller cleared aircraft X direct destination; which appeared to keep the flight north of the route of aircraft Y; as well as clear of the thunderstorm. Aircraft X was cleared by the section 27 controller to 190b210. The trailing aircraft was lower than the lead aircraft; who was the only one in the flight with his mode C turned on. Aircraft Y checked on with the sector 27 controller climbing to 150; as per standard procedures. When aircraft Y stated that he needed to deviate for the thunderstorm; the section 27 controller immediately started calling traffic with the trailing aircraft in the aircraft X flight; as well as ask the aircraft X flight lead what the altitude of the trailing aircraft was. Aircraft Y reported the aircraft in sight; and the flight lead reported that the trailing aircraft was above aircraft Y by more than a thousand feet. The targets of the trailing aircraft and aircraft Y merged; or were very close to merging.it seems that oma approach cleared aircraft Y to an altitude and on a route that did not guarantee that necessary separation would remain between the aircraft X flight and aircraft Y. Both aircraft were cleared on a route that put them very near or into a known thunderstorm; for which many aircraft had already deviated from their assigned route. Also; because the aircraft X flight was significantly slower in comparison to aircraft Y; this was an overtake situation. Additionally; the aircraft X flight was non standard; many miles in trail. This had the potential to put aircraft Y in very close proximity with one or more of the aircraft in the flight. My suggestion is that; in this case; oma approach should have assigned aircraft Y and aircraft X flight either headings or routes that clearly met approved separation standards; as well as keep the aircraft clear of the known thunderstorm. This would allow the section 27 controller to climb aircraft Y above the flight; at which point the aircraft could join their assigned routes. The attempted use of vertical separation; with the large difference in speed (much faster behind) and when the trail aircraft of a flight of six is not squawking mode C; in this case; caused an uncontrolled situation and had the potential to put these aircraft too close together.

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Original NASA ASRS Text

Title: ZMP FLM and Controller report of a non-standard flight being overtaken by speed of a trailing aircraft.

Narrative: Small area of thunderstorms about 20 miles SE of OVR. Aircraft X; flight of six; departed OMA; and appeared to be on course direct IRK. Aircraft Y departed OMA a few minutes later; on course direct JALAP. Both routes put aircraft in close proximity to thunderstorm. Aircraft X flight was about 7 or so miles in trail; non standard; with the trailing aircraft squawking 4000; but no Mode C; and was significantly slower than Aircraft Y. The aircraft between the lead and trail aircraft were primary only. Controller cleared Aircraft X direct destination; which appeared to keep the flight north of the route of Aircraft Y; as well as clear of the thunderstorm. Aircraft X was cleared by the Section 27 Controller to 190B210. The trailing aircraft was lower than the lead aircraft; who was the only one in the flight with his Mode C turned on. Aircraft Y checked on with the Sector 27 Controller climbing to 150; as per standard procedures. When Aircraft Y stated that he needed to deviate for the thunderstorm; the Section 27 Controller immediately started calling traffic with the trailing aircraft in the Aircraft X flight; as well as ask the Aircraft X flight lead what the altitude of the trailing aircraft was. Aircraft Y reported the aircraft in sight; and the flight lead reported that the trailing aircraft was above Aircraft Y by more than a thousand feet. The targets of the trailing aircraft and Aircraft Y merged; or were very close to merging.It seems that OMA Approach cleared Aircraft Y to an altitude and on a route that did not guarantee that necessary separation would remain between the Aircraft X flight and Aircraft Y. Both aircraft were cleared on a route that put them very near or into a known thunderstorm; for which many aircraft had already deviated from their assigned route. Also; because the Aircraft X flight was significantly slower in comparison to Aircraft Y; this was an overtake situation. Additionally; the Aircraft X flight was non standard; many miles in trail. This had the potential to put Aircraft Y in very close proximity with one or more of the aircraft in the flight. My suggestion is that; in this case; OMA Approach should have assigned Aircraft Y and Aircraft X flight either headings or routes that clearly met approved separation standards; as well as keep the aircraft clear of the known thunderstorm. This would allow the Section 27 controller to climb Aircraft Y above the flight; at which point the aircraft could join their assigned routes. The attempted use of vertical separation; with the large difference in speed (much faster behind) and when the trail aircraft of a flight of six is not squawking Mode C; in this case; caused an uncontrolled situation and had the potential to put these aircraft too close together.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.