BluSkyi·Engineering Insight Article
Flight Testing · Commercial Derivative Aircraft · Handling Qualities
Four Safety Gates Stand Between a Civil Turboprop and the Forward Air Control Role
A military-standard flight test of the LX7-135 found strong climb, endurance and dynamic stability, but egress, stick clearance, pitch stability and secure radio shortfalls must be fixed before any tactical use.
- Source Journal
- Aerospace
- Lead Institution
- Istanbul Technical University
- Published
- 25 July 2026
- Discipline
- Aerospace
- Focus Area
- Aircraft, UAV & Aviation Systems
- Research Type
- Applied
- Licence
- CC BY 4.0
- Authors
- Köroğlu et al.
The Bottom Line
A light civil turboprop can do much of what an airborne forward air controller needs: it climbs hard, loiters for hours and has well-damped dynamics that forgive a pilot looking outside the cockpit. But a single exit door, a sidestick that fouls the cockpit wall, weak pitch-force cues and no secure radio are hard gates, not trade-offs. For buyers of commercial derivative aircraft, the lesson is that civil airworthiness says little about military suitability until someone flies the mission.
Key Findings · At a Glance
- Of 26 assessed parameters, 12 were rated satisfactory, 10 tolerable and 4 unsatisfactory by a single experimental test pilot.
- Two-person ground egress through the single left-side door took 60 seconds, double the 30-second limit set by MIL-STD-1290A.
- Full lateral sidestick deflection struck the cockpit wall or the pilot's leg, breaching the 1.5-inch clearance required by MIL-STD-1333.
- Longitudinal static stability showed no free return speed and a stick force slope of about 1 lb per 20 KIAS, meeting only Level 2.
- Corrected climb rate reached 2,900 ft/min at 168 KIAS, even with engine torque limited to 1,900 of an available 2,400 ft-lb.
- A 150-gallon tactical fuel limit gives 5.7 hours total endurance, including 4.7 hours on station, at an optimum 132 KIAS.
- Simulated rocket-delivery ratings improved from CHR 7 to CHR 3 over eight runs as the pilot adapted to the unaugmented airframe.
Opportunity
Close air support has traditionally relied on expensive multi-role jets, which the authors argue are logistically and economically inefficient for prolonged holding patterns. Procurement has been shifting toward lower-cost, high-endurance turboprops under the commercial derivative aircraft (CDA) model, which adopts civil airframes with proven aerodynamics and established supply chains to shorten development, contain cost overruns and reduce technical risk.
Forward Air Control (Airborne), or FAC(A), is among the harshest tests of that model. The crew deconflicts airspace, controls terminal attacks and guards against fratricide, often marking the target with smoke rockets 60 to 90 seconds before a strike aircraft arrives. The job fuses precise manoeuvring with heavy battle management.
Süleyman Murat Köroğlu and colleagues put the LX7-135, a single-engine, pressurised carbon-composite turboprop with a 750 shp engine and an off-the-shelf glass cockpit, through four sorties and 6.5 flight hours. They benchmarked it against military flying-qualities, cockpit-geometry and crash-resistance specifications, rated handling on the Cooper–Harper (CHR) scale and workload on the Bedford scale, and flew simulated 9-Line briefings and a Box Pattern smoke-rocket delivery. With the yaw damper unserviceable and the autopilot failing on the third sortie, much of the data describes the raw airframe.
Why Commercial Derivatives Have Not Yet Delivered
The authors locate the obstacle in what they call the airworthiness seam. Civil certification is built around passenger safety and benign flight profiles, and mission equipment that could endanger the aircraft, including weapons, rockets and active countermeasures, is ineligible for civil approval. Everything beyond the civil baseline must instead be judged against military criteria. Empirical evidence on how civil design choices behave on the military side of that seam is, in the authors' words, a significantly underexplored gap.
Results
Performance
Climb was the standout. Normalised to a standard weight, the aircraft achieved 2,900 ft/min at a best-climb speed of 168 KIAS, with a stated uncertainty of plus or minus 85 ft/min. That came with torque held to 1,900 ft-lb, roughly 79% of the engine's 2,400 ft-lb, because full power produced a left yaw that exceeded available rudder authority. A quadratic fuel-flow model (R² of 0.982) placed optimum endurance at 132 KIAS and 164.2 lb/hr. Capping fuel at 150 gallons frees weight for a two-person crew and 200 lb of mission equipment within the 3,850 lb maximum take-off weight.
Dynamics
The unaugmented airframe proved forgiving where it matters for target tracking. The short-period mode was deadbeat with no pilot-induced oscillation; Dutch roll damping was estimated at 0.4 controls-free and 0.5 controls-fixed; the phugoid was convergent at 120 KIAS with periods up to 200 seconds; and stalls were benign, with clear buffet and a progressive angle-of-attack tone.
The gating defects
Four findings were rated unsatisfactory. Egress took 60 seconds because the right-seat occupant had to cross the console to the only door. The sidestick could not reach full lateral deflection. Pitch stick forces were too shallow to cue airspeed. And the aircraft carries neither UHF nor secure voice radio, despite clear VHF out to 40 nautical miles.
Mission tasks
Flying a holding pattern by hand while relaying a 9-Line briefing, the pilot held altitude and route within tolerance and finished the briefing in 2.5 minutes, but heading wandered by up to 7 degrees against a 4-degree limit, and workload rated Bedford 6. In the Box Pattern, the first two of eight runs failed, one aborted after exceeding the never-exceed speed. By phase, the downwind and base legs rated CHR 4, gross acquisition CHR 5, fine tracking CHR 4 and the 3G dive recovery CHR 2. The team distilled an aircraft-specific profile: a 180 KIAS downwind, an idle glide to 110 KIAS, two segmented 45-degree turns into a 10-degree dive, and a 12 to 13 second tracking window.
The Practical Applications
Defence procurement
For buyers weighing turboprop CDAs for FAC(A) or close air support, the study maps where civil design falls short of military standards. The four unsatisfactory items are the kind that surface late and cost most to fix.
Light aircraft manufacturers
Builders of pressurised composite turboprops seeking a military market get specific design targets: a second exit, sidestick clearance, a steeper pitch-force gradient and military coordinate entry in the avionics.
Flight test organisations
The non-intrusive instrumentation suite, built from the avionics logger, a handheld force gauge, a clip-on gunsight, a phone clinometer and cockpit cameras, shows how much can be learned without disturbing a civil certification baseline.
Aircrew training
The refined Box Pattern parameters give training organisations a starting point for a type-specific syllabus.
Assessment
The authors are explicit that this is an early developmental assessment, not an operational validation. All ratings came from one test pilot on one prototype over 6.5 hours, in daylight visual conditions. The aircraft flew clean, with no pylons, pods or real stores, so drag, store separation and electromagnetic compatibility are untested. The 1 Hz data logger could not resolve short-period or roll-mode dynamics, so several favourable results are qualitative. Nominal-configuration workload with the yaw damper and autopilot working is an analytical estimate, not a measurement. The authors call for three to five operational pilots in later testing, biometric workload sensors, and aft centre-of-gravity stability checks.
The more important caution concerns how the scorecard is read. The 12 satisfactory ratings do not offset the four failures, which the authors treat as "absolute gating requirements for future airworthiness certification". Their conclusion is correspondingly measured: the aircraft "shows potential suitability for further development". Several fixes, notably a new exit and a redesigned sidestick, are structural rather than procedural, and each must be flight-tested again.
About the Research
Conducted by Süleyman Murat Köroğlu and İbrahim Özkol at the Faculty of Aeronautics and Astronautics, Istanbul Technical University, Turkey, with test pilot İlker Ünlü of the International Test Pilot School, Canada. Published open access in Aerospace under a Creative Commons Attribution 4.0 licence.
Citation
Köroğlu, S. M., Ünlü, İ., & Özkol, İ. (2026). Forward air control mission suitability case study: Tactical and airworthiness evaluation of the LX7-135 turboprop as a commercial derivative aircraft. Aerospace, 13(8), 667.