Design and construction of the Gavilán prototype (HK-3500-Z) began in the first half of 1987, and first flight was made on 27 April 1990. After a 50-hour development test flight programme, the fuselage was lengthened by 0.305 m (1 ft 0 in) forward of the wing and max T-O weight was increased by 136 kg (300 lb). The lengthened Gavilán first flew on 7 November 1990, and domestic (Pt 9) certification was expected in December 1991, to be followed by US FAR Pt 23 type approval in mid-1992.
Aero Mercantil planned to begin series production of the Gavilán in the third quarter of 1991. The standard civil aircraft is priced at US$230,000.

TYPE: Eight-seat utility light transport.

WINGS: Braced high-wing monoplane. Wing section NACA 4412. Dihedral 2°. Incidence 1° 30'. Unswept, two-spar structure of 2024-T3 aluminium alloy, with constant chord and single-strut bracing each side. Piano hinged, mechanically actuated ailerons and offset hinged single-slotted trailing-edge flaps also of 2024-T3. No tabs.

FUSELAGE: Mainframe of 4130N steel tube, with 2024-T3 aluminium alloy skin.

TAIL UNIT: Two-spar fin and fixed incidence tailplane. Mechanically actuated elevators and rudder; trim tab in starboard elevator. Entire structure of 2024-T3 aluminium alloy.

LANDING GEAR: Non-retractable tricycle type, with elastomeric shock absorption and single wheel on each unit. Tyre sizes 700 x 6-6 (main) and 600 x 6-6 (nose). Cleveland mainwheel brakes. Float/ski options to be offered later.

POWER PLANT: One 261 kW (350 hp) Textron Lycoming TIO-540-W2A flat-six engine, driving a three-blade constant-speed Hartzell propeller. Fuel tank in each wing, combined capacity 378.5 litres (100 US gallons; 83.3 Imp gallons). Refuelling point in each tank. Oil capacity 11.4 litres (3 US gallons; 2.5 Imp gallons).

ACCOMMODATION: Pilot and co-pilot or one passenger at front. Two rows of three seats to rear of these; middle row faces aft. Door at front on each side, plus larger cargo double door at rear on port side.

SYSTEMS: Pneumatic system, vacuum pump driven at 0.37 bar (5.5 lb/sq in). 12V 70A battery. Gaseous oxygen system optional.

AVIONICS: Bendix/King KX 155 VHF/VOR and KR 87 ADF, and blind-flying instrumentation, are standard.