Overview

The Rockwell Commander 685 is the rare piston-engine derivative of the Commander 690 turboprop airframe, built by Rockwell at the Bethany, Oklahoma factory and FAA type-certificated on September 17, 1971. It pairs the larger, pressurized 690 fuselage with twin 435 hp Continental GTSIO-520-F/K geared-and-turbocharged piston engines turning three-blade Hartzell feathering propellers, giving the late-Commander family its only fully pressurized piston cabin twin in the 9,000 lb gross-weight class. Production ran from 1971 through 1976 for a total of about 66 airframes.

The 685 occupied an unusual showroom slot. It was offered alongside the turbine 690 with the same cabin, the same systems philosophy, and a markedly lower acquisition cost than its turboprop sibling, but with the GTSIO-520 powerplant economics of a high-output geared turbocharged piston rather than the simpler PT6 or TPE331 turbines that increasingly defined the cabin-class market by the mid-1970s. Buyers who chose the 685 over the 690 chose acquisition price over operating simplicity; the market made the opposite choice often enough that the 685 was discontinued after six model years while the 690 series continued in production for another decade.

Set against its siblings, the 685 reads as a niche acquisition today. The cabin and the airframe are excellent; the 690’s later refinements are largely absent because the 685 predates them; and the GTSIO-520 powerplant requires specialist support that is meaningfully harder to find than King Air PT6 or Twin Commander TPE331 support. Type-club support runs through Twin Commander Aircraft LLC alongside the rest of the line, which keeps parts and recurrent training reachable.

Key Features for GA Buyers

• 690-cabin pressurization without turbine acquisition cost. Roughly 5 psi differential cabin in a 9,000 lb piston airframe, sitting well below the 690’s price band on the used market.
• Cabin-class capacity. Eight-seat interior (1-2 crew, 6-7 passengers), 600 lb baggage capacity, walk-in airstair door, and a flat floor courtesy of the high-wing carry-through.
• Long range with the auxiliary fuel option. Standard 256 USG gives roughly 1,251 NM; the optional 322 USG configuration extends that to about 1,655 NM at economy cruise, competitive with several contemporary turboprops on book numbers.
• High service ceiling for a piston twin. A 25,000 ft service ceiling with comfortable long-range cruise in the low-to-mid 20s puts the 685 above weather more often than typical pressurized piston twins.
• Twin Commander Aircraft LLC type support. The same Service Center network that supports the 690, 695, and earlier Commander variants covers the 685; recurrent training and AD management are actively maintained.

Trade-offs

• GTSIO-520 powerplant economics. Geared, turbocharged, and fuel-injected, the GTSIO-520-F/K is among the most operationally demanding Continental piston engines: hot-start discipline, careful power management, expensive overhauls, and a shrinking pool of shops with current experience.
• Tiny production run, thin parts pool. 66 airframes built across six model years means airframe-specific parts often require salvage sourcing or fabrication; the 690 next door has parts commonality on many systems but not on the powerplants or engine mounts.
• Piston operating costs at turbine-airframe utility levels. The 685 carries a 690-class hangar footprint and pressurization maintenance domain on a piston-twin operating budget; buyers often find the total-cost-of-ownership envelope unflattering compared with a King Air 90.
• Frequently misattributed in listings and databases. The 685 is regularly described as an “Aero Commander 685” or conflated with the 680T/V/W turbine series or the 690 turbine variants; verify engine fitment, model year, and TC details on any candidate airframe.
• Aerodynamic refinements postdate the 685. Several of the 690’s later-production improvements (winglet kits, engine-nacelle refinements, larger windows on later -A/-B variants) are not retrofittable to the 685; the airframe represents the early-1970s state of the platform.

See Also

• Commander 690 – Direct turbine successor on the same airframe; the TPE331-powered 690 is what the 685 became when Rockwell committed to turbines for the family. Compare
• Cessna 421C Golden Eagle – Direct period peer and Wichita’s dominant pressurized piston cabin twin of the same era; twin TSIO-520-N turbocharged Continentals, a smaller and lower-MTOW cabin, and the deepest support network in the bracket. Compare
• Piper PA-31P Pressurized Navajo – Direct period peer with the closest powerplant economics: Lycoming TIGO-541-E1A geared turbocharged sixes whose maintenance domain and shrinking-support profile most closely mirror the 685’s GTSIO-520. Compare
• Beechcraft Duke 60 – Pressurized piston cabin twin from the same era in a tighter cabin; TIO-541-E1A4 geared turbocharged Lycoming with a similar operational-demand profile to the 685’s GTSIO-520 – both engine families have shrinking support networks. Compare
• Aero Commander 680FP – Sister pressurized piston Commander on the earlier 680F airframe; IGSO-540 powerplant rather than GTSIO-520, and a smaller cabin. Compare
• Cessna 340 – Smaller pressurized piston cabin twin in the same general market segment; a step down in size, performance, and operating cost. Compare
• Aero Commander 680FL – Predecessor stretched-cabin Commander; unpressurized, IGSO-540-powered Grand Commander on the prior airframe lineage. Compare