The Twin Commander Turboprops

The Twin Commander Turboprops

The Twin Commander Turboprops

A Commanding Presence

During the winding down of massive wartime aircraft production at the end of World War II, military airplane builders and workers were suddenly faced with finding something else to do. Quite a few went to work on new civilian designs, employing techniques and materials that had been developed for the War effort. Aviation, it was expected, would find ready acceptance in post-war society.

Among these visionaries was Ted Smith, a Douglas Aircraft engineer who had most recently worked on the A-26 Invader light bomber. Smith could the see the need for a modern twin-engine business airplane, to be flown by executive pilots newly returned from wartime service. They were going to want fast, comfortable, all-metal airplanes, not tube-and-fabric types powered by radial engines. There was nothing available in that category, so Smith formed a team to design the first modern executive twin.

His rough prototype took to the air in 1948, powered by a pair of 260-hp geared Lycoming flat-six engines, the most powerful lightweight engines available at the time, and riding on borrowed maingear from a Vultee BT-13 basic trainer. It wasn’t pretty, but it proved the concept; a high-wing twin with a tall tail and low-slung cabin. The only problem was finding venture capital to put it into production.

Smith’s investor search finally led him to Oklahoma, where George Pew teamed up with brothers Bill and Rufus Amis, successful earthworks contractors with an enthusiasm for aviation. The Aero Design and Engineering Company was formed and a plant was built just outside Oklahoma City in Bethany, Oklahoma, adjacent to the present Wiley Post airport. With considerable modification from the old “guppy”-shaped prototype, the first Aero Commander model 520 was certificated and put on the market in 1952.

dia_0105The Commander immediately shook up general aviation’s Big Three – Beech, Cessna and Piper – who were suffering from the collapse of a post-war aviation boom that had burned out in only a year. It turned out that the market was not in light trainers and runabouts; it was in business aircraft, which weren’t available as war-surplus or prewar leftovers. Beech rushed to develop the Twin Bonanza, Piper came up with the Apache and Cessna brought out the Model 310. But leading the pack was the Aero Commander, the original postwar light twin.

I learned to fly many-motored aircraft in a 1954 Commander 560, and it wasn’t until I moved into other twins that I found out most of the other designs weren’t nearly as gentle and forgiving as the tubby old Commander. The Aero Commander’s simple systems demanded little of the pilot, and even a gear-up landing (particularly unforgivable, since the gear wells were visible right outside the cabin) left the airplane more-or-less flyable as soon as it was jacked up and the wheels were extended. The Commander had its quirks, but it was loved by its pilots.

Aero Design and Engineering was acquired by Col. Willard Rockwell’s Rockwell Standard company in 1958, anxious to turn the Commander line into a power player in general aviation. Rockwell proceeded to gobble up several small G/A manufacturers, seeking to build a broad line of singles and agplanes, among which were Volaire, Meyers, Call-Air and Snow. The hoped-for amalgamation wasn’t successful, but it wasn’t for lack of trying.

Meanwhile, the twin-engine Rockwell Commander’s star shone brightly and the Commanders were developed into a full line, even evolving into a Jet Commander in 1965 to compete with the Lear Jet. The J-C was forced to be divested by an anti-trust ruling after Rockwell merged with North American Aviation, builders of the Sabreliner. Most observers could never understand how combining the disparate Jet Commander and Sabreliner could constitute a monopoly.

One of the most-attractive Commander developments was 1964’s stretched Grand Commander 680FLP, which was a full six feet longer than the standard Commander. It sported a forward entrance door, mechanical pressurization and a 14-foot cabin behind a flight deck. Powered by 380-hp IGSO-540 Lycomings, the sleek 8,500-pound Grand Commander had everything but the muscle needed to make it fly, particularly on one engine.

By 1966, it was evident from the Beech King Air’s success that turbine power was the logical next step for the Grand Commander. The model 680T, powered by brand-new 575-shp Garrett AiResearch (now Honeywell) TPE-311-43A turboprop engines, was certificated in September, 1965, beginning two decades of evolution in Turbo Commanders. The 1967 680V yielded 450 pounds more gross weight, wheelwell doors and larger exhaust ducts. The -V was followed in few months by a 680W with clipped wingtips and -43BL engines. A 681 was approved in March, 1969, dubbed the Hawk Commander as Rockwell tried putting bird names on all its lineup, finally graduating to the definitive model 690 in July, 1971.

TC9fThe 690 benefited from the 717.5 shp of a TPE-331-5-251K engine, flat-rated from 840 shp, and 30 inches of additional inboard wingspan, with 850 pounds more gross weight. Through the ‘70s, the 690 series grew into 690A, 690B, and 690B-I and 690B-II versions, with engine TBO stretching out to 5,400 hours. Adding to the types confusion, a 690C appeared in 1980, but was marketed as the model 840, along with a 980, with slightly more shp, certificated as the 695. These JetProp models featured wet-wing fuel bays outboard of the nacelles to supplement the standard bladder cells, boosting optional fuel capacity by nearly 100 gallons. Wingspan was increased by 5.5 feet and small winglets were added. By now, North American-Rockwell had thrown in the towel as the G/A market imploded, selling off the Commander line to Gulfstream Aerospace in 1981.

In the early 1970s, Rockwell engineers had designed a from-scratch single-engine Commander series, planned to be adaptable to fixed-gear, retractable, turbocharged and high-performance variants, but after production lapsed under Gulfstream that project was sold to a Commander Aircraft company in the 1990s, and thus was born the “Twin Commander” label, to differentiate the widely-different Commander-named airplane lines.

During its tenure, Gulfstream marketed 900 (690D) and 1000 (695A) Commander turboprops, powered by engines of 748 and 820-shp respectively, before production ended in 1985. Because the airplane was simply too good to go away, the type certificate was eventually acquired by Twin Commander Aircraft Corporation, where it resides today. Support continues to be available from Twin Commander through 17 service centers, 12 in the U.S. and 5 overseas. Most impressively, a Grand Renaissance Commander upgrade was developed by Twin Commander that essentially takes the airframe down to its bare essentials, effectively remanufacturing the airplane.

Whether or not one wants to pursue the Grand Renaissance rebuild, it makes sense to upgrade to the TPE-331-10 engine configuration, as used in the 980, which retains horsepower to higher cruise altitudes. For older airplanes, a recurring-inspection AD on the center spar cap can be eliminated by installing a desirable mod. Also, AD #2013-09-05 mandates compliance with Service Bulletin 241, an intensive inspection for cracking in the aft pressure bulkhead, picture window frames and attachments. Accordingly, the relatively few Grand Renaissance Commanders that have been completed are highly sought-after airplanes.

Legacy0441Twin Commander Support

All through the Commander legacy, there was always one aftermarket go-to place relied on by Turbo Commander owners. Downtown Airpark, Inc. in Oklahoma City, located across town from the factory, offered complete service for the airplanes, from interiors to painting to avionics to overhauls. Shoehorned in among downtown Oklahoma City’s buildings, between a railroad and power lines, DTA’s 3200-foot runway saw much history during its existence. The little field closed in 2005, threatening to take a noted Twin Commander support base with it.

Fortunately, R.J. Gomez, himself a legend in Commander circles, was encouraged to relocate his DTA service crew to suburban Yukon’s Clarence E. Page airport, where a 6,000-foot runway outside the Class C airspace offered room in a welcoming environment. Reborn as Legacy Aviation Services, Downtown Airpark’s heritage lives on, about to celebrate its 10th Anniversary under the Legacy banner.

In its spacious new location, Legacy Aviation is in a position to be more than a Commander specialist. It is already servicing King Airs and has taken on Citation service as well. Twenty-four hour fuel service is available, part of an acquisition of neighboring Mid-Continent Airmotive in September 2014 that added 17,000-square feet of extra space and full avionics capability. Legacy now has nearly 50,000 square feet of space in four hangars, with 22 employees.

Flying The Twin Commander Turboprops

On the ramp, Commanders are both intimate and commanding in stature. The airplane is low-slung, allowing single-step boarding into a fuselage riding only a foot or so above the pavement, but the broad 52-foot high-mounted wing and tall tail assert its presence. Nothing else looks like a Commander.

Preflighting is a mix of convenience and arduousness, in that the contents of maingear wheel wells are easily accessed, but the empennage must be observed from afar. The nacelles housing the Honeywell TPE-331 engines are sleek, faired into the big propeller spinners. Swing-down doors on the right side of the nacelles allow oil checking, with adjustable oil cooler doors located on the inboard sides. The exhaust flow from the engines is routed outboard into large augmenter tailpipes for noise suppression.

The Commander is a hydraulic airplane, using engine-driven fluid power for the landing gear, flaps, brakes and nosegear steering. A nitrogen blow-down system backs up hydraulic extension. The beefy maingear uses 8.50 x 10 tires and rotates 90-degrees as it retracts into the aft portion of the nacelles, lying flat beneath sequencing clamshell wheelwell doors. An STC is available to remove the doors. However, Legacy Aviation has noted some cracking in the nacelle’s trailing edge from turbulence behind the unfaired tire when the doors are removed.

The nosewheel is a 6.00 x 6 size, often using a 15-6.00 x 6 low-profile tire. It steers by applying hydraulic power through pressing the tips of the rudder pedals, just short of brake actuation. The trick is to simply hold the pedals even and wiggle your toes, rather than fan the rudder. Preflight normally includes removing an external rudder gust lock, since there’s no nosegear linkage.

TC9fThe nosecone holds only an avionics bay, radome and wheel well; baggage is easily loaded into a 600-lb compartment behind the cabin, reached through a door on the left side of the fuselage. Batteries and environmental apparatus are in the aft tailcone. The 15-foot vertical fin and steep-dihedral horizontal stabilizer are far overhead. Both elevators and the rudder are fitted with trim tabs.

The fuel system was always operationally simple, using center tanks to feed the engines, replenished from interconnected fuel cells and bays in the wings. Refueling is accomplished by overwing ladder access.

Entering the aircraft through the left-side door, forward of the propeller, is facilitated by a step that extends with the door’s opening. An STC to remove the step allows the door to fold flat for easier loading of bulky objects. A side-facing seat is usually installed across from the door, with club seating aft and a lavatory in the rear. The large picture windows introduced on the 690 returned to small rectangular panes when pressurization differential was increased on the JetProp 900 and 1000. An emergency exit is incorporated into the forward right-side window.

The cockpit is snug but convenient, once ensconced. Big transport-style yokes sprout from the floor, with rams-horn handlebars replacing the control wheels in the later 690-series airplanes. An overhead panel contains starting, fuel, lighting and manual trim wheels, flanked by eyebrow windows to add light and vision. The central pedestal holds power and condition levers, plus the autopilot head. An increasingly popular mod, developed by Twin Commander service center Eagle Creek Aviation Services, installs a Garmin 950 glass cockpit in the older Commanders, essentially a Garmin G1000 three-screen package paired with an S-Tec 2100 autopilot.

The TPE-331 engine is a single-shaft powerplant, so the entire engine must rotate to compress air for starting. A check of battery condition is essential, to make sure stored power is up to the task, and most operators will avail themselves of a cart start if it’s offered. The sequence is fuel pump on, start, ignition on at 10% rpm, guarding the condition lever as acceleration continues and ITT rises; ignition goes off at 50% rpm and the engine stabilizes at 65%. The second engine start benefits from the generator-assist of the first engine, of course.

The Honeywell engines operate at a howling 41,000 rpm, so it’s well to be a good ramp neighbor and move out as soon as idle is established. The propellers park in fine pitch, so a touch of reverse is applied to the power levers to unlock them for taxi, then eased forward to the edge of Beta range. Swinging the big Commander around comes naturally once getting accustomed to the power steering. Just pay attention to the position of the main tires back there, some 17 feet aft of the nosewheel.

Pretakeoff duties are short; overspeed and underspeed governors tested, flaps up and yaw damper off, avionics set, trim and controls checked and we’re ready to go. The TPE-331’s NTS (negative torque sensing), which drives propeller pitch toward near-feather if an engine fails, greatly relieves the pilot of fighting yaw in an emergency. NTS and torque limiter checks are done during start.

With the immediate response of the Honeywell engines, acceleration is strong, linked directly to throttle movement. The torquemeter target is initially low, then tweaked as airspeed builds. The big-tail Commanders have relatively low 90-knot Vmc and 100-knot rotation speeds, so the takeoff happens fast, accentuated by the feeling of speed from the low seat position. Bringing the nose up with a positive pull, then relaxing aft pressure to stabilize climb attitude, gets one airborne is short order.

IMG_1191Climb power of 700 shp is max continuous with the -5 engines; once the gear comes up, there’s little to do but climb at 140-160 knots. The Commanders are pilots’ airplanes, stable as a brick when trimmed up, responsive and powerful. They are fun to hand-fly, lacking surprises, although the ailerons stiffen up at high cruise.

Initial rate of climb can be 3,000 fpm or more at 140 knots, but the deck angle is best moderated to less than 10 degrees for the sake of the passengers, using 180-190 knots. Quickly reaching the upper 20,000’s, where best efficiency is found, cruise speed settles in at 260-280 knots or so, burning 600 pounds for the first hour, 500 pph thereafter. The TPE-331’s are efficient turboprops, delivering excellent miles per gallon numbers. Many Commander turboprop owners seek RVSM certification, especially the later JetProp models that can reach the mid-30s.

Handling remains excellent, even at low speeds; if you want to stall the big Commander, feel free to do so. In landing configuration, the airplane will hang on into the seventy-knots regime and will unload predictably to regain normal flight. Normal maneuvering at 140 knots or so with 30-percent torque sets the aircraft up for an approach; gear speed is 200 knots for the 690 models, with half-flaps allowed at 180 knots and all the flaps below 140.

As in all flight conditions, the stable Commander rides down final at 120 knots on rails, with power response immediate from the Honeywell turbines. Threshold reference speeds are low, 100 knots or so, and power can be reduced to idle as you flare. The aft-mounted maingear trundles on and then you fly the cockpit on down to the runway, at which point ground idle can be selected. With 100-inch propellers and Beta on tap as desired, getting stopped in 3,000 feet or less is easy.

Once parked, the power levers are nudged aft to regain the start locks and shutdown is done. The door cannot be opened until the left engine stops, for safety purposes. If a quick turn is expected, the Honeywell-engine pilot will manually rotate the propellers a foot or so to turn the engine shaft during cooldown, when the shaft can otherwise be in a “bowed” condition. Some pilots spin the prop vigorously to draw cooling air through the engine. No restart is permitted until ITT is below 300 degrees.

The Commander turboprops remain viable, attractive haulers that deliver a lot of capability for their price. If serviced and flown properly, in accordance with Twin Commander’s recommendations, nothing else will do their job.

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