AAU Editor, Bill Lavender, prepares to depart from Mid-Continent Aircraft Corp.’s Hayti, Missouri airstrip for an evaluation flight in the Thrush 550 powered by a PT6A-60 turboprop engine.

by Bill Lavender
HAYTI, MISSOURI — At the 2005 Arkansas AAA convention in North Little Rock, Dick Reade of Mid-Continent Aircraft Corporation invited me to Hayti to fly a new Thrush 550 powered by a PT6A-60 engine. I gladly accepted the invitation.
There are several significant changes to the Thrush 550, compared to the earlier 510-version (Thrush is also building a 510 version powered by a PT6A-34. AAU is scheduled to fly it when one of the first ones exit the assembly line). Of course, of the most notable changes are the larger hopper and PT6A-60 engine. The hopper capacity has been increased by a more precise vacuum bag manufacturing process that saves weight with its even thickness throughout and incorporating the hopper top of the 660, raising the capacity to 535-550 gallons, depending on the hopper throat arrangement.

The wing has an all-new spar cap that virtually eliminates spar cap maintenance. The new spar cap design can be retrofitted to older model Thrushes. The Thrush factory guarantees the new lower spar cap for 3,650 hours. However, the FAA assigned a 29,000-hour safe life to the wing spar. This was done very conservatively by using a factor of 8 (or, 1/8 the engineered life). Furthermore, the wing has been tested under FAR 23 structures to 11,500 pounds. This is more weight than the aircraft is capable of carrying. The Thrush 550 I flew was certified to 10,500 pounds. The factory plans to take this certification to 11,500 pounds in the near future.

No more landing gear shock biscuits for Thrush aircraft (only the tripod gear is available for the 510 model). With the spring steel landing gear, shock from working off rough strips is reduced, providing a smooth ride during takeoffs and landings.
The empty weight of the Thrush 550 is 5,250 pounds. It is a heavy-duty aircraft. The maximum certified takeoff weight, per FAR 23, is 10,500 pounds, with a maximum landing weight of 8,800 pounds. The factory has pulled the airframe to 11,500 pounds and is waiting on FAA approval. The Thrush S2R HG G-10 (powered by the TPE-331-10 turboprop) is currently in certification with the optional 11,500-pound gear that will be approved for a 11,500-pound takeoff, fly and landing weight. Once approved, with the optional 11,500-pound gear, the heavier weight will be available for all S2R HG models. The meaning of this is the aircraft will be certified per FAR 23 without restrictions to “G” loading or airspeed, the primary contributing factors to wing spar cracking.

When I flew the –60 powered 550, its gross weight was about 6,900 pounds; with an empty hopper, full 228-gallon fuel tank and pilot’s weight. It was a cool and windy January day. The wind was from the south at 20-25 knots with gusts. Other than an operator being behind with his dry work, this would not be a typical day to fly an ag-plane. However, my time was short with only one day between the Arkansas and Texas conventions. The flight would have to be made regardless of the wind.
There is one well-known feature about a Thrush and that is its ruggedness. This carries over into its ability to remain stable even in the worst wind conditions. Today, this feature proved itself.

Upon departure, into the wind, the power lever never made it to redline torque (with the cool outside air temperature, there was no limiting TIT). The aircraft literally leapt from the ground. With a strong wind on the nose and an empty aircraft, this came as no surprise.

I set up the aircraft with 25 pounds of torque and 1,500 rpm. This is a relatively low power setting for the –60 engine that is capable of developing 1,050 shaft horsepower. However, any more power and I would have been exceeding the appropriate application airspeed. This power setting produced airspeeds between 140 and 150 mph.

With the wind from the south, I entered the field on the north end flying from east to west. In the beginning of the application, I made the typical procedure-type turn known as “back and forths”. I was not trying to hot dog the aircraft, but turns were easily in the 25-second realm. Remember, this was an empty aircraft with a strong crosswind.

Next, I set up for a series of racetrack-type turns. The Thrush aircraft excels with these type of turns. I am a racetrack nut, making racetrack turns even if the field requires only four passes. I start on the downwind side making the first turn into the wind, splitting the field at the middle. The next turn is a little trickier with a slight break upwind for the smaller fields, then a cautious downwind turn to line up for the third pass back on the downwind side of the field. In a smaller field, you lose a little time in the downwind turn, but you make it up on the following upwind one. Day in and day out, the Thrush will perform better (for me) doing its job with racetrack-style turns.

Even in the gusting winter winds, the Thrush tracked a straight line across the field. Whether going across the field or making the turn, the feel of the aircraft was rock solid. It flew like a Thrush was supposed to fly, with only a little rudder pedal pressure in the turns, good aileron response and no need for flaps while turning.

If a pilot is flying any type of Thrush aircraft, the transition to the Thrush 550 with the -60 engine will be an easy one. The aircraft goes in the direction you point it when you point it. It is as easy to land as a J-3 Cub. With the –60 engine, it should not have any problems working loaded from short strips, no matter how hot, humid and/or windy the day is.