Donald Dierks (wing), manager for Farmers Aerial Seeders, Corporation in
Stuttgart, Arkansas, with Thrush 660 pilot, Phil Eichler.
Photo by Bill Bracewell, acrosign@classicnet.net

by Bill Lavender
ALBANY, GA — It’s not your daddy’s Thrush, nor the one you’re flying today for that matter. It is a better Thrush in many ways, the new Thrush 660. The brochure designates this aircraft as such, although the 660 can grow to 775 gallons with the Hatfield fire bombing gate.

What a great performer and thrill to fly is Thrush Aircraft, Inc.’s new Thrush 660. Powered by a PT6A-67 engine derated to 1,300 shp (normally rated at 1,350 shp), this aircraft will leap off the ground and accelerate its way to the field to be sprayed, or fire to be extinguished. I’d like to thank Farmers Aerial Seeding Corporation in Stuttgart, Arkansas for allowing me to fly N204PE. The PE obviously represents chief pilot, Phil Eichler’s initials. Farmers Aerial Seeding is a farmer-owned and operated cooperative. Ten years ago they bought two 510 Thrush. They traded one of them with Thrush dealer, Dick Reade at MidContinent Aircraft Corp. for the new 660.

At the time of my evaluation flight, the January weather was nice, about 60°F. However, due to it being winter, there was no water supply available for filling the hopper. However, there are no doubts, this aircraft can easily carry any load that’ll fit in its hopper and under any conditions. Nothing takes the place of horsepower, especially when it is mounted to a magnificent steed like the 660. By derating the engine, there’s an extra 50 shp that you’ll probably never need, but working at a high altitude and/or during a hot day, the horsepower is available.

When I went for my joy ride, oops, I mean evaluation flight of the 660, I used about 25 pounds of torque and 1,500 rpm. Rpm is limited to 1,700. Many pilots use 1,600 rpm, but I found with the empty hopper the aircraft felt better with the prop set at 1,500 rpm. With torque settings of 35 pounds and 1,500 rpm the fuel burn was 80 gallons an hour and I was zipping along at 175 mph. Thirty pounds of torque slowed to only 155 mph, while I worked it at 150 mph and 25 pounds of torque and 1,500 rpm. The red line on the torque is way up there at 48.03. To figure the amount of horsepower I was using, simply multiply the torque by the rpm and divide by 62.79.

After departure from ABY, I flew about five miles west to find a field to simulate spraying. En route I climbed to about 2,000 feet for a few stalls. Not unlike a heavy Cub, the 660 has a gentle stall, stalling at 72 mph without power. The airspeed gauge is marked for a stall at 90 mph, but this would be at gross weight. Plan on the 70-something mph stall for a normal, empty hopper landing.

Very comfortable with the 25 pounds and 1,500 rpm set up (597 shp), I found my ideal spray field with long west-east runs on the south side and short east-west runs on the north side. I fell into the long runs first at 150 mph, burning 75 gallons of fuel an hour. Flying the aircraft was a piece of cake: very, very stable and easy to handle. I played with the flaps in the turns, but I could not see any real advantage to using them.
Next came the standard race track turns flying west to east in the long runs, winging over to the short runs on the north end flying back westward. The 660 is ideal for race tracking. You can lay it over, pull back and just point it where you want to go. I flew a squeeze pattern to see how tight the 660 would turn. Cheating, I added power to pull tighter, but that only upped my airspeed across the field. At 90 mph the 660 stall horn would go off just as the stick would shake hands, warning enough was enough.

Popping up, adding power, and pointing back toward ABY, I visualized the 660 was on a firefighting mission. The airspeed jumped easily to 175 mph with only 35 pounds of torque and 1,500 rpm. There was plenty more horsepower remaining for more airspeed, if needed. The red line on the airspeed indicator is 220 mph! I didn’t go there.

With the larger vertical stabilizer and rudder, landing the 660 is easy. If an ag-plane handles well on an asphalt runway with a locking tailwheel, it is assured to handle even better on a grass strip. I had no problem with visibility. Pulling the power back from the ferry speed setting immediately slowed the aircraft and I found myself pointed toward the runway. I’d recommend not pulling the power all the way back to flight idle unless the runway was made. Whatever you see off the end of the nose is where you are going, more or less. I set up for landing at about 80 mph with full flaps. The 660 will land slower. And, of course, there’s that wonderful reversing prop to really bring it to a short landing with a very quick stop.

I flew the 660 on a cool day, empty, with 228 gallons of fuel and spray gear. Of course it handled well. However, anyone who flies this aircraft will quickly note it has the capability to work at high altitudes and under hot conditions. There was plenty of excess horsepower that went unused during my flight. The aircraft is certified for a maximum takeoff weight of 14,150 pounds. No matter how you load the 660, with eight-pound material (like water), you can’t overload the certified weight. Empty, the 660 weighs 6,300 pounds. Add another 1368 pounds for fuel and 200 pounds for the pilot and an estimated 150 pounds for the spray gear and you are up to about 8,000 pounds. Subtract this from the certified gross weight of 14,150 pounds and you have a useful hopper load of 768 gallons of water, or over 600 gallons of fire retardant (at 10 pounds per gallon). Depending on the fuel load, the 660 probably can’t be overloaded, except under the most adverse conditions. Cutting the fuel load in half would allow for another 650+ pounds of fire retardant or water. Now you would have more load carrying capability than hopper capacity, unless you installed the Hatfield gate and “grew” the 660 to 775 gallons.

The 660 is sturdily built for years of service. It has a spar life of 26,625 hours, no worries there. Optional fuel capacity can be added, raising the fuel capacity to 346 gallons. It has a certified landing weight of 12,500 pounds that makes it a great fuel hauler and also reassures other type operators it can handle returning home, if needed, with a large portion of its load still in tack.

As advertised in the brochure, the 660 can takeoff at 12,500-pound gross weight on a standard day in 1,500 feet. On that same day, it can land (as usually landed, empty) in 600 feet. The climb out at 10,500 pounds is 1,250 fpm, while at 12,500 pounds it is 1,025 fpm. There is nothing slouchy or sloppy with that performance. I believe it.
If you think the –67 engine is too much power (you can never have too much horsepower), then the –60 with its 1055 shp is available, along with the –65 as well.

With a new rudder and vertical stabilizer, the 660 is easier than every to handle. The new cockpit forward lines improve the looks and visibility. With no airworthy directives on the wings, there is no scheduled down time for wing repairs. Redesigned engine exhaust stacks respond to operators’ concerns about smut building up on the right side of the aircraft. Dry applications are improved with vanes directing the flow of air into the spreader, a 660 owner’s idea. Operator comments are considered, and often implemented. Thrush Aircraft, Inc. is listening and in the process is building one fine ag-plane.

Addressing exhaust stack residue build up on the right hand side of the 660, the factory worked with Pratt & Whitney Canada engineers to lower the exhaust stack 7° on the aircraft’s right side and raise it 7° on the left side.

The 660 has clean, professional lines with its new rudder, vertical stabilizer, repositioned tailwheel spring and exhaust stack. Hidden inside its 54-foot wingspan is twelve feet of massive spar caps for each wing.

Barry Wilson submitted his 660 to dry application tests in Arkansas after designing a set of vanes that better direct the airflow under the aircraft and into the spreader. This design has been incorporated into all of the new 660s.

(L-R) With 660 pilot Phil Eichler on his right and Farmer’s Aerial Seeders Corporation manager Donald Dierks on his left, Thrush Aircraft Corporation president Larry Bays delivers the company’s first Thrush 660.