Home-United StatesTWENTY YEARS AGO — AT-401 Upgrades to Walter

TWENTY YEARS AGO — AT-401 Upgrades to Walter


Daryl Johnson makes turn over storage barns in Garfield, WA in Cascade Flying Service’s AT-401/Walter.


GARFIELD, WA — During a recent trip to the Pacific Northwest, I had the pleasure of exercising an invitation to not only visit Cascade Flying Service, but to also perform an evaluation flight of his newly converted AT-401 to Walter turbine engine power; the same aircraft on display at NAAA’s Reno 2000.

By anyone’s standards, it was an IFR day, not the most pleasant for an evaluation flight. Cold, wet, and windy, with low ceilings amongst the Palouse Hills. If you’ve never seen the Palouse Hills, I highly recommend you take a drive through them. Your respect for the ground tracking abilities of local ag pilots will be increased several notches.

Driving up to Doran Roger’s Cascade Flying Service in Garfield, Washington, located in the eastern part of the state, I was greeted by veteran ag pilot (has been with Cascade Flying Service for 26 years) Daryl Johnson squeaking the mains of the AT-401/Walter on the rain-soaked asphalt runway.

This visit turned out to be about more than just another evaluation flight. I discovered, nestled in the hills, a company that does excellent work rebuilding wrecked ag aircraft. A company that can work with fiberglass to design and install an efficient air inlet system for the PT6 turbine engine, and a company that its credibility is certified as an authorized Air Tractor repair facility and Walter engine installation center.
Slipping out of the misting rain into a warm hangar, remember this was June, Doran Rogers introduced me to the crew at Cascade Flying Service. A tightly run operation, with Daryl serving as spray pilot, test pilot and all-around helper in the shop, Mark Scarbourgh handling the sheet metal and fiberglass work and Brad Akin also doing sheet metal and welding. Doran’s wife, Sandy, handles the bookkeeping end of business.

Doran immediately began telling me about his two primary projects; turbine engine air inlet mod and the Walter conversion. The air inlet mod addressed all the obvious points of design that one would have thought anyone else building an air inlet would do. The Walter conversion was mounted to an AT-401 airframe after a fresh overhaul.

With the Cascade Flying Service air inlet, air is directed in such a manner as to flow with a minimum of internal turbulence. Other inlets that I’ve seen have boxed off internal edges that undoubtedly must create an air disturbance, disrupting a smooth even flow to the filter. Whereas, the Cascade air inlet is aerodynamically smooth from the opening all the way back to the end at the K&N air filter, offering the least amount of air disturbance as possible.

Doran Rogers by newly manufactured belly skins.

Another item I noticed about the Cascade air inlet was the opening’s position in relation to the prop. The opening sets back from the prop area by five inches. Other installations are much closer. I believe the significance of this is the potential of the prop to create a buffeting against the inlet that could transmit throughout the airframe.

Cascade builds up its own fiberglass inlets in their shop in Garfield. The fiberglass shop is headed by Mark Scarbourgh. Here, from a mold, the air inlets are formed and fitted to work on Air Tractor models 402s and 502s with the PT6 engines, -11 through -34. The air filter is the popular K&N filter.

According to Doran, at 82°F outside air temperature, the Cascade Air Inlet lowers the ITT by 27°C at 2% less Ng. The inlet installation also includes a new all aluminum plenum. With the air inlet, the underside of the cowling is cleaned up and should improve cowling aerodynamics, an assumption of mine. However, the Cascade Air Inlet eliminates the nose bowl lights. If an operator needs these lights, Doran says a retracting light can be installed.

The Cascade Air Inlet for the Air Tractor 402 and 502 took three years to develop.

The Cascade Air Inlet for the Air Tractor 402 and 502 took three years to develop. It now has an STC effective March 2001.

Now for the primary reason I was visiting Cascade Flying Service, to fly their 1993 AT-401. Its airframe had installed on it a factory remanufactured Walter 751 shp M601-E11 and a three-blade Hartzel 106-inch propeller. The aircraft (N9091Q) looked and even smelled new. Of course, it only had 460 hours total time. When 9091Q was powered by a R-1340 it had been damaged. Cascade rebuilt the aircraft from the ground up.

I don’t know the extent of the damage to the aircraft. But, I did see in Cascade’s shop where they had the jigs to rebuild wings and fuselages, as well as “remanufacture” belly skins.

During the preflight, Daryl briefed me on cockpit procedures. Cascade designed the cockpit to follow the PT6 installation design as much as possible. The throttle quadrant was typical Air Tractor-style. Except with the noted ease of the Safe-Start System, the cockpit was for all practical purposes like any other turbine powered Air Tractor.

The power settings I used were based on torque. With the cool outside air temperature, the engine easily developed 751 shp and was limited by torque and not Internal Turbine Temperatures (ITT). Most likely, on a very hot day, the pilot would need to monitor the ITT needle as it approaches redline.

Daryl advised me the typical torque setting for working to be 50 pounds. That’s what I used with the first empty flight, but found 40 pounds to be plenty of power with working speeds at 145 mph. Returning for a half load of 200 gallons, still 40 pounds was plenty of torque. Only on the last full load, did I feel better using 50 pounds of torque while working the aircraft.

The internal design of Cascade Flying’s air inlet allows for a smooth flow of air into the K&N air filter. Also, the inlet is set back from the prop approximately five inches.
The internal design of Cascade Flying’s air inlet allows for a smooth flow of air into the K&N air filter. Also, the inlet is set back from the prop approximately five inches.

With little or no wind, maybe a knot or two downwind, even with the full load, the Air Tractor used only about three-quarters of the 2200-foot asphalt strip. I’m sure Daryl, with much more experience in this aircraft, breaks ground sooner.

An interesting fact about the Walter turbine engine is its resistance to damage from a wire strike. Walter policy is to simply check the crankshaft for trueness, repair the prop as needed, and go back to work. Why this engine doesn’t have the problems a wire strike causes other turbine engines is beyond me. I’ve known of other instances where the Walter engine was stood on its nose, albeit idling, inspected in the field and sent back to work!

Maintenance-wise, the Walter engine is about as good as it gets. There are no fuel nozzles to clean. There are no hot section inspections due until overhaul at 2,000 hours for the -E11. Three hundred-hour inspections are recommended, but they are only filter changes, check the starting and rigging adjustments, if needed, the basic stuff. Keep jet fuel in it, and an operator can expect about a $40 per hour operating expense during the 2,000 hour exchange or overhaul period.

The Walter powered AT-401 turns easily and manages airspeed surprisingly well. The reason for that last comment is, you must remember I’m attempting to literally ride the Palouse Hills with better than 100-foot rises. A mile run could have a half dozen of these rises, giving a roller coaster effect. I think the rise and falls (mini-climbs and descents) were close enough together the overall effect to the airspeed was minimal, a consistent 140-150 mph across the field.

One thing I noticed not related to the Cascade Walter-powered Air Tractor was the depth perception exercise that is continually going on while spraying the Palouse Hills. You really have to pick out the tops of your hills and focus upon them, making sure the one you are looking at is actually the next hill! I can see where a hill could get disguised along a pilot’s path, especially when the field is a solid color, like with wheat. Of course, I’m sure experience makes this a simple maneuver.

Departure, climb-out, cruise and working, the Cascade Walter powered Air Tractor is smooth, just like a turbine ag plane should be. I attribute this to two things; the Hartzel three-blade prop and its position in relation to the air inlet and the 0° angle of incidence for the engine mount. The aircraft tracks straight (yea, a little rudder on takeoff) in flight, and it still needs flaps in the turns and on takeoff. But, it flies like a turbine AT-402; quick, easy and fun.

Yes, you can have fun while working, it is OK. Just be careful with this aircraft. It has the ability to put you in an unusual attitude that you may not be able to fly out of, with its excellent power to weight ratio and aerodynamic design. It’s almost like driving a high speed sports car with 55 mph speed limits.

Doran Rogers is no new-comer to the ag aviation game. He started ag flying in 1962. He formed Cascade Flying Service in 1966 and has been operating from the same location in Garfield, Washington since 1969. The shop is a factory-authorized repair station for Air Tractor (since 1992) and more recently, a Walter engine installation facility, as well as running a spray operation.

Like other Palouse Hills operations, Cascade Flying Service sprays primarily grain cereals, as well as lentils and canola. The season starts in March with dry fertilizer applications, followed by herbicides on grain. In June and July, the flying is mostly insecticide work with most of the work coming to an end in mid-August with preharvest herbicide applications. This leaves a nice long winter for Cascade’s shop to perform STC’d Walter installations and STC’d Turbine Air Inlet conversions, as well as other ag aircraft rebuilding projects.

An ag pilot can climb into the cockpit of a Cascade Flying Service’s AT-401 converted to Walter power as its new owner for less than $300,000. The aircraft will be low time, new paint, new instrument panel and a Walter factory overhauled engine with new Hartzel prop. That’s about as an affordable priced turbine ag plane, in like-new condition, that I’ve seen on the market. Well worth looking into for 400 gallon-ship operations.

An overview of Cascade Flying’s shop. Note jigs in foreground.


Daryl Johnson taxis in from a load. Note the rise and fall of the Palouse Hills in the background.




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