Orsmond Aerial Spray

By Mike Williams

BETHLEHEM, SOUTH AFRICA- Over the years we at Orsmond Aerial Spray, the

engineering and maintenance division of Orsmond Aviation, have tried many

various engine retrofits to Thrush S2R, Ag Cats, etc. and have generally

been very disappointed with the conversions. The Walter engine conversion

idea started back in the mid-1990's when I noticed the similarity of the

M601"Z" model Walter engine (512 takeoff SHP/328 SHP continuous, installed

on a Zlin ag plane) and the P&W PT6A. Later, we began to operate a few PZL

Turbo Kruk aircraft powered by the M601D.

I toyed with the idea of installing the Walter engine onto the Thrush. It

made sense to install this engine, which proved itself in reliability and

low maintenance with the Kruk, onto a lighter and more productive airframe.

We decided to go our own route with an Ayres Thrush conversion. Our goal

was to design an installation matching the quality of the Ayres aircraft. I

wanted the engine mount and installation to be interchangeable and to

accept both the PT6A and the Walter engines. The engine mount can accept

both engines with only minor changes, including cowlings, baffles, hoses,

etc. The air filtration uses the standard barrier filter with cockpit

filter-condition indicator.

I redesigned the Walter exhaust stacks to face outward to reduce cockpit

warming and exhaust fumes contamination. The exhausts, approved by Walter,

lower the max ITT at take off.

The ignition and start system selected is the Auto Start, with Override

selected. This system provides ITT protection during start, automatically

regulating fuel supply at start, overspeed protection in beta and reverse

conditions with max overtemp recording. Utilizing this system meets with

Walter's warranty requirements.

The instrument layout could be mistaken for current production PT6A

installations, using 90% of PT6A original instrumentation and a

conventional throttle quadrant.

Certification was complied with under FAA Part 23 regulation using the

M601D. All modifications were carried out in-house by the technical staff

of Orsmond Aerial Spray workshops, with the exception of the engine mount

stress analysis which was carried out by Springer Aviation Consultants.

Walter engineers visited during the conversion and worked closely with us.

Technical training courses have been attended in the Czech Republic and

in-house with instructors from Walter.

Soon, a Walter M601E-11 will be installed on an Orsmond Aviation Thrush to

carry out comparison evaluations with the M601D and certification. The

M601D has 27 SHP less at takeoff rating, but the same continuous power

rating.

Performance and handling

The M601D has performed beyond our expectations. The airframes used are

early piston model S2R's which were previously modified to PT6A

installations. The installation weighs-in at approximately 4,200 lbs.,

requiring no ballast with standard spray gear and SATLOC equipped. We are

now completing the third aircraft, due this fall.

The aircraft continues to meet all expectations, operating far from home

base. One aircraft is operating in remote areas of Zambia, north of our

borders. The other aircraft is now in firefighting operations and will be

preparing for our upcoming crop spraying season.

We have taken the best ideas from the various Thrush S2R models, put them

together into this conversion and feel that this is a conversion of

exceptionally high standards, marketable to the ag aviation industry,

either in a firewall forward kit or complete aircraft. If you are looking

for quality and don't want to change a proven design, but also have

interchangeability, consider this route. We know our Thrush S2R aircraft.

In the pre-firefighting and spraying season, we operated and had 23 Thrush

aircraft on the apron, ranging from a piston to a PT6A-65. An impressive

sight!