By Boran Pivčić
All photos (and some screwdriver work) author
About a month or so back, I got the almost-unique opportunity to peek into the internal workings of two Air Tractor AT-402 cropdusters (9A-DKG and -DKJ) sitting around at Lučko. Having been parked there for a good part of eight years, they finally went up for sale and some prospective buyers wanted to have a looksee under the hood. Through a long chain of events, I ended up being there – and of course didn’t miss the opportunity to have a photo field day :). Going to the airfield without my camera… bah!
A bit about the planes themselves first – as their name suggests, these are rough-and-tough utility machines, designed for continuous 24/7 back-water, dirt-strip torture. As such, they’re built to last, utilising proven, classic technology. Designed to be dismantled with little more than a screwdriver (and liberal amounts of WD40 in our case), ATs of all marks – 300s, 400s, 500s and 800s – hold few surprises under the skin. But they’re simple and uncluttered and a good showcase of aircraft structural solutions.
The 402 version came about when someone decided to ditch the earlier versions’ 600 HP Pratt&Whitney R-985 9 cyl piston radial and replace it with a more reliable – and far simpler – turboprop. Since the 400 series, all ATs have been produced in this configuration, with engines of varying outputs to cater for increasing weights. The current standard is the AT-802 wheeled model and the AT-802AF Fire Boss amphibian. Despite being designed primarily for cropdusting, most 802s today are used for firebombing. Indeed, the Croatian Air Force operates both variants down at the coast (one wheeled, three Fire Bosses) with notable success.
And now, a step-by-step condensed lesson in aircraft structures :).
The naked plane. A general overview of the 402 (9A-DKG) with all side panels removed. Despite its imposing size, the AT is basically full of hot air :). Of note is the thick and juicy wing profile, providing a lot of lift at low speeds. The consequence of this increased lift - drag - is not so important here, as speed and cruising efficiency were not high on the design priorities list
Rear quarterview showing some of the internal structure. Like most light aircraft, the AT series uses a frame construction, much like the one you see on construction cranes. The frame - which absorbs all inflight loads and holds the structure together - is covered by panels to make the whole thing aerodynamic. These "panels" can be made from a wide range of materials, wood and fabric in the olden days and aluminium today - though composites and glass and carbon fibre are becoming increasingly common
Removing the other side panel adds some clarity to the shot. Easily visible now is the control linkage, linking the control stick and pedals in the cockpit with the rear control surfaces. The rod you can see going through the structure controls the elevator, while the thin gray cables running along the outside of the structure are linked to the rudder
Turbo power! Up close with the Pratt&Whitney PT6A-15AG 715 HP turboprop. Like all turboprops, the PT6 is a small package for the power it delivers, with most of the space in the back taken up by auxilliary and ancilliary devices such as the starter, generator, oil pumps, control links, air intake and the odd cooler or two
View from a different angle. Again like most turboprops, the PT6 is a free-turbine reverse-flow engine, meaning it's installed ass backwards :). To avoid going into detail, this makes the engine lighter and hence more efficient - and explains why the exhaust pipes are located up front: the back of the engine is there. Because its front is now deeper in the engine bay, it has to be fed by the air intake visible under the nose
A wider view of the nose. Despite looking thin and whimpy, the landing gear is QUITE strong. During factory testing, the designers mounted a four-ton cement block on top of the landing gear assembly (just the gear, not the whole plane) and let it drop from a height of two-three meters (4 tons corresponding to the maximum takeoff weight of the plane). After spreading out and absorbing the weight, the gear sprung - sprung, with a four-ton block on it's back! - back into its original shape... on another note, the brownish thing between the engine bay and cabin is the hopper, with a capacity of 1,500 liters. During cropdusting, this would have contained the cropspray solution, while during firefighting either water or, more commonly, fire retardant
A more recent photo of DKG with its engine and prop removed for overhaul. Only when you disconnect all the pipes, cables and wires do you realise how big a mess the engine bay can be
Closeup of the engine mount. This is basically all that holds the engine connected to the rest of the plane :). But, like the landing gear shown previously, this is built to last. Also visible is the back of the bay is the firewall, "the part of the plane specifically designed to let in fire and smoke" as the joke goes :). In serious-world, it prevents any fire in the bay from reaching the rest of the plane, be it the chemical hopper, cabin or any part of the structure
And last but not least, a view down the lower engine bay. The air filter that feed the engine with nice, clean air is the most imposing feature. Despite the turboprop's somewhat higher resistance to dust than piston engines, bad filtering can lead to a rapid decrease in engine performance and more often than not serious damage to the compressor