Airtest – Cessna 208 Caravan I

The inception of the C208 Caravan came about when Cessna’s engineers were endeavouring to develop a follow-on aircraft for their very successful 180, 185, 206 and 207 workhorses way back in 1980. General feedback from operators confirmed great satisfaction with the Cessna products but pointed towards an aircraft with greater capacity and preferably burning Avtur. Avgas was becoming increasingly difficult, if not impossible, to procure in the more remote regions of the world that are home to the Cessna workhorse.

Their first attempt involved the splitting of a Cessna 207 fuselage down the middle to accommodate three abreast seats. Unfortunately, this hybrid airframe coupled to a Pratt & Whitney PT6A-112 engine fell somewhat short of their stated goals and by the time they had made all their paper modifications, it became increasingly evident that the best results would be achieved with a clean sheet of paper. Enter Russell Bannock and Richard Hiscocks. Bannock was the former president of de Havilland Canada and Hiscocks the former vice president of engineering. These men joined the Cessna team as consultants. Their contribution also explains the slight resemblance the Caravan has to other de Havilland aircraft.

Bannock and Hiscocks had extensive experience with utility operators and were well aware of the potential for a Caravan-type aircraft. In fact the pair had wanted to build just such an aircraft when they left de Havilland but were unable to secure sufficient backing. Now, together with Cessna they were destined to succeed. The team gathered monthly for their design review meetings, developed and discussed their objectives and gradually the Model 208 evolved.

Official Board approval for the project was granted at the end of 1981, the team had a prototype flying by December 1982 and the Cessna 208 Caravan 1 obtained its Type Certificate on the 22nd of October 1984. An orderly and well-planned operation. The launch customer, the giant Federal Express, received the first of 30 aircraft on the 27th of February 1985. By September 1985 Federal Express had exercised its option on a further 79 aircraft and took up an option on a further 90 aircraft. By early 1986 the total Caravan fleet, world wide, had amassed a staggering 12,000 hours of flight time, with an Alaskan operator leading the way with over 1,000 hours. All this must give any prospective purchaser confidence in the product.

Taking a closer look at the aircraft, it is obvious that the designers had easy access and easy maintenance foremost in their minds. Most of Cessna’s later aircraft are built to the ‘failsafe’ method and the Caravan is no exception. They have built in numerous features that will ease the maintenance burden and give operators comfort in the knowledge that their aircraft has exceptional strength and ability to cope with the rough and tumble of everyday bush operations.

The fuselage is of conventional semi-monocoque design giving a slab-sided but cavernous cabin. The main cargo door is made up of two halves. The upper portion folds upward and the lower portion swings forward 180° to clip against the fuselage, affording access to items such as 44-gallon drums, 8′ x 4′ sheets of building material or virtually any type of cargo that will fit through a 49 inch (1. 2m) wide by 50 inch (1. 3m) high opening. There is a replaceable steel cap door sill for the inevitable damage caused by overzealous cargo loaders and the floor has been thoughtfully laid out with a 200Ibs per sq ft loading, ample provision for tie-down rings and strategically located nut plates to secure a plywood floor to spread the load of those very heavy items, such as a truck differential or bulldozer track. The cabin volume is a generous 341. 4 cubic feet and if you require more you can add an ‘underbelly’ cargo pod that will accommodate a further 83. 7 cubic feet. Federal Express have the cargo pod fitted to their version.

Should you desire to use your Caravan as a people mover, you can accommodate up to fourteen souls on board. Lightweight commuter seats are fitted to a ‘Douglas’ type track giving a true quick-change capability. Passengers board through a clam-shell airstair door that is 50 inches (1. 3m) high and 24 inches (60cm) wide located on the right rear fuselage allowing easy access for all shapes and sizes. The cabin is wide and airy, I doubt that anyone would suffer from claustrophobia once inside. Both the pilot and co-pilot doors are large and swing forward 180° to clip onto the forward fuselage, one can then lower the inbuilt step ladder to climb onto the flight deck. The step ladder is necessary as the flight deck floor is at chest level when one stands next to the aircraft. The flight deck is very spacious, the instrument panel is enormous giving sufficient room for any kind of instrumentation, a totally dual avionics fit, radar, navigation system and full co-pilot panel. Everything is clearly and logically laid out. Even the circuit breakers are located in a large, self-contained panel, located to the left of the pilot giving clear identification and easy access. The only criticism I would have of the interior was that the noise from the cabin cooling system was rather excessive. However, I am advised this problem has been cured on later versions of the Caravan. Access to the Pratt & Whitney PT6A-114 engine is superb. The large cowls swing up and expose all areas of the engine and accessories. The battery is also located in the engine compartment on a base that swings out giving excellent access for removal or servicing. All this easy access must reflect in lower maintenance manhour costs.

The wing is based on the venerable NACA 23000 series. It is a fail-safe structure built around a front and rear spar and externally braced by wing struts which are also fail-safe. You might consider this a belt and braces attitude but it is comforting to have all this redundancy when one operates in a turbulent tropical environment. Each wing, between the front and rear spars, also forms a fuel tank giving a total capacity of 335 US gallons (1268 L). Another desirable wing feature is that the outboard portion of each wing has been designed and constructed so that should one taxi a Caravan between two trees spaced at a distance of somewhat less than the wing span, the damage would be limited to an outboard deformable structure that can be easily removed, and repaired in the field, and minimise the damage to the critical fuel bay area. The skin and spars are spliced at a point six feet in from the tip and the leading edge is riveted and spliced into three sections to facilitate field repairs. The wing struts are made of two extruded channels bonded back to back with a formed leading and trailing edge bonded to them to give an aerofoil shape. These struts are interchangeable from left to right.

Whilst the Caravan is not designed as a STOL aircraft it can take off and land in a remarkably short distance. Much of this low-speed agility comes from the power of the turbine engine and the aerodynamic contribution of a set of wide Fowler flaps. The latter presented the designers with a new challenge. With so much of the wing span taken up by the flaps, very little span was left for the ailerons. So to improve the low-speed handling qualities they opted to supplement the ailerons by incorporating a spoiler on top of each wing working in conjunction with the aileron. This system is very effective giving the Caravan an 80°per second roll rate at 120 kts.

The empennage is of conventional design, except that the stabiliser has vortex generators just ahead of the elevators for better pitch control.

The fixed undercarriage is a masterpiece of utility design.

Requiring minimum maintenance, it is the essence of simplicity. The nose gear is based on a vertical oil-only filled leg (not the usual air/oil). This is connected to a spring steel drag brace, which is actually a C182 main leg. This combination is simple and can accept tremendous punishment without transmitting the shocks back into the airframe. The main gear is a conventional three piece steel tube structure. Four bearing caps and eight bolts allow the main wheels to be easily removed for float installation. These bearing caps are designed to shear off in the event of a severe impact (approx 9g) thus minimising damage to the fuselage and passenger area and absorbing much of the impact shock. The Australian Caravan will have the larger, 47 psi tyres giving quite a soft footprint. However, Ron Basten of Rex Aviation, advised that they are looking into a modification that will allow the fitting of Twin Otter tyres giving an even lower footprint and fleet commonality for some of our island utility operators.

The powerplant is an interesting combination in the Caravan.

The engine is a Pratt & Whitney PT6A-114 turboprop derated to a reliable 600 shp at 1,900 prop RPM. The time between overhauls for this model is 3,500 hours. This installation would have been quite a challenge for the engineers as the PT6 engine draws air into its aft end and exhausts out the front. Had the exhaust system been left in its usual place as on the King Air and Bandeir ante type installations, exhaust fumes would have found their way into the passenger cabin. So the Cessna engineers devised some clever ducting that placed the air intake on the left-hand side and took the exhaust out on the lower right-hand side and under the fuselage. The air intake ducting also incorporates an inertial separator consisting of two doors, which under normal operations, route air into the plenum through a gentle turn. However, during potential icing conditions, the doors can be placed to create a sharper turn for the air thus forcing the moisture particles out of the air and ducting them overboard through a vent located on the left rear of the engine cowl.

Pilots who have flown turbine engines before may have experienced a Fuel Control Unit problem and whilst they will testify that this is rare, a turbine engine with a faulty FCU will not get you home. Cessna have addressed this problem by incorporating an emergency lever. This lever is connected through a series of linkages to a manual override lever on the FCU. So in the event of an FCU malfunction, the pilot can now manually control the fuel into the engine. This certainly won’t be as effective or smooth as a normally functioning FCU but it will get you back to home base where repairs can be effected.

Engine instrumentation is of the standard turbine type. There is a torque indicator, a propeller RPM indicator, an inter-turbine temperature (ITT) indicator, a gas generator% RPM indicator, a fuel flow indicator, an oil pressure gauge and an oil temperature gauge. The start limit temperature is 1,090 °C, but a normal start is affected by 600 °C. During the start cycle fuel is fed into the engine when it has achieved a stable air flow at aroung 12%, light off occurs shortly thereafter and self-sustaining speed is reached at around 46% and ground idle is at 54%. Quite a cool and smooth engine.

Most people are aware that Cessna owns McCauley Propellers and one is accustomed to seeing that propeller either pushing or pulling a Cessna through the air. So it is quite surprising to note that the three-bladed, fully reversible, composite propeller on the front of the Caravan is a Hartzell unit. Apparently, Hartzell had been working on a wide-chord composite prop for some time and when they heard about the Caravan, they asked Cessna to try out their new prop. Well the combination proved to be an immediate improvement and quickly accepted as the standard fit. Only a minor modification, in the way of vortex generators on the upper surface of the stabiliser, was necessary due to the additional thrust delivered by the new prop.

When I saw the prop, the aircraft had completed extensive rough field demonstrations, but there was little evidence of this in the form of prop erosion. The blades are made of Kevlar epoxy and have a nickel alloy leading edge. Any damage suffered out in the field, such as a large stone chip can be easily repaired by simply sandpapering the affected area smooth, mixing up a batch of epoxy filler and re-surfacing that area. A task that can be expertly performed in a very short time.

The systems employed on the Cessna Caravan are generally conventional. The aileron system is a combination of cables, quadrants, bellcranks and push-pull rods. The lip spoilers, located above the outboard portion of the flap are interconnected to the ailerons via a pushrod connected to the aileron bellcrank. Spoiler travel is directly proportional to aileron travel for deflections in excess of 5° up. The spoilers remain retracted for aileron movements of less than 5°. The rudder control is effected through a conventional cable system, as are the elevators. Elevator trim is accomplished through two elevator trim tabs connected by cable to the elevator trim wheel located on the control pedestal. The single slotted fowler flaps are electrically driven and feature a primary and standby motor which drives the flaps through a system of cables, bellcranks and pushrods. The standby electric drive motor, through a selector switch in the overhead panel, provides immediate back-up flap actuation should the primary system fail. The flap system is of the preselected type and has the limit speeds marked on the selector quadrant.

The fuel system consists of two vented integral fuel tanks (one in each wing) which gravity feed into a reservoir located in an isolated compartment below the cabin floor. The fuel selector, located on the overhead panel, allows the pilot to select fuel from either the left or the right tank, but under normal operations the fuel selector will be on both tanks, thus minimising any pilot finger trouble. Fuel in the reservoir is pumped by the ejector boost pump or by the auxiliary boost pump (which will cut in automatically any time the ejector boost pump doesn’t do its job) to the engine.

There are two Low Fuel warning lights, one for each wing tank, located in the annunciator panel that will illuminate any time that their respective tank contains less than 25 gallons of fuel (about 30 minutes flight time at 10,000 feet). If that is not enough there is a third Low Fuel warning light in the annunciator panel that will illuminate any time the reservoir is less than full. If this third light should illuminate it would be wise to be in the close vicinity of a landing ground as you will be landing within ten minutes.

Electrical energy, once the engine is operating, is supplied by a 28 volt, 200-amp engine-driven starter generator. Starting and standby power is supplied by a 24 volt, 45 amp-hour lead-acid battery, or you can install an optional Ni-Cad. Personal experience has shown lead-acid to be quite adequate and less costly. The electrical system incorporates two general buses, two avionics buses with a normally open avionics tie switch and a battery bus for memory keep-alive, clock and courtesy light functions. Cessna can also offer an optional standby electrical system which is fed by its own 95 amp alternator, although this system is standard if you order the de-ice package.

Despite having 600 shp mounted in the front the Caravan requires very little torque correction on take-off. This is thanks to an excellent engine installation that has been canted down and to the right to minimise torque effect. Any single-engine Cessna pilot could do an excellent job of flying this aircraft first time out. Our takeoff ground run was surprisingly short with torque set at 1,550 lbs, prop at 2,000 RPM, ITT at 690°, Ng at 100% and a fuel flow of only 380 lbs/hr the Caravan climbed out at 1,000 feet per minute and indicated 125 kts. The noise level for this type of aircraft was quite low and the ride was smooth. The flight control forces were so light and precise that it was difficult to imagine that we were flying in an 8,000 lb aircraft. At an altitude of 4,000 feet with the prop RPM set at 1,750 and torque at 1,600Ibs the fuel flow was showing 364 lbs/hr and the Caravan was returning a TAS of 170 kts. Quite efficient for a single-engine aircraft capable of uplifting over a tonne. At an altitude of 10,000 feet with the prop RPM at 1 ,750, torque at 1,400 lbs and the fuel flow reduced to 314 lbs/hr the Caravan had almost the same TAS at 171 kts. These power settings would be typical of most operators but if required the Caravan can TAS at 183 kts. I am advised that with a cargo-pod installation, the speed only drops by 6 kts. The stall speed with 30° of flap is just under 50 kts giving the aircraft a very wide operating envelope. With the power lever set to zero thrust and the prop feathered, the best glide speed for our weight was 96 KIAS and this gave us a gentle 730 feet per minute rate of descent. If a power failure occurred at 10,000 feet, and one used the correct technique, the aircraft’s nil-wind glide distance would be 22 nautical miles and the aircraft could remain airborne for almost 14 minutes. This would give a pilot ample time to select a suitable landing field and perhaps find the cause of the power failure.

The Caravan 1 was a very different aircraft from the ones I usually fly, but I found it very responsive and enjoyable and foresee a very strong future for it in the Australasian region. The initial market would appear to be with New Guinea operators, outback stations and even the military. Eventually, the DoA will bring their rules up to date with other countries and allow single-engine aircraft with adequate backup systems into Primary Control Zones under IFR conditions. This would clear the way for a very efficient, low-cost overnight freight aircraft operation a la Federal Express in the USA. This is an excellent workhorse capable of many tasks. The Kiwis already have one aircraft operating with a second one following close behind and I would expect to see several Caravans operating in Australia before the end of 1986.