The Turbo Caribou Program
PEN Turbo Aviation of Cape May, NJ, undertook the program of re-engineering the DHC-4A Caribou to the turbine powered variant, now designated DHC-4A Turbo Caribou.
As discussed later, conversion to turbine power plant offers improved STOL performance, much lower long term costs of ownership and, of course, turbine reliability. The selective application of modern avionics whether to military or commercial standard, will restore the aircraft to the front rank of cost effective ramp loading STOL transports.
The turbine conversion program replaces the Pratt & Whitney R2000 piston engines with the Pratt & Whitney, Canada PT6A-67T Turbine Power Plants. Overall performance has improved and "new" basic weight is reduced while maximum normal take-off weight remained at 28,500 lbs. Maximum payload is 10,000 lbs.
PEN Turbo believes that the Turbo Caribou is a cost effective solution to military or commercial requirements for low cost STOL transport operations.
REQUIREMENTS FOR ENGINE UPGRADE
The operating characteristics of the Caribou aircraft make it particularly suitable for operation in remote areas and from unprepared strips. The airframe of this aircraft is known to have a high residual fatigue life, however the R2OOO piston engines adversely affects the aircraft reliability and operating costs. Contributing factors are as follows:
· Piston engines inherently have poor maintainability characteristics when compared with turbine engines.
· The reliability and time between overhaul of piston engines are far lower than for turbine engines
· The AVGAS fuel for piston engines is substantially higher in cost than AVTUR required for turbine engines. In addition, aircraft fleets using both types of fuel require cost-adding duplication of fuel storage and handling facilities.
OUTLINED BELOW ARE THE ENGINEERING AND COST CONSIDERATION INVOLVED IN THE OPTIONAL PROGRAMS FOR MODERNIZING THE EXISTING FLEET:
· Extension of the operational life of a valuable asset for a relatively low cost.
· Absence of any competitive aircraft at anywhere near Turbo Caribou price.
· Increase in aircraft disbatch reliability and ease of maintenance.
· Changeover to the more readily available fuel and elimination of duplicate storage and handling facilities for AVGAS
· Improved operating costs substantially improved payload capabilities and productivity. The entire program is designed to be conducted with minimum change to the existing airframe.
BASIC ENGINEERING CHANGES TO DHC-4 CARIBOU
The design objective achieved the following:
· Utilize existing nacelle mounting hard points and two additional hard points totaling six.
· Engine mounting structure design to failsafe criteria.
· Inclusion of vibration isolator, four each per engine.
· Provide ease of access for servicing, four large engine nacelle accesses, two swinging doors (also removable), two large removable panels.
· Quick engine change (QEC) capability. QEC can be performed without removing top cowling. Left/right interchangeable.
· Enclosed exhaust system with dual augmentation design is also used to induce nacelle ventilation and reduce IR signature.
. Fixed vane composite one piece unit.
Aircraft System Changes
Changes were made only to those systems that were incompatible with the change of power plant. These are summarized as follows:
· The aircraft fuel system is the same design except for additional boost pump, new type fuel filters and a fuel totalizer system.
· The DC electrical system changes to generators and starter motors required to suit the turbine engines.
· Cockpit instrumentation was changed to state of the art "EMIS" engine monitoring system.
· Engine control system modified for PT6-67T engine needs utilizing 'ball bearing' type cable throughout the system.
· Original hydraulic pumps driven electrically.
· Power plant fire selection changed to single zone dual loop.
Engine and Propeller Controls
With a revised engine and propeller the Turbo Caribou has had the controls modified, but still keeping the overhead console between the pilot and co-pilot. The existing rope type engine control cable system has been replaced with ball bearing type cable system throughout the airframe and nacelle run.
Redesigned for PT6-67T use are as follows:
Hydraulic system balance maintained by utilizing existing pumps and driven electrically through auto logic which allows deactivation of system when not required during flight.
Fire Detection System
The Turbo Caribou fire detection system uses a single zone dual loop type. Its purpose is to prevent any false indications and offer redundancy in case the two loops fail.
Bleed air from the right engine is used to supply heating for the cockpit and cabin. Regulated Bleed air from the left engine is used to supply air pressure for the operation of the deicer boots, the right engine can also supply de icing air pressure in standby mode.
The fitting of a combined DC starting generating unit to the engine gearbox for the re-engined Caribou offers the simplest and the most cost effective means of incorporating these functions. The control and protection circuitry is based on the existing control panel which has been retained as far as practicable. Also updated are dual transverters (inverters) and new dual generator control units (GCU) which incorporate a self paralleling feature. To provide a positive and efficient starting capability a second lead acid battery is provided, which is paralleled to the existing lead acid battery system.
The existing center panel in the cockpit engine instrument display is changed to suit the new power plants.