Any aircraft mechanic that has watched a turbine engine slowly spool up on a cold winter day has likely experienced anxiety relating to possible engine temperatures exceeding published limits during start. Exceeding the published limits can result in damage to the combustion chamber or hot section of a turbine engine caused by extended spool up times — a real concern to those operating in cold climates. The combination of cold thick oil being asked to lubricate driveline components and gearboxes, and the effect of low temperatures on battery efficiency is a formula for achieving a temporary spike in engine start temperature or, worst case, a hot start.
Aircraft mechanics and pilots both know that the best solution is a heated hangar, but this is not always possible. In the case of a helicopter, preheating of specific components such as gearboxes, transmissions and other complex components can eliminate these potentially damaging conditions by lowering the viscosity of cold soaked fluids. One method of preheating is to install an electric preheat system.
The electric preheat system
To learn more about electric preheat systems, AMT turned to Tanis Aircraft Products in Glenwood, MN. Tanis electrical preheat products have been used in reciprocating aircraft engines for decades. Much of the same technology is currently used on helicopters to transfer heat from an electric heat pad into an engine’s oil, oil sump, hydraulic reservoir, reduction gearbox, accessory case, fuel control unit, tail rotor gearbox, or most any complex component.
Jim Conn, business development manager for Tanis Aircraft Products, says, “The preheat technique is quite simple to explain in words: that is to raise the temperature of all helicopter critical fluids and components to a warm steady state. Over time, the temperature will stabilize similar to the internal temperature of a nominally heated hangar.”
Conn goes on explain, “Electric preheat systems are shoreline (hangar or ground outlet) powered by 115v or 230v AC and designed to draw between 5-13 amps per helicopter. Because the system operates on shoreline power and is not operated in flight, the installation does not require Supplemental Type Certificate (STC) approval but is regarded rather as a minor alteration, even though we maintain a current Parts Manufacturing Approval (PMA) from the FAA. Additionally, battery heaters can be installed into the main cabling for the component heat pads which will improve the efficiency of cold batteries during engine start.”
Thomas Judge, executive director for LifeFlight of Maine, says, “We work in an extremely cold climate and have the heaters installed on the main rotor gearbox, tail rotor gearbox, both engines, hydraulic oil tanks, and battery on our Agusta fleet. The electric preheat system increases our operational readiness and shortens take off times.”
Design, maintenance, and inspection
Conn says, “The heat pads contain resistance elements which are sandwiched and vulcanized between two sections of flexible heat conducting pads. These pads are affixed with a cold cure bonding agent to specific heat conducting locations on fluid reservoirs and components that if preheated will enhance the starting process.”
Conn explains the older style harnesses used high-temp silicone insulated wiring and spade type connectors. Feedback from aircraft mechanics in the field indicated that technicians were not always satisfied with the original spade type connectors. As a result of this type of feedback, they upgraded the design to use Mil-Spec Teflon insulated cable, the connectors are environmentally sealed and include a self-locking feature, and a quick disconnect feature was included using MIL-C-39029 connector technology. The latest SWAMP (severe wind and moisture problem areas) design parameters are used to develop and manufacture all helicopter systems.
A full set of installation instructions are shipped with each system, which makes for easy installation by any technician experienced on that model helicopter. Ongoing maintenance and inspection instructions are specifically called out in the Instructions for Continued Airworthiness (ICA) that accompanies each Tanis electric preheat system. These ICAs should be made part of the aircraft’s maintenance program. General maintenance and inspection practices for 100-hour or annual inspections include such tasks as:
- Examine the cabling system for abnormal wear and integrity at attachment points and bulkhead penetrations, and the shoreline power plug and each individual cable lead for continuity to its respective heat pad element.
- Heat pad elements should be visually checked for cable lead integrity and security of the bonded pad to its heated component. If any portion of a pad heat element comes loose, it may be re-bonded using only factory specified bonding compound. The use of standard tool crib adhesive is not acceptable for use as a bonding material as it will interfere with heat transfer properties of the heat pad element.
- If a pad heat element exhibits a gray discolored area, it is likely due to an air bubble under a poorly bonded element and will require replacement.
- Once the visual inspection is satisfied, verify with an ohmmeter proper heat pad element resistances as published in the installation handbook.
Wendell Stadig, ERA Helicopter lead maintenance technician for LifeFlight of Maine’s two Agusta 109E helicopters, says, “The electric preheat systems are really trouble free and require very little attention. The only trouble I ever had was trying to remove heat pads from engines or transmissions being removed from the helicopter for overhaul. The pads do not come off very easily! Now I just leave them on and install new pads on the replacement component.”
A few safety tips worth passing on regarding the handling of heat pads are, never apply electrical power to heat pads until they have been properly installed to the designated aircraft component. Applying power prematurely can potentially destroy a pad element before it is installed. Also, resist the temptation to touch a heat pad with your bare hand to see if it is working. Heat pad elements performing properly will radiate noticeable heat, so with electrical power applied touch the component adjacent to the pad to determine that it is working correctly.
Helicopter operations in severe winter conditions can create a special set of mechanical stresses on critical driveline components and engine starts. One method of overcoming these stresses is through the use of electric preheating systems. Stadig concludes, “The best part is that we just plug them in shortly after the last flight while the oils are still warm. It’s easier to keep the components warm than to get them warm.”
Information for this article was provided by the following:
Jim Conn, business development manager for Tanis Aircraft Products. Jim holds a commercial pilot certificate and has been a user of Tanis products since the 1980s. He has established and managed aviation departments for small businesses, and in 1995 was cited "Wisconsin Aviation Person of the Year." He was also part of a flying adventure in 1994 named “PolarFlite.” Jim can be reached at [email protected] or (320) 634-5149.
Dirk Ellis, engineering manager for Tanis Aircraft Products. Dirk has been part of the Tanis team for 20 years and he holds an Airframe and Powerplant certificate. Dirk can be reached at [email protected] or (320) 224-4425.
For more information: http://www.youtube.com/watch?v=vPwI_A7yUE0&feature=youtu.be
