AIRFRAME
Fuel Control Unit
Damage Caused by Starting Starting the helicopter engine causes more stress to the engine components than just about any other operation. This is due to thermal shock and wear. When starting a cold-soaked engine there are a multitude of damages that can occur, from having excessive wear on a bearing/shaft from poor lubrication, to a seal that has become frozen to a shaft, or even damage to a shaft from excessive stresses due to back pressures on it. Some water will work its way into the gear boxes and
engines and might form ice on bearing surfaces that can force bearings to slip on shafts, or not move effi ciently. Any blockage to the oil system might also cause issues with a bearing running dry, too. Many of these damages do not occur on a single start but happen over time, slowly chipping away at a component’s health. A new gear box seal might be perfect at normal temperatures, but because it hasn’t worn in, it might be too tight on a cold shaft. That is why there is normally a wear-in phase for an engine or main drive component on any vehicle. When drag on the accessory gears and drive components is
reduced signifi cantly, the acceleration of the engine is greater/ faster. Peak gas temperatures experienced during start are then shorter in terms of duration and are cooler.
Hot Starts When starting, it is common practice to warm the engine with a run up, but what is happening when a helicopter is forced to start when it is cold soaked? Often much larger forces are created and they push much harder on the engine. This creates localized heat, which can raise the temperature of that particular engine section drastically in a short time. This leads to thermal stresses in the materials and can cause cracking, fatigue and blown seals. A “hung start” can also happen at this point if the engine
fails to accelerate and the rpm stays constantly low. This uses up battery power. If the battery runs out of power then the engine slows down. Less and less cold air is drawn through the engine and subsequently the temperature in the combustion chamber becomes really hot within a few seconds, causing a lot of damage.
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HelicopterMaintenanceMagazine.com December 2014 | January 2015
MRGB Oil Filter Housing Preheating lubricants, accessories and driveline components
results in a reduction in drag, impact torque, etc., during engine start and run-up procedures. Fuel vaporization is also improved. The results are a reduction in start times, less wear and tear on the starting system (i.e., starter/generator, battery and combustion section). Thermal stress in the C-section is a function of both peak temperature and time of exposure. Proper preheating reduces both and yields a second order reduction.
Diminished Battery Capacity Preheating aircraft batteries eliminates freeze point depression. This condition causes unheated batteries to lose their load function and ability to receive a charge. Many operators have concerns regarding batteries. Their
concerns are valid not only with regard to starting capabilities, but more importantly the battery’s ability to receive and maintain a charge when temperatures are in the extremes. There are also a number of other electrical concerns centered around the starter generator, power surge and increased loads imposed on electrical systems during cold weather starting.
Blown Out Oil Filter Seals and
Hydraulic Lines Due to Congealed Oil The most common place for an oil system to leak is where the oil fi lter is — because it was designed that way. Most oil systems are set up to use a pressure fi tting around the oil fi lter that will allow excess pressure or gasses to escape the system. But what happens when you have congealed oil in your large cold oil fi lter? It causes a pressure rise until the oil can warm enough to fl ow properly. This also can aff ect lines in much the same way because the lines can be made impossible to have proper fl ow from congealed oil. If the pump is adding pressure it has to go someplace.
Mechanical and Electrical
Stress to Systems Many people believe that it is better to keep computers cold and they are correct, but a monitor might not be able to handle the cold the same way a computer does. For instance,
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