ENGINES
Cavitation Briggs tells Helicopter Maintenance magazine that there was a technical bulletin issued for Bell Jet Rangers in the ‘80s that addressed potential fuel pressure indication surges at altitude. “An operator of an older Jet Ranger contacted us up and asked us if we knew anything about fuel pressure indication surges at altitude,” he says. “He was operating at 4,000 feet field elevation, so he was starting off pretty high. But he said when he got to around 6,000 feet AGL, he started getting a fuel pressure light. I told him there was a bulletin published by Bell addressing the issue. With the centrifugal fuel pump, when the helicopter got higher, the engine fuel flow went lower because the engine would operate more efficiently at altitude – it would burn less fuel. Meanwhile, the fuel pump cartridge was still trying to pump the same amount of fuel to the engine when the engine wasn’t demanding it any more. When that starts to happen, if the impeller can’t continue to push fuel out, it gets back pressure on it and then it goes into cavitation.”
The solution for the problem called out in the technical
bulletin (206-82-60) was to replace the plug adapter in the fuel pump with a new adapter that would allow for better pressure relief at altitude. In addition, the removed plug adapter could be modified to be reinstalled by drilling two additional holes into the plug adapter as called out in the bulletin.
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HelicopterMaintenanceMagazine.com February | March 2014
“The China Syndrome” Mike Lotzer, national sales manager for Rapco, says pump cavitation can lead to an even bigger problem of fuel pump degradation they call “The China Syndrome.” “This example isn’t a helicopter, but a twin Cessna operator, but it can happen to any fuel pump,” Lotzer shares. “Around 15 years ago, an operator was going numerous aux/boost fuel pumps on his older Cessna 402B tip tanks. We had a lot of warranty claims come back from him and they were all burned up – the armatures were melted, the commutators were melted together, and the brushes were smoked. I finally went down to visit him and asked him if I could inspect the aircraft. He operated flights to and from the Bahamas daily. I noticed in the cockpit that the fuel selectors were on “Aux,” and the aircraft had just landed. It’s normal procedure in the check list on that plane to take off and land with the fuel boost pumps on low. Both fuel boost pumps were on low, but they weren’t on the main tanks, they were on the Aux tanks. The Aux/ Boost fuel pumps on twin Cessnas with tip tanks are located within the tip tanks (main tanks). These pumps could not push fuel to the engines because of the fuel restriction at the fuel selector valve. The fuel continued to cavitate within the system thus overheating the pumps causing complete pump & armature failure.” “If a fuel pump is running at rated voltage and amperage, and fuel is flowing through the pump, the fuel passing through and around the armature is enough to cool the pump,” adds Briggs. “But if you have a scenario where you shut off the engine, or the engine is at minimal power like at idle, and the pump is still running, it cavitates. Other than the relief valve on the pump, there is no way to relieve the pressure. So even if fuel exits through the relief valve, it re-enters the pump inlet and circulates back through the pump components. The pressure builds up, it exits the relief valve again and re-circulates through the pump. The fuel gets hotter through each cycle, causing this “China Syndrome.” The copper in the windings gets hot and expands. When the copper expands, it causes spitting the potting compound out between the bars of the laminations and off the ends. The epoxy gets into the pump and causes vane damage, and can even seize up the comp. That pump is done at that point.” The worse-case scenario for the China
Syndrome is when the engine is not running, but the pumps are,” Briggs adds. “There is absolutely no flow. If the engine is at idle, and the pump is left on for an extended period of time, the damage will still happen, but not as quickly.”
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