One fuel nozzle and one ignitor — that’s not much for redundancy when it comes to a Honeywell GTCP 36-100 APU fuel atomizer. Failure of either one of these components is not an option. For aircraft maintenance professionals tasked with inspecting and maintaining these APUs, the fuel atomizer inspection is critical. This article will give some tips on maintaining and inspecting the fuel atomizer assembly on these APUs.
The 36-100 APU is a constant speed turbine engine that utilizes a single centrifugal compressor and a single radial turbine. It has a single duplex fuel atomizer that is installed at the bottom of the combustor assembly. A single ignitor is installed on the side of the combustor.
The APU provides air for main engine starts. It also provides air for heating and air-conditioning systems on the aircraft. With a generator mounted to the APU gearbox, it supplies electrical power to the aircraft as well.
Air enters the APU through the inlet and goes through an impeller which compresses the air. The compressed air then enters a plenum in the back of the APU. From there, the air has two places to go. It either goes through the load control valve on the top of the APU assembly, or it enters the combustor assembly. Air that enters the combustor gets swirled by swirl tabs welded on the bottom of the assembly. The swirling air gets mixed with fuel from the fuel atomizer. The compressed air/fuel mixture is then ignited. The expanding hot gases then go through the turbine nozzle and spin the turbine wheel which is another impeller on the back of the APU, continuing the process of combustion before exiting at the tailpipe.
The 36-100 APU is controlled through an electronic control unit (ECU). The ECU monitors APU operating indications and adjusts the fuel control unit and load control valve to maintain rpm and temperature within limits.
There is also a drain valve located on the bottom of the combustor assembly. It allows any residual fuel left in the assembly after shutdown to drain overboard.
Combustor assembly inspection
Inspection interval of the combustor assembly depends on the type of aircraft it is installed on. Jeff Clarke, national APU sales and service engineer for Dallas Airmotive, shares. “Most aircraft call for inspection of the combustor assembly at 450-hour intervals. The inspections on some aircraft are set at 300 hours, but most are at 400 to 450 hours.”
Be sure to follow the maintenance manual for inspection requirements. The inspection requirements are called out in chapter 49-30-51 of the maintenance manual. It’s important to follow the inspection and maintenance instructions called out there.
The combustor assembly inspection includes the fuel nozzle, ignitor plug, fuel drain valve, combustion cap, and combustion assembly.
Visual inspection first
One thing Clarke points out is the importance of performing a visual inspection upon disassembly of the APU combustor assembly. “The maintenance manual says to inspect the nozzle face for burrs and scratches or carbon buildup which may disrupt airflow or spray pattern,” Clarke shares. “They want you to look at it and inspect it in the as-received condition. Inspection of the nozzle before cleaning is critical to get an assessment of the health of the fuel atomizer.”
If you think about it, this seems contrary to what we are accustomed to when inspecting aircraft components. We are used to removing a component, disassembling it, cleaning it, and then inspecting it. But with the 36-100 APU, inspecting it in the as-received condition is important. Clarke adds, “The fuel atomizer inspection is a little different from common inspection practices. The maintenance manual is pretty specific. It tells you to inspect it like you see it. Check for carbon, shroud looseness, blockage, and carbon buildup. And then it says to clean it. What is key here — if it is not messed up, don’t disturb it. If it looks good and passes inspection, don’t touch it. That’s a good fuel nozzle, and you put it right back in. If you see dirt or carbon buildup, then you clean it, and put it back in.”
Obviously, proper operation of the fuel atomizer is critical to APU operation. That’s probably why a lot of maintenance personnel don’t mess with fuel atomizer inspection. “The nozzle is so critical to proper operation of the APU, a lot of operators don’t even mess with it,” says Clarke. “They just make it a 100 percent replacement item. They’ll send it in and put an overhauled nozzle in its place.”
But you don’t have to be afraid of the atomizer inspection. “Again, follow the maintenance manual,” Clarke tells AMT. “If a proper inspection is performed, there should be no reason to automatically replace a nozzle.”
One thing Clarke cautions about is the debris contamination. “Be very careful not to allow debris to get introduced in the nozzle when cleaning it. The manual tells you to apply 30 psi of air to the fuel inlet while cleaning it. It also instructs you to hold it upside down, face down, when you clean it. Gravity is your friend and allows debris to fall down and not re-enter the nozzle. You really need to follow the manual when cleaning these nozzles.” Clarke also warns that the same 30 psi of air that is meant to keep contaminants out of the nozzle when cleaning it can cause debris to re-enter the side of the nozzle because of a venturi effect. See photo on page 10.
During your combustion assembly inspection, be sure to look not only for carbon buildup, but for evidence of a piece of carbon breaking off. “When a piece of carbon breaks off, you’ll see a flat spot with points that stick up and are sometimes kind of white on the edge,” Clarke shares. “That’s an indication that a piece has broken off and has gone downstream.”
If a carbon piece breaks off, it will follow the same path the ignited air/fuel mixture does. It will exit the combustor assembly and go through the turbine nozzle and on to the turbine wheel and out the APU. But if the debris is large enough, it can get stuck in the turbine nozzle. This can lead to compressor stalls.
These are a few tips on 36-100 APU fuel atomizer and combustor assembly inspection. For more information about 36-100 APUs or for information on APU services Dallas Airmotive provides, call Jeff Clarke at (856) 455-0539 or e-mail firstname.lastname@example.org.