Troubleshooting the Honeywell 36-150W APU
By Quentin Saarinen, Augusta crew chief
Troubleshooting an aircraft on ground (AOG) takes a tremendous amount of experience, patience and a keen eye. Our team of expert Aircraft Maintenance technicians recently relied on all three while tackling a challenging Honeywell 36-150W auxiliary power unit (APU).
The Situation
Installed on a Cessna 650, the APU was shutting down after several minutes of operation or rolling back and hanging uncommanded. The customer called StandardAero for AOG assistance saying that he was unable to restart the APU intermittently, and occasionally during the rollback period, the APU would surge, pop and stall. No bite codes were displayed at the APU electronic sequencing unit (ESU) after shutdown.
Early Troubleshooting
Previous experience led us to suspect a chafed wire at the fuel control inside the APU cover at the starter/generator. We have had several issues of the cover chafing the fuel control torque motor wire harness, causing similar symptoms.
We dispatched a technician to troubleshoot the problem. No faults were found at the suspect area but several indications and findings led us to believe a possible fuel control issue, fuel shut-off valve (SOV) problem, ESU fault or monopole. The ESU bite indicator tripped a “loss of speed signal” after four of the uncommanded shutdowns also prompting the troubleshooting direction.
After swapping the ESU, replacing the fuel control, replacing the fuel shut-off valve and swapping the monopole, the issue still remained. Each time, six to eight minutes into a normal APU run, the unit would either shutdown or rollback RPM and exhaust gas temperature (EGT), and hang for several seconds and then start back up by itself to normal speed. The longer the APU master switch and power remained on the worse the cycling interval of the APU would get.
Digging Deeper
The aircraft was brought to StandardAero’s Augusta service center and we explored other areas. There is an airframe-mounted APU fuel shut-off valve in line with the fuel supply to the APU. We disconnected the connector P10U at the SOV and monitored voltage supply with the APU master switch in the “on” position.
While monitoring, we found that the voltage disappeared at the six- to eight-minute mark. Investigating the wiring prints for items controlling power to the shut-off valve led us to several items in the circuit. There is a varistor in parallel with the wires at the connector, a diode in series with the APU master circuit breaker (CB), the APU master CB, the APU master switch, an APU master relay and an APU fire relay. Suspecting the varistor, diode and CB possibly breaking down and causing power loss, we investigated, but no defects were noted. We also checked the APU master switch and the fuel SOV for any faults with none found.
The Solution
We finally moved to the APU power distribution box with the relays mounted in it. We heard a relay clicking and cycling when power was applied to the circuit. Isolating the relays, we found the APU master relay to be at fault.
Replacing the relay K11U fixed the issue because it was overheating as power was applied to the circuit. As the relay overheated, the coil began breaking down and relaxing the contacts supplying voltage to the SOV.
We know from experience that using the troubleshooting charts from the manuals, relying on assistance and directions from manufacturer technical support, and analyzing the symptoms seen during a fault, do not always lead technicians in the right direction. It takes diligence and thorough inspection of all interfacing systems to successfully troubleshoot an aircraft.