Aircraft AC GPU Calibration & Repair

Aircraft Ground Power Units (GPUs) are used to supply power to aircraft on the ground and aid in starting aircraft engines. These are useful, especially during cold weather operations and for maintenance activities within hangers. The FAA advises airline operators to strictly adhere to the manufacturer’s recommendations for engines starting using a GPU.


Aircraft GPUs – Operation Principle and Types:

Aircraft GPUs deliver a specified power output using a standard electrical system, battery pack, and power source (often an engine). The following section details the types of GPUs based on their operational principle:


  1. Classification based on power source: GPUs are classified as Internal Combustion (IC) Engines, and Gas Turbine (GT) Airline operators commonly use IC Engine GPUs (diesel or gasoline fuelled) for portability. A gas-powered ground power unit is also highly portable; however, fuel and turbine blade maintenance should be accounted for.
  2. Classification based on Current systems: GPUs are classified based on their power output – either Direct current (DC GPUs) or Alternating current (AC GPUs). GPU selection shall depend on the type of current compatible with the aircraft. DC GPUs typically provide 12 VDC or 24 VDC and are often compatible with smaller jets and turboprop airlines. AC GPUs provide current in the 400 Hz range, with 115 VAC or 120 VAC voltage output. AC GPUs are compatible with larger business jets and commercial airliners.
  3. Classification based on direct power or power conversion: Engines (IC) based GPUs are portable systems, whereas Electrical based systems use converters to transform standard electrical power to AC or DC. Electrical-based GPUs are often quieter in operation, while Engine based GPUs are noisy units and emit toxic pollutants.


AC (Alternating Current) vs. DC (Direct Current) GPUs:

Aircraft operators maintain a combined fleet of AC and DC-powered aircraft. Typically, small and medium jets such as Piston and Turboprop are DC powered, and the larger/heavier aircraft such as the Challengers, Falcons, Globals, and Gulfstreams are AC powered. To power these high-powered AC engines, it is necessary to maintain AC GPUs, typically costing more than $60,000. However, AC GPUs can power both DC and AC-powered aircraft. Whereas a DC GPU can only be used with smaller DC-powered aircraft.


Although AC GPUs are compatible with DC and AC aircraft, utilization of AC GPUs for heavier AC aircraft is infrequent, primarily to avoid the potential risk of damaging aircraft electric components. Pilots often choose to run the inbuilt Auxilary Power Units (APUs) over GPUs to assure a clean and steady power supply. Therefore, AC GPUs are less common amongst operators when compared with DC GPUs.


AC GPUs further require a full-time operator to carefully monitor the aircraft powering progress until the unit is unplugged, along with an annual inspection and certification. Hence the operating and maintenance cost of an AC GPU is higher compared to a DC GPU, which is relatively quick and easy to deploy.

Read More: DC GPUs – Preventive Maintenance and Calibration.


Load Bank Testing of AC GPU Systems:

Load bank testing is an essential preventative generator maintenance plan, which operators conduct annually. Load bank testing ensures that GPUs are operational and can handle the highest possible load at any critical time. This mainly involves firing up a standby or prime power generator and running it under an artificial load at its maximum capacity for a specified period.


The following is the procedure for load bank testing of a standard 400 Hz 115 / 200 V AC rated GPU:


Step 1 – LOAD raising – within 30 sec. from 0 to 100% LOAD (a linear increase of load)
Step 2 – HOLD on – 100% LOAD for 5 min
Step 3 – LOAD decreasing – within 10 min from 100 to 0% LOAD (linear decrease of load)
Step 4 – LOAD raising within 30 sec. from 0 to 100% LOAD (a linear increase of load)
Step 5 – HOLD on 100% LOAD for 45 min
Step 6 – LOAD decreasing within 15 min from 100 to 0% LOAD (linear decrease of load)


As mentioned above, load testing and GPU servicing comprise a steady increase of load from 0 to 100% within a stipulated time (decided based on the capacity of the GPU), measuring performance at 100% load and gradually decreasing the load. At all three conditions, parameters of the GPU are studied. Any deviations from the standard limits are corrected with preventive maintenance activity. The load bank testing procedure for military applications is highly precise and often tedious than passenger aircraft.


Preventative Maintenance Program for AC Powered GPUs:

FAA Considers GPUs as critical ground support equipment, and proper maintenance and inspection are vital. GPUs’ improper handling can result in dangerous consequences and be extremely expensive for airline operators. Therefore, a robust GPU Preventative Maintenance program (PM Program) should include regular visual inspections, tests, and calibration. Following are specific guidelines followed across the industry as standard GPU PM program:


  1. Filters: Lubricating oil and fuel filters should be inspected at 300 running hours. Lube Oil should also be changed after 300 hours of operation.
  2. Cooling System: The radiator cap should be calibrated to the correct pressure per OEM guidelines. The rubber seal should not have visible dents or cracks. In case of damage, they should be replaced. The radiator fins should be both clean and straight. Green stains on fins indicate leakages, in the event of which a pressure test should be performed.
  3. Coolant level: The coolant level should remain 100%, with a homogeneous mixture of ½ water and ½ antifreeze. Hoses should be inspected to ensure that the clamps are sufficiently tightened without leakages. A pressure wash is recommended to keep cooling efficiency at its peak.
  4. GPU Battery Maintenance: Batteries should not be overlooked during ground power unit maintenance. Electrolyte levels should be checked, and the cables and terminals should be both secure and clean. Also, verify that the clamps are suitably tightened.
  5. Engine Maintenance: Engine and GT should be checked for oil or gas leaks. All connections, sensors, and wiring should be carefully inspected. Engine shafts, lubricating system, transmission, valves, piston rings, and mounts should be checked.
  6. GPU Wheels, Tires, and Brakes: It is recommended to replace the wheels, tires, and brakes every two years or 2,000 running hours. Tire pressure, damage, axle bolts, brake actuation, and efficiency must be checked.
  7. Tow Bar Maintenance: The tow bar should not have excessive sideways play. The latch should firmly lock the tow bar into place. In case of play, necessary repair work has to be done.
  8. Electrical Systems and the Generator: Worn-out cable plugs are one of the primary causes of GPU power interruption. Cable plugs are often scraped or abraded if they are dragged on the ground and should always be checked during ground power unit maintenance. These scrapes and abrasions eventually result in a loose plug connection. The cable itself should be inspected for the presence of cuts or abrasions. The terminals should be checked for roundness and wear. The contactor cover should be removed for inspection if it is not a sealed unit. If pitting is evident on the contact surfaces, contact cleaner and fine sandpaper can be used to restore them.


GPU PM plan should also include load bank testing. Load bank tests simulate the aircraft connected to the power unit, which is required to carry out adjustments and test activities on the GPU under different electric loads, both as a part of preventive maintenance work and following service repairs.


Why are AC GPU Calibration and Preventative Maintenance essential?

AC GPUs should follow certain maintenance adjustments (described below) to supply steady, clean power to the aircraft. These GPU adjustments should be under OEM guidelines and accurate. If neglected, these adjustments can potentially damage the aircraft. Therefore, the annual calibration of an AC GPU is of prime importance:


  1. Voltage setting (400 VAC): Depending on the type of aircraft, load, and operating voltage, GPUs have a provision to adjust voltage output from 96 to 136 VAC in 0.5 increments. To ensure this standard resolution is maintained throughout the service life of GPU, annual calibration is necessary.
  2. Line drop (resistance) compensation – AC 400 Hz: Typically, AC GPUs are factory set for 2% compensation, assuming a standard 30’ cable. These settings can vary with the cable length, and GPUs have a provision to adjust between 0-5%. To ensure that the right line compensation adjustments, GPU calibration is essential.
  3. Line drop (resistance) compensation – DC: Similar to AC line compensation, DC voltages are adjustable between 0-50%. To ensure that a steady 28.5 VDC power is supplied to aircraft, calibration and load bank testing should be performed with a 1000 amp load, with a standard 4/0 30’ cable.


Aircraft operators should therefore assimilate that a GPU is not just a generator. Whenever a GPU is connected to the aircraft, power is being supplied to a multi-million-dollar machine, and there is a potential risk of damaging the aircraft components. Therefore, it is essential to deliver clean, carefully regulated power every time a GPU is connected to the aircraft. An accidental power spike or unregulated surge could destroy sensitive and expensive electronic and avionics systems.


e2b calibration offers industry-leading consultancy and certified PM services for your aircraft GPUs. At e2b Calibration, we maintain and troubleshoot your GPUs so that you can concentrate on maintaining your aircraft. Our labs are ISO/IEC accredited and operated by a team of qualified experts providing training and consultancy services on Aircraft Cabin Pressurizer maintenance and inspection. Our verifiable services are unmatched in the industry. Contact e2b calibration for all your equipment calibration needs.



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