The Air Starter Unit (ASU) is a crucial piece of ground support equipment (GSE) used at airports to provide the compressed air necessary for starting aircraft engines. Like any mechanical system, an ASU is vulnerable to various faults and malfunctions that can disrupt aircraft ground operations. Understanding how many faults can occur in an ASU and how to diagnose and resolve them ensures smooth, safe, and efficient engine starts.
In this guide, we cover all common ASU faults, their causes, and practical solutions for ground technicians, maintenance engineers, and airport professionals.
Table of Contents
What is an Air Starter Unit (ASU)?
An Air Starter Unit is a mobile or stationary machine that delivers high-pressure air to help start jet or turboprop aircraft engines. This is particularly important when the aircraft’s Auxiliary Power Unit (APU) is not functioning or absent.
ASUs are powered by either a diesel engine, electric motor, or in some advanced setups, a battery-powered system. They include key components like:
- Compressor
- Air filters
- Pressure regulators
- Control systems
- Storage tanks
- Air hoses and nozzles
Total Faults That Can Occur in Air Starter Units
Below are 15 of the most common faults that can occur in Air Starter Units, divided by system and function, along with solutions for each.
1. Low Output Air Pressure
Faults:
ASU delivers air pressure below required threshold (often <30 PSI).
Causes:
- Worn compressor
- Air leaks in hoses or connections
- Dirty or clogged air filters
- Malfunctioning pressure regulator
Solution:
- Replace or service compressor piston/rings
- Tighten or replace faulty air hoses
- Clean or replace air filters
- Adjust or replace regulator
2. No Air Flow from ASU
Faults:
ASU runs, but no air is discharged.
Causes:
- Severely clogged air intake
- Compressor failure
- Shut-off valve stuck
- Hose disconnected or blocked
Solution:
- Check and clear air intake
- Inspect and repair compressor internals
- Replace or lubricate valve
- Inspect hose connection and airflow path
3. Air Leaks
Faults:
Air hissing or pressure drop during operation.
Causes:
- Damaged hoses
- Loose fittings or quick couplers
- Gasket wear at joints
Solution:
- Perform leak detection using soapy water
- Replace hoses and seals
- Tighten fittings or use Teflon tape for leaks
4. Compressor Overheating
Faults:
Compressor temperature exceeds safe limit.
Causes:
- Insufficient lubrication
- Overuse without cooldown
- Blocked cooling fan or fins
- Faulty temperature sensor
Solution:
- Refill or change compressor oil
- Allow rest cycles
- Clean cooling components
- Replace faulty sensors
5. Engine Fails to Start (Diesel ASU)
Diesel engine won’t crank or start.
Causes:
- Dead battery
- Clogged fuel filter
- Air in fuel system
- Faulty starter motor
Solution:
- Charge or replace battery
- Clean or replace fuel filter
- Bleed air from fuel lines
- Replace or repair starter motor
6. Electric Motor Not Working (Electric ASU)
Faults:
Electric ASU won’t turn on.
Causes:
- Low battery voltage
- Blown fuse or circuit breaker
- Faulty ignition switch
- Motor coil damage
Solution:
- Recharge or replace battery
- Replace fuse or reset breaker
- Test and replace switch
- Inspect and repair motor windings
7. Erratic Pressure Readings
Faults:
Fluctuating or unstable pressure output.
Causes:
- Unstable regulator
- Valve flutter
- Air trap or moisture in lines
Solution:
- Replace or adjust regulator
- Inspect and clean valve
- Drain air moisture traps regularly
8. Emergency Stop Engaged or Faulty
Faults:
ASU won’t operate; E-stop may be engaged or stuck.
Causes:
- E-stop pressed or defective
- Broken contacts inside switch
Solution:
- Reset emergency stop
- Test continuity and replace if faulty
9. Contaminated Air Supply
Faults:
ASU delivers air with moisture, oil, or dirt.
Causes:
- Dirty intake filters
- Oil separator failure
- No air dryer or faulty dryer unit
Solution:
- Replace or clean filters
- Inspect and service oil separator
- Install/repair air dryer unit
10. Incorrect Hose Connection to Aircraft
Faults:
ASU connected but aircraft engine doesn’t start.
Causes:
- Loose coupling
- Wrong size connector
- Valve not opened
Solution:
- Verify proper fit of connector
- Use compatible coupling
- Open valve before supplying air
11. Control Panel Not Responding
Faults:
No response from switches, dials, or indicators.
Causes:
- Blown control circuit fuse
- Faulty wiring
- Malfunctioning circuit board
Solution:
- Replace fuse
- Inspect and replace damaged wires
- Replace PCB if unrepairable
12. Excessive Vibration or Noise
Faults:
Unusual noise during ASU operation.
Causes:
- Loose mounting bolts
- Worn bearings
- Unbalanced fan or motor
Solution:
- Tighten all mounting hardware
- Replace worn bearings
- Balance fan blades
13. Back Pressure from Aircraft
Faults:
Pressure flows back into ASU or fails to build in aircraft.
Causes:
- Check valve failure
- Aircraft bleed valve malfunction
Solution:
- Replace or clean check valve
- Coordinate with aircraft maintenance to inspect bleed system
14. Oil Leak in ASU
Faults:
Oil stains or dripping seen near compressor or engine.
Causes:
- Damaged gasket
- Overfilled oil tank
- Seal failure
Solution:
- Replace gaskets and seals
- Drain excess oil
- Regularly inspect after use
15. ASU Shuts Down Automatically
Faults:
Unit shuts off mid-operation without manual intervention.
Causes:
- Thermal overload protection triggered
- Engine oil pressure too low
- Safety relay fault
Solution:
- Let unit cool, reset thermal switch
- Check oil level and pressure sensor
- Replace safety relay if needed
Diagnostic Tools for ASU Faults
- Digital multimeter – For electrical continuity tests
- Leak detection solution – To spot air leaks
- Infrared thermometer – For overheating components
- Pressure gauge kit – To verify air output levels
- Diagnostic software – For advanced electronic ASUs
Maintenance Checklist to Prevent ASU Faults
| Frequency | Task |
| Daily | Check fuel, oil, and battery level |
| Weekly | Inspect hoses, clean filters, test control panel |
| Monthly | Load test, check compressor pressure and airflow |
| Quarterly | Replace air filters, tighten fittings, clean air dryers |
| Annually | Full system inspection, compressor overhaul, valve maintenance |
Safety Tips During Troubleshooting
- Always turn off and isolate the unit before inspection.
- Discharge residual pressure before disconnecting hoses.
- Use PPE (gloves, safety glasses, ear protection).
- Avoid hot surfaces and spinning components.
- Never bypass safety relays or pressure regulators.
Conclusion
The Air Starter Unit is the unsung hero of aircraft ground operations. Knowing how many faults can occur and exactly how to fix them ensures that flight schedules remain uninterrupted, and safety remains uncompromised. From minor air leaks to major compressor failures, every issue has a solution—provided it’s addressed with proper knowledge and regular maintenance.
By implementing a preventive care plan and understanding the root causes behind each fault, you can extend the life of your ASU, minimize downtime, and support efficient, on-time flight operations.