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Troubleshooting Common Helicopter Gas Turbine Engine Problems

I have spoken with numerous operators and discussed what they consider to be some of the more common gas turbine engine problems they have encountered, and the solutions to those problems. Since I am in no way, shape or form an engine specialist, I gladly accept these operators’ input and share it with you.

If you consider yourself to be a gas engine turbine specialist, please feel free to skip this part of the article. However, if you are not an engine person, I encourage you to read on. You just might learn something.

Jet Engines

Jet or gas turbine engines, as they are commonly called, are usually thought of as producing thrust for aircraft and not used as the driving force for gear boxes and rotor blades as they are in helicopter applications.

Turboshaft or turboprop engines are gas turbine engines that do not produce thrust. They take power from the production of thrust and turn it into mechanical power to drive rotors or propellers. The way the turbine blades are shaped determines whether they change thrust into great amounts of power to drive a shaft, or if they allow most of the thrust to be used as propulsion and take a small amount of drive power away to drive the compressor section of the engine.

In just about all helicopters where a gas turbine engine is used, the engine is a turboshaft engine. These engines are light weight, powerful and more reliable than reciprocating engines as there are fewer moving parts. 

How does a gas turbine turboshaft engine work? Four words are all it takes to describe its operation: suck, squeeze, burn and blow.

The front of the engine is the compressor section which “sucks” in air and “squeezes” it to make it denser and better for combustion. Air is brought into the compressor by the turning compressor blades that are shaped like small airfoils. It works like a very powerful fan to move air into the engine. Between the moving rows of compressor blades are stationary blades called stators. The stators help shape the direction of the airflow and help in the compression process.

As the air moves through the compressor section the volume the air can occupy gets smaller. This helps to heat the air on its way to the diffuser and then on to the combustion chamber. Here the air is mixed with fuel and ignited and we get the “burn”. The burning is continuous, like a gas furnace, barbeque or plumber’s torch. This is a continuous, controlled combustion process. The burning air/fuel mixture expands tremendously into a hot gas at high pressure within the combustion chamber. This high-pressure gas is then directed into the turbine section where the pressure is converted into a combination of drive power (torque) and thrust (exhaust). This is the “blow” part. Since, in this case, the pressure is converted mainly into power to drive a transmission, it is referred to as a turboshaft engine. Obviously, we have oversimplified the operation here, but you should have a feel for what is going on.

Getting Power from the Engine to the Rotor System

Now that we have power, we need to get that power to our rotor system. The turboshaft is spinning at many thousands of rpm and the rotor system cannot spin that fast for many reasons. We need to reduce the engine output rpm down to a useable number that is typically measured in hundreds of rpm. This gearing down is accomplished through the main transmission gearbox.

In the process of reducing the rpm for the rotor system, the torque is increased. The transmission drives the mast assembly to which the main rotor blades are attached. Often another drive shaft will come out of the transmission to drive the tail rotor assembly directly. An accessory gearbox mounted on the engine draws a small amount of engine power to drive the oil pump, electrical generator and fuel control for the engine.

Abbreviations

If you have worked in aviation for any period of time, you have come to learn that we have a language all our own. (I have even thought about doing a dictionary on aviation terminology, but that is for another time.) To further complicate matters, many of the terms used with regard to gas turbine engines have no industry agreed-upon standard. Working with these engines, we see abbreviations such as N1 (low-speed compressor) and N2 (high-speed compressor). These N speeds indicate what speed the compressor and main rotor blades are spinning as a percentage of maximum allowable rpm. Likewise, we see temperature or pressure readings designated as T1 or P1, respectively, and T3 or P3 accordingly. NR is the speed of the main rotor. We also have inlet turbine temperature (ITT), turbine outlet temperature (TOT), exhaust gas temperature (EGT)and turbine gas temperature (TGT).

            

When working with gas turbine engines, faults (problems) can generally be divided into two types. One type occurs during the engine start cycle and the second type occurs while the engine is running. Here is a listing, in no particular order, of some of the maintenance issues that were found to be the cause of the fault listed. These were found among many different engines and not one in particular.

Gas Turbine Engine Starting Problems

No RPM (N1) During Start Attempt

•  No electrical power to starter/generator

•  Starter shaft sheared — starter only spins

•  Starter shaft spline worn out — starter only spins

•  N1 indication defective

- Is oil pressure rising? Could be defective indicator or sheared shaft on tach drive

•  N1 rotor system seized

– Remove starter and attempt to turn drive

Insufficient N1 RPM at Start Attempt

• Insufficient voltage to starter?

– Check the power source and leads. Are the batteries low?

•  Check N1 rpm indicating system

– The indicator may read low, be hung up or sticking

Delayed Start

•  Improper start technique

– Check the flight manual for the correct procedure

•  Fuel manifold adapters are positioned improperly

– Adjust to the proper position

•  Fuel nozzle flow restriction

– Check and remove restriction 

•  Air in the fuel system

– Bleed the system; check on why and where air is entering the system

•  Check spark igniters

– Replace as necessary

•  Check ignition exciter

– Check on air start while motoring; replace as necessary

•  Check voltage to ignition exciter

– Low voltage may be accompanied with low cranking rpm; this can cause a hot start

Engine Fails to Light

•  Improper start technique

– Check the flight manual for the correct procedure

•  No fuel to the engine

– Check aircraft valve on; check fuel boost pump pressure

•  Excessive air in fuel system

– Bleed fuel system

– Check the reason for air entering the system

•  No ignition — gives wet stacks

– Caused by open circuit; let the engine drain and dry motor run the engine

– Attempt starting the engine using air start igniters

– If the start is accomplished, the problem is in the aircraft electrical system

– If there still is no start, change the igniters and/or ignition exciter

– Caution: Allow ignition exciter of snap type ignition to bleed off energy.

• Fuel manifold adapters improperly installed

– Install manifold adapters properly

•  Fuel pump failure

– Attach direct reading line in fuel pump to fuel control unit line

– Check for proper pressure while cranking the engine

– Check the fuel control unit filter

•  Contaminated fuel system

– Check all filters; clean fuel system as necessary

•  Fuel control unit bypass valve stuck open

– Check bypass valve for closing

– Remove fuel control unit cap fuel outlet and bypass fittings

– Apply 5 psi air pressure to the bypass return port

– If an air leak is detected at fuel the inlet, replace the fuel control unit

•  Start control defective

– Check the primary fuel line for flow; if no flow, replace the start control unit

•  Flow divider and dump valve stuck

– Replace the flow divider

Engine Starts But Slow to Idle

•  Improper start technique

– Early removal of starter; this will also cause hotter-than-normal starts

•  Leaking or restricted P3 signal lines

– Check all lines for obstructions, cracks and security; check the P3 filter for contamination.

•  Corrosion and/or ice in fuel control unit

– Replace the fuel control unit

•  Contaminated or defective fuel control unit

– Check all fuel filters; check the fuel control unit bypass valve

Hot Starts

•  Improper start technique

– Usually caused by entering fuel too early and/or early starter removal

•  Insufficient voltage to the starter

– This causes lower-than-normal cranking rpm and loss of starter assist

•  Defective starter

– Replace unit

•  Delayed igniters

– Check the system

•  Minimum fuel flow stop setting too high

– Replace the fuel control unit

•  Fuel nozzle restriction

– Check the fuel nozzles; if the spray pattern does not reach the igniters, this will cause a flame out

•  Fuel manifold adapters in the wrong position

– Confirm proper position by referencing the installation manual

•  Start control transfer valve stuck open

– Remove valve, clean and polish

Dump valve in flow divider stuck closed

– Replace unit

•  Defective N1 indicator

– Reads high, which causes early entry of fuel; replace N1 indicator

Operating Problems

Compressor Overspeed (N1)

•  Sheared fuel control unit drive coupling

– Replace the fuel control unit and coupling

•  Defective fuel control unit bypass diaphragm

– Replace fuel control unit

•  Fuel control unit bypass valve stuck closed

– Replace unit

Vibration

•  The compressor is out of balance and causes a steady hum

– Check for FOD

– Check bleed valve for being stuck shut; this will cause a higher-than-normal ITT at idle

•  The compressor is out of balance and causes an intermittent hoot 

– This condition is okay if hoot goes away by 60-percent N1 rpm; check turbine balance.

•  The power turbine out of balance

– Check power turbine for balance and failure

High N1 RPM

•  Check the indicating system

– Replace the indicator as necessary

•  The compressor is dirty

– Perform a compressor wash

•  Possible compressor FOD

– Check the first-stage blades for FOD/damage

•  Compressor bleed valve(s) are open

– This will give an accompanying high ITT; have the bleed valve(s) bench checked

•  Excessive P3 air leaks

– Check all gas generator gaskets for integrity and security

Uncontrolled Acceleration

•  The fuel control unit drive is sheared

– Replace coupling and fuel control unit

•  Fuel control unit bypass diaphragm is ruptured

– Replace the fuel control unit

•  Fuel control unit bypass valve is stuck closed

– Replace the fuel control unit

Stall During Acceleration

• The compressor bleed valve(s) are stuck shut

– Replace bleed valve(s)

• The compressor bleed valve(s) are out of calibration

– Have the unit(s) bench checked and calibrated

•  A defective fuel control unit is overfueling

– Replace the fuel control unit

•  Compressor FOD

– Check first-stage compressor blades for FOD

Failure to Accelerate Properly

•  The P3 sense line is restricted or leaking

– Check the line for any obstruction(s)

•  Corrosion or ice in the fuel control unit bellows section

– Remove the fuel control unit for cleaning; check the P3 line for water

•  Defective fuel control unit

– Replace the fuel control unit

•  Dirt in pneumatic section of fuel control unit

– Clean the fuel control unit

•  Contaminated P3 filter

– Check and change filter; this will also cause a slow start

•  Defective temperature compensator

– Replace the unit

Failure to Decelerate

•  Disconnected or improperly-rigged fuel control unit

– Repair or re-rig as necessary

•  Fuel control unit bypass valve is stuck

– Replace the fuel control unit

•  Binding control linkage

– Check the control linkage and repair as necessary

Flameout

•  Interrupted fuel supply

– Aircraft fuel valve shut off inadvertently

•  Fuel pump drive failure preceded by slight increase in N1 rpm

– Check fuel pump cranking pressure

• Air in fuel system

– Flameout at idle after a start; bleed fuel system and check reason for air entering the fuel system

•  Contaminated fuel control unit

– Check all fuel filters; clean the fuel system and replace filters as necessary

•  Incorrectly-rigged fuel control unit linkage, not contacting maximum stop at full throttle

– Re-rig fuel control unit

•  Fuel nozzle restriction, partially plugged

– Clean and flow check fuel nozzles; this will cause hot section distress

•  Contaminated or defective fuel control unit

– Disconnect fuel control unit outlet and check for flow while motoring; if no flow, remove fuel control unit for bypass valve check.

•  Dirt in pneumatic section of fuel control unit caused incorrect signal effect

– Clean or replace unit

•  Contaminated P3 filter-restricts P3 signal, causes slow acceleration

– Change P3 filter

Temperature Limited

•  Instruments are out of calibration

– Have all instruments calibrated

•  Selected wrong torque from graph

– Recheck

•  Dirty compressor

– Perform a compressor wash

•  Excessive accessory load

– Place the generator off line; check accessories for extra loading

•  Compressor FOD

– Check compressor first stage blades

•  Air leaks

– Check all gas generator case gaskets and flanges for leaks

Low Oil Pressure

•  Low oil level

– Check the tank oil level; check oil consumption

•  Defective oil pressure indication

– Check with direct reading gauge

•  Pressure relief valve malfunctioning, probably stuck open

– Replace unit

•  Internal oil leak

– Will cause oil smoke on start or shutdown and oil smell in cabin area

•  Failed heat shield in power section

– Causes excessive heating of oil; oil cooler might not be able to handle the added temperature.

High Oil Pressure

•  Defective oil pressure indication

– Check with direct pressure gauge

•  Pressure relief valve malfunctioning

– Stuck closed; oil pressure will follow N1 rpm

High Oil Temperature

•  Insufficient supply

– Check tank level; replenish supply, check consumption

•  Defective cooling system

– Check oil cooler thermostat

•  Failed heat shield in power section

– Causes excessive heating of oil; remove and repair power section

There you have it, some of the more common problems and solutions in working with gas turbine engines. My thanks to those of you who contributed to this article. If you have some issues and solutions not discussed here that you would like to share with our readers, please send them to my attention. You can even remain anonymous if desired.