Eng 3 and 4 RPM decays/bogdown/flameout

[Mike Philippines]

Hi everyone,
I’d like to open a discussion regarding an issue we recently experienced on a C-130H.

During taxi, No. 3 and No. 4 engines bogged down/decayed and eventually flamed out. Both engines were operating at LSGI at the time. No cockpit indications showed abnormalities prior to the event.

I’m looking for insights on probable causes.

One factor I’m considering is the wind effect—specifically a tailwind or a sudden increase/decrease in wind entering the inlet, which could disturb compressor airflow. At LSGI, the compressor stall margin is already low, so any airflow disruption might affect the air/fuel ratio enough to cause decay or flameout.

Additional note on engine condition:

No. 4 propeller has an above-minimal hydraulic leak, and I’m considering whether prop hydraulic fluid entering the inlet could influence compressor performance.

No. 3 prop has a minimal leak as well.
Not sure if this could contribute to compressor airflow disturbance or contamination, but it’s worth adding to the discussion.

Has anyone experienced something similar, or have inputs on other possible causes (bleed air setup, inlet distortion, FCU behavior at LSGI, environmental factors, etc.)?

Appreciate any feedback or shared experiences. Thanks 🫡🫰

[Lkuest]

On 11/18/2025 at 6:05 AM, Mike Philippines said:

No. 4 propeller has an above-minimal hydraulic leak, and I’m considering whether prop hydraulic fluid entering the inlet could influence compressor performance.

Hydraulic leaks could only affect performance by increasing dirt build-up on the blades. I wouldn't even consider this a possibility unless the compressor efficiency was measured as "low." I would be more concerned about aircrew hydraulic fluid inhalation and A/C system contamination.

I would look at your fault isolation manuals for guidance, which has you checking fuel system delivery, fuel control settings, throttle rigging, (and yes compressor condition too). Unfortunately, operational checks don't have the maintenance personnel operate the engine in the way you describe, which makes me think it is "normal" to have unstable RPM at LSGI while manipulating throttles and loading the engines down with multiple bleed air demands while simultaneously dumping air out of the bleed collectors.

Just do a full operational checkout of the engines and see if anything is out of adjustment. I would also ask the aircrew to show you where their manuals allows them to taxi with all 4 engines in LSGI. As far as I know, that's not a thing.

Edited November 30, 2025 by Lkuest

[Mike Philippines]

On 11/30/2025 at 12:18 PM, Lkuest said:

Hydraulic leaks could only affect performance by increasing dirt build-up on the blades. I wouldn't even consider this a possibility unless the compressor efficiency was measured as "low." I would be more concerned about aircrew hydraulic fluid inhalation and A/C system contamination.

I would look at your fault isolation manuals for guidance, which has you checking fuel system delivery, fuel control settings, throttle rigging, (and yes compressor condition too). Unfortunately, operational checks don't have the maintenance personnel operate the engine in the way you describe, which makes me think it is "normal" to have unstable RPM at LSGI while manipulating throttles and loading the engines down with multiple bleed air demands while simultaneously dumping air out of the bleed collectors.

Just do a full operational checkout of the engines and see if anything is out of adjustment. I would also ask the aircrew to show you where their manuals allows them to taxi with all 4 engines in LSGI. As far as I know, that's not a thing.

Thanks for your input.

No. 1 is on NSGI, the rest at LSGI. Engine efficiency is also quite good. Engines 3 and 4 were running at LSGI for like 5mins before it decayed, probably the only movement they made was moving the throttles slightly forward due to the tailwind factor during taxi that might cause oil temps to rise.

[Lkuest]

On 12/1/2025 at 12:18 PM, Mike Philippines said:

No. 1 is on NSGI, the rest at LSGI...

My assumption then is that the #1 engine is the only one supplying bleed air for aircraft systems.

Assuming everything with the engines check out to be within normal limitations (rigging, air leaks, efficiency, propeller blade angle, etc...), I'd recommend you adjust the Fuel Control LSGI RPM towards the upper limit. Other things you can do if that doesn't work is to re-seal the 5th and 10th bleed air valves and blow out the CIP line from the fuel control fitting into the air inlet housing. Do not adjust the Minimum flow adjustment as a misadjustment there should only cause a flameout when transitioning from HSGI to LSGI.

If none of this works, (again assuming all rigging and other checks are correct), the most likely culprit is the fuel control. This is unlikely the cause due to multiple fuel controls involved, but it's not unheard of for multiple components to share the same defect if they were overhauled at the same facility around the same timeframe. Could also just be bad luck.

I'm still not sold on taxiing with one single engine in HSGI, especially with a tailwind. I'd recommend only 2 at a time during taxi. If oil temps are difficult to keep under control in LSGI during normal taxi operations and minimal tailwind, you may need to look at that issue separately. If cleaning the coolers with fresh water doesn't fix that issue, either straighten the radiator fins or replace the coolers. Oil cooler augmentation is a great addition if you can afford the modification.

[superbigbig]

Quote

I have previously encountered a situation where all four engines experienced a simultaneous RPM decay. The scenario was that all engines were operating in LSGI with Oil Cooler Augmentation ON, and during a turn we entered an area with different wind conditions. I consulted the TCG team, and they confirmed that under LSGI with Oil Cooler Augmentation ON, an engine flameout or RPM droop is possible.

 

[Lkuest]

What does your flight manual say about operating the oil cooler augmentation systems in LSGI?

[pjvr99]

Been thinking about this problem .... i dont know the aug system, never worked engines that had them. However, taxiing

in LSGI you do run the risk of popping the LSGI button(s) setting up an eventual rpm decay and flameout. Hot n humid

conditions will make it worse.