How to Prevent PT6T Compressor Surging in Extreme Conditions

Even when a PT6T engine’s bleed valves are adjusted properly and working perfectly, compressor surging sometimes occurs in extreme operating conditions. Here’s why it happens – and how to prevent it.

Conditions Where Compressor Surging Is a Concern

All PT6T engines are equipped with bleed valves to prevent compressor surging. This phenomenon happens when the engine sucks in more air than it can handle due to the difference in the efficiencies of the axial and centrifugal compressors.

How do you know when a surge occurs? “There’s no mistaking it,” says Stéphan Michon, Senior Technical Specialist, Customer First Centre. “The gauges go back and forth, you lose power for a split second, and you’ll hear a noise like a shotgun blast.”

The bleed valve is designed to prevent surging by venting out extra pressure from the axial compressor to match the capacity of the centrifugal compressor under varying speeds. But in certain extreme conditions, surging could still occur, as Stéphan explains.

When you have a combination of high altitude and cold temperature, compressor surging is sometimes seen in cases of rapid engine acceleration, or collective pull. These conditions could affect operators flying in colder countries or helicopters transporting heli-skiers to remote mountain locations.

- Stéphan Michon, Senior Technical Specialist, Customer First Centre

When a surge occurs, the pilot needs to reduce and modulate the aircraft’s power. And while PT6T engines are designed to withstand surging, it puts them under considerable strain.

Basic Precautions to Help Prevent Surging

“Make sure you have a clean compressor section in good shape and that the bleed valve is working fine,” Stéphan advises.

To keep the compressor clean, simply wash it regularly. An in-situ bleed valve check and inspection can be done to see if the closing point has shifted from its original setting and detect air leaks, stiffness in the piston movement, etc.

A Simple Hardware Solution to Prevent Surging

For operators who are exposed to conditions conducive to compressor surging, a simple, effective fix is available: a bleed air case.

Stéphan was part of the team that developed this accessory – also known as a “belly band” due to the way it wraps around the power section’s belly. It’s easily installed with a couple of bolts and requires no special maintenance other than routine checks of its condition.

How does it work? Stéphan explains: the bleed air case assembly wraps around the power section bleed valve area, creating a chamber where the temperature and pressure of the hot air escaping the open valve are equalized before re-entering via the intake.

A localized streak of very hot re-ingested air could greatly disturb the colder airflow, often leading to surging on collective pulls. The bleed air case eliminates this situation, bringing the surge margin back to a similar level to when it was adjusted on the ground.

- Stéphan Michon, Senior Technical Specialist, Customer First Centre

Some aircraft do not need this equipment, such as those powered by the PT6T-6 engine or its derivatives. For these engines, the airframe installation is different and the bleed valve exit is ducted overboard, therefore eliminating localized re-ingestion of hot air.

Bleed Air Case Benefits Other Operators Losing Surge Margin

It was later discovered that the bleed air case can benefit certain operators even if they’re not flying in cold-temperature, high-altitude conditions. These include aircraft operating in harsh environments that are losing surge margin due to compressor deterioration, foreign object damage (FOD), normal wear, etc.

Installing the bleed air case can sometimes allow a power section to stay installed a bit longer before the need for a shop repair due to loss of surge margin.

- Stéphan Michon, Senior Technical Specialist, Customer First Centre

The bleed air case is available through our Component Solutions division or authorized distributors. For more information, talk to your customer manager or field service representative or contact us at [email protected].

Want more practical tips on PT6T/PT6A maintenance? Check out our expert’s advice on preventing salt air corrosion.