Brownout is a dangerous condition for helicopters. In a sandy environment, the downwash from a helicopter’s rotor kicks up a cloud of dust during take off and landing, completely engulfing the aircraft. Concentrations of dust up to 2.5 g/m3 of air have been measured during desert landings.
Under these conditions the engine can ingest vast quantities of sand. The intense, blinding dust clouds that are stirred up by rotor wash during near-ground flight cause:
“Brownouts have claimed more helicopters in recent military operations than all other threats combined.”
-Lt. Col. Steve Colby in Rotor & Wing, 2005)
The particle size distribution of the dust penetrating the engine air intakes is similar to a reference silica based laboratory test contaminant: ISO Coarse Test Dust. This particular test dust is extremely fine, with 12% of the particles between 0 and 5 microns and a median particle size of 30 microns. For a medium size helicopter, this could mean as much as one kilogram (2.2 lbs) of dust getting into the engine per minute!
This explains why in desert operations, unprotected CH-47 Chinook engines did not survive more than fourteen (14) landings before their removal for erosion damage.
“Helicopter Brownout is a $100 million per year problem, leading to significant hardware loss, injuries, and fatalities.”
(USAF Central Command)
The Centrisep engine advanced protection system (EAPS) was initially designed to respond to hazardous brownouts. It maintains excellent dust separation efficiencies (96% or better) at concentrations of up to five g/m3 of air, as proven by extensive wind tunnel testing.
The Centrisep EAPS can handle very high dust concentrations and increase the erosion life of critical turbine components by substantial amounts compared to an unprotected engine, without introducing any additional maintenance burden on the operator.
Note: The manufacturers of oil wetted IBFs advise the following “When operating in an environment of high sand and dust levels, frequent servicing of the filter assembly may be required based on the time exposure and severity of the environment. Any operations in an environment that can result in “brownout” conditions should therefore be minimized or avoided to the maximum extent possible within the constraints of the operation.”
As part of a qualification of a helicopter engine, a dust endurance test was carried out in a wind tunnel. The engine was protected by a Centrisep EAPS.
The engine was to run continuously for 10 hours at maximum continuous power with SAE Coarse Test Dust at a concentration of 1.5 g/m3 supplied to the engine’s air intake. A total of 156 kg (343 lbs.) of dust was fed into the Centrisep air intakes. Upon completion of the 10 hours, the engine survived the test with only a 3% power loss due to erosion. Note that less than 1/20th of this total dust fed into an unprotected engine would have been enough to cause severe power loss through engine erosion.
No maintenance was carried out on the filter or the engine throughout the entire 10 hour test.
