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)