Incident Description
An emergency medical service helicopter was approaching a rooftop hospital helipad in a congested metropolitan area at about 800 feet above ground level when the left side engine cowling opened. The cowling partially separated and impacted the bottom of all four main rotor blades, causing substantial damage. The pilot described the event as a brief shudder similar to flying through the rotor wash of another helicopter and was unaware that damage had occurred. The helicopter landed uneventfully.
Post-Flight Discovery
After landing, a third medical crewmember boarded through the right rear door for a local flight to a suburban hospital. During engine shutdown at the destination, another medical crewmember exited the helicopter on the left side and was the first to notice the damage.
Preflight Inspection Omission
The pilot stated that he failed to complete a thorough preflight inspection because the crew was assigned a medical mission just after their shift started.
Previous Pilot's Actions
The previous pilot who flew the helicopter the evening before the accident stated that he had opened the left side engine cowling to check the oil level and became distracted. He also stated he could not remember if he had secured the cowling latches.