Stalling of engine compressor due to excessive ingestion of dust particle while hovering for landing caused the fatal Marine Corps MV-22 Os...
Stalling of engine compressor due to excessive ingestion of dust particle while hovering for landing caused the fatal Marine Corps MV-22 Osprey tiltrotor aircraft crash on May 17, 2015, according to the Judge Advocate General Manual (JAGMAN) investigation statement released by U.S. Marine Corps Pacific Forces.
This tragic mishap occurred at Marine Corps Training Area Bellows during a training sortie and led to the death of two Marines and injuries to 20 other Marines onboard, destroying the aircraft.
The investigation found the main contributing factors to this mishap were pilot performance and an improper site survey of Landing Zone (LZ) Gull. The pilots did not violate any regulations or flight standards; however, pilot decision-making failed to take into account the contributory events that led to the mishap.
The first landing attempt indicated that the Reduced Visibility Landing (RVL) level of LZ Gull was much higher than anticipated. The investigation indicated that a proper risk assessment should have prompted the pilots to choose an alternate flight profile, path, or landing site that would have minimized or avoided the severe brownout conditions.
The investigation found that repeated, sustained flight time in brownout conditions (an in-flight visibility restriction due to dust or sand in the air) while attempting to land caused the left engine to stall, resulting in a loss of power that placed the aircraft in an unavoidable freefall to the ground.
Specifically, Engine Percent Power (EPP) decreased on both engines each time the aircraft entered a low-altitude hover over Landing Zone Gull as dust and sand particles increased in the air due to rotor wash.
The sand and dust ingestion caused a buildup of material on the turbine blades and vanes leading to a compressor stall in the left engine, which decreased lift and resulted in the hard landing.
Many of the recommendations from the investigation focus on aiding the pilot decision- making process, such as: displaying engine performance and stall margins on the Multi-Functional Display, more advanced brownout technology, advisories alerting pilots when engine power declines below 95%, and reconsideration of Naval Air Training and Operating Procedures Standardization (NATOPS) recommendations regarding flight times in brownout conditions.
As a direct result of this mishap, a NATOPS Interim Change was published on 17 November, 2015, which reduced exposure time in Reduced Visibility Landing Profiles to a maximum of 30 seconds.
As a part of recommendation Marine Corps will also field an improved engine air filtration systems that is being developed for the CV-22 variant operated by U.S. Air Force.
The investigation recommendations also include the potential for disciplinary and administrative actions.
Furthermore, the investigation concluded that the mishap did not occur due to any misconduct or negligence of duties or training on the part of the air crew.
Unlike large rotors in helicopters, Osprey have small low inertia rotors which have poor autorotation ability, hence the sink rate during an engine failure is not reduced.
