Investigation Summary
The accident occurred as Flight 2311 was maneuvering for a visual approach to Runway 7. Witnesses observed the aircraft flying at an unusually low altitude, between 100 and 200 feet, before it suddenly rolled 90 degrees to the left and plunged nose-down into a wooded area. The NTSB investigation focused on the Hamilton Standard 14RF-9 propeller system. Physical evidence from the wreckage showed the left propeller blades had moved to a 3 degree pitch angle, far below the flight idle stop. This condition created massive drag on the left side, effectively acting as a speed brake while the right engine continued to provide normal thrust. Testing and simulation demonstrated that the crew would have had no warning of the impending failure until the propeller reached the mechanical stop, at which point the aircraft would become nearly impossible to control. The failure was traced to a worn titanium spline in the propeller control unit. The NTSB criticized the design of the PCU, noting that it did not account for this specific failure mode, and faulted the FAA for approving a design that lacked a redundant protection against uncommanded fine pitch in flight.
Final Conclusions
The NTSB determined the probable cause was the loss of control in flight resulting from a malfunction of the left engine propeller control unit, which allowed the propeller blade angles to go below the flight idle position. Contributing factors included the deficient design of the propeller control unit by Hamilton Standard and the FAA's approval of that design, which failed to evaluate the failure mode that led to uncommanded and uncorrectable blade movement.
Video Analysis
The Senator's Final Flight | Atlantic Southeast Airlines Flight 2311 — A detailed look at the mechanical failure and design flaws that led to the crash of ASA Flight 2311 in Georgia.
If playback is blocked, the owner has disabled embedding — use the link above.
Airframe & Maintenance
Crew Experience
Captain
Mark Friedline, aged 34. Described as having extensive knowledge of the EMB-120.
First Officer
Hank Johnston, aged 36. Qualified flight instructor.
Systems & Failure Modes
Propeller Control Unit (PCU)
A Hamilton Standard unit responsible for adjusting the pitch of the propeller blades. A design flaw allowed a worn spline to cause the blades to move below the flight idle stop.
Hamilton Standard 14RF-9 Propeller
The four-bladed propeller system used on the EMB 120. The investigation found the system lacked sufficient redundancy to prevent catastrophic pitch reduction in the event of a PCU failure.
Interesting Facts
- 01The aircraft had been in service for less than four months at the time of the accident.
- 02The left propeller blades were found at a 3 degree angle, having bypassed the flight idle stop of approximately 22 degrees; the commanded angle for feathering was 79.2 degrees.
- 03The aircraft was not equipped with a Cockpit Voice Recorder (CVR) or a Flight Data Recorder (FDR), as they were not required for this class of aircraft at the time.
- 04Among the fatalities were former U.S. Senator John Tower and astronaut Sonny Carter.
- 05The NTSB found that the pilots had received only 5 to 6 hours of sleep, though this was not cited as a direct cause of the crash.
Safety Actions & Advisories
Propeller Control Unit Redesign
Following the investigation, the NTSB recommended and the FAA mandated modifications to the Hamilton Standard propeller control units to prevent uncommanded blade movement below the flight idle stop.