- Eastern Airlines 375 at Boston
- Accident Overview
- Accident Board Findings
- Accident Board Recommendations
- Relevant Regulations
- Prevailing Cultural / Organizational Factors
- Key Safety Issue(s)
- Safety Assumptions
- Resulting Safety Initiatives
- Airworthiness Directives (ADs) Issued
- Common Themes
- Related Accidents / Incidents
- Lessons Learned
- Eastern Airlines 375 at Boston
Photo of Eastern Airlines Flight 375 wreckage
History of Flight
A few seconds after becoming airborne, at an altitude of approximately 120 feet, the airplane flew through a large flock of starlings. A number of these birds were ingested into engine numbers one, two, and four. Following the crash, the investigators recovered approximately 75 starling carcasses on/near the presumed area on the runway where the bird encounter occurred. The engine number 1 propeller was feathered by an autofeather system, as designed. Engine number 2 and 4 experienced substantial losses of power, but, by design, those propellers were prevented by the airplane’s system from also feathering automatically, since only one propeller is permitted to autofeather when the autofeather system is armed. The abrupt and intermittent loss and recovery of power and associated thrust asymmetry caused the airplane to yaw to the left and decelerate below the speed at which directional control could be maintained. When the speed decayed below that where the yaw could no longer be controlled, the left wing dropped, the nose pitched up, and the airplane rolled left and fell almost vertically into Winthrop Bay near the end of the runway.
Wreckage photos of Eastern Airlines Flight 375
Allison 501-D13 engine on engine maintenance stand
The L-188 Electra was powered by four Allison 501-D13 turboprop engines, each rated at a takeoff rating of 3,750 shaft horsepower. The Allison 501-D13 is a single shaft, modular design turboprop engine with a 14-stage axial flow compressor driven by a four stage turbine. Power is supplied to the propeller through a reduction gearbox driven by a driveshaft connected to the power turbine. Gear reduction is through a series of planetary gears resulting in a 13.54 engine-to-propeller gear ratio. A complete description of the engine, propeller, and cockpit controls is contained in two documents available at these links: (Engine System Description 1) (Engine System Description 2)
Cross-section of Allison 501-D13 Engine
Photo of Electra propellers
Photo copyright Aris Pappas - used with permission
The Aeroproducts A644FN-606 propeller design is a self-contained hydraulically operated propeller incorporating an integral propeller control system. Propeller rotation supplies the power through the hydraulic governor to change blade pitch, maintaining constant rotational speed by varying blade angle through a range of varying engine and aerodynamic loadings. Primary changes in propeller operating regimes, which involve governing, negative pitch, feathering, negative torque signal (NTS), and taxi operations, are initiated by mechanical signals from the cockpit control levers or engine sensors. This design concept provides an isolated propeller which functions independently of the other propellers, engine, or aircraft systems.
Secondary changes in propeller operations, which concern synchronizing, phase synchronizing, or auto-feathering, are controlled by electrical power supplied to the propeller. In the case of synchronizing or phase synchronizing, electrical power is used only to adjust or trim the self-contained hydraulic system.
Diagram of propeller internal mechanisms
Photo showing engine inlets
(on top of nacelle) for Allison engines
Photo Copyright Wolodymir Nelowkin – used with permission
In conjunction with these primary and secondary concepts, certain design and safety features were incorporated to increase reliability, maintainability, and functional capability of the propeller. The functions include:
- The hydraulic system includes four mechanically driven pumps and one electrically driven pump;
- A hydraulic low pitch stop is provided to complement the mechanical low pitch stop as part of the over-speed protection system;
- The spinner and blade cuffs incorporate electrical anti-icing elements;
- A beta (blade angle associated with propeller reversing) range indicator is provided; and
- An air shutoff is activated when the propeller is feathered to prevent the flow of cooling air through the spinner.
The propeller converts engine torque to thrust, regulates this thrust to absorb engine power under varying conditions, and maintains a constant propeller rotational speed. The integral hydraulic system supplies the power required for changing propeller pitch to compensate for various loading conditions. The hydraulic system is controlled by a mechanical linkage from the power lever in the cockpit. An electrical control system trims propeller speed for synchronizing and phase synchronizing to a master propeller.
A reduction gearbox reduces the engine RPM to a useable propeller speed. For example, at takeoff and maximum continuous power, the engine operates at 13,820 RPM, while the propeller is governed to 1,020 RPM. Safety devices incorporated into the reduction gear box include a propeller brake and a negative torque system.
Photo of an Electra L-188 with
a feathered number 4 propeller
Propeller reduction gear mechanism
Lockheed P-3 (military variant of the Electra)
in flight with Number 1 propeller feathered
Photo copyright Wolodymir Nelowkin - used with permission
The autofeather system is intended to automatically reposition the propeller blades to a minimum drag angle, i.e., feather, for low speed, high power phases of flight, such as takeoff. Since the “windmill” drag (drag associated with a freely rotating, unpowered propeller assembly while in flight) of an unfeathered turbo propeller is extremely high, resulting thrust asymmetry can result in an airplane loss of control during low speed flight phases. In order to reduce this risk, autofeather systems were introduced on most turboprop airplanes, including the L-188, in order to protect airplane takeoff performance and not rely on manual feathering by the pilot during critical phases of flight. Since takeoff performance is only predicated on a single engine failure, safeguards are necessary in the autofeathering circuitry in order to prevent more than one propeller from feathering during a single event.
Photo of General Cockpit Arrangement of L-188 Electra
Photo copyright Petr Volek – used with permission
The system automatically feathers the propeller anytime the autofeather system is armed, the power lever is above 75°, and the engine is producing less than 500 pounds of thrust. Engine power loss is sensed by a system that monitors torque produced by the engine drive shaft. If a sudden loss in torque is sensed (negative torque system) the autofeather system interprets this torque loss as an engine failure and triggers an autofeather command to the propeller. A bird ingestion event, such as occurred on Eastern Flight 375, results in a momentary disruption in the gas flow path and a momentary drop in engine torque.
The autofeather system is made up of a network of inter-locking circuits through contacts in the cockpit feather switches. The arrangement is such that: (1) if any single propeller receives an auto-feather signal, or if it receives a signal in advance of signals from other propellers, it will autofeather and disarm the autofeather system for the propellers on the other three engines; and (2) if thrust switches on more than one engine close simultaneously, the inter-lock of the auto-feather circuits establishes a priority sequence (4, 1, 3, 2), allowing only one propeller to autofeather. The other propellers must be feathered manually, just as if the signals had been triggered sequentially rather than simultaneously. See cockpit diagram showing propeller controls, including autofeather system.
This accident was the first major transport airplane accident with substantial loss of life attributed to engine failure following bird ingestion. Turbojet and turbopropeller engines were newly introduced into commercial service in the 1950’s. As such, little was known regarding threats that would soon be experienced in service, such as failures due to bird ingestion. The engine standards associated with the Allison 501 D-13 and other turbine engines of that era did not include requirements to continue running after any type of bird ingestion event.
As a result of this accident, there was an industry-wide recognition that some level of bird threat capability would be necessary for all turbine engines. This capability has evolved into two safety objectives associated with bird ingestion threats. The first involves the assumption that an ingestion event occurs on a single engine and will fail in a manner (e.g., burst, catch fire, etc.) that will not threaten the continued safe flight of the airplane. This threat is assumed to be a large bird (goose, eagle, etc.). The second bird ingestion scenario assumes that all engines of a multi-engine airplane ingest birds simultaneously, and the combined power or thrust loss associated with each engine is no greater than the loss of a single engine on a four-engine airplane. This threat is assumed to be from small or medium size birds such as starlings, small gulls, pigeons, etc.
Flock of small birds near a Boeing 777
Photo Copyright Darrell Morell – used with permission
The birds encountered by Eastern Flight 375 were common starlings, which tend to congregate in huge flocks in flight, known as “murmurations.” It was believed by investigators that engines 1, 2, and 4 ingested numerous starlings. Engines 1 and 2 were primarily affected, with the propeller for engine 1 being automatically feathered and then manually shut down, and engine 2 experiencing a partial power loss and subsequent recovery. Engine 4 also experienced a partial power loss, but the investigation stated that it was not as severely affected as engines 1 or 2.
The starling was identified as a small bird for the purposes of engine ingestion standards, and would later be identified as an eight ounce threat. Because of its small size, a single bird ingestion event on large turbine engines often result in little, if any, damage. However, the major threat from birds such as Starlings comes from their tendency to congregate in large, dense flocks, which poses a particularly high threat to aviation safety.
Flocking birds, such as Starlings, are a particular hazard to aircraft due to their natural tendency to fly in close groups of several thousand individuals. This behavior has frequently led to large numbers of birds striking aircraft and engines, causing engine malfunction by gas turbine fan blade damage and/or core ingestion, as with this case in point.
Electra Passenger Cabin
Photo Copyright Air Nikon - Used with Permission
Cabin & Seats
Investigators determined that seat restraint failures jeopardized passenger survivability in this accident, and they recommended that research be conducted which could lead to enhancements in the L-188 seat restraint structure. This accident was one of the first in a series of accidents which would lead to the development of enhanced seat, seat retention, and seatbelt standards. Although no regulations resulted as a direct consequence of this accident, it was a catalyst for advancements in the area of cabin safety.