2. Analysis
Table of Contents 2.1 General
Table of Contents The flight and cabin crewmembers were properly certificated and qualified under federal regulations. No evidence indicated any preexisting medical or physical condition that might have adversely affected the flight crew's performance during the accident flight.
The accident airplane was equipped, dispatched, and maintained in accordance with federal regulations.
The LGA departure controller chose to display only correlated targets on his radar display. If he had chosen to display both the correlated and uncorrelated targets, he would not have been able to effectively control traffic because a large amount of extraneous information would have been shown on the display. Additionally, he would not have been able to determine whether the additional targets were birds, boats, precipitation, or any other item the radar detected. Therefore, the NTSB concludes that the LGA departure controller's decision to display only correlated primary radar targets on his radar display was appropriate.
Examinations of the recovered components revealed no evidence of any preexisting engine, system, or structural failures. The airplane met the structural ditching certification regulations in effect at the time of its certification, and the engine met the bird-ingestion certification regulations in effect at the time of its certification, as well as an anticipated additional regulation that it was not required to meet at that time.
The investigation determined that the airplane's descent rate at the time it impacted the water was 12.5 fps, more than three times the descent rate of 3.5 fps assumed during ditching certification, resulting in external pressures on the aft fuselage, primarily from FR55 to FR70, which significantly exceeded the values established to demonstrate compliance with the certification criteria. These external pressures were sufficient to cause the large-scale collapse and failure of the aft fuselage frames, cargo floor, and passenger floor struts and to initiate cracking of the lower fuselage skin, allowing water to enter the airplane. Further, the water ingress and continued forward motion of the airplane through the water resulted in postimpact pressures and suction forces that caused additional damage, including the failure of the lower fuselage skin panel and aft pressure bulkhead. Therefore, the NTSB concludes that the airframe damage was caused by the high-energy impact at the aft fuselage and the ensuing forward motion of the airplane through the water.
The term, "ditching," is not defined in federal regulations. The NTSB addressed this issue previously in its Safety Study 85/02, "Air Carrier Overwater Emergency Equipment and Procedures," which stated the following:
[ditching] usually means a planned water event in which the flight crew, with the aircraft under control, knowingly attempts to land in water. In contrast to an inadvertent water impact, in which there is no time for passenger or crew preparation, ditching allows some time for donning life preservers, etc.
The NTSB considers this accident to be a ditching because the pilots clearly intended to ditch on the Hudson River. The accident event falls between a planned and unplanned event in that, although the pilots did not have time to complete each step of the applicable checklist, they did have sufficient time to consult the QRH, begin checklist execution, transmit radio calls, determine a landing strategy, configure the airplane for the ditching, and alert the flight attendants and passengers to "brace for impact."
Although the airplane impacted the water at a descent rate that exceeded the Airbus ditching parameter of 3.5 fps, postaccident ditching simulation results indicated that, during an actual ditching without engine power, the average pilot will not likely ditch the airplane within all of the Airbus ditching parameters because it is exceptionally difficult for pilots to meet such precise criteria with no power. Further, the water swell tests conducted on Mercure airplanes indicated that, even with engine power, water swells and/or high winds also make it difficult for pilots to safely ditch an airplane, and these factors were not taken into account during certification. (See section 2.6 for a more detailed discussion of this issue.)
Although both engines experienced an almost total loss of thrust after the bird encounter, the flight crew was able to ditch the airplane on the Hudson River, resulting in very few serious injuries and no fatalities. Further, all of the airplane occupants evacuated the airplane and were subsequently rescued. Consequently, this accident has been portrayed as a "successful" ditching. However, the investigation revealed that the success of this ditching mostly resulted from a series of fortuitous circumstances, including that the ditching occurred in good visibility conditions on calm water and was executed by a very experienced flight crew; that the airplane was EOW equipped even though it was not required to be so equipped for this particular flight; and that the airplane was ditched near vessels immediately available to rescue the passengers and crewmembers. The investigation revealed several areas where safety improvements are needed.
The analysis discusses the flight crew performance and safety issues related to the following: in-flight engine diagnostics, engine bird-ingestion certification testing, emergency and abnormal checklist design, dual-engine failure and ditching training, training on the effects of flight envelope limitations on airplane response to pilot inputs, validation of operational procedures and requirements for airplane ditching certification; and wildlife hazard mitigation. Also analyzed are survival-related issues, including passenger brace positions; slide/raft stowage; passenger immersion protection; life line usage; life vest stowage, retrieval, and donning; preflight safety briefings, and passenger education.
2.2 Engine Analysis
Table of Contents 2.2.1 General
FDR data indicated that, during ground operation and takeoff, the N1 and N2 speeds of both engines accelerated in unison during the throttle advancement to full takeoff power and that these speeds were similarly matched and stable during takeoff and initial climb until about 1 minute 37 seconds into the flight. Although the right engine had recently experienced an engine compressor stall, US Airways had corrected the problem in accordance with maintenance manual practices, and no FDR evidence indicated that a compressor stall occurred before the bird encounter.
2.2.2 Identification of Ingested Birds
The Smithsonian Institution analyzed the feather and tissue samples from both engines and determined that the left engine contained both male and female Canada geese remains, indicating that the engine ingested at least two geese. (The average weight of a male Canada goose is from 8.4 to 9.2 pounds, and the average weight of a female goose is from 7.3 to 7.8 pounds.) The Smithsonian Institution report stated that only male Canada goose remains were found in the right engine, suggesting that it might have only ingested one bird; however, a comparison of the physical features and quantity of the damage in the two engines, which will be discussed in the following sections, indicated that the right engine ingested at least two Canada geese.132
2.2.3 Engine Damage
The engines were certificated to withstand the ingestion of birds of a specified weight in accordance with the certification standards and still produce sufficient power to sustain flight. (The certification requirements are discussed in section 2.2.5.) However, during this event, each engine ingested at least two Canada geese weighing about 8 pounds each, which significantly exceeded the certification standards, and neither engine was able to produce sufficient power to sustain flight after ingesting these birds. This section will discuss the progression of the damage to the engine parts, starting with the engine spinners and moving to the cores, to explain at which point in the bird-ingestion sequence the damage occurred that prevented the production of sufficient power to sustain flight.
2.2.3.1 Engine Spinner, Fan Blade, and Fan Inlet Case Damage
If a bird enters the engine inlet near the inner radius (near the spinner), a portion of it may be ingested by the engine core because of the radius' proximity to the core. Both engine spinners on the accident airplane exhibited soft-body impact damage, indicating that both engines ingested a bird very near the inner radius of the engine inlet and that some of that bird mass entered the engine core. Although all of the left and right engine fan blades were present and intact, three of the left engine fan blades and five of the right engine fan blades exhibited damage indicating that both engines ingested a second bird near the fan midspan, but, because it was ingested near the edge of the fan blades, none of that bird mass entered the core.
When a turbofan engine ingests birds and no fan blades are fractured, the damage to the fan blades is generally localized because the bird will affect only those fan blades that actually impact or slice it as it passes through the fan plane. The number of fan blades affected by the impact is determined by the bird size, the relative bird velocity with respect to the airplane, the rotational fan speed, and the bird-impact angle. As the fan blades impact...
