Hudson A320: Partial engine power aided textbook ditching

Washington DC
Source: Flightglobal.com
This story is sourced from Flightglobal.com

Sources close to the on-going investigation of US Airways Flight 1549 say that the Airbus A320's number 1 (left) engine continued to run at approximately 35% fan speed (N1) during the three-minute window between striking birds at 3,200ft and ditching in the Hudson River on 15 January. All 150 passengers and five crew safely exited the Charlotte-bound aircraft in the river, aided by nearby ferry boat crews in a spectacle that captured global interest and fueled renewed admiration for airline training and professionalism.

Though limited, the left engine's speed would have been adequate to keep the aircraft's generators and hydraulic systems on-line, providing "normal" flight control laws and communications as well as giving pilots ability to deploy flaps and slats, elements that proved critical to performing a low-speed water landing. To maintain altitude on a single engine however, experts say the powerplant would have had to been running at 70% N1 or more.

Investigators planned to retrieve the left engine, which broke from the aircraft during the ditching, from the river bottom Friday or Saturday to perform an inspection. Flight International has learned that the aircraft touched down at 125-130kt airspeed with flaps and slats both in the "2" position, or midpoint, position. An A320 normally lands at 120-125kt with fully deployed flaps and slats.

us airways a320 crash

The right engine, which remained attached to the aircraft after ditching, was apparently operating at only 15% fan speed, according to sources. Investigators afterward determined that the engine had received "soft body impact damage" to its first stage fan blades. In addition three variable guide vanes were fractured and two were missing. The engine's electronic control unit was missing and "numerous" internal components were "significantly" damaged, said the National Transportation Safety Board (NTSB) in a 21 January update on the accident.

The NTSB says the organic material that was found in the right engine and on the wings and fuselage after the aircraft was lifted onto a barge will be identified through DNA analysis, and a feather found attached to the flap track on the wing was sent to the Smithsonian Institution for identification.

Officials have also confirmed that the A320's Hamilton Sundstrand-built ram air turbine (RAT) had deployed from its compartment near the root of the left wing during the event and that the Honeywell auxiliary power unit in the tail had been operating. Though the RAT will deploy automatically when engine or electric power drops below a threshold, pilots can also manually deploy the propeller-driven emergency system. The device provides power to one of three hydraulic systems onboard which would have given pilots the ability to deploy slats but not flaps.

Sources tell Flight International that the first officer had tried to relight the left engine during the descent, which averaged 1,000fpm rate, but that the engine did not respond other than to continue spinning at 35% N1. It's possible the pilots restarted the APU in order to have bleed air available to help restart the left engine as the A320's airspeed was relatively low.

Pilots review ditching procedures in textbooks during recurrent training but do not practice the events in simulators as there are no test-verified models available. Pilots do however practice double engine-out scenarios with a re-light afterward. NTSB has completed its interviews of the pilots, flight attendants and air traffic controllers and was working to wrap up its fact finding with passengers and a crew that two days earlier experienced a compressor stall on the accident aircraft. That event does not appear to be related to the 15 January accident. 

Though US Airways provides all seats in its pre-America West merger A320s with life vests, which includes the accident aircraft, the former A320 captain says there would not have been time to fly and diagnose the aircraft in addition to alert the passengers to the situation and asking them to don the vests before landing. Pilots also did not activate the A320's "ditch" button which automatically configures the various valves and openings on the aircraft for ditching, a preparation the former A320 captain says is about three pages into the ditching checklist. "Some of the flight attendants didn’t even know they were going into the water," he says of the short window of opportunity during the descent. "There was not enough time for any of it."

Picture below (from Hamilton Sundstrand) shows the position of the RAT on an A320