As images of the Costa Concordia blinked across the globe, the sight of the huge cruise ship stranded upon the rocks was almost beyond comprehension. And yet, as controversy raged about the Captain's actions, one could not help but recall the day three years ago when a US Airways Airbus A320 ditched in the Hudson River. In this case the crew were applauded as the seemingly impossible had been achieved. Those images showed an intact airliner with itsprecious human cargo stood huddling on the wings. Even so, almost as quickly as the Airbus engines had spooled down, a single question surfaced, “How?”
There is no denying that New York is truly the city that never sleeps. From the town that burnt scenes of airliners plunging into the Twin Towers into our consciousness emerged vastly contrasting images of survival and hope. Amidst a flotilla of ferries, the Airbus bobbed gently in the frozen water as its passengers were evacuated to shore in an orderly manner.
US Airways Flight 1549 was not an isolated incident in history. Land-based aircraft have been forced to use water as an emergency runway almost as long as there has been powered flight. Whilst the instances of ditching airliners are relatively rare, a range of aircraft from Super Constellations to Convairs have ultimately ‘landed’ on water. In 2002, a Garuda B737-300 was forced to ditch near Java, whilst a chartered DC-3 put down in Botany Bay in 1994 after both engines failed after take off out of Sydney, Australia.
What made the Hudson River episode so intriguing was the relatively unscathed appearance of both man and machine as it became evident that the ditching had been made with minimal or no thrust. In such a time critical emergency, there are so many factors at play working against the crew. However, US Airways Flight 1549 proved that sometimes the planets can align.
Against the Odds.
The variables confronting a crew in any ditching situation are enormous. The available aircraft systems, the water’s swell, aircraft design, time of day and proximity to landfall are just a mere handful of considerations that must enter the mind of a pilot as the inevitable nightmare of ditching looms in the windscreen.
Achieving clarity amongst this background hash can prove a key achievement in itself and critical to a successful outcome. As in all emergency situations, the overwhelming priority must be to fly the aeroplane. Even so, there will be factors beyond the crew’s control with which they will have to deal on the day.
A variety of aircraft have ditched over the years and a number of those have been void of thrust. The reasons are varied from flameouts in ice and heavy rain, to the serial offender of fuel starvation. Whether a ditching is to take place with or without power affords the crew a very critical component; time. The powerless Airbus on the Hudson had no such luxury as inertia and gravity dictated its time frame once it had ingested the flock of geese.
Even when an aircraft’s remaining power may not be sufficient to maintain level flight it may provide enough to time for the crew to limp closer to landfall, sustain various aircraft systems, or control the rate of descent in the final stages prior to impact. Meanwhile in the passenger cabin, this time can afford the crew an opportunity to prepare the people and equipment for the ditching.
The availability of power will also determine the availability of aircraft systems from hydraulics to electrics. While most airliners have at 30 minutes of battery power to power minimal electrical systems, the resultant loss of hydraulic pressure may limit the amount of flaps and powered controls that are available to the pilot, depending on the particular system. A number of types, including the A320, are equipped with a Ram Air Turbine, or RAT, that extends into the airflow and spins over to provide and alternate source of power and pressure.
Ironically, some of those that have been forced to ditch without power have unwittingly exhausted their fuel supply whilst attending to a lesser in-flight emergency. Once again it gets back to the cardinal rule of aviation: Fly the aeroplane.
Aircraft design can also play a hand. Whether the wings are mounted high or low and if the engines are under slung on the wings, or aft mounted on the fuselage. All of these design features play a part, particularly in how the aircraft first contacts the water and decelerates. Once on the water, the low wing may assist egress, such as the images of passengers standing on the wing in the Hudson, or hamper it in such types as the Cessna singles where the door lies below the wing.
Whether the selected water mass is a river, lake or ocean can greatly influence the outcome of a ditching. While this can determine the proximity of the aircraft to landfall and rescue, it also can also dictate the water surface conditions. The prevailing swell is a major factor in the execution of a ditching and, generally speaking, the ocean is the home to the big swells. The American Great Lakes may have phenomenal seas, but generally a river or lake will present a more level surface for the aircraft to land on.
If an aircraft is forced to ditch at night, many of the visual clues may be lost unless a bright full moon can illuminate the white caps. Early assessment of the water surface and swell may be impossible and a ‘best guess’ plan must be devised on the planned prevailing winds at a lower level. Descending in the darkness will also make the actual approach and touchdown far more difficult to judge as there will be no runway lights in the peripheral vision and no horizon ahead to assist in assessing the rate of closure. Flight instruments and radio altimeters will be critical in guiding to the aircraft to the final moments and support calls from the ‘pilot not flying’ (PNF) will free the flying pilot to divert some attention out the windshield to catch the first glimpse of the waves in the landing lights.
Similarly, poor weather and a low cloud base will greatly reduce the visual information available to the pilot to execute the manoeuvre. Breaking out beneath the cloud base at low level with rain thrashing against the windscreen may leave the pilot with less than a minute of poor visibility to guide his aircraft to the surface.
The crew of US Airways Flight 1549 had daylight, reasonable weather and a relatively smooth water surface working in their favour. But -and it’s a BIG ‘but’- they had lost both engines and were effectively a 60 tonne glider. The crew had only minutes from losing the engines to touching down on the Hudson in which they had to decide, plan and execute their course of action.
A Landing with a Difference.
Landing on water is not the natural environment for aircraft not equipped with floats or a flying hull. Each manufacturer will make recommendations on how to best ditch their aircraft, though thankfully it is often not the voice of experience. The advice may stem from computer models, similar past ditching or pure hypothesis.
Generally speaking and where possible, the goal is a controlled touch down at the minimum safe speed, with the wings level, landing gear retracted and a minimal sink rate. Healthy conjecture will always flare amongst pilots as to the best means to achieve this. Thankfully, it will remain theoretical discussion for the great majority.
To achieve the lowest speed over the water, a landing into wind would normally be the prime choice. However, the swell also needs to be considered as landing head on to a rolling wave will exclude the chance of a smooth touchdown. Hence, landing parallel to the swell is advisable.
The selection of flap is also often debated. While the ditching procedure in certain manuals calls for the use of full flap to minimise the speed and afford better visibility at touchdown, these checklists are also predicated on having thrust available to control the rate of descent. (And enough time to carry out the checklist.) Often a mid flap setting is suggested to offer a compromise between reduced speed, sink rate and wing configuration on first contact with the water, particularly if the approach is without power.
The flare, hold off and touchdown is critical. Judging the flare over water is void of the normal cues associated with landing and can prove difficult, particularly over calm, glassy water. Similarly, not too many pilots are routinely judging the landing flare with their wheels retracted and the associated changes in height and ground effect. Keeping the undercarriage retracted is designed to prevent the wheels ‘digging in’ on touchdown and pitching the aircraft violently into the water.
Maintaining a wings level attitude will also guard against one wing striking the surface first and slewing the aircraft around in an uncontrolled manner. This was graphically illustrated in the case of the widely circulated footage of the Ethiopian Airlines 767 of Comoro in 1996.
Approaching touchdown, at around 500 feet, the flight crew will warn the cabin to brace for impact. Dependant on the aircraft type, the ideal nose attitude is cited to be in the realm of 10-12 degrees as the water looms close and contact with the surface is imminent.
A smooth, single touchdown on the aft fuselage is the goal, though often not the case. ‘Skipping’ along the surface will inherently expose the aircraft to a series of impacts, but in reality, touching down in a relatively ‘clean’ configuration there is a high chance of more than one contact.
In the end, on the day there will be compromises and judgement calls based on the remaining aircraft systems, time available and prevailing conditions. If it all comes together successfully, there will be some very grateful passengers and the possibility of a slot on the talk show circuit.....
Check back soon for part two of 'Sink or Swim'.
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