Source: International Airline Passengers Association
The procedure is used in the event of an engine failure. It has successfully brought troubled aircraft back to the ground (or water) from the Azores to the Hudson River. Now, the "engine off" approach is being used to give passengers more comfort and to reduce noise and air pollution. Just to ease your minds, the engines aren't actually "off."
Passengers often fear that if a plane's engines give out, it immediately dooms the flight to disaster. Not so. Pilots have known from early in their training that altitude is your best friend. The higher up you are the better chance your airplane has to glide to a safe landing, no matter how heavy it is. Aircraft have an ideal airspeed to maintain that will give it the longest gliding range should it find itself powerless. Pointing the aircraft nose downward to ensure sufficient air is flowing over the wings, pilots use time and distance measurements to find the safest, flattest landing zone possible. Ideally, this would be a nearby airport. With that in mind, airlines are testing the plane-as-glider concept to make your flight smoother and more environmentally friendly.
Using optimal distance and time measurements, more and more aircraft are testing what it known as continuous descent approaches (CDA). Typically, when your aircraft is ready to make its descent, engine power is used to "step" the plane down from one designated altitude to the next. This requires the on-again, off-again application of thrust – a procedure that burns more fuel – for each flight level, or altitude, that controllers designate. You can best hear this application of thrust as the plane approaches a runway. A greater amount of engine power is applied when the plane needs to keep its altitude as it approaches the runway. Conversely it is reduced when the plane needs to sink a little to keep its trajectory. Much like stepping on your gas pedal excessively, this procedure can waste fuel and can be noisy. The CDA procedure allows a plane to maintain a steady trajectory toward the runway from much higher altitudes. This allows the engines to operate at "idle" (actually at minimal thrust) as the plane steadily descends with barely a sound or stomach-churning drop. An aviation blogger describes this technique as going down a ramp versus going down stairs.
Airlines throughout the world are testing the CDA technique. It can't be used in all circumstances (think stormy weather), however the fuel savings and pollution reduction puts the commercial aviation industry on a straighter path toward achieving carbon emissions targets set by various governments. It takes careful coordination between airlines, airports, controllers and agencies, but the implementation of this technique, combined with the adaptation of next-generation navigational aids, should give passengers a smoother, more environmentally guilt-free ride. As standards are developed under the guidance of the International Civil Aviation Organization (ICAO), agencies from the Federal Aviation Administration in the U.S. to Eurocontrol are developing procedures to take advantage of greener procedures and technology. With no known drawbacks, you can consider this technique a "slam dunk", meaning a sure winner. However, if you are an aviation geek who knows the typical definition of the term "slam dunk"1, you may not want to mention it in this case.
1The term "slam dunk" often refers to a procedure used by air traffic controllers to hold a plane at a steady altitude over a busy slice of airspace then bring it down rather quickly once the congested area is passed. This often leads to a rapid and steep descent and that funny feeling inside you would soon rather forget.
Photos courtesy of Alaska Airlines (www.alaskaair.com)