A folding wing-tip on a commercial airliner is never supposed to fail.
With the first such mechanism scheduled to enter service on the 777X family in two years, Boeing set a goal to design a folding device so elegant and reliable that its workings would be as unnoticeable to the operators as flap tracks and thrust reversers today.
But that doesn’t mean Boeing engineers assumed the 777X’s folding wing-tips will operate flawlessly forever. In a newly-published, 13-page annex to an airport planning document, Boeing for the first time details how airports and 777X operators should cope with a list of potential, albeit highly unlikely, “non-normal” scenarios for the Liebherr-supplied folding mechanism.
The preliminary planning document, dated March 2018, is aimed at helping operators cope with any risks that come with Boeing’s embrace of folding wing-tips for the 777X.
For the manufacturer, a folding wing-tip’s appeal is straightforward. In flight mode, the mechanism stretches the 777X aircraft’s composite wings to 71.8m (236ft) from wing-tip to wing-tip. On the ground, the wings fold by 3.5m on each side, allowing the aircraft to fit on ICAO Class E airport taxiways, parking ramps and gates. That means the 777X can gain access to the same airports as the 777-300ER, but with a far more efficient wing.
In normal operations, Boeing expects 777X pilots to taxi the aircraft with the wings folded until some point before reaching the hold position near the runway, with the exact location determined by each airport. After the flightcrew commands the 20s procedure for the wing-tips to extend and lock, the flightcrew will confirm the aircraft is ready to fly and receive clearance from air traffic control, according to Boeing’s planning documents.
Boeing’s engineers identified a conceivable problem with a wing-tip folding failure prior to take-off.
In this “non-normal scenario”, the flightcrew might command the wing-tips to extend, but one or both tips don’t move or only partially unfolds, Boeing says in the document.
The crew will receive a “wing-tips drive” message on the engine indication and crew alerting system (EICAS) display, plus see a master caution light and hear an aural beeper. If both wing-tips fail to extend, the crew can simply return to the gate. If only one wing-tip extends but the other doesn’t, the air traffic controllers and flightcrew must agree on a route to return to the airport gate or parking stand.
For landing, Boeing has designed the wing-tips to fold automatically within 20s as soon as the 777X slows below 50kt. In the improbable situation that the automated command fails, a “wing-tips position” message will appear on the EICAS display, along with a master caution light and an aural beeper. That tips off the flightcrew to manually push the “folding wing tip pilot control module” lever.
If one or both of the wing-tips fail to move, the EICAS will display a “wing-tips drive” caution message, showing which wing tip isn’t working. Again, the aircraft won’t be in compliance with a Code E airport wingspan, so the air traffic controllers and flightcrew must agree on the “non-normal” route to gate or parking stand, according to Boeing’s documents.
“Boeing designs its airplanes so that redundant systems are independent, and failures or external threats cannot compromise both primary and backup systems simultaneously," Boeing tells FlightGlobal. "The folding wingtips are simple and highly reliable with redundant deploying, retracting and locking mechanisms. We are designing the folding wingtips like every other flight critical system so they meet safety requirements.”