The US Federal Aviation Administration is set to publish a final list of 10 special conditions on certificating the airworthiness of Boeing’s unique folding wingtips for the 777X family.
The list of 10 requirements will be published in the Federal Register on 18 May, even as Boeing has started building the wings for the first four 777X test aircraft in Everett, Washington.
Boeing designed the 777-9 and 777-8 aircraft with longer, carbonfibre wings to improve aerodynamic efficiency compared with the 777-300ER. To keep the 777X family compatible with airport gates and runways used today by the 777-300ER, Boeing has added a hinge mechanism that allows the wingtip to fold upward shortly after landing on a runway.
Boeing has designed the 777X folding wingtips to meet the same certification requirements as other moving surfaces on an aircraft, such as the flaps, ailerons and landing gear doors.
No rules for folding wingtips exist in the FAA’s certification criteria under Part 25 of the Federal Aviation Regulations, so the agency developed a list of 10 special conditions for Boeing to meet to prove they are safe to operate.
The rules cover a range of worst-case scenarios. For instance, Boeing has to prove that the power to the folding wingtips is isolated while in flight, so that they can’t rotate upward due to a hardware or software malfunction, according to a notice published on 17 May by the Office of Public Inspection..
The FAA also requires that Boeing provide multiple to alert the flight crew if the wingtips are folded up or not properly secured before take-off. If the crew somehow misses the alerts, Boeing also has to prove the aircraft won’t take-off if the system detects that the wingtips are not properly secured in the flight position.
Another concern by the FAA is how the folded wingtips perform in high wind gust conditions. Boeing has to prove the wingtips are safe in horizontal gusts up to 65kt from any direction and in any position, the FAA says. As the wingtips fold upward after the aircraft lands, Boeing also has to demonstrate “acceptable” handling qualities during crosswind conditions, even if one wingtip fails to completely fold.
In a comment submitted to the FAA about the rules for the folding wingtips, the Air Line Pilots Association (ALPA) focused on the demonstration requirement, saying that “no exceptional piloting skill is required during these operations”.
In a response to FlightGlobal, Boeing says the company has worked with the FAA for several years on developing the special condition for the folding wingtip.
“As with any critical airplane systems, like flight controls, fuel systems, or brakes, Boeing uses a hazard class determination for certain extremely improbable airplane level failure events to drive the appropriate high-integrity system architectures, design redundancies, and safety features to preclude such events,” Boeing says. “This same safety and certification methodology was used with our new folding wing tip to ensure a robust systems architecture with mechanical and electrical redundancy, high integrity control and monitoring architecture, and structural fail-safe load paths from wing tip to fixed wing.”