Boeing and the FAA have been unwilling to identify the extent of the software and hardware actuator redesign needed on the 777X, which CEO David Calhoun confirmed in the January 27 results webcast. However, looking at topics on the big twinjet that have been discussed in the past between the airframer and regulators, one issue stands out: the folding wing tip mechanism. Most likely this is the part that will get the redesign Calhoun has referred to.
The folding wing tip is the most visible and prominent feature that makes the 777X stand out from its predecessor. As the new carbon fiber wing has a longer (65 meters) span, Boeing came up with the solution to fold the tips in order to keep the 777X within ICAO’s Code E airport classification. The folding mechanism reduces the span by 3.5 meters as soon as the aircraft has touched down after landing and parks at the gate.
The mechanism has been thoroughly designed and manufactured by Liebherr Aerospace. It has a motor and rotating actuator that fit into the restricted confines of the wingtip. When designing the system, Liebherr has used its experience from the primary flight control actuators that also drive the leading and trailing edge devices on the 777X.
Topic raised in May 2018
Still, the mechanism featured prominently in a public consultation by the FAA in May 2018. The discussion focused on the reliability of the system, hence its fail-safe status. “Boeing has determined that a catastrophic event could occur if the Model 777–8 and 777–9 airplane wingtips are not properly positioned and secured for takeoff and during flight”, the FAA said in a May 11, 2018 document. “Consequently, the FAA has determined that the level of safety in protecting a misconfigured airplane from takeoff with wingtips folded should be the same as taking off with the gust locks engaged. (…) The applicant must show that such an event is extremely improbable, must not result from a single failure, and that appropriate alerting must be provided for the crew to manage unsafe system operating conditions. In addition, the applicant must ensure that the wingtips are properly secured during ground operations to protect ground personnel against bodily injury.” The FAA issued a dedicated policy statement on ‘‘Certification of Structural Elements in Flight Control Systems’’ to address structural elements in systems that act as both structures and as part of a system.
The FAA issued a ruling on the system, saying: “More than one means must be available to alert the flight crew that the wingtips are not properly positioned and secured prior to takeoff. Each of these means must be unique in its wingtip-monitoring function. When meeting this condition, the applicant must add a function to the takeoff warning system (…) to warn of an unlocked or improperly positioned wingtip, including an indication to the flight crew when a wingtip is in the folded position during taxi. In addition to a takeoff warning, a means must be provided to prevent airplane takeoff if a wingtip is not properly positioned and secured for flight.”
Boeing also had to consider the effects of folding-wingtip freeplay when evaluating compliance to the design load requirements and the aeroelastic stability (including flutter, divergence, control reversal, and any undue loss of stability and control as a result of structural deformation). “Thus, the effects of normal wear, and other long-term durability conditions (such as corrosion) of the folding wingtip operating mechanism on freeplay, and its impact on loads and aeroelastic stability, must be considered.”
Designed for 65 knots gusts
The FAA also required that “the folding wingtips and their operating mechanism must be designed for 65 knots, horizontal, ground-gust conditions in any direction. Relevant design conditions must be defined using combinations of steady wind and taxi speeds determined by rational analysis utilizing airport wind data.” It specified that the folding wingtip “is not a control surface.”
“The wingtips must have means to safeguard against unlocking from the extended, flight-deployed position in flight, as a result of failures, including the failure of any single structural element. (…) The wingtip latching and locking mechanisms must be designed so that, under all airplane flight-load conditions, no force or torque can unlatch or unlock the mechanisms. The latching system must include a means to secure the latches in the latched position, independent of the locking system. It must not be possible to position the lock in the locked position if the latches and the latching mechanisms are not in the latched position, and it must not be possible to unlatch the latches with the locks in the locked position”, the May 2018 FAA document concludes.
MAX crashes changed regulatory oversight
The two MAX crashes in October 2018 and March 2019 have resulted in a review of the 777X design amongst regulators. After details emerged about flaws in the FAA’s oversight, other regulatory agencies have become wary of following the agency’s recommendations on aircraft certification. We have seen in recent months how certification has changed, with the FAA, ANAC, CAA, GCAA, and EASA all taking their own position and role when approving re-entry into service of the MAX.
In this context, the 777X has become part of its own review. In the 2019 Dubai Airshow week, the FAA, UAE’s GCAA, and future operators discussed the type’s certification at length, resulting in the Emirati’s requesting their own role in the process. At the time, Emirates still had 150 aircraft on order, with Etihad 25 and Qatar Airways sixty, so the 777X featured prominently in the Gulf region.
No details have emerged about what has been agreed since and if test flights of the 777X, which began on January 25, 2020, have identified any flaws or deficiencies in the folding wing tip mechanism. But David Calhoun’s remarks this week are an indication of a recent wish from regulators to further improve the reliability and redundancy of the system. It is significant the redesign is happening quite late in the program. The redesign is one of the reasons why the delivery of the first 777-9 to Emirates has slipped by a year to late 2023, with market changes following the Covid crisis another and more worrying one for Boeing.
Airinsight has asked Liebherr-Aerospace for commenting on the redesign but the Lindenberg-based (Germany) company refers to Boeing for further questions.