In the wake of two recent crashes seemingly caused by Boeing’s design decisions for their 737 Max 8 and Max 9 airplanes, is it any wonder that people are concerned about Boeing’s new models, the 777-8 and 777-9, that feature folding wing tips?
The new planes are scheduled for delivery in 2020, and like their big sister the 777-300ER, the 777-8 and 777-9, are twin-engine, wide-body planes with carbon fiber wings. The 777-8 will seat up to 395 people, and the 777-9, up to 425 people, making it the world’s biggest twin-engine jetliner.
The new plane’s range will be over 14,000 kilometers (8,699 miles) and the 777-8 will sell for $360.5 million, the 777-9, for $388.7 million. Boeing has logged 326 orders from eight customers: Lufthansa, Etihad Airways, Cathay Pacific, Emirates, Qatar Airways, ANA, Singapore Airlines and an unidentified customer. Boeing expects to deliver the first 777-9 to Emirates in mid-2020.
Why the folding wing?
The reason for the folding wing tips is that the wingspan of the new aircraft, at 235 feet, is too long to fit at the gates of most airports. Once folded, the wings will be only 212 feet in length. The purpose of the longer wing is to reduce drag caused by vortices, or wake turbulence, that form at an aircraft’s wing tips. The less drag, the greater the fuel efficiency, and the more cheaply the aircraft can be operated.
According to Boeing, the wings will only fold when the plane is on the ground, and jet fuel will not be stored within the folding sections. Normally, jet fuel is stored within the wings of an airplane. For years, military planes on aircraft carriers have had folding wings so that they take up less space.
To get approval for the folding wing from the U.S. Federal Aviation Administration, Boeing submitted a “Special Conditions” request, and the final decision by the FAA was made public on May 18, 2018 in the Federal Register, a daily journal of the U.S. government.
This document has some pretty sobering language regarding the 777-8’s and the 777-9’s new design:
“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. In service, numerous takeoff operations with improper airplane configurations have occurred due to failures of the takeoff warning systems, or inadvertent crew actions.
…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 also raised concerns about the following issues:
“With wingtips in the folded position, the conventional airplane-wingtip-position lights may have reduced visibility due to the upward position of the wingtips, possibly impacting ground-operation safety. Light placement may require special consideration to retain the current ground-operation safety, and mitigate any adverse impact this light position may have on pilot visibility during night-lighting conditions.
Due to upward wingtip positioning on the ground, significant loads may be imposed by wind gusts combined with taxi speed during the transition from the unfolded to the folded position.”
The FAA’s Requirements
To receive approval, the FAA mandated the following to Boeing:
“1. More than one means must be available to alert the flightcrew that the wingtips are not properly positioned and secured prior to takeoff. Each of these means must be unique in their wingtip-monitoring function. …the applicant must add a function to the takeoff warning system …to warn of an unlocked or improperly positioned wingtip, including indication to the flightcrew when a wingtip is in the folded position during taxi.
2. …a means must be provided to prevent airplane takeoff if a wingtip is not properly positioned and secured for flight.
3. The applicant must consider 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, … If lubrication is required to control excessive wear, lubrication intervals must be established. …The effects of freeplay on wing-joint torsional and bending stiffness, as well as wing frequencies, must be evaluated when showing compliance to loads and aeroelastic stability requirements. Also, the effects of freeplay on fatigue and damage tolerance must be considered …
4. The folding wingtips and their operating mechanism must be designed for 65 knot, 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. … Runway roughness … must be evaluated separately up to the maximum relevant airplane ground speeds…
5. The airplane must demonstrate acceptable handling qualities during rollout in a crosswind environment, as wingtips transition from the flight-deployed to folded position, as well as during the unlikely event of asymmetric wingtip folding.
6. The wingtip-fold operating mechanism must have stops that positively limit the range of motion of the wingtips…
7. The wingtip hinge structure must be designed for inertia loads acting parallel to the hinge line…
8. … The forward position lights … must not impair the vision of the flightcrew when the wingtips are in the folded and transient positions.
9. The applicant must include design features that ensure the wingtips are properly secured during ground operations, to protect ground personnel from bodily injury as well as to prevent damage to the airframe, ground structure, and ground support equipment.
10. 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. All sources of airplane power that could initiate unlocking of the wingtips must be automatically isolated from the wingtip-fold operating system (including the latching and locking system) prior to flight, and it must not be possible to restore power to the system during flight. 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.
When we board an airplane, we’re putting our trust in the expertise of the engineers who designed, built and tested that aircraft. But, today’s business climate seems different – with compressed timeframes, rushed decisions, shortened design cycles, and bolt-on solutions, such as the Maneuvering Characteristics Augmentation System (MCAS) system. It was this system that Boeing added to its 737 Max airplanes to offset the nose of the aircraft pitching up due to its heavier engines. And, it is this system that is suspected in the crashes of Lion Air Flight 610 and Ethiopian Airlines Flight 302.
Source: Interesting Engineering