Flying with a missing winglet is now part of the flight test programme for the 737 Max, says Boeing vice-president and general manager Keith Leverkuhn.
The wingtip-mounted devices reduce fuel consumption by cutting down on the swirling vortices linked to higher induced drag, but they are also susceptible to damage by wayward servicing trucks.
The 737 Max is equipped with a Boeing-designed, dual-feather winglet, which includes a downward-pointing ventral strake that is even more at risk to accidents on airport ramps and taxiways.
“It’s not ramp-ant, if you’ll forgive the pun,” Leverkuhn says. “But it’s happening.”
Winglet damage is a costly annoyance for airlines, especially when it occurs at a remote “out-station” with no maintenance crews. Airworthiness rules dictate that an aircraft with winglet damage cannot carry paying passengers. It is allowed one ferry flight to an airport where it can be repaired.
To avoid that complication on the 737 Max, Boeing hopes to clear the re-engined aircraft to be safe to fly a load of passengers even if some or all of the winglet is missing because of an accident, Leverkuhn says.
“We’re going to go test different configurations,” he says. “We’re going to fly it full-up on one side and we’re going to take the lower [panel] off on the other side and go fly it. And then we’re going to do the opposite. And then we’re going to fly them with both out.”
The tests will be used to validate a proposed condition on the minimum equipment list (MEL) for the 737 Max, Leverkuhn says. The MEL must be approved by the US Federal Aviation Administration, but it is not part of the Part 25 airworthiness certification.
Boeing’s analysis has already suggested that a missing winglet or winglet panel should not pose a safety risk, Leverkuhn says, but those results need to be verified in flight test. Two possible concerns are introducing a flutter condition and causing a change to the stability and control of the aircraft, he adds.
Flutter occurs when aerodynamic, inertial and structural forces interact in a way that, in a best-case scenario, produces a buzzing sound. In the worst case, the interactions cause uncontrolled oscillations that rip the structure apart.
“I’m really not too worried about flutter,” Leverkuhn says, noting the next-generation 737 and re-engined 737 Max share very similar winglet structures. “While we’ve gauged-up [for the Max], I wouldn’t expect that to drive flutter.”
A slightly higher concern is the impact of a missing winglet on directional control, with the outboard misalignment causing a yawing force. But pre-flight analysis suggests the stability issue won’t be a problem, Leverkuhn says.
“We can’t just pencil-whip it,” he says. “We have to go demonstrate it.”