One of the great success stories of the last decade in aviation has been the development of winglets, an Aviation Partners success. Now factory installed on virtually all Boeing 737s, these devices are becoming ubiquitous with “modern” commercial aircraft and are often thought of by the public as differentiating a newer model from an older aircraft.The “conventional wisdom” in the industry is that winglets significantly increase fuel economy, and provide about a 5% benefit in fuel economy for aircraft that utilize them. The reality is that those benefits occur only in specific high altitude flight regimes, and that a more realistic average fuel savings benefit, for typical domestic operations, is about 0.5%.
The reality certainly doesn’t match the hype, and even Airbus will soon be offering “sharkets” on its A320 because competitively its airplanes, despite being more efficient with their small wingtip fences for most flight regimes, don’t look like the new Boeing’s.
So why is everyone installing winglets on their aircraft? Hype, and not fully analyzing the advantages and disadvantages associated with them.
Winglets change the operational characteristics of an aircraft, and while they operate well at high altitude, above FL390, they require a slower climb to be as efficient as a non-winglet equipped aircraft in that regime. As soon as you speed up during climb, the fuel efficiency advantages of a winglet equipped aircraft turn negative, and rather than climbing at 330 knots after 10,001 feet, a winglet equipped aircraft needs to climb at 250 knots at FL100, 260-270 knots at FL150, 270-280 knots at FL180 to match climb fuel economy of a non-winglet equipped aircraft.
While more efficient at high altitude, for many flights on the US east coast and west coast, where heavy traffic areas exist, a flight may not get above FL270, and never reach the altitude at which the efficiency of winglets kicks in. And at those lower altitudes, cruising at more than M0.71 will result in an increase in fuel consumption because of the weight of the winglets and parasitic drag.
Airlines operating aircraft in a mixed fleet of winglet equipped and non-winglet equipped aircraft need to modify their operational speeds to maximize efficiency as well as procedures, as aircraft that are winglet equipped handle differently than those without winglets. This can be particularly important on landing, particularly on wet or snowy runways.
Winglets add lift to the wings, which results in faster speeds for the aircraft in ground effect. While jets typically increase speed by 7-8 knots in ground effect, winglets exacerbate that process, with speed increases of 10-12 knots once power is off. Without different operating procedures, it is easier to overrun a runway in a winglet equipped aircraft than one without winglets, as the speed change in ground effect can increase landing distances. And because winglets generate additional lift, the effectiveness of braking can be minimized unless pilots pull back on the yoke to firmly transfer weight from the nose wheel to the main gear and wings, which is not a part of normal operating procedures at many airlines. The potential for a runway overrun is more significant with a winglet equipped aircraft unless operational procedures are modified, as evidence by a couple of recent events.
In addition, winglets add weight to the wingtips of an aircraft, more than 1,000 pounds for a typical Boeing. This has an impact on the strength of the wing, effectively weakening wing torsional rigidity and introducing aeroelastic issues that must be compensated for, currently accomplished by the installation of about 100 pounds of depleted uranium in the leading edge to reduce the twist on the wing generated by the winglet installation. A basic 737 has a high gust tolerance, well in excess of regulatory standards, but one modified with winglets will lose much of that margin above regulatory requirements.
A rule of thumb for most aircraft is that its cost is about 3% of any additional weight carried in fuel each hour. For a winglet equipped aircraft, operating on short-haul routes with virtually no fuel efficiency benefit, a winglet equipped aircraft could, depending on operating procedures, have a negative, rather than positive, impact on fuel economy.
If you don’t believe the numbers, run a flight plan for a short-haul flight that doesn’t go above FL330 for a winglet and non-winglet equipped aircraft. The fuel differential will be about 0.5% better, not the 5% commonly repeated by pundits in the industry.
The reality is that the payback for winglets will take about 10 times longer than marketing brochures would imply, unless all your flights are long-haul operations operating above FL390. For Air New Zealand’s 767s, winglets make perfect sense for their long-haul operations. For short-haul domestic operations, one must question the payback of winglets, which certainly isn’t going to be quick with a 0.5% fuel benefit. We wonder whether taking on 1,000 pounds of revenue belly cargo, rather than saving 0.5% of fuel with winglets, might generate a better economic result for an airline.