Why can’t more older 737s be retrofitted with more newer winglets?

Question

Some background: there are four different types of winglet that have been used on various 737s over the years.

1. The mini winglet is used to retrofit 737-200s (one of the two 737 Original variants, the other being the 737-100). It is basically a tiny trapezoidal tab riveted to the rear portion of the wingtip.

(Image by Julian Whitelaw at airliners.net, via meisam nemati here at AvSE.)

2. The blended winglet is what most people think of when they hear the word “winglet” - an attachment that looks like someone simply extended the wingtip and then bent it up to near-vertical. It was factory-installed on all 737 Next Generation variants (the 737-600/-700/-800/-900) for the first two decades of production, and has also been retrofitted on most of the 737 Classic models (the 737-300/-400/-500) that remain in service.

(Image by Ralf Roletschek at Fahrradtechnik auf fahrradmonteur.de via Wikimedia Commons, modified by Altair78 at Wikimedia Commons.)

3. The split-scimitar winglet has replaced the blended winglet on the last few years of the Next Generation production line; it is shaped somewhat like the blended winglet, but a) is swooshed rearwards at its upper tip, as opposed to the simple trapezoidal tip of the blended winglet, and b) has an additional scimitar-shaped, rearward-swept fin projecting from the outside of the upbend in the winglet.

(Image by Mnts at Wikimedia Commons.)

4. The MAX winglet is used on the 737 MAX series (737 MAX 7/8/9/10); it, too, is a split winglet,¹ but of a simpler shape than the split-scimitar winglet, looking much like someone extended the wingtip outwards, then split its upper and lower surfaces away from each other and bent them upwards and downwards, respectively, to form what looks remarkably like a glorified Airbus wingtip fence.

(Image by Aka The Beav at Flickr, via Helmy oved at Wikimedia Commons, modified by Altair78 at Wikimedia Commons.)

Now for what I’m curious about:

1. Although the 737-100 (the original Original) was produced in smaller numbers than the 737-200 and is no longer in service, it still hung on with various operators past the turn of the millennium; why weren’t any -100s also retrofitted with mini winglets, given that the -200 is basically just a stretched -100 with more fuel capacity?
2. Given that the blended winglet, despite being originally designed for the Next Generation wing, was retrofitted on the vast majority of the Classic fleet with little to no trouble, why couldn’t it also be retrofitted on the remaining Originals, given that the Classics are essentially CFM56-engined, (sometimes) stretched -200s?
3. Seeing as the Next-Generation-designed blended winglet was easily adopted for the earlier Classics, what keeps the split-scimitar winglet, likewise designed for the Next Generation wing, from also being retrofitted to the Classics?

(Yes, I know that this is three questions rather than one, but I felt it was better to have one question asking three closely-related questions than to clutter up the homepage with three separate but near-identical questions.)

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¹: As for why Boeing went to the trouble of developing two different styles of split winglet for the two in-production 737 families, that’s a different question.

Answer 1

Winglets change the spanwise loading on the wing. By increasing the effective span of the wing, lift is increased towards the tips of the wing. With more lift there, you get an increased bending moment on the wing.

All this means is that you can't just put winglets on or change the ones you have. You must retrofit the structure of the wing which is very expensive and requires significant downtime. It also adds a weight penalty which decreases the benefits of the winglet.

For example, on the A320, which was already wingtip fence equipped, going to the sharklets requires removing the skin and reinforcing both the tip and the center wing box near the fuselage. This takes the aircraft out of service for 15 days and involves, I've heard, in excess of $1 Million in parts alone.

On the 737, the list price for the blended winglets is $1.06 Million.

So the answer for why is because it often is not economical to do such a huge modification given the limited lifetimes on older models.

Answer 2

In order to develop and test a new product, you have to be sure that there is a sufficient market for it. Because of how aircraft certification is handled, it doesn't make sense to test a modification for a model that can't sell enough units to pay for itself.

As you noted, the 737-100 is the most extreme example, where only 30 of the 737-100 were ever built, and the 737-200 saw over 1000. This gave them a market roughly 30 times larger. It probably didn't make sense to certify the mini-winglets on the 737-100 if there were only a handful of airplanes that would use it.

By the time the makers of the blended winglet were ready to look at derivatives, the mini-winglet was already available, or soon would be. The development cost may not have supported splitting the market, especially since the mini-winglet was part of a more extensive flap modification that provided other benefits.

As for the split scimitar, while airlines have seen the benefit of retrofitting it to the relatively new NG line, they may see less benefit of putting on the older classic line, especially if they are still trying to reach the payoff for paying to put the blended winglets on.

Answer 3

Because there is not much point.

Winglet increases effective wing span. It is not as good as just increasing the span, but it is simpler to redesign a shorter wing by adding winglet than designing a longer wing, and keeping the wingspan down is useful so the new version can still usie all stands and taxiways the previous version could.

However, if the aircraft was designed with appropriate span for its weight and cruise speed, increasing the span is not what you want. While it would decrease the drag at the best lift/drag point, it would push that point to lower speed, and the drag at the original cruise speed would increase.

Winglets are added to the newer versions, because they are also given more powerful engines, which allow higher take-off mass. The higher weight requires more lift, which increases the best lift/drag speed, so increasing the span is needed to compensate. And modifying the wing by adding winglets is much easier than making it larger, because while it may still need strengthening the spars, as the bending moments do increase, all the other systems like flaps and ailerons and deicing and fuel system can stay the same.