25 July 2019

Boeing Boeing

No, not the silly movie from the 60s starring the 707 (which, by the bye, the US taxpayer paid for because Boeing just took the KC-135 and added windows and seats), but the current issues with the 737Max. Reporting all over the place today that Boeing may stop making any. Until it's approved, again. What none of the usual pundits have asked is, what's the Worst Case Scenario? I'll ask.

First, why is the 737 such a funny looking plane? It's got dwarves' legs, aka landing gear that's obviously shorter than any other commercial aircraft. How come? Turns out that the original plane was intended to be usable at the smallest airports, typically without jetports, those moving articulated passageways from plane to terminal. IOW, airports where passengers have to walk off the plane to the tarmac. And, worse, airports that don't have a fleet of movable staircases. What to do? Boeing decided to make the original plane with internal stairs, just like your average business jet. In order to do that, the fuselage has to be not much farther off the tarmac than a business jet.

If you do the wiki, you can see a picture of the 737-100 with its stair extended. Really.

This design requirement also made it necessary to embed the JT-8D (a low-bypass turbofan type) into the wings. This is an engine with a max diameter not much more than military turbojets. This cozy relationship between fuselage and tarmac has persisted ever since.

And all was well for years. If you've flown a 737 in the last couple of decades, it's had the CFM56 engine (high-ish bypass) with that flat bottom front. This engine is mounted on a pylon hanging well out in front of the wing.

The latest engine which makes the Max special, the CFM LEAP-1B, has even a bigger max diameter than the '56. What to do? The '56 has a fan 60 inches, while the LEAP is 69 inches. Oops. Among other things, Boeing was determined to slide the Max through the existing certification, which is to say no additional training on type-specific simulator. Them dings be expensive. Among other things, the LEAP had to be placed even higher and more forward than the '56. The upshot was that the flight characteristics of the Max differed from previous 737s. What to do? Cheat.

Ok, that's harsh, but what Boeing really did. We know, now, that they slipped in this MCAS software to make the Max behave like previous 737s in conditions where the testing showed discrepancies in behavior. And now we know that didn't work. We also now know that a simple change to the software isn't in the cards. If that kind of fix would fix the problem, we'd be there by now. There be dragons in that plane.

What to do? What's the Worst Case Scenario?

In simple terms, the Max can't be certified as is, MCAS or otherwise. Software can't always rescue hardware. In other words, my suspicion is that Boeing has been lying about the degree of aerodynamic problems the LEAP engine does to the airframe. If that's the answer, Boeing has to go back to the drawing board. At the very least, the landing gear and wings have to be redesigned. From the original plane, the main trucks just tuck into the fuselage, without doors. Longer legs mean the trucks have to be anchored farther out on the wings, which brings all kinds of aero questions. Can the existing wing form support landings where the hinging has to be? Does the existing wing form even have room for trucks farther out? Will the longer legs interfere with engine mounting? That's a really big oops. And so on and on.

Worst Case Scenario.

No comments: