Flying the Hump


So I’m driving home from work the other day, with my usual wait at the traffic light along runway 10L/28R here in Columbus. Being a professional airplane geek (i.e. stupid enough to work in the business instead of doing something that makes real money), I of course spend that time watching the departing airliners stacked up behind the hold-short. This day, something unusual caught my eye, namely this unwieldy-looking hump on a Southwest bird:

Nope, that’s not a luggage carrier back there. Photo: gTarded/flickr

The last time I saw one of these was on a different 737, one that my employer was preparing to sell, and we had been tasked with running the test flights for a brand new satellite antenna housing. That’s what that hump is, though I see how it could be mistaken for a luggage container like you’d put on top of the old Griswold family station wagon. Or maybe a carrier for Mitt Romney’s dog.

So anyway…airplanes have to be put through all sorts of wild and wooly maneuvers before they can be certified to carry passengers. It’s not just for the pure-dee hell of it, either. There’s all sorts of predictions that have to be validated outside of the wind tunnel, which means you have to get air over the wings and under your butt.

It may not look like much, but this was a major modification which would affect the plane’s aerodynamics. The designers had run a lot of CFD (computational fluid dynamics, basically a virtual wind tunnel) predictions but the FAA would never let anyone fly that plane again without making sure it would, in fact, fly. It may not seem like much, but look where that upside-down bass-boat-looking thing is: right in front of the vertical stabilizer. In straight and level flight, it could disrupt the airflow around the rudder. In turning flight, it could disrupt airflow around the horizontal stabilizer. Either condition could make the airplane uncontrollable.

This is why real engineers make really good money.

Handling quality tests call for lots of steep turns, dutch rolls, skids, slips, edge-of-stall slow flight, and high-speed dives at maximum Mach. In other words, bank and deck angles that place the outside world in some unfamiliar positions, the kind that would make your average white-knuckle passenger puke like a teenager who broke into his old man’s liquor cabinet.

This is why experienced pilots also make really good money.

They also have to be flown in what’s known as the “least favorable” center of gravity. Every airplane has a balance point, which has to remain in a certain area on the airplane for it to remain flyable. Look at the wing, just ahead of the main landing gear…CG is always going to be in that general vicinity. At least it better be – if it moves behind the gear, the airplane will end up on its butt:

Now, imagine if that happened in flight (and it has, with horrendous results). This “least favorable CG” thingy is typically at the far aft range of the envelope, so have another look at that picture. We equip airplanes in a way which generally prevents that from happening – great for an airline with who-knows-who loading it, but not so much for flight testing. And that’s essentially what we were doing, replaying the certification test flights that Boeing did years ago.

Our problem was that this wasn’t a “green” airplane. The manufacturer can place all kinds of ballast weight wherever they need it for test flights. We didn’t have that option – in fact we couldn’t put anything in the baggage pits because they’d been replaced with extra fuel tanks. Using fuel as ballast was the only solution, but that presented its own problems – namely, it meant the fuel would have to be loaded and burned off outside of normal conditions, which is a tad unpredictable. Temperature plays havoc with density, which would determine how much we could load and where. And it’s a liquid, which doesn’t tend to stay exactly where you’d like it to be.

On top of that, the CG was going to start moving around the moment we started flying. As fuel was burned off, the balance point wouldn’t stay in the same spot. In fact, if the pilots didn’t manage fuel just right that point would jump clear out the back of the envelope. Didn’t mean the airplane would suddenly become uncontrollable, but we’d likewise have no guarantees that it wouldn’t.

For that reason, yours truly was fortunate enough to ride along as the onboard weight & balance engineer, which was really cool. I’d always wanted to play flight test hero. My role consisted of devising the in-flight burn sequence and giving the pilots correct points to switch tanks that would keep us at the back of the envelope without tearing it wide open. So before flight, we noted the temperature and determined specific gravity of the fuel, and I rode along and took their levels from the totalizer (fancy word for a gas gauge), recalculating CG every couple of minutes through all of those chandelles and high-speed dives.

One of the gallant aviators up front was our chief pilot, who told me afterwards the contract test pilot flying with us had been determined to make me throw up. I’m happy to say I disappointed him and quite enjoyed the whole experience. That’s right, beeches: Lindbergh, Yeager, Armstrong, and me.

If you’re ever flying Southwest and enjoying the airborne WiFi…you’re welcome.

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