The details of the British Airways 747 near-accident in South Africa are out, and man, was that scary for the pilots. They did a great job of keeping that bad boy in the air, but it could have ended very differently. Here’s what happened.
On May 11, 2009, BA flight 56 prepared for its evening departure to London/Heathrow. Afternoon rain had cleared out and it was a clear evening with light northerly winds and temps in the mid-50s (something like -358 degrees Celsius, I’m told). Boeing 747 G-BYGA was ready to bring 265 passengers and 18 crew members back to the UK, so it was about 80 percent full. It probably looked a lot like this one (though this was in Cape Town, not Jo’burg):
They buttoned up and headed for the runway. Engines spooled up as usual and they started rocketing to the north on runway 3L for the long flight home. When the airplane hit 167 kts, just about the time for it to rotate, all hell broke loose. Somehow, due to a technical fault, the airplane showed that thrust reversers had been deployed. Thrust reversers deflect the air within the engine to push it forward instead of backward. This is generally only a good idea when you want to stop the airplane, so it happens with wheels on the ground during the landing rollout. Here’s what they look like on a Lufthansa 747.
Fortunately, the thrust reversers didn’t actually deploy and it was merely a faulty warning, but it did bring with it some unintended consequences. When the thrust reversers deploy, the slats Krueger flaps automatically retract. What the heck is a slat Krueger flap? I’m glad you asked.
See those little things hanging over the front of the wing? Those are slats Krueger flaps. Like flaps behind the wing, they’re meant to help increase the surface area camber of the wing to provide more lift. When you’re cruising, you don’t want this because it provides drag and slows you down. But when you’re taking off and landing at slow speeds, it makes it more stable and allows you to fly slower. That’s good.
When it’s not good is when they retract just when you need them most. So picture a 747, just about reaching take-off speed, that suddenly loses its slats Krueger flaps because they think it’s time to retract. Lift goes away and the pilots see less and less runway ahead. Holy crap. So what happened? Well, they took off and sat at about 40 feet above the ground trying to pick up speed. It kind of looked like this:
Ok, so I lied. It looked nothing like this. Instead, replace that airplane with a hulking, slat-less 747 barely clearing the terrain below. Yeah, I’d freak out too. Ultimately, the slats Krueger flaps were back in their deployed position a mere 23 seconds after they ran away, but those were the 23 most critical seconds of the flight. The airplane then started climbing, but the pilots weren’t content to continue on. They dumped fuel and eventually returned with everyone safe.
My guess is that there might have been some people in the back wondering what was going on, but it happened so quickly that they unlikely would have had a chance to even register that this was a real issue. The pilots, however, must have absolutely flipped. Fortunately, they did a fantastic job. The pilot in command happened to have aerobatic training and was well-versed in how to fly at near-stall speeds. There’s no question that those guys saved that airplane and all the people onboard.
But it’s not just them. There was some serious luck here. Johannesburg sits a mile high, and that reduces aircraft performance. But had this been in summer instead of winter, it would have been much worse. Hot weather makes it harder for airplanes to gain altitude, so the mild temperature undoubtedly helped here. It’s also a blessing that the airplane was only 80% full instead of 100%. The added weight would have hurt. On the other hand, it certainly didn’t hurt that they had a slight headwind and the the weather was good.
Anytime there’s an accident, it always requires a handful of things to go wrong. In this case, while one awful thing went wrong, everything else went right. And that’s why the airplane was saved. One other thing going wrong could have resulted in disaster. Fortunately, that didn’t happen here and changes required by the FAA mean this particular incident shouldn’t happen again.
Update at 917p on July 9 – Thanks to the readers who corrected me here. There are no slats on the 747 but rather Krueger flaps. Wikipedia has a good explanation of the difference:
While the aerodynamic effect of Krueger flaps is similar to that of slats or slots, they are deployed differently. Krueger flaps, hinged at their leading edges, hinge forwards from the under surface of the wing, increasing the wing camber and maximum coefficient of lift. Conversely, slats extend forwards from the upper surface of the leading edge.
