Saturday, June 27, 2009

The Flaps That Make It Rise

Recently someone forwarded this article to a mailing list I read. In excerpt:

Jumbo jet packed with British tourists seconds from disaster after it fails to rise on take-off

By Daily Mail Reporter Last updated at 2:21 AM on 01st June 2009

Hundreds of passengers narrowly avoided disaster when their plane nearly crashed after taking off.

The British Airways plane shook violently and did not rise more than 30ft above the ground as it set off from Johannesburg to London.

The pilot has been praised for his quick actions in keeping the Boeing 747 in the air, saving the lives of the 256 passengers on board. Miraculous escape: The British Airways Boeing 747 is thought to have gone into landing mode so that the flaps that make it rise did not work.

Travelling at 200mph, he dumped enough fuel for the aircraft to eventually gain height, before returning it to the airport.

It is believed that a technical fault caused the plane to go into landing mode so that the flaps that normally make it rise did not work.

An investigation is under way as to how the jet came so close to crashing.

A BA spokesman said: 'As a precaution BA56 Johannesburg to Heathrow flight on Monday May 11 returned to the airport shortly after take-off due to a suspected technical problem.

'The Boeing 747 aircraft with 256 passengers on board landed safely and the customers disembarked as normal into the airport.

'We are cooperating fully with the South African aviation safety authority's investigation into the flight.'

Referring to the pilot's quick actions, he added: 'Our crews are trained extensively to deal with all eventualities.'

So "the flaps that make it rise" weren't working. Before reading part on, what would you think this term referred to?

It could be the actual flaps, and there are certainly accidents that result when flaps are mistakenly not extended, but flaps are normally used for landing, so 'going into landing mode' doesn't make sense. If the airplane did get off the ground with no flaps and into level flight, I'd expect it to have accelerated rapidly. They say 200 mph. That's slow, but is it in the slow flight regime for a B747? I don't know. I also don't know which orifice they pulled the 200 mph number out of.

I considered that "the flaps that make it rise" were the two sides of the elevator. It does flap like the flukes of a whale, and certainly when it is raised, the airplane normally pitches up. But operation of the elevator is not suppressed during landing.

I thought also of things like spoilers or leading edge devices: anything tab-shaped that protrudes from the airplane could be a flap. But nothing matched well enough to seem like more than a wild guess.

Fortunately a member of the mailing list found an NTSB report on the flight. Without the identifying details of type, airline and airport I wouldn't even have matched the tabloid story to the report.

On May 11, 2009 at 18:37 UTC, a British Airways Boeing 747, powered by Rolls-Royce RB211-524H2-T engines, experienced a No. 3 thrust reverser unlock light illumination during the takeoff roll from the Tambo International Airport (FAJS - formerly known as Johannesburg International Airport) while the airplane was traveling at 124 knots. The No. 2 engine thrust reverser unlock light came on at 163 knots and just prior to rotation the slats retracted. The airplane rotated and climbed at a 200 foot per minute rate. The flight crew dumped fuel and did an air turn back to FAJS where a safe and uneventful landing was made.

So the flaps that made this particular airplane rise appear to have been the slats, which the NTSB report must be using as a generic term for leading edge lift devices. The B747 uses Krueger flaps instead of slats to modify the wing for takeoff. The Krueger flaps fold out from under the leading edge of the wing, creating a barrier to air incident there. This makes the airflow behave as though the leading edge of the wing were thicker and rounder, just like the wing on an airplane designed to go slowly, thus giving the wing more lift at low speeds.

Mind you, if forced to condense that into six words I'm not sure I'd come up with anything much more meaningful than "the flaps that make it rise." Anyone?

I'm scheduling this to post a week or so after I'm writing it, because everyone is talking about Air France 447 at the moment. Although when you see this, I doubt the voices will have resolved the divided opinions:

  • a thunderstorm alone could have done this to a perfectly good airplane
  • there must be a flaw in the aircraft for a thunderstorm to do this
  • the pilots or the software must have reacted badly to exacerbate the situation
  • It was terrorists/a meteor/aliens that did it

People I respect and who are more knowledgeable than I inhabit each of the first three camps, so I'm not putting up my tent anywhere. My traffic is way up from people finding this blog by searching on terms like "coffin corner" and "stall recovery." That means that instead of just talking to the people I hang out with, my regular readers, I'm also addressing a lot of spectators who don't "know" me. I do think it's possible we'll never have a satisfactory answer to what happened on that flight.


dpierce said...

"The forward flaps that assist takeoff"? (Everyone I know just calls them slats, whether it's technically correct or not. (Not that everyone I know is an excellent measure of anything.))

And it wasn't a terrorist, meteor, or alien. It was Mothra.

nec Timide said...

Sigh, Godzilla is never around when you need him.

Aluwings said...

"... and just prior to rotation the slats retracted..." HOLY CRAP, Batman! That'll change your stall speed significantly at the worst possible moment.

They did well to survive that one imho.

May it at least put an end to Airbus critics jabbering all the time about how Boeing's aircraft never do bad things because of the automation.

Callsign Echo said...

I'm not quite clear on the NTSB's language. Did the thrust reversers deploy, and were those perhaps the the "flaps" in question?

ganloth said...

"No. 3 thrust reverser unlock light illumination during the takeoff roll ...while the airplane was traveling at 124 knots."

Frankly, I'm surprised that this warning did not result in the pilot aborting the takeoff. 124 knots is still slow enough that it would probably be below V1 ... and the possibility of a reverser deploying with an engine at takeoff thrust is pretty frightening...

I'll be watching how that investigation unfolds.

Unknown said...

Awww, C'mon, this is the Daily Mail that we're talking about, you know, the readership-profile that's just graduating from crayons to felt-tip pens.

" take-off lift enhancement devices"
would be totally alien vocabulary to them and their "story-tellers" (what grown-up newspapers refer to as"reporters"
So, on balance, they probably got it right. ;-)

Geekzilla said...

HAHAHAHAHA!!!! @ Mothra

david said...

It may be that we have to treat AF 447 (loosely) like Schrödinger's cat -- since there's no one alive who witnessed the accident, and (so far) no flight data recorder that the living can read, we treat the accident as unobserved, and accept all probable situations as true.

And anyone who doesn't believe a severe thunderstorm could tear apart a 747 hasn't spent time close to them.

Cirrocumulus said...

Just about everything that appears in the Daily Mail is a load of complete horsedroppings.

Mothra? This was an African takeoff. Maybe they had Ananse as a first-class passenger. He has caused trouble on BA flights over the years.

FBW Airbuses have been around for 20-odd years. If they fell out of the sky on a regular basis somebody might've noticed. I live under the queue for Heathrow and I don't think any amount of conspiracies could've hidden the smoking craters from us locals.

Anonymous said...

there must be a flaw in the aircraft for a thunderstorm to do this

No disrespect to the folks you know Trix, but I'll side with David. To think that we can build a flying aluminum tube that nature can't destroy on a whim, to me is nonsensical. Kind of like that "unsinkable" ship...

And how about "flaps for extra lift at takeoff?" Ignores their use when landing, but isn't quite so 6th grade English class.

No slats, and possibly pulling power on two engines to avoid / minimize deployed reverser damage makes for some Herculean flying. Kudos to the crew.

Anoynmous said... that the flaps that normally make it rise did not work.

It sounds like they worked as designed. It's not the slats' fault if the plane told them to go stow themselves.

When I'm describing things to a non-jargon-savvy audience, I tend to avoid mentioning components by name or even by function. I instead try to give a higher-level description, and go into further detail only if asked.

"It is believed that a technical fault caused the plane to go into landing mode and take the wings out of the high-lift configuration used for takeoff."

Anonymous said...

It will be interesting to see what the eventual report on the BA incident reveals, the Daily Mail article gives little of the detail.

The information that I have was that:-
- Between V1 and VR the T/R interlocks on engines 2 & 3 triggered.
- As the aircraft was still on the ground this resulted in the auto retraction of the inboard L/E flaps
- After rotation stick-shaker activated @ 12ft (radar) until 30ft (radar)
- Significant positive rate of climb was not achieved until after the gear had been retracted, which triggered the extension of the L/E flaps. (Think about the consequences of the additional drag from the gear doors whilst flying a 747 on the edge of the stall, 30ft off the ground, in the dark!)

mattheww50 said...

People forget that high weight takeoffs from JNB are ALWAYS challenging. There is a reason the runway is about 14,500 feet long.

JNB is about 5500 feet above MSL, so things that wouldn't be a serious performance problem with more favorable density altitude, can cause serious difficulties at JNB.

Also the Slats (Krueger Flaps) are often referred to as flaps because selecting flaps 1 deploys them. They are always used for takeoff.

In addition flaps (other than the slats) are not used on the 747-400 for most takeoffs, however the Boeing technical manuals call for flaps 20 at typical JNB density altitudes.

The density altitude at JNB is a double whammy on aircraft performance. You get both reduced engine performance, and increased Vr, hence the very long runways.

Sarah said...

I 2nd the "holy crap" comment of Aluwings. At first, I pooh-poohed the "cheated death" tone of the article, but... no. Losing the slats just after rotation and a 200 ft/min climb at night in a heavy 747 .... yikes!

I'll have to ask the few 74 pilots I know about the automation issues. I'd have thought flap/slat retraction would be a little less automatic than that.

As far as AF447 --- yes, without more data we may never know for sure. We'll have to let the wave function remain uncollapsed and consider all possible causes. Thanks David for that image. Information hints at airdata errors and a chaotic overspeed/stall inflight breakup. But we don't know.

GPS_Direct said...


If I remember correctly, the Kruger flaps retract when the WoW switch is closed and the reversers are activated - so as to keep crud from being blown into the mechanism.