Wednesday, July 01, 2009

Taking a stab at Air France 447

Interesting story by Alan Levin in Monday's USA Today that suggests National Transportation Safety Board investigators could look to a plane crash from 35 years ago to perhaps explain what happened to Air France 447.

What's the connection?

Early focus of the Air France investigation has centered on the plane's airspeed indicators, which could have malfunctioned, causing the pilots to tragically misinterpret the readings of their most-needed instruments.

That sounds similar to what caused a Northwest Orient 727 crash in Bear Mountain, New York in 1974.

The USA Today story offers some nice play-by-play of the Northwest crash, but doesn't really get into the guts of the most important part -- the "why." Why did the pilots reacted the way they did to a malfunction, effectively stalling the plane and sending it into a graveyard spiral.

So we'll do that here.

A quick primer on the pitot-static system

There's six basic flight instruments that makes up what's known as the "six pack" in the instrument panel. Three of these -- the airspeed indicator, altimeter and vertical speed indicator -- receive their information from the pitot-static system.

(In bigger jets, the machmeter also receives its information from this system, which is important to note, given the accidents we're discussing. But in the interests of keeping this relatively readable, I'm not going to get into details that will put you to sleep).

In its simplest form, the pitot-static system is comprised of a pitot tube (rhymes with speedo) and a static port.

The pitot tube is typically mounted on a wing or the fuselage, depending on the aircraft, and looks like a little stick with a hole at the tip jutting into the wind. It measures the direct pressure of the air blowing into it. The static port, which is a little hole on the side of the plane about the size of a pinhead, measures atmospheric pressure.

Airspeed is measured by the difference between the direct pressure and atmospheric pressure is compared.

When the pitot-static system fails

Blockages in the pitot tube and static port, while not common, aren't particularly rare either. Ice can easily gunk up the pitot tube, so there's a heater on most pitot tubes. The static port can often get bug juice in or around it, so there's an alternate static source.

Even with those runarounds on potential problems, all pilots must know how those three key instruments are affected when the static port or pitot tube -- or both -- are blocked.

What happens when the static port is blocked?

Well, this is serious because it affects all three instruments. The altimeter will stop at the altitude at which the blockage occurs. The vertical speed indicator will show level flight, no matter if the plane is climbing or descending. The airspeed indicator will show a slower-than-actual speed in a climb and a faster-than-actual speed in a descent.

What happens when the pitot tube is blocked?

In a way, it's simpler, because only the airspeed indicator is affected. But it's also a more nefarious problem. The airspeed indicator will function as an altimeter, showing an increase in speed as the plane climbs, even if actual airspeed is constant.

Northwest Orient, 1974, Bear Mountain, N.Y.

So far, I've given you a lot of basic theory. Here's how it actually applies in the case of the Northwest Orient flight referenced in the USA Today piece:

Crews are usually trained in some capacity to maintain a constant-airspeed climb at such-and-such a power setting.

So imagine you're the pilots aboard this flight, thankfully a repositioning flight with only three crew members aboard. As you continue your climb through 16,000 feet, you notice that you're climbing at 300 knots when you should be at 200 knots.

(I don't know the actual figures for the 727 climb; I'm just using them as an example).

What are you going to do?

Keep in mind one of the basic rules of flight: Pitch plus power equals performance. These guys did what makes sense. They decreased their power and pitched the plane up in an effort to slow down to 200 knots.

Unfortunately, the Northwest pitot tube had iced over. Their airspeed indicator was showing a faster-than-actual indication, essentially functioning as an altimeter and increasing as they climbed. In reality, the pilots were on their target climb speed.

By decreasing their power and pitching the plane up, they slowed down to something slower than their stall speed -- remember that from our original Colgan post? -- and induced an aerodynamic stall and subsequently spun the plane into the ground.

AeroPeru 603

There's another crash not mentioned in the original article that's worth mentioning here, an accident involving an AeroPeru flight in 1996 that had multiple instruments fail because the nimrods washing the plane beforehand taped over the static ports and forgot to remove the tape.

It was a night flight in instrument conditions. These poor folks didn't know which way was up, how fast they were going, or whether they were headed up or down. They crashed into the ocean 25 minutes after takeoff.

There's a fascinating National Geographic special on AeroPeru 603 that I recommend watching on YouTube if you have a half-hour to kill.

While speculation centers on the pitot tube in the Air France crash and AeroPeru involves the static ports, this could nonetheless be a really strong comparison, in the sense that you have false instrument readings caused by massive problems in the pitot-static system ultimately leading to disaster.


Crews are trained how to spot anomalies between the instruments that would lead a pilot to realistically catch the error. On a typical flight in instrument meteorlogical conditions, there's a constant cross-check of the instruments in the six pack to verify and confirm information.

In the heat of the moment, could you miss something that leads to a crash? Absolutely. There has to be a lot that goes wrong to get to that point, but yeah, it is feasible.

You could make the argument that if the Air France 447 pilots had been distracted by a vicious thunderstorm and alarms buzzing about incorrect airspeed readings that there was enough confusion that they did precisely the wrong thing.

That's an awfully big leap to make at this point. I'm sticking to what I said the other day -- there's so much information still missing from the Air France puzzle, that it's not prudent to even make an educated guess as to what brought it down.

But since USA Today is offering up a theory, we'll dissect how it might have applied to Air France 447. Right now, it's as good a guess as any.

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