
Report on Air France 447 crash deepens mystery - ColinWright
http://www.newscientist.com/blogs/onepercent/2011/05/blaring-alarms-confused-doomed.html
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crikli
I figure a quick and dirty explanation of basic flight systems might not be
unwelcome.

In most aircraft and certainly in jets, you have six primary instruments:
airspeed indicator, attitude indicator (artificial horizon), altimeter, turn
indicator, heading indicator, and a horizontal speed indicator.

The attitude, turn, and heading indicators rely on gyroscopes that are
propelled by a vacuum pump and/or electric motors.

Airspeed, altimeter, and vertical speed indicators rely on a pitot-static
system. The pitot tube must be exposed to the air that is uninterrupted by the
plane's passage. The static port(s) must be positioned where the air is calm
and undisturbed.

Each instrument of the pitot-static system relies on pressure differentials,
but only the airspeed indicator is reliant on both the pitot tube and the
static port. The other two instruments rely on the static port and vents or
calibrated leaks.

Part of a pilot's primary training and certainly part of instrument training
is in recognizing failures in this crucial system. For example, let's say you
take off and your airspeed slowly drops to zero but you remain flying. There's
a good chance you've got a blocked pitot tube. This is a simple example, but
pilots are trained on this and other failure scenarios during primary and
instrument courses. Pilots that want to die of old age train themselves beyond
what's required by these courses.

At lower altitude in visual flying conditions, these failures aren't as
deadly, primarily because one can still see the horizon. But high altitudes
remove most of the visual cues, requiring pilots to trust the hell out of
those instruments...except when they can't.

Someone else said it, but these guys lost situational awareness; I'm guessing
that all of the warnings going off just overloaded them and caused them to
overlook the clues that would have saved them. I've personally never flown
anything bigger than a six-place Beechcraft, but I've had enough scary moments
with buzzers and lights flashing to appreciate just how much pressure that
situation would have caused.

~~~
epochwolf
NOVA did a program on this. They do a detailed look at this type of failure.
They also had trained pilots demonstrate how to fly that specific plane
without airspeed indication. It wasn't difficult to do. There are specific
throttle and angle settings you can use to maintain stable flight. The program
is available on netflix: <http://www.netflix.com/WiMovie/70148706>

Edit: This may not be the best place to add this but I don't feel like
replying to several comments.

If I recall correctly, the pitot tubes on 447 were defective models known to
be prone to icing. They met existing safety standards but they had still
caused a number of failures other aircraft of the same model before this crash
happened. The aircraft had been due for pitot replacement but Air France
hadn't gotten around to it yet. The aircraft had flown directly into a
thunderstorm system capable of making super cooled water which could easily
overwhelm the defective pitots.

~~~
jonknee
It's also available for free on PBS.org:

<http://www.pbs.org/wgbh/nova/space/crash-flight-447.html>

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wazoox
Interesting analysis in this post on the PPrune forum:
[http://www.pprune.org/rumours-news/447730-af447-wreckage-
fou...](http://www.pprune.org/rumours-news/447730-af447-wreckage-
found-30.html#post6477527)

~~~
lotharbot
My simplified summary/explanation [0]:

Several flight instruments rely on air pressure. If one of your pressure
gauges gets blocked, you can get very misleading data.

This guy's theory as to what happened in this case is that the forward
pressure tube (pitot tube) had an ice blockage, which led to a low airspeed
reading. The autopilot tried to correct by accelerating the plane. Since
loaded jetliners normally fly slightly nose-up to maintain altitude [1], the
autopilot kept that orientation as it accelerated, which led to a climb and
ultimately a stall.

When the plane stalled, it started dropping altitude rapidly -- but with a
blocked pressure line, this could misleadingly report a high airspeed due to
the increase in external pressure. As the pilots took over, they were
(according to this guy's theory) trying to reduce airspeed by cutting the
throttle and nosing up. Rather than fixing an overspeed condition, they were
actually making the stall worse. They never would have realized what was
happening because their instruments kept reporting overspeed.

[0] I'm not a pilot, but I worked in an aerospace museum's education
department, and learned a lot from the pilots and engineers who volunteered
with us.

[1] According to one of the volunteers, who was also an engineer for one of
the big jetliners, loaded jetliners get about half of their lift by keeping
their wings 3-6 degrees above level. The exact angle is selected based on load
and airspeed; get it wrong, and you'll accidentally climb or descend.

~~~
theycallmemorty
Why wouldn't they have felt themselves dropping as they plunged towards the
ocean?

~~~
epochwolf
At night, in the middle of a thunderstorm, at over 30,000 feet? Those are
perfect conditions for vertigo. That's the reason pilots learn to ignore what
they are feeling and trust their instruments. Unfortunately, some of those
lied.

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bugsy
I don't think the report deepens the mystery, it answers questions. The flight
data recorder information was recovered and tells what happened. The flight
speed sensors weren't working and so autopilot shut off while the captain was
taking his rest period. Then the two copilots didn't know what to do and
called him. When he came they all continued doing random things indicating
none of the three had any flight training that they remembered. This behavior,
of not knowing how to deal with flight problems because they are used to
running everything on autopilot, is not uncommon in airlines based in third
world countries where pilots are the nephews of autocrats, but is unusual for
French pilots.

~~~
rbanffy
The question not answered is why three trained pilots made wrong guesses as to
what was going wrong with the plane.

BTW, I imagine a pitot tube placed on the body of the engine would not need
extra heating and would be very unlikely to ice. You can even place it inside
the exhaust and compensate the reading for engine flow.

We have to figure out what went wrong, what mistakes the crew made in order to
prevent future accidents like this.

And I, coming from a third-world country, take offense at your statement.
Pilots here endure very thorough training. One would have to be a lunatic to
trust his nephew to a flying computer. The pilot must _always_ know what's
going on.

And that brings the main point: people die when the machine surprises its
operator. "Smart" planes have become so smart pilots have trouble
understanding what they are doing under all that software. And that's assuming
the software is not buggy and they didn't got a very unfortunate
NullPointException in the worst possible moment.

~~~
icegreentea
I would a pitot tube in exhaust work? There's very little 'normal' air in the
exhaust, just the engine exhaust, and unless I got my physics completely
wrong, will always give the same relative speed relative to the plane(pivot
tube) at any given engine setting... which doesn't tell you anything about the
actual airspeed (which is what you need).

Pivot tubes are 'tubes' located away from the body of the airplane for the
reason that they get away from the boundary airflow to give accurate airspeed
readings. Attempting to 'compensate' for other factors just introduces more
uncertainty, which is exactly what you don't want when you're 5 knots above
stall.

~~~
rbanffy
Is the engine airflow at any given regime completely independent of airspeed?
I would never use it as a primary source, but it could be used to flag
abnormal readings of other instruments.

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tzs
I wonder if you could accurately measure airspeed acoustically, instead of
using pitot tubes and pressure? For instance, suppose you placed microphones
near the trailing edge of the front wings, and near the leading edge of the
rear wings.

Compare the sound of the engines as picked up by the front microphones and the
rear microphones, and figure out how far behind the latter is shifted in time
compared to the former. The faster the plane is going relative to the air, the
shorter that time should be.

If the engine sound is too uniform to provide any way to match up the sound as
recorded from the two locations, some kind of sound source could be added.

~~~
jonsen
Acoustic Doppler velocimetry:

<http://en.wikipedia.org/wiki/Acoustic_Doppler_velocimetry>

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simon_weber
Anybody read Airframe by Michael Crichton? Similar thing happens, and the
recordings show an experienced pilot makes questionable moves to correct the
problem. In the end, it turns out that the pilot's son was flying the plane
when it happened.

