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Home > Air Crash > Air France Crash: How could they have gotten it so wrong?
Air France Crash: How could they have gotten it so wrong?
Aviation - Air Crash
Sunday, 04 December 2011 04:39

Air France Flight 447's harrowing end
Hi Capt Lim,
I was wondering what your take is on the Air France 447 crash where a high altitude stall caused the plane to lose lift and plunge.
I would have thought that it is basic knowledge for all pilots that pulling out of a stall requires pointing the nose of the plane down to increase air speed and lift. However, they did the opposite and pointed the nose up, making the situation worse.
How could they have gotten it so wrong?
Also, I didn't think turbulence could cause the plane to stall. How did this happen?
Jim
Hi Jim,
The Air France Flight 447 crash was a tragedy that could have been prevented if pilots were given more training on recovery from a jet upset. Most pilots were trained on stall recovery technique during their basic flying school days but it appears that this exercise was no longer required because it was thought that modern planes have been designed to make it unnecessary. Well it appears to be a mistake.
In the instance case, it was unfortunate that besides getting into a high speed stall, the plane had unreliable airspeed due to the pitot tubes being blocked by ice. There were conflicting information of aural stall warning and erratic airspeed. The nose up was caused by the pilot applying full power - this was not the correct technique for initial recovery from a stall for a jet plane.
It is easy in hindsight to ask why it had gotten so wrong. It seems there were no good visual horizon for a proper recovery due to the weather and aggravated by conflicting aural and airspeed information.
Turbulence did not cause the plane to stall. It was the combination of many other unfortunate circumstances – mainly the loss of reliable airspeed caused by the frozen pitot tubes.
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procedures are completely changed!
You write "by the pilot applying full power - this was not the correct technique for initial recovery from a stall for a jet plane" Yes and NO. We know this after the accident. You must don't forget the Airbus procedure was completly opposite at the time of the accident: in case of stall warning: Go around id Max power! Now after this accident and the Perpignan accident, it is officially completely opposite...
Jean(John) E., France , 06 Dec, 2011
GPS-system on board
In this case, like in my cases, where commercial airline crashes happen, it's usualy not just one thing that causes the crash.
In the movie shown above we are able to see a tiny part of what went wrong during this accident.
The truth is, it's far more complicated than that.
Due to a hardware error ( frozen pitot tubes ), the plane's software malfunctioned as well, leaving the pilots in total mystery about the alltitude, cruising speed etc. The complete software system of airbus simply wasn't designed to cope with this situation aswell as the pilots, who weren't trained for a stall situation.
A simple GPS-system could have made the difference in this case.
Because at high altitude ( flying trough thin air) a difference of some 50 km/h can mean the difference between flying regular or stalling.
beerza , 11 Jan, 2012
...
One more possibility as to the crash is that the pilot have been put to mistrust by the airplane. I saw in one documentary about the AF447 crash, the procedure for pitot tube failure is to apply 80% power and 5 degrees nose up to maintain speed. But since the autothrottle was acting upto the final disconnect, the engine power could have been lower as per the auto throttle systems to get through the turbulence, but it was not reflected in the thrust levers. So, the pilot may have assumed he was having 80% thrust when in fact he was having insufficient thrust after disconnect, causing him to lose speed with the nose up attitude and hence stall.
Venkatesh , 20 Jan, 2012
Pitot Tube Anti-ice
I was just an amateur pilot but how the... Frozen pitot tubes? One of the most important things we learned at the ground-school was never to mess up with snow and ice. Did they forget to turn on the "anti-ice"? Or false indication the pitot tube "anti-ice" was on (similar to DC-10 cargo bay doors)? Or the heater was too weak for the icing conditions?
MikeVictor , 23 Jan, 2012
Pitot Tube Ice
Well, it's very possible that flight 447 encountered SLD icing and the pitot heat just couldn't keep up... If you look at the flight path and altitude in relation to the precipitation they were flying through and around at the time of the accident it's easy to see how Supercooled Liquid Droplets could have been present and overwhelmed the pitot heat system.
CaptainSmitty , 26 Jan, 2012
There are more than few cases when pilots mishandle a stall
And that completely baffles me. Colgan Air Flight 3407 -- a stick shaker activates and pilot pulls on the stick resulting in a stall and crash. Birgenair Flight 301 -- stick shaker, pilot pulls the nose up. West Caribbean Airways Flight 708 -- plane stalls at a very high altitude, pilot keeps pulling on the stick all the way down.

All this really makes a case for an airbus style FBW -- at the very least it would prevent gross pilot errors. But we need to make the autopilot more smart, relying on different independent sensors (GPS, etc.)
Yuri , 12 Apr, 2012
Bachelor of Aerospace Engineering, Georgia Tech; Private Pilot Single Engine Land, Federal Aviation Administration
There are two "morals" of Flight 447:

1) Avoid thunderstorms at all costs even if it adds 200 nm to your trip or requires you to divert or abort and return to the starting point and regardless of what your aircraft type is, you are no match for a thunderstorm with icing.

Before this I can point to two U.S. crashes Southern Airways 242 in 1977, and Delta Airlines Flight 191 in 1985 - both were brought down by the consequences of flying in or near a thunderstorm cell.

2) Your flight management system is no substitute for remembering and practicing the BASICS of aviating: PITCH AND POWER!

http://faaflighttest.us/AC61-50A.pdf

AC61-50A states: Angle of Attack is the primary control for airspeed and Power is the primary control for altitude (this is for normal region of command.)

At a constant altitude then there is only one airspeed for a combination of airspeed and altitude in the normal region of command. (See my detailed explanation below).


Douglas J. De Clue , 07 May, 2012
PPSEL, BSAE
As for the accident, it appears that in flight 447, the pitot tubes iced up from supercooled rain drops that froze on contact with the aircraft. Pitot tubes are heated but there is a limit to how much ice they can melt and flight 447 lost this battle. This ice then led to inaccurate airspeed indication and then incorrectly applied pilot attempts to correct for this led to a stall/spin accident.

This yet another crash where too much reliance on digital flight management systems and unthinking application of written procedures led to a totally avoidable accident had the crew thought through the fact that their Mach-meters (air speed indicators) were unreliable and then applied some basic aerodynamics / aircraft performance principles to deal with what should have been a temporary emergency situation as the pitot tube heaters would have eventually melted the ice and if necessary the crew could have descended to a much lower altitude after gaining control of the situation to aid in melting the ice.

If the airspeed cannot be accurately determined it is useful to remember that an aircraft only has one airspeed for a particular power and attitude (pitch) combination in the normal region of command so you can get to a known airspeed by setting that power and attitude and the airplane will fly that airspeed even if you can't properly check that airspeed with your airspeed indicator.

You can figure this out by plotting thrust required (drag) curve vs. thrust available curve for a particular angle of attack and altitude to come up with an known airspeed.

http://www.lets-go-fly.com/drag-and-power-v-airspeed.pdf

This link is for piston powered aircraft but the drag curve is still the same for jets with a component for parasitic drag and another for induced drag and in the "normal" (as opposed to reversed) region of command for a particular angle of attack there is a particular amount of drag that will be generated that will be a function of lift/drag ratio (induced drag) which is a function of the design of the aircraft and a particular amount of parasitic drag (which is normal skin friction of the airframe which is a function of the design) which is generated.

Drag=[1/2*(ro*Velocity^2)*Cd]+[2*(Lift^2)/(ro*Velocity^2*WingArea*pi*efficiency*AspectRatio)]

ro = the Greek letter used to indicate density of the air.

pi = the Greek letter used to indicate the ratio between the perimeter of a circle and it's diameter. Usually approximated as 3.14 but you can use 355/113 for a highly accurate easy to remember fractional ratio that approximates pi.

At low speeds the second or induced drag term dominates..this is the region of reversed control. At high speeds the first or parasitic drag term dominates.. this is the region of normal control.

For any given airspeed and density altitude then at a particular angle of attack there is an amount of engine power input which is required to equal the drag force being generated while maintaining level steady state flight (lift=weight and thrust=drag).

In piston aircraft this thrust would basically be power (HP or kW) divided by speed. In jet aircraft, thrust is relatively constant for a particular throttle setting with speed so there is generally no need to consider speed in calculating available thrust.

Thus if you know the power or thrust required curve for the aircraft and can set a known angle of attack (pitch) and power setting you should get a known airspeed at a particular density altitude. This information should have been available to the AF447 crew in a manual or via their flight management system but a lot of times in these accidents the crew forgets a basic aerodynamic fact and relies too much on the procedures and when the procedures run out they fail to consider the basic aerodynamics of the situation and make a mistake.

Things are mathematically a bit more complicated for an airliner flying at high subsonic speeds (probably between Mach 0.76 and Mach 0.82) with compressibility but with a slow enough airspeed they wouldn't have to worry about transonic shocks on the upper portion of the wing although they would have to account for compressibility effects on the power required but then this would have been built into any performance tables they would be using in a manual or the FMS anyways so in reality they wouldn't have to calculate it out anyways, just find a good airspeed in the table and figure out the right pitch and power and fly it until the ice melted which it would have eventually done and treat the aviating and the navigating as two separate tasks with the navigating being done by GPS while the minute to minute aviating would have been done by flying the PITCH AND POWER.

Doug De Clue

http://en.wikipedia.org/wiki/Pitot_tube

http://en.wikipedia.org/wiki/Machmeter
Machmeter

Doug De Clue FYI: I am an FAA licensed private pilot w about 200 HRS TT and I am a degreed aerospace engineer from Georgia Tech.
Douglas J. De Clue , 07 May, 2012
Mr
If you understand the construct of pitot satic instruments, you will know that in the event of a pitot line blockage, the ASI will act like an altimeter (ie. increase when climbing and decrease when descending). This create confusion to the pilot when he is trying get out of stall. His nose down recovery action will caused the ASI to decrease. Knowing that he needs to get the speed increasing, he might apply the opposite action instinctively, which caused the aircraft to stall even further.
Tan , 11 Jul, 2012
Why dropping when planes are supposed to glide
So it appears that the plane dropped into the ocean, belly-on, within 3.5 minutes. I keep reading that normally, a plane can glide for about 10-20 minutes and make a controlled crash landing even when both turbines are malfunctioning. So why did this AF flight just drop like a stone then? Thanks
Ben , 31 Aug, 2012
traveller
Yes, why didn't the airplane glide??????????????????????????????????
Una , 24 Oct, 2012
Why didn't it glide ?
I am no expert but as a Private Pilot I think you need to recover from the stall first before you can glide at a controlled rate. In this case the aircraft stalled at high altitude and never recovered and started rapid loss in altitude with the nose pointing up.

The documentary have commented about the Pitot being frozen I wonder if the Static port were frozen too. If it was not frozen then the Altitude Indicator should be working and Pilots would have noticed the loss in Altitude besides their bodies should also be feeling the downward G forces. Since reliable airspeed info were lost by turning off all automation going back to flying 101 they could have stood a chance.

Sorry Guys...Easy for a guy sitting in a nice warm room playing "Monday Morning Quarter Back" to say.
CAVOK , 19 Jan, 2013
...
Please let me understand the following, if someone knows: Given that the autopilot detected the pitot tube failure, why it didnt follow the standard procedure? (adjust the angle of attack and put the engine power to the right level?) What the documentary states is that it just disengaged :S
Dionisis , 04 May, 2013
Why the autopilot was disengaged?
You can see the answer here http://tinyurl.com/caoulwu
Captain Lim , 05 May, 2013

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