PITOT TUBE: Investigators are looking into the possibility that ice build-up on the airspeed sensors of Air France Flight 447 led to the May 31 crash
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As Brazilian authorities retrieved 16 more bodies from the Air France Flight 447 crash this morning, investigators were focusing on airspeed indicators called pitot tubes as the cause of the disaster.
On May 31, the Airbus A330 jet was flying from Rio de Janeiro to Paris when it vanished during a thunderstorm with 228 people on board. Initial speculation centered around the possibility of a lightning strike, but last week Airbus released a memo stating that "there was inconsistency between the different measured airspeeds" coming from different pitot tubes. The pitot tube is a classic fluid dynamic sensor named for its inventor, Henri Pitot, who in the 18th century developed it to measure the speed of rivers and canals in France.
Airbus's pitot tubes were known to have icing problems, and Air France had begun replacing them on April 27 when an improved version was released. Flight 447, however, had not received the new sensor. On Monday, Brazil revealed that it had found the plane's vertical stabilizer, suggesting that it been sheared off due to high speeds.
To find out more about pitot tubes, we spoke to Ken Powell, an aerospace engineer at the University of Michigan at Ann Arbor.
[The following is an edited transcript of the conversation.]
What is a pitot tube?
Basically, a pitot tube is used in wind tunnel experiments and on airplanes to measure flow speed. It's a slender tube that has two holes on it. The front hole is placed in the airstream to measure what's called the stagnation pressure. The side hole measures the static pressure. By measuring the difference between these pressures, you get the dynamic pressure, which can be used to calculate airspeed.
On an airplane, the pitot tube can be mounted in a number of ways, including jutting out from the edge of the wing or sticking up from the fuselage.
How can ice cause the pitot tube to malfunction?
Basically, if there's a blockage of that tube, then you will get an incorrect reading for the pressure difference and your airspeed. Ice accretion on wings is a big problem, and it can also build up on a pitot tube. Pitot tubes must be calibrated to work properly, and if ice changes the shape of airflow around the tube, then it will give an incorrect reading.
Why is measuring airspeed important?
Planes have a certain operating envelope, and it's dangerous to fly at too low a speed because you can stall, and it's also dangerous to fly at too high a speed because of structural reasons.
Can't pilots just use Global Positioning System (GPS) to measure their airspeed?
That's a very good point. I'm not a pilot so I don't to what extent commercial pilots double-check airspeed readings with ground speed readings from GPS. But you don't need something to give you a completely wrong reading to throw you off, it could be off by less than 25 percent and still create a dangerous situation.
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33 Comments
Add Commentit is a shame that:
Reply | Report Abuse | Link to thisa- GPS is not regularly used to check velocity, and 25 percent is an enormous error amount, as Prof. Powell mentioned
b-a Pitot tube is responsable for the downing of a modern airplane.
BTW:What does Boeing use?
Pilots could use GPS to measure their speed, but not in the same way as pitot tubes do. The main problem when pitot tubes malfunction is most modern aircraft have some type of emergency autopilot functions, which will cause the aircraft to automatically do certain things in certain situations. Say the plane thought it was going way too slow (but really wasn't); it could, depending on how the flight computer is designed, automatically put the aircraft into a dive to recover airspeed.
Reply | Report Abuse | Link to thisSo, basically, if the pitot tube malfunction theory is correct, the crash was caused by the aircraft thinking it was going too slow, not the pilot.
I work on helicopters, not jets, so I might be wrong, but I think that if an aircraft did a 100mph maneuver at 500mph, there's a pretty good chance that it could break apart in midair.
A pitot tube measures speed through the air, while GPS measures speed over the ground. At altitude, the speed through the air and speed over the ground can differ significantly. If a plane is flying in one of the jet streams, for example, that difference can be as much as 300 mph. (See http://www.centennialofflight.gov/essay/Dictionary/Jet_Stream/DI68.htm)
Reply | Report Abuse | Link to thisThe min and max speed limits of a plane refer to it's speed through the air, and using GPS can cause the pilot to exceed those limits, especially at altitude where the min and max speeds that a plane can fly converge. (See http://en.wikipedia.org/wiki/Coffin_corner_(aviation) )
Pitot tubes measure air pressure based on the area of the opening, air density thus air temperature. I have never flown an airplane that did not have a heated pitot tube except a J-3 Cub and a Luscombe, two antique airplanes. Usually, in my experience, when there is an occlusion of the opening of the pitot tube, the speed is indicated low but there is room for the indication of airspeed stuck at high speed. GPS is too slow to be used for slow flight.
Reply | Report Abuse | Link to thisMy guess would be that the ice accumulation reduced aerodynamic lift and caused a stall at a higher than expected velocity. NTSB will arrive at the root of the problem...they are very good at that aspect of investigation.
While the pitot may have been blocked by ice and given an error , the real problem is that Airbus is a fly by wire airplane and the computer over rides what the pilot may want if the software thinks it knows better. Does anyone remember the Airbus that crashed at the Paris Airshow a few years back? Same issue, the pilot made a low pass down the runway then commanded full power and the computer over rode him. The result was a big fireball. Very probably this was why 1549 couldn't make anything but the river after the bird strike, too. If it ain't a Boeing, best not be going.
Reply | Report Abuse | Link to thisrickofudall: I used to have a private pilots license (single engine VFR), but havent flown (as a pilot) in more than twenty years. Ive never flown anything that didnt have a mechanical connection between the controls and the flight surfaces and dont think I would want to (background so you won't think I'm a complete neophyte).
Reply | Report Abuse | Link to thisI heard an interview with "Sully" Sullenberger, the pilot of 1549, and he said that almost immediately after the bird strike he knew that with the loss of power in both engines he would not be able to make it back to the airport. I dont think it had anything to do with the fly-by-wire configuration of the plane, but I dont know enough about the system to make a definitive statement. My understanding of the process is that the computer only acts as the connector/translator between the controls and the flight surfaces. I would think this would be especially true during the most critical portions of the flight, take off and landing. Perhaps someone with more technical knowledge about the fly-by-wire system could enlighten us.
However, I do agree with your sentiment about Boeing
i'm a complete layman - but wouldn't relying just on GPS (even if the readings were accurate and quick enough) not be able to take into account other factors (like wind blowing against the direction) which could also add stress to the airplane, while the pitot tubes can?
Reply | Report Abuse | Link to thiskuchung,
Reply | Report Abuse | Link to thisI'm not a pilot, but it seems to me you need both the GPS and the pitot tube. The GPS will tell you your speed relative to the earth. The pitot tube will tell you your speed relative to the surrounding air stream. For purposes of stall and structural integrity, the airspeed is all important. Let's say the plane had a 100 mph headwind and a groundspeed of 400 mph. The GPS would show 400 mph and the pitot tube would show 500 mph. From that, the pilot and autopilot would infer the correct wind speed of 100 mph headwind. If the tube starts to ice over at the tip opening, the pitot tube reading would drop, leading to application of more power to the engines or a dive to gain speed. Pretty soon the plane might be actually going 600 mph or more on an airspeed basis and be unable to withstand severe turbulence.
This airplane broke apart in flight before the pilots had a chance to look and compare the Airspeed on their respective instruments. If they were flying too slow or too fast they would have gotten automatic stick action (shaker, pusher..). Those were not a part of the information that was received by their maintenance base. Also, the autopilot system is modern enough to include reasonability checks that would have warned the pilots of an impending malfunction. This information (from the Autopilot) would have been sent to the maintenance base without pilot's intervention.
Reply | Report Abuse | Link to thisThe little phrase concerning the fact that "for structural reasons" pilots of an Airbus300 have to pay close attention to exactly how fast they are going highlights the reality that this airplane has a very fragile carbon composite rudder and vertical stabilizer that can't hold up to real world conditions.
Reply | Report Abuse | Link to thisWhen the Airbus 300 crashed in Queens in late 2001 it was because the co-pilot tried to use the rudder to correct for loss of control in extreme turbulence. Actually, a pilot should be able to do that in a properly designed and constructed airplane. But when you use the rudder in an extreme situation in an Airbus 300, the rudder breaks off and takes the vertical stabilizer with it. The solution that European airplane maker Airbus has tried to get by with is simply to warn pilots to never use the rudder over a certain speed or in extreme turbulence or in some combination of the two factors.
Airbus is trying to obfuscate the situation by blaming the pitot tube. The only reason the pitot tubes are important at all is because airspeed indicators are the band-aid that Airbus has put over the real problem--the vertical stabilizers and rudders of the A-300 airplanes are much too fragile and need to be replaced what ever the cost. Until these real fixes are made, the whole fleet of these airplanes world wide should be grounded.
As a pilot of Boeing and McDonald Douglas equipment, I see a possibility of the following: as the pitot system fails due to ice, the airspeed will appear to the pilots and on board computers start to decrease. The apparently decreasing airspeed will result in the auto throttle system increasing thrust to maintain the desired airspeed. The aircraft will soon be at a much higher Mach number than desired or indicated on the airspeed indicators. At the same time, the continued apparent decrease in airspeed will cause the rudder limiter to increase the authority of the rudder. Soon you have an aircraft traveling extremely fast with a very overpowering rudder. Now enter into an area of heavy turbulence in the vicinity of a thunderstorm. The autopilot will start to battle the turbulence with its control surfaces to maintain flight. The rudder is tied into the yaw dampener system which will be trying to compensate for the yawing of the aircraft in turbulence. If this scenario is played out, it is easily seen that the rudder can overcompensate and the vertical tail be ripped off the aircraft as is apparently the case in the photos I've seen.
Reply | Report Abuse | Link to thisI would think the AirBus is triple redundant like Boeing and McD and this scenario should never be able to happen ...But I wholeheartedly agree....If it ain't Boeing, I'm not going!
I think that based upon these preliminary findings the crash of a propeller plane in upsate NY in icing conditions should be re-visited. Additionally, I have heard no calls from the US pilots for upgrades to the Airbus fleet here!!
Reply | Report Abuse | Link to thisAs a pilot of Boeing and McDonald Douglas equipment, I see a possibility of the following: as the pitot system fails due to ice, the airspeed will appear to the pilots and on board computers start to decrease. The apparently decreasing airspeed will result in the auto throttle system increasing thrust to maintain the desired airspeed. The aircraft will soon be at a much higher Mach number than desired or indicated on the airspeed indicators. At the same time, the continued apparent decrease in airspeed will cause the rudder limiter to increase the authority of the rudder. Soon you have an aircraft traveling extremely fast with a very overpowering rudder. Now enter into an area of heavy turbulence in the vicinity of a thunderstorm. The autopilot will start to battle the turbulence with its control surfaces to maintain flight. The rudder is tied into the yaw dampener system which will be trying to compensate for the yawing of the aircraft in turbulence. If this scenario is played out, it is easily seen that the rudder can overcompensate and the vertical tail be ripped off the aircraft as is apparently the case in the photos I've seen.
Reply | Report Abuse | Link to thisI would think the AirBus is triple redundant like Boeing and McD...But I wholeheartedly agree....If it ain't Boeing, I'm not going!
sorry for the double post
Reply | Report Abuse | Link to thisWell, first of all, hurray to the good sense of the "if it ain't Boeing I'm not going crowd." I only hope that the 787 Dreamliner (which is all-composite materials) does not run into similar problems as the Airbus airplane is experiencing with carbon composites. At least one very senior Boeing engineer was blackballed right out of the company for raising warning signs in that regard. . .
Reply | Report Abuse | Link to thisBut on the Inspector Closeau front, the French are now putting out the smokescreen that there were two names on the passenger list who may have been terrorists! Let us never, ever forget that literally hundreds of billions of dollars are at stake here should the world suddenly wake up and realize that every Airbus A-300 needs to be grounded until a major sdtructural rebuild is done on its tail section!
All aircraft are required to pass certain certification criteria for their category, in this case the transport category. This applies to Boeing, Airbus and all other manufacturers. All aircraft have maximum never exceed speeds and are tested during certification flight tests. This applies to carbon fiber, aluminum, fiberglass, etc.
Reply | Report Abuse | Link to thisFor those who prefer Boeing aircraft, they don't seem to be aware that Boeing builds aircraft with composite materials and, in fact, the Boeing 787 will have more composites that any aircraft to date so perhaps they will want to scratch Boeing off their list. As far as flight controls, after the DC-9 and Boeing 727, there are no large aircraft with mechanical connections to the flight controls. They are all hydraulically actuated. Fly by wire systems add a layer of complexity but have proved to be reliable with appropriate backup schemes. These designs are subject to a rigorous risk analysis by the appropriate authorities (FAA in the US, EASA in Europe).
The final details of the cause of the accident are certainly a ways off from determination.
Severe weather has caused numerous accidents. And some times not. The Boeing 737 had a a series of accidents and incidents some of which were attributed to weather that eventually turned out to be an anomaly in the rudder control servo - no weather (mountain waves as NOAA would have liked), just a very rare failure mode.
The certification authorities and manufacturers do not want any aircraft in the air with serious design short comings. The overall safety of the aviation system is proof of that. With tens of thousands of flights worldwide daily, its pretty clear that is the case and the job is being well done. In this case, it sounds possible the pitot tube issue is involved but no one really knows as yet, period.
Regarding GPS: GPS is not a ground-speed technology. It determines speed and direction using radio signals from satellites and determines speed from some kind of signal timing differentiation, not from the relationship of the signals to the ground. It is not subject to speed aberration resulting from head winds or tail winds, either.
Reply | Report Abuse | Link to thisGPS is likely susceptible to rain and cloud presence in that it may impede signal reception. This might cause inconsistencies in speed determination.
Regarding what the pilots knew or didn't know regarding the aircraft's air speed (or anything else, for that matter), and whether they received any indication of a problem from the "stick-shaker" mechanism: that seems to be pure speculation, unless there has been a news update recently that indicates that the stick shaker was activated. I suspect no one will know for sure what the pilots knew or didn't know unless and until the FDR and the CVR are recovered.
In this kind of incident, who are to blame? Pilots or Engineers constructing massive amazing airbus plane or Pitot tubes mentioned in the above article or weather conditions like thunderstorm depicted above. Whatever anything like above ones happens, all passengers aboard the plane lost their lives. I think France government is the most responsible for this cause.
Reply | Report Abuse | Link to thisAll we can do now is say "May all souls boarding the plane rest in peace". Min Aung Soe, Burma
In this kind of incident, who are to blame? Pilots or Engineers constructing massive amazing airbus planes or Pitot tubes mentioned in above article or weather condition like thunderstorm depicted above. Whatever anything like those happens actually, all passengers aboard the plane lost their lives. I think France Airways is the most responsible for this cause. All we can do now is to say " May all souls boarding the plane rest in peace".
Reply | Report Abuse | Link to thisgood question
Reply | Report Abuse | Link to thisI am curious to know.
dhisrael writes: "NTSB will arrive at the root of the problem...they are very good at that aspect of investigation."
Reply | Report Abuse | Link to thisYes, they are the best, but I doubt they will be involved. This was a european aircraft flying from Brazil to France and never crossed US airspace. When a US biz. jet hit a Brailian airliner, Brazil threatened to arrest NTSB inspectors when thier findings contradicted the Brazilian government's story.
GPS can basically measure speed through space (most relevantly expressed as ground speed) but, since what counts is the relative velocity through a fluid (air), the ground speed is meaningless. I have flown very well into a strong headwind with near zero ground speed. And it would be possible to stall aerodynamically while moving rapidly across the ground - most frequently this happens when there is awind shear.
Reply | Report Abuse | Link to thisBut in defense of the pitot tube argument I seem to recall a Boeing airplane some years ago (it may have been a 757) that another South American ground maintenance crew had covered the pitot tubes with duct tape while they were working on something else, to keep debris out.
Reply | Report Abuse | Link to thisProblem was, they forgot to take the tape off. The airplane went up at night and the aircrew quickly became completely disoriented. They slapped off the surface of the water, struggled back into the air, and eventually ditched. It seems to be quite hard to fly a modern airliner when you don't know how fast you are really going.
Yup, you can get 'downwind stall' for want of a better phrase. In the light planes I fly the cruise speed and stall speed can differ by about 40 knots, say. So while traveling with a strong tail wind you'll move with that body of air and have an impressive ground speed. A pilot could think he's going too fast if he looks down to the ground track and hence throttle back, thus dropping his speed relative to the air he's within. Actual flow over the wings drops and one can stall.
Reply | Report Abuse | Link to thisIt's imperative to control the plane with regard to the relative airflow and hence pitot tubes, stall warning devices and the like. A GPS does not primarily reconstruct/deduce that speed of the air over the wing surface, however realtime it might be.
In any case the relative airflow is in the ( vector ) opposite direction of centre of mass motion of the plane. So if one is climbing vertically up or down, say, your ground track can be a motionless point on the Earth's surface. The airflow over the wings also depends on the attitude of the plane with respect to it's direction of motion. Hence a plane pointing up but traveling down, as is often done in aerobatics, will have no lift generated as the airflow is going from back to front across the wing.
Oh, and I'm an old-fashioned 'steam gauge' type of pilot. 'Glass cockpits' give me the willies. Whilst one can remove workload from pilots with various devices, you can also inadvertently reduce fly-a-bility.
Reply | Report Abuse | Link to thisDo they have guard dogs in the cockpits yet? The pilot is there to feed the dog, and the dog bites him if he touches the controls.
I do not believe the hypothesis is correct. There was no voice communication indicating there was anything wrong. Since the aircraft was flying at an altitude of over 30 ooo feet, the pilots would have had at least 30 seconds to indicate a problem. Unless the aircraft broke up in the air, the pilots certainly have made an attempt to alert the ground to their problem.
Reply | Report Abuse | Link to thisI cannot see how anything short of catastrophic air frame failure or a fire in the cockpit would have prevented this.
The two air crashes and the Italy train blast all in June 2009 are interconnected and the root cause is, according to my hypothesis, global electric load demand. This is causing the earth to heat up causing persistent weather changes and hence global climate change so rapid as to make existing designs of our airplanes and railroads to experience states beyond their design basis and hence fail. For details of the hypothesis visit my website: http://collateralsofclimatechange.blogspot.com
Reply | Report Abuse | Link to thisThe governments must come together and restructure the civilisation, it definitely appears. If this is not done in a professional manner, at least 4 billion lives may be lost in the transition to a normal way which assures our survival.
GPS can also be used as a velocity measuring device, but it will not give as much accuracy as we axpect from the pitot tube,
Reply | Report Abuse | Link to thisbesides when it comes to the air travelling it becomes necessary to ensure all possible risk aspects to have safe journey. so we cant go with gps with inedequate safety measures.
I would like to ask if in storm conditions,with lightning, the Pitot tubes give the same speed measurement as in the conditions during their calibration (without lightning). As the speed measurement is related with the air density (inverse of the air density square root) and in a storm the air density change, the speed measured in such conditions will be different. The Pitot tube can function as a scape point of the electrical charges in the airplane surface and a strong electrical field can be created at the air entrance of the tube. In such conditions ionized air will have a different density. Also ozone can be formed and the gas density will increase. If this happen the velocity given by the Pitot tube will be different of the real density (lower if the gas density is higher than normal air density).
Reply | Report Abuse | Link to thisLuis Bento
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Reply | Report Abuse | Link to thisIt is fascinating to see the bias in favour of Boeing within the dominantly American readership of this magazine. It would not be so ludicrous were it not for the fact that Boeing has a very similar unsolved problem regarding the rudder of the 737, of which several have detached in flight. Google it and see.
Reply | Report Abuse | Link to thisBy the way, I have nothing against either of these companies, which both build fine aircraft.
I'm a real americaphile and found the thread fascinating to read. However, the pro Boeing comments are totally unnecessary and trivialise the tragedy of the Air France crash.
Reply | Report Abuse | Link to thisAll airplanes, regardless of manufactures, country, whether military or civilian use Pitot tubes, although it is easier to see them on military airplanes, but only if you know what to look for (they tend to look like spikes or tubes that protrude from the wing or the tip of the aircraft)
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