Technical Alternatives for Pitot Tubes?

[greek-freak]

Given that pitot tubes apparently play some role in the Air France accident,
I was wondering if there are technical alternatives to pitot tubes.
To the non-expert it's rather surprising that redundancy for measuring such
important parameters is not implemented via at least one additional device
that uses a different technique and may be less susceptible to
icing/contamination etc.

So my question to the tech gurus is whether there exists an alternative.

Thanks

[OutOfRunWay]

I wonder if it would be possible to measure the dynamic perssure using some sort of piezo-electrical device; a flat patch head-on to the slipstream on the airframe somewhere where the airflow is clean?

I frankly have no idea wether anybody ever considered searching for an alternative to the pitot, which is tried and tested technology, and does not go wrong a lot.

Regards!

[parabellum]

Pitot tubes, properly heated, have worked for nearly 100 years so why change now? But I agree, an alternative method of measuring pitot pressure wouldn't be a bad idea.

[leewan]

Measuring airspeed through the GPS could be another alternative although it's accuracy at such high speeds has not been validated, AFAIK. Another alternative is to put a small propeller in the windstream and get the airspeed through the propellers rotation. Not very accurate as the windspeed could mess it up, but better than nothing.

[vodmor]

How would you like to measure airspeed with GPS? You can calculate GS, i don`t really see the technical possibility to get airspeed with GPS. The propeller method was already used, I think even Wright Flyer had one;) I guess it didn`t proove accurate and reliable enough.
You can measure airspeed with variety of means, some aircrafts had Venturi airspeed indicator,

[TyroPicard]

Some modern anemometers use ultrasonics for measuring wind velocity - perhaps that techology could be applied to the problem.

[csd]

How about measuring doppler shift from a laser aligned to the aircraft centreline? Should work well in IMC and hence as a backup for iced up pitots but would probably be ineffective in a clear sky.

Regards

csd

Why is pprune putting a "@" in laser instead of an "a"?

[mrangar]

I agree with the need for a pitot tube alternative. While the pitot tube may have been around for a 100 years, and works without incident most of the time (esp. with pitot heat), when conditions for icing, etc exist, all the redundant pitot tubes on the plane are susceptible to the the same kind of failure.

It would be great if aircraft, in addition to the pitot tube, had some other mechanism (as suggested by posters earlier) which can be used to cross-verify pitot tube readings in failure scenarios.

[JABBARA]

A mandatory AOA indicator as apart of stby instruments can be a solution.

If you lost everything fly the AOA.

In fact, a version of this is in use and can be purchased as an option for A 330.

If All ADRS are gone, PFD displays a green band, simply adjust speed to fly within.

Ironically, T 38 Talon manufactured 1968, we used to fly green AOA light on approach regardless weight and Flap.

[muduckace]

There is no other method to read actual airspeed. Most above posters negate the factor of air pressure. GPS etc,etc calculates ground speed only, not the speed of air in relation to the aircraft.

All other options would have to be mounted on the aircraft and be subject to the environmental forces a pitot tube is capable of reading.

[Intruder]

It is possible the ultrasonic devices mentioned above might be adaptable to aircraft use. The ones I have seen on sailboats have 3 vertical probes oriented in an equilateral triangle that measure wind speed as well as direction. I suppose a static input could be mixed in with it to get an equivalent "indicated" or true airspeed readout...

I don't know if such a system is adaptable to high airspeeds or if it would be more or less susceptible to icing, though...

[leewan]

Isn't it possible to get the airspeed via GPS through calculations or through the known windspeeds or air pressure from ATC or the MET chart ? I'm not saying GPS should be the primary means of measuring airspeed but as a backup, it's better than flying blind. Morever, most commercial a/cs are fitted with GPS, so not much of a retrofitting to be done.
I wonder what the QRH states for in case the pitots fail ?

[vodmor]

To calculate airspeed with GPS you would need PRECISE wind data. And you don`t have it.
In small airplanes I was told if you experience pitot tube failure you need to fly the aircraft with the seat of your pants and use the power setting to guess the airspeed, but obviously you need to know the aircraft you`re flying pretty well, and be lucky ;)

[NZScion]

How about a venturi system? The surfaces around the throat could be heated to ensure ice does not build up, and the system built large enough to ensure that water and other debris do not cause difficulties. I know some (older?) light aircraft use a venturi to drive the suction driven instruments, surely the pressure in the venturi could be measured (and a correction applied) to give dynamic pressure - or would this be subject to the same issues as a pitot tube?

[vodmor]

Venturi system was used in old aircraft. It is susceptible to many problems. Of course icing is the issue, just the same as carburetor icing. Then comes the accuracy. It is dependent on geometry of the whole system more than pitot tubes. You have to keep the surface smooth or large error will build up.

[Storminnorm]

Or even a calibrated plate acting against a spring of known strength.

[muduckace]

I wonder what the QRH states for in case the pitots fail ?
In the cast of pitot failure all you have other than GPS for G/S ref. are usually a set of generic power settings for each phase of flight. This is an extremely rare case.

[muduckace]

Or even a calibrated plate acting against a spring of known strength.

Ice is still a factor in both weight and now friction.

[Old Smokey]

Sorry to take the wind out of the sails of the venturi enthusiasts, but at typical Jet Transport aircraft speeds, the venturi would "choke" with supersonic flow.

OutOfRunWay, the last flat plate airspeed indicator that I saw was on a Gipsy Moth (DH60), even it's successor, the Tiger Moth (DH82) had Pitot tubes.

The AoA indicator is the only suggestion that bears some merit.

Regards,

Old Smokey

[mvsb1863]

Given that pitot tubes apparently play some role in the Air France accident,
I was wondering if there are technical alternatives to pitot tubes

YYYEEESSSS of course there is an alternative:


BOEING AIRPLANES !!!!!!!!!!!!

[727gm]

How about just sticking with the good old reliable double(or triple) pitot system, but coupled with an airplane that is designed from the outset to be safely flyable (in weather) under pilot manual control, in the event of A/P failure.

[vodmor]

And how do you think Boeing airplanes know what their airspeed is?

[Obi Offiah]

I actually did some research (layman research) on air data systems a few months ago and discovered that laser based optical air data systems had been developed.

Here are a few interesting links:
Optical air data systems (http://www.ophir.com/oads.html)
Michigan Aerospace Corporation - Molecular Optical Air Data System (MOADS) - FAQ (http://www.michiganaerospace.com/business_units/oads/moads_faq.shtml)
Michigan Aerospace Corporation - Molecular Optical Air Data System (MOADS) (http://www.michiganaerospace.com/business_units/oads/moads.shtml)
http://www.michiganaerospace.com/pdfs/airborne_lidar.pdf

I hope this helps
Obi

[Dani]

laser laser laser laser laser laser

funny...

Is it because Laser is a trademark?

[jackharr]

Way back in the late 1960s when I was instructing on the Varsity, one exercise practiced by instructors was to make an approach to land without an ASI (simulating pitot icing or some other failure). You would be under the “hood” while the safety pilot had full visual and full instruments. You flew by attitude, power, etc, and if all was in the grove by around 200 feet, you were allowed to look up and land.

Frankly, it wasn’t difficult given adequate practice, but a very useful exercise nonetheless. The Varsity was a primitive design (direct development from the WW2 Wellington) so failures were not unknown. However, only once did I have to carry out a “no ASI” landing for real.

I guess such training exercises are totally alien to the modern generation of pilots.

Jack

[Obi Offiah]

LOL. Dani I don't no whats happening, but I can't seem to type laser with an 'a' on this board for some reason.

[khorton]

Airbus actually developed a very expensive and heavy laser airspeed measuring system for use on their flight test aircraft. Initial flight testing is a special case in airspeed measurement, as you don't yet know what the errors are in the aircraft's pitot and static pressure systems. The determination of the errors in the pitot and static pressure systems is one of the test phases that must be done early in the flight test program.

The system that Airbus developed is a very large unit mounted next to a cabin window. It shoots three orthogonal laser beams out the cabin window. Some of the laser light bounces off dust particles, and is reflected back to the device. It is able to measure the aircraft velocity on each of the three orthogonal axes, and thus determine the true airspeed, angle of attack and angle of sideslip. They combine the TAS with static pressure from the static ports and outside air temperature and then calculate calibrated airspeed.

I don't know if they are still using this system, but it was used back in the days of the A340 flight testing. It was found to be superior to the usual trailing cone + pitot pressure system in some parts of the flight envelope.

It would theoretically be possible to develop a lighter, smaller version of this system, but it would be fairly expensive.

[LH2]

For those wondering about why laser gets changed to l@ser, the bulletin board software behind this site has the capability to replace certain terms, which the site's administrators have taken advantage of.

This would have probably been done to stop people Googling for laser pointers finding this site.

[w1pf]

Icing is a tough taskmaster.

In a prior life I worked beside a group that did way-excellent ultrasonic anemometers .. but they'd be useless iced over.. yes, they can be heated, but so can pitot tubes..

I have not spent a lot of time in the technology of laser anemometers, but I can see how things might get challenging with particles of varying aerodynamic parameters flying every which way in an MCS..

one needs to be careful when looking for technological silver bullets.. PCBs, anyone?

..tom

[Mad (Flt) Scientist]

The AoA indicator is the only suggestion that bears some merit.

Of course those ice up too, if things aren't going well. And it's hard to provide accurate airspeed to AOA relationships at high speed, because the change in AOA for even moderate speed changes is small, and you start getting into instrument accuracy ... it's hard to get much under 0.5 deg total AOA system accuracy, but 0.5 deg out at Vmo-type speeds is a LOT of speed shift. AOA isn't bad at low speeds, but its pretty useless at cruise speeds. Especially if you need accurate data for other systems, like rudder limiters and the like.

AOA alone at low speed has its own problems too - with just AOA it's harder to know how oversped one might be - so landing distances might become more problematic. Also, failure cases where we today routinely apply speed increments become more interesting when we have to think in AOA increments. Not saying thats impossible, but its certainly different.

[JABBARA]

MS. Not saying thats impossible, but its certainly different.


Airbus BUSS (Buck Up Speed System) simply relies on AOA.

As I mentioned before, we used o fly final App with AOA on T38.

This system has been in use in Military Airplanes for many years.

[Mad (Flt) Scientist]

It's one thing for a back-up to rely on AOA. You've already got system degradation and you're now in the "continued Safe Flight and Landing" element of certification, where the assumption is that we're simply trying to recover the a/c to any safe base. It's a whole other thing to plan flight on that basis. How are you going to respect MMO/VMO based on AOA? It's almost impossible, because depending on weight, VMO will be a different value.

Again, your T-38 LANDINGS were on AOA. How did you do a descent from cruise alt? I'm willing to guess, not by reference to AOA.

[JABBARA]

MS,

Airbus BUSS, as it named it is a back up system, only displays in case of triple failure of ADRs; and better than nothing.

I respect definitely your posts in this forum, however I disagree with the comment "Vmo/Mmo is dependent on weight." No it is not. it is a fixed speed regardless of weight. Below crossover altitude where dynamic pressure is dominating it is given as KEAS; above crossover altitude where shock waves are dominating it is given as Mach No.

It was old days I do not remember all details about T 38, but another thing I remmeber, if Flaps are Up, the green light was on at L/D max. We never used for descent but in the worst case L/D max speed can be used as descent speed safely, takes too much time though.

I agree with you, for the accuracy of AOA. In one of the bulletins of A 320, it mentions 0.1 degree AOA error causes around 3t weight calculation error
(some systems are fed with calculated weight depending on AOA, CG, THS and N1)

[Mad (Flt) Scientist]

however I disagree with the comment "Vmo/Mmo is dependent on weight." No it is not. it is a fixed speed regardless of weight. Below crossover altitude where dynamic pressure is dominating it is given as KEAS; above crossover altitude where shock waves are dominating it is given as Mach No.

Afraid you've misunderstood me.

My point is that VMO and MMO are, as you say fixed absolute speeds.

But if I'm flying on AOA I don't know my speed, just my AOA.

Lets say I'm actually right at VMO (or MMO). If I'm heavy, my AOA may be +1 deg. If very light, maybe -1 deg? So, what is my AOA limit for VMO/MMO? It ends up being a function of weight - alphaMO=f(weight). I dont think anyone would appreciate having to know their weight and look up a chart to know if they had a speed exceedance.

Basically, its very hard to respect a defined speed limit in Knots or Mach if all you have is AOA.

[JABBARA]

MS,

Apology,

Now I understand what you mean.

In that sense, I have no idea to determine Vmo/Mmo by AOA.

Meanwhile, from my previous observations for A 330-200: F3 landing, at Vapp AOA is around 7.5 degree regardless of weight. This can be read from one maintenance page of FMGEC.

I was keeping this in memory for the worst case scenario, even before the BUSS is invented:E

[greek-freak]

Many thanks for your answers guys :ok:

What about using some piezzoelectric device which was mentioned in the very beginning
of this thread? Would that make sense or be applicable.

[falconer1]

personally think, that pitots as they are , are good enough, or let's better say, should be good enough..

it worries me a bit that we do find in today's highly sophisticated airplanes pitot / statics sensors, maybe also TAT probes that may not conform to specs..

what I do not find ideal is that such a vital system component, considering that a lot of the airplane's flight control system and pilot interface depends on correct pitot / static / tat probe data, that, first, the most modern designs seem at times to be defect by design ( i mean that technology has been out there for a long time, what happened in the engineering departments of the suppliers and OEMs??,) and second, it may not be ideal that OEMs use more than one supplier and obviously design for such an important subsystem..

seems on the A330 for instance, and I am not fluent on that aircraft, so please correct me if I'm wrong, there are two suppliers of those probes. Only one of the suppliers probes obviously may have a problem at times, that's why upgrades are available,...

So while its fine if an airline can chose powerplants, if I were an aircraft OEM, I would limit myself to one supplier and one model of such probes.

Why?, simply stated, I would like my statistical sample rate as big as possible, to find out rather sooner than later if there may be a glitch..

I mean considering how far reaching the consequences are to my flight control system, if that basic probe goes south ( by design..ice, humidity whatever, you name it)..

and, OK, in hindsight, it surprises me a bit, that the upgrades had not been implemented by a "mandatory" SB, reviewing the history of actual and possible malfunctions..and considering what it does to the flight control system, and what difficulties a crew may face by being forced to fly a plane in alternate law in the flight levels....( probably already highly demanding in smooth air, but may be near to impossible in weather conditiions which actually may "fail" those probes..)

So from a total system integrity point of view, that may not be OK..

any opinions on that??

[Dont Hang Up]

It shoots three orthogonal laser beams out the cabin window. Some of the laser light bounces off dust particles, and is reflected back to the device. It is able to measure the aircraft velocity on each of the three orthogonal axes, and thus determine the true airspeed, angle of attack and angle of sideslip. They combine the TAS with static pressure from the static ports and outside air temperature and then calculate calibrated airspeed.


Laser anemometers can become unusable in very clean air (such as high at high altitude) due to the lack of particulate matter to scatter off. Having said that, water droplets in clouds would do nicely. And the fact that such a device can operate from behind glass could remove the problems of icing faced by a pitot or ultrasonic (or direct pressure) solution.

The appeal of the time proven pitot static method is that it is a direct physical measurement of the very dynamic forces that are keeping the aircraft flying. If the only real problem is icing, then that suggests simple redesign rather than technology replacement.

[ampclamp]

piezo elec can measure pressure.but it still must take a reference pressure from outside the aircraft in the immediate flt environs.

There is often a yawning gap between ground speed and airspeed.

The airbus (and boeing) air data modules do something just like that.
A pitot head externally and a short length of tube within the fuselage transfer a dynamic air pressure to a small electronic box (the ADM or similar) that converts the info to an elec signal for processing at the adiru or similar for use and distribution to a variety of other computers and indicators.

[Obi Offiah]

Laser anemometers can become unusable in very clean air (such as high at high altitude) due to the lack of particulate matter to scatter off. Having said that, water droplets in clouds would do nicely. And the fact that such a device can operate from behind glass could remove the problems of icing faced by a pitot or ultrasonic (or direct pressure) solution.

Hi Dont hang Up

There are laser devices that can provide air data in clean air, without the presence of particulate matter in the atmosphere. I think these are perhaps cutting edge technologies?
In terms of costs, I'd be very surprised if the initial installations prove to be cheaper that the current systems but over the life of an airframe it could be?

personally think, that pitots as they are , are good enough, or let's better say, should be good enough..
what I do not find ideal is that such a vital system component, considering that a lot of the airplane's flight control system and pilot interface depends on correct pitot / static / tat probe data, that, first, the most modern designs seem at times to be defect by design ( i mean that technology has been out there for a long time, what happened in the engineering departments of the suppliers and OEMs??,) and second, it may not be ideal that OEMs use more than one

I think this situation is a little like the mechanical vs FBW issue, they both have their advantages and disadvantages but for larger/expensive airframes FBW seems to be the way to go.

As you mentioned air data systems are critical components of an aircraft. The only thing that can be done is to make them as reliable as possible and install redundancies.

BTW how are posters able to use 'a' instead of '@' when typing laser?

Cheers
Obi

[Blacksheep]

please correct me if I'm wrong, there are two suppliers of those probes. Only one of the suppliers probes obviously may have a problem at times, that's why upgrades are available,...

So while its fine if an airline can chose powerplants, if I were an aircraft OEM, I would limit myself to one supplier and one model of such probes.
You're quite correct. There are multiple suppliers, however the aircraft manufacturer owns the design. Probe vendors are given the drawings and manufacture to the aircraft manufacturer's design specification.

To complicate matters further, there are in addition, approved repair stations that can "repair" a probe that has been eroded out of specification or where the heater has failed. They don't actually repair the probe at all, they rebuild it to the original drawings but at a lower price than the original probe manufacturer charges for a brand new replacement.

The result is not a PMA part, it comes back with the same Part Number and Serial Number as the probe that was sent for repair. A bit like restoring an old aeroplane using nothing but the original data plate.

[falconer1]

another question..

we may need a dedicated pitot / air data system specialist to answer that after some soul searching..

am i correct??

heated probes are part of the anti-ice protection of any transport aircraft..

having said that, "anti-icing", like in "heated" are preventive anticipating systems..

(or "have been")

we all know the old probes, that had been heated to the max from take-off to touch down..same heat as in "very hot"..may not have been the most elegant way, they did look "burnt" after awhile, but as long as they worked one could be pretty sure that whatever nature would throw at them, those things kept free of ice..

now, with the newer probes, as I understand, those are controlled by thermostats.. for all kinds of good reasons maybe..

however, as we know, problems with newer type probes always surfaced in a highly dynamic environment, i.e. sudden temp & humidity changes..

could it be that by the philosophy of the newer designs, that their heating circuits by nature simply do not respond fast enough to sudden temp & humidity changes, i.e. perverting the "anti - ice" philosophy to the point that an original preventive system becomes a reactive system..??

not sure whether I worded my question correctly..

[ampclamp]

Falconer,
in short is the 'kiss' principle a good idea?
many would say yes of course.others would disagree.In the end one can only judge by failure rates.
Whats more reliable, a s/w, a few relays, current sensors and a probe versus a clever box some wire s/w and a probe.
Done a bit of time on buses and I have changed a few PHC's.
I'd say more PHC's than failed probes on that type.
Most probes I've changed were the result of physical damage or altitude splits (in combo pitot /static probes as used on older 737 and 767 etc.)That is anecdotal evidence, you'd need to collate quite a history to have any scienctific conclusions drawn.

I'll leave any design critique to others that are more informed of the guts of the boxes in control. Been a while since I was on type and my notes are stashed somewhere.

[milsabords]

What about a design change: connect the closed end of the Pitot tube to a high pressure pump. Whenever erratic air speed indications are suspected, push a burst of fluid via this connection to dislodge ice, dirt or leftover scotch tape.

[Mad (Flt) Scientist]

Chance are that any attempt to blow a blockage off the probe would destroy the sensing device long before it managed to clear all but the smallest blockage. After all, it's the overpressure in the tube that's going to clear the obstruction, and that same excess pressure will be affecting the sensor.

[Michael Birbeck]

Surely we should be asking "how do we prevent the pitot, a device that has accurately helped measure airspeed for decades, from icing up in extreme conditions"?

The pitot design is relatively simple and cheap (in comparison to some proposed alternatives) and it works.

[JRBarrett]

What about a design change: connect the closed end of the Pitot tube to a high pressure pump. Whenever erratic air speed indications are suspected, push a burst of fluid via this connection to dislodge ice, dirt or leftover scotch tape.

I don't think that would be a workable solution. The amount of pressure required would be enormous in the case of a solid blockage - especially a plug of ice.

I once had to replace a pitot tube on a Bell Jet Ranger helicopter which was completely blocked. We suspected that the blockage was caused by an insect.

The maintenance manual specified that a maximum pressure of 20 psig could be used to attempt to purge a blockage (after disconnection all downstream sensors). We tried the procedure, without success.

It turned out that the tube had been blocked by a mud dauber wasp, which had built its nest within the tube over a 5-day period that the aircraft had been parked outside (in mid-summer) with no pitot tube cover installed.

Once the new tube was installed, we connected a regulated nitrogen tank to the old tube to determine just how much pressure it would actually take to clear the blockage.

We had to apply almost 200 psig of pressure before the wasp nest finally dislodged, and when it did, it came out like a bullet fired from a rifle.

The point being that trying to remove a solid blockage from air data probes with reverse air pressure would only be likely to cause major damage to the probes themselves, or to the associated plumbing - to say nothing of the extremely sensitive sensors to which the plumbing is attached.

JR Barrett

[Obi Offiah]

Some of the later probes such as the SmartProbe by Goodrich, don't require pneumatic lines or plumbing so there is less chance of a blockage.
http://www.goodrich.com/portal/goodrich/images/Goodrich%20Content/Business%20Content/Sensors%20and%20Integrated%20Systems/Products/Literature%20Listing/4083%20SmartProbe.pdf

Obi

[tonytech2]

Its a great idea and certainly eliminates the web of tubing we have on many aircraft, BUT, it is still a pitot-tube except the sensor portion is built right into it instead of being remote in an ADC. I still like it though. If you want to have fun, try trouble-shooting a leaking system with the plumbing running all over the place.

[JRBarrett]

Its a great idea and certainly eliminates the web of tubing we have on many aircraft, BUT, it is still a pitot-tube except the sensor portion is built right into it instead of being remote in an ADC. I still like it though. If you want to have fun, try trouble-shooting a leaking system with the plumbing running all over the place.

"Amen" to that. I recently had a G-IV which had developed a serious in-flight data miscompare between the L/H and R/H air data computers. Ground testing quickly revealed a major leak in the static line feeding the #2 ADC.

We eventually found the leak to be caused by a cracked plastic B nut at the point where the static line feeding the #2 ADC connected to the R/H static port.

Fixing the actual problem took less than 30 minutes. Getting access to the point of failure however was another matter entirely. Six man-hours to remove the overhead ceiling panels in the entranceway to gain access to the tee fitting where the L/H and R/H static port lines joined together, (in order to isolate which side was leaking) - then an additional 35 man-hours of labor time to disassemble and remove the forward lav and galley, and then put it all back together afterward.

Remoted air data sensors are definitely a major improvement, which eliminate
a weak link in the classic "plumbed" pitot/static system.

JR Barrett

[Gergely Varju]

I dont think anyone would appreciate having to know their weight and look up a chart to know if they had a speed exceedance.

While you might say, such calculations are hard to do for a pilot, and noone memorizes a chart, there is one advantage of computer technology: Your computers can remember the weight, can run "SELECT" queries to look up data quickly and efficiently.

Where pilots are limited by how much math can do, how much data they can analyze, it might be possible to use computers to do the math. Both in case of calculations like this, and when predicting probable / possible future.

[ij_]

What about a design change: connect the closed end of the Pitot tube to a high pressure pump. Whenever erratic air speed indications are suspected, push a burst of fluid via this connection to dislodge ice, dirt or leftover scotch tape.

This gives me another idea... How about reversing the airflow completely, ie., push compressed air out of the sensor chamber continuously and measure the difference to the expected in-chamber pressure.

[ampclamp]

but if is just measuring expected air pressure (there is no free flow as such) why not do away with the pitot altogether? Just guesstimate it without all the convolution.No I cant see how it would work.

The aircraft must know what the immediate static and dynamic air pressures acting upon the aircraft are at any time and it must be accurate.

All sorts of bits on an aircraft are ice protected from pitot/static probes,angle of attack sensors, engine cowls, leading edges, windows, drain masts, temp probes,propellor leading edges engine intakes etc.Some electrically some by hot bleed air.
In general they work well.They must
The important ones are monitored for failures.Operations are limited if flying in icing conditions.
If there is a practical, sensible, affordable alternative that works better than the current type I would hazard a guess and say the manufacturers would be using it.
Newer aircraft have done away with much of the tubing which makes maintenance easier (less scope for leaks) but the basic needs have not changed.

[Loose rivets]

I had a post deleated in R&N so rather halfheartedly posted this in JB. Despite everything, I think it's germane.

Skip the first couple of paras.


http://www.pprune.org/jet-blast/375943-air-france-jet-missing-18.html#post4993312


The computers can't interpret long periods of 'Rubbish in' so they gave 'Rubbish out'. quote was well covered in a post earlier.

[milsabords]

Could heated Pitot tubes be subject to vapor locks, i.e. hot bubbles of air/steam trapped inside and yielding erratic pressure measurements ?

[ampclamp]

No I dont believe this could happen.

[bob.arctor]

What about using some piezzoelectric device which was mentioned in the very beginning
of this thread? Would that make sense or be applicable.Piezo-resistive material could literally be sprayed on to the surface of the aircraft, facilitating multiple local pressure zones. The IAS could safely be calculated and the data could further be used for advanced anti-shake control and for computed compensation while flying in strong side-wind etc.

[ampclamp]

and when the skin ices up?
the pitot is simple its provides an accurate means to enable airspeed measurement.
I really cannot see what the fuss is about.
IF there is any link between with the air france prang and pitot problems it is likely a manufacturer issue not principle issue. In the vast majority of cases they work and work well.
Many critical parts of an aircraft have more failures than pitot probes.

[bob.arctor]

and when the skin ices up?The ice would still need to hold on to the piezo-resistive layer. Also there should be sufficiently many sensor zones so that it wouldn't matter if a few of them stopped working properly.


Edit - Removed sensor heating feature...:cool:

[Pugilistic Animus]

You could always ask the birds for the ''ps and 'q' because they know what they are doing!!!:}

seriously, keep it simple and safe


it seems we have left behind good simple thinking for all of the wonderful 'new ideas' in aviation like that bring'em down year after year for the same old reasons since 1903 :suspect:

and no I don't mean technological breakthrough or innovation we can't return venturi driven DG's on airbus or boeing airplanes, nor am I refereing to any accidents that remain unsolved

PA

[bob.arctor]

it seems we have left behind good simple thinking for all of the wonderful 'new ideas' in aviation like that bring'em down year after year for the same old reasons since 1903 http://static.pprune.org/images/smilies/cwm13.gif

Apple should start making airports and airplanes IMHO :)

[decurion]

To come back to the original question are there ttechnical alternatives for pitot tubes...The answer is yes, optical air flow meters have been developed and tested on aircraft with success. They accurately measured the airspeed of the aircraft. It could take some time until they become available on the commercial market.

[bob.arctor]

A few nanometers thick layer of piezo-resistive skin could perhaps also be seen as an alternative ;)

Processing the data would require some dense continuous computing but this method should give a very reliable and precise IAS reading.

[Graybeard]

Until the present Thales/Airbus situation, pitot tubes have been very reliable.

It just shows that manufacturing even the simplest device can be screwed up. What could possibly go wrong with making and maintaining piezo-electric skin?

GB

[Mr Optimistic]

a few years ago I believe work was being done on speed sensors which relied on 'vortex shedding ' (why telephone lines 'sing' in the wind). These don't rely on dynamic pressure. Don't know what, if anything, became of them.

[Mr Optimistic]

Vortex Shedding Flowmeters and Pitot Tubes: Vortex Meter and Pitot Tube Information Form/Questionnaire (http://www.omega.com/toc_asp/subsectionSC.asp?subsection=g&book=Green&all=1)

[bob.arctor]

What could possibly go wrong with making and maintaining piezo-electric skin?

1) 'Print' film with micro connectors onto the wing surface

2) Paint job

:)

The sensor surface shouldn't require any more maintenance than say a carbon fiber airframe.

[bob.arctor]

...As a secondary and auxiliary meter, the IAS could be derived from an internal 3D reading of the wing geometry together with weight, static pressure and accelerometer data.

[ampclamp]

Pls tell me you are having a lend of us :hmm:
or from what galaxy you lobbed in from?:eek:

[bob.arctor]

Ha ha :)

(I'm from Concerned Non-frequent Fliers Anonymous)

Advanced automation within aviation is the future and thus the need for a more sensible airframe.

[Mad (Flt) Scientist]

Are you seriously suggesting to try to back out the external wing loading from FEM type measurements within the structure? That would require a degree of accuracy that simply does not exist, and may not feasibly ever exist in any cost-effective sense.

[bob.arctor]

Are you seriously suggesting to try to back out the external wing loading from FEM type measurements within the structure? That would require a degree of accuracy that simply does not exist, and may not feasibly ever exist in any cost-effective sense.First, a piezo-resistive matrix on the wing surfaces would give data that eventually could be used to make flying in strong turbulence experienced as rather comfortable.

There would be a combination of inputs needed, with advanced realtime computing involved, surely. A carbon fiber airframe would have more flexing / movement that would make the measurements more 'visible' to the various sensors.

The main 3D physical motion of a wing could be measured from one strategic single point, affordably.

[Mad (Flt) Scientist]

You can't deduce airspeed from the data you are suggesting, though.

At least, not with any expectation of accuracy.

A wing flexes the same under load to support the same weight, independent of airspeed (to a first approximation). So the wing shape at 100knots and the wing shape at 200knots would be essentially identical for the same weight. Even then, backing out the weight would not be trivial, especially given the unknowns of the inertia distribution due to fuel and payload distribution. (The wing flex for a heavy a/c with low fule state will differ from that with a high fuel load but correspondingly lower payload)

So, let's assume nevertheless that somehow we back out a reliable estimate for the weight. What's left in the lift equation - 1/2 rho V squared, S and CL. OK, S we can define, but to get closer to V we need CL. For that we're going to need an accurate model of the aerodynamics - not at all trivial, especially for an in-service aircraft not given the TLC a test vehicle gets. (Indeed, this methiod has been used with a KNOWN V to estimate degradation of CL compared to standard, and it's not always small).

I think if you do the error budgets in that process you struggle to get any kind of accuracy for airspeed - especially given that we have a pretty good way of measuring it right now that's a great deal simpler.

The alternative - Well, you could attempt to deduce the airspeed by looking at very small differences in wing loading due to low order Mach effects - not something terribly accurate at low Mach numbers. But that's even more error prone I suspect.

[bob.arctor]

The model would be complex, and this 'synthesized IAS' could be thought of as a complementary reading, used when there was an eventual problem with the main meter.

So the wing shape at 100knots and the wing shape at 200knots would be essentially identical for the same weight.The resistance will here be quadrupled and this will bend the wing accordingly.

The accelerometer data is meant to make the reading / the modeling more accurate.

(I dropped out of school long before any of the above and we only had access to an ABC 80 btw :bored:

[toolboxstickers]

A friend, John Talbot, has designed a simple system where a valve shuts off the ASI from the pitot tube and engine bleed air is blown back through the pitot system to the pitot head, clearing any ice, moisture, insects, etc. If this is done on each pitot head one at a time by a sequence system of some kind the ASI reading will not be lost.
The patent for this is now in the public domain so a developer would not have to pay royalties, etc. but I`m sure John would appreciate an acknowledgement. We have tried sending the idea to most of the manufacturers but no luck so far.
Publication number:
GB2418739
Publication date:
2006-04-05


Details can be found on

http://v3.espacenet.com/textdes?DB=EPODOC&IDX=GB2418739&F=0&QPN=GB2418739 (http://v3.espacenet.com/textdes?DB=EPODOC&IDX=GB2418739&F=0&QPN=GB2418739)

[Mad (Flt) Scientist]

The resistance will here be quadrupled and this will bend the wing accordingly.

No. Although the dynamic pressure is 4 times greater at 200knots compared to 100 knots, the wing loading is actually the same. Because the wing is supporting the same aircraft weight, and THAT is what causes the bending.

[funfly]

xMillion dollars worth of electronics being fed information by a couple of holes in the side of an aircraft.
I know its not as simple as that but its a question of balance.
Takes me back to my days in industry when the time and motion engineer would measure operations to the nearest minute then produce costings to a thousanth of a penny! He didn't understand the problem but common sense......
It is actually quite serious because we now have a very good indication that something wrong with these 'holes' can totally disable a large aircraft and that 'something' can be a problem encountered regularly by all pilots i.e. ice.

[Graybeard]

Why is it only Airbus receiving attention for multiple pitot problems?

[augustusjeremy]

laser l@ser l@ser l@ser l@ser l@ser

funny...

Is it because l@ser is a trademark?Customs...... "The Traditional Monkey" (http://worldupdates.tripod.com/newupdates10/id84.htm) ;)

P.S.: I wonder if it could be applied to machines somehow...

[Mad (Flt) Scientist]

Why is it only Airbus receiving attention for multiple pitot problems?

In industry terms, it isn't "only Airbus".

EASA Safety Information Bulletin 2009-17, issued June 09, contains "recommendations" to operators regarding unreliable airspeed procedures and the applicability is:

All aeroplanes operating in commercial air transport.

Now obviously the more general media are concentrating on AB because that's the simple story. And to be honest, would it really help matters if the press started harping on "it could happen to Boeing too, or Douglas, or ..."? There's enough unnecessary anguish already.

[Hyperveloce]

Hi there
There are multiple projects of airborne laser anemometers, some are used by Airbus on a plane to study the turbulences in the wake of its airliners, so, provided it can be certified for commercial usage and proven reliable, maybe a different technology like laser velocimetry can be goog for the overall navigation chain of airliners.
Speaking about the anemometry, I have a question for those who knows in flight mechanics/control and navigation: if the autopilot has the slightest doubt about the reliability of the airspeed measurements or AoA, would it be possible for the autopilot to add to the normal law/orders, very small predefined orders on the control surfaces (the goal is not to induce an upset) to see how they translate into load factors (linear/rotational accelerations via the IRUs) and estimate in another way via a numerical model of the flight mechanics of the aircraft, these suspicious air parameters (airspeed, AoA,...) ?
Jeff

[Graybeard]

Quote:
Originally Posted by Graybeard http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/buttons/viewpost.gif (http://www.pprune.org/tech-log/376881-technical-alternatives-pitot-tubes-4.html#post5015639)
Why is it only Airbus receiving attention for multiple pitot problems?
-------

Mad Flt Scientist:
"In industry terms, it isn't "only Airbus"."

"EASA Safety Information Bulletin 2009-17, issued June 09, contains "recommendations" to operators regarding unreliable airspeed procedures and the applicability is:

Quote:
"All aeroplanes operating in commercial air transport."
-----
Now obviously the more general media are concentrating on AB because that's the simple story. And to be honest, would it really help matters if the press started harping on "it could happen to Boeing too, or Douglas, or ..."? There's enough unnecessary anguish already."
--------

The generic safety memo aside, I guess I should be more blunt: Is there a history of Boeing or McDouglas multiple pitot icing problems?

GB

[Turbavykas]

Cars use MAF sensor. It measures amount of the air getting into the engine and probably this is the speed of the air. They are cheap and uses different methods. My Toyota Supra 1988 has an optic one. I think there are dozens of other ways to measure air speed without mechanical devices.

[falconer1]

we need no alternatives to pitot tubes..

just well heated ones..

[ampclamp]

the voice of reason. Could not agree more.

Its not airbus its a vendor and airline issue.

[alex_ledin]

I've lurked on this forum for years, but for the first time I think that I might have a valuable idea to contribute. (I'm just a PPL, but as this conversation is more about engineering than pilotage...)

In regards to the idea posited by bob.arctor (http://www.pprune.org/members/303266-bob-arctor) and commented on by Mad (Flt) Scientist (http://www.pprune.org/members/58150-mad-flt-scientist) - if I understand it correctly - to take a model-based approach to determine airspeed, I have this to suggest. Instead of building a complete flight model, or using a new sensor technology (eg. covering the skin of the aircraft with piezoelectric devices), why not just create an database with prior good data?

Presuming that you've had at least a few minutes of reliable indication off of a pitot source, you now have some reasonable data on aircraft performance. Put another way, on any given flight, you know what airspeeds have resulted with recent combinations of AoA, power, vertical speed, and temperature. That should be sufficient to backdrive at least an approximate aircraft mass. If you wanted to get fancy you could incorporate flight control positions as well, but I'm primarily thinking about level, unaccelerated flight.

If airspeed data then become unreliable due to any factor - sensor failure, icing, impact damage, whatever - you should already have a reasonably close estimation of present aircraft mass (perhaps supplemented with data from a fuel totalizer) and, if I'm not mistaken, should then be able to calculate airspeed given present AoA, power, vertical speed, and temperature. This system should also make it possible to resolve disagreement between multiple pitot / static systems.

If each airframe had a onboard database of prior combinations of the same types of data, then it should be possible to deal with the failure of one sensor, whether it was an AoA vane, pitot tube / static port, thermometer, or whatnot. This strikes me as something that would be quite valuable for pilots, especially because it would eliminate a lot of guesswork as to what sensor or system was providing bad data. It such a system was incorporated into flight logic, it might have prevented recent upsets due to sensor or software failures. (And no, I'm not just referring to AF447, I'm also thinking of the Malaysia Airlines 777 upset over Perth in 2005.)

Obviously, for a system such as I've described to be useful, a single sensor failure should not make two variables unknown. For example, you'd want vertical speed to come from a gyroscopic source, not from the static system, since static port blockage could simultaneously render both airspeed and vertical speed unreliable.

Also, it's worth noting that I'm not sure how good the temporal resolution of this sort of system would be, but I'm reasonably certain that it would be good enough to avoid overspeed / stall during cruise and to maintain Va on approach. If its response were made fast enough it might also be able to take care of cruise gust load alleviation.

Perhaps someone's already implemented such as system? Thoughts and comments would be appreciated.

Cheers,
Alex

[Hyperveloce]

we need no alternatives to pitot tubes..

just well heated ones..

Ok, but it's been nearly two decades that the EASA has been issuing airworthiness directives to replace Pitots by Pitots: the problem is still here and seemingly worsening, more and more frequent. If it was just about augmenting the efficiency of the heaters of the Pitot probes, why hasn't it been done in two decades ? When the responsible of the EASA is being asked why the last Pitot replacement was not mandatory, he suggests that there is no point to replace Pitots by other Pitots which will not offer any improvement over the previous ones. And even after Air Caraibe or the AF447, the Pitot replacement is not mandatory (even given the suspected consequences of a loss of the flight assistances and alternate law 1 or 2). About responsabilities: when air transport companies ask their aircraft manufacturers about more resilient Pitot probes, the manufacturer answers that their probes are up to the specifications and even beyond. They don't improve their products in that direction since they are already compliant to the spec. : isn't it the responsability of the regulatory bodies (EASA, BEA,...) ?
When an inconsistancy is detected on air references (a doubt arises), what about testing the authority of control surfaces as previously suggested ? It does not require any new instrument (or the instrument is the entire airframe)
Jeff

[falconer1]

first, for better or for worse, EASA did not exist two decades ago..it is a fairly recent "invention"..but that's besides the point..

1) The way I understood it, and worldwide the media has not reported correctly on that, it was in fact operators like Air France and others who confronted the manufacturers and demanded a "better" probe for the A330 / A340 family..

the probe upgrade for the A320 was not originally done for the same reasons as was deemed necessary by the operators for the A330/340 family..

so obviously the manufacturers seem to have told the operators that they don't believe the A320 probe upgrade would help with the A330 situations..

(that's why it obviously was not classified mandatory, but sort of optional, like in "nice to have" but not that necessary..)

at least that's the way I understand it from our industry media reports..

2) specs......well you can design a no nonsense old fashioned probe that is glowing like a frying pan, and it would fulfill those specs probably to the point..only problem, as a manufacturer, what do you want to charge for that?? it does not cost that much and the OEMs would not pay that much..

then again, if you design a super - duper space shuttle type probe, that has all kinds of technology in there, it may not be a common sense product, but it would still be up to specs... and guess what, for what that probe is sold for and for what the OEMs are ready to pay for it, you probably could buy a real nice 4 seater private airplane...we are talking big bucks here..

a product with "value added".. so to say...

3) regs.. well I am of the old fashioned variety in that I DO NOT believe that more regs and more mandatory stuff actually will improve a product, nor will it ultimately improve safety..

so maybe I'm wrong, but once you start to make some wild high tech thing out of a pretty basic pitot tube, well, it WILL fail more often, simply because more failure modes are designed into it..

look at an old 737 probe, or from whatever 60's or 70's era plane and look at the new stufff

times are a 'changin..

the only thing that still is fairly predictable and has not changed for a long while, guess a couple of million years by now, ( contrary to what all the climate change folks want you to believe ) IS the weather..

so maybe we should revert back to old "cheap" probes for the same old "expensive" weather..

[Hyperveloce]

first, for better or for worse, EASA did not exist two decades ago..it is a fairly recent "invention"..but that's besides the point....

True, but the BEA and other regulatory bodies did exist:
_______________________
AIRWORTHINESS DIRECTIVE
released by DIRECTION GENERALE DE L’AVIATION CIVILE

Translation of ‘Consigne de Navigabilité’ ref. : 91-227-021(B) R1
In case of any difficulty, reference should be made to the French original issue.

AIRBUS INDUSTRIE
A320 aircraft
Pitot tubes

The present Airworthiness Directive applies to AIRBUS INDUSTRIE A320 aircraft, MSN 002 to 122, 124 to 179, 183 to 194, 196 to 228, 230 to 245 and 247 to 255 included without AIRBUS INDUSTRIE Service Bulletin A320-34-1024 R3.
In order to prevent wrong speed indication - one or several speed indications lower than the real aircraft speed - due to water accumulation in flexible hoses betweeen pitot tubes and air data modules (ADM), the following measure is rendered mandatory on the effective date of this Airworthiness Directive :

accomplish AIRBUS INDUSTRIE Service Bulletin A320-34-1024 R3 at first opportunity or before
January 31, 1992 at the latest.
Ref. : AIRBUS INDUSTRIE Service Bulletin A320-34-1024 R3
EFFECTIVE DATE : JANUARY, 03 1992
_________________
And I don't have the means to make a more comprehensive search in the BEA archives.

the only thing that still is fairly predictable and has not changed for a long while, guess a couple of million years by now, ( contrary to what all the climate change folks want you to believe ) IS the weather..

so maybe we should revert back to old "cheap" probes for the same old "expensive" weather..

Well, I don't want to debate about the climate change and about the GIEC, but a pilot (now being a specialist of aeronautics consulted by most of the French TV) mentionned the effect of climate change at high altitudes to try to explain why the frequency of Pitot problems seem to increase. I am not a met specialist, but Pitot problems have been here for long and it is not only about Airbus.

But maybe prevention/maintenance can also help: between two flights in the airports, are there systematic inspections of the pitot drain hole and of the static pressure lines ? (drainage of water/humidity/moisture in the pressure lines)
Jeff

[bob.arctor]

* Sorry for going on about this :bored: *

Here follows some more on the 'synthesized IAS'. The software could work much like in face recognition, assuming that every [relative speed level vs altitude] make its own distinct pattern.

The noise could possibly be attenuated by incorporating broad audio spectrum analysis in addition to the accelerometer and piezo-electric readings from points within the main carrying structure.

About cost, the computing could possibly be done on an affordable modern server, and the software could be open sourced.

Mac OS X and Linux are solid operative systems and not prone to crash / output BSOD's.

The system structure could be made to accept multiple failures and still output reasonable data.

THAT is what causes the bending.I meant the full motion of the wing, not just the vertical.

[falconer1]

sorry, but you are proving my point, maybe without wanting to..

the story about the A-320 probes seems to have been about water drainage..

(stuff that could affect the pitots when flying in heavy rain for instance in an approach situation..)

The problems that Air France and Air Caraibes for instance have had with the A-330 /340 probes seems to have been during "dynamic convective" situations at high altitude...and that does not seem to be related to the water accumulation as such in the probes but seems more related to sudden freezing of the probes by super cooled droplets..so seems to be related to the heating elements..

That's the reason, according to what I have heard, that Airbus themselves had been skeptical whether the updated A-320 probe would solve the A330/340 issues..

and in regards to any reports that may correlate the high altitude freezing issues with any "new" climate we may experience..

well, I would be very careful with these theories..

the simple reason may well be that we have a lot more long range flights today which cross well known convective zones on a regular basis, and therefore we may see a significant increase of such incidents / accidents..

and like some others here in the radar threads maybe rightfully suspect, that we may not be as conservative in regards to heavy weather avoidance as in earlier times, maybe because some new high tech stuff in the cockpits, while being an enormous improvement in many regards, on the other hand may lead to a false sense of safety at times..

and may have changed in ever so subtle ways our threat perceptions..

[bob.arctor]

... as this is the Pitot Tubes Alternatives thread ;)

One very good place to put piezo-electric sensors for the 'IAS Synthesizer' would be at the engine mounts.

This [synthesized] reading could be thought of foremost as complementary to the conventional old-school and normally reliable pitot tube-based meter.

[falconer1]

One very good place to put piezo-electric sensors for the 'IAS Synthesizer' would be at the engine mounts.

This [synthesized] reading could be thought of foremost as complementary to the conventional old-school and normally reliable pitot tube-based meter.

grand idea, how about if we'd also couple it to the N1 indicator or EPR then subtract the ITT and put it into relation to the pressure altitude corrected by the "freezing point" of supercooled water droplets, if present, and divide it by the mean aerodynamic cord of fan blades and finally subtract the thickness of the windshields..and then

the EAS could determined with a poll in the cockpit taking into the equation the tilt of the engine mounts and the age difference between PF and PNF..

man, how about we stay with straight ( don't even mind if they're bent) PITOT TUBES..nice hole up front and a short circuit built into them that draws MEGAAMPS for a real nice hot temp at the hole..??!!

just my two cents..;)

[bob.arctor]

@alex_ledin

What you have described, calculating with tedencies could certainly purposefully be incorporated in the IAS Synthesizer :ok:

@falconer1

The concept of an IAS Synthesizer apparently sounds far fetched to you.

I'm quite sure that such a device can be built, although maybe not exactly the way I have described it (The story about the car Homer Simpson designed for one of the Big 3 comes to mind). Some uni could possibly make it work.

About the new weather, flying trough strong turbulence in an airplane today can well be likened with traveling in a speeding bus on a very neglected dirt road, with no suspension and all tires blown.

If the cockpit, cabin and baggage compartment together were made floating in relation to the airframe (as with electronic F1 car suspension technology), the general safety and comfort could be way much better.

[bob.arctor]

THAT is what causes the bending. About the vertical motion / wing oscillation, analyzing vertical accelerometer data over the wings alone should give an IAS reading as air speed and air density are two main parameters affecting the wave damping.

Surely the analysis would be complex in turbulence conditions but then there'd be all the other data.

[Graybeard]

Goes to show there are a lot of complicated ways to measure airspeed.

GB

[Mike-Bracknell]

Just my 2p:

Alex_ledin - Like the idea of a database-driven approach, but it's maybe susceptible to other inputs giving false readings and exacerbating the feeling of a loss of control of your senses (no pun intended).

bob.arctor - Linux and MacOS don't crash?? Don't make me laugh!

"Laser" anemometers - good idea for a backup that works in a totally different way - shouldn't be too expensive to mass-produce either? Also, if you have a problem with finding particles that far up, create your own! Just stick a smoke generator a few feet in front if necessary.

And now on to my off-the-wall idea for measurement:

Sensors in each jet engine in the compressor stage, measuring the amount of air resistance occurring at the fan blades.

Not sure how sensitive it might be (or rather how insensitive the jet engine is for a given state of fuel entry), but it's one place to try?

[VnV2178B]

Mike,
most engines have their own pressure and temperature sensors in the intakes already - but I guess they are pitot/statics too!.

The A380 has all the engines reporting their parameters to the central system which then sends information back to the other engines as well as the airframe values.

More use might be made of all this data, but certification of any computer system on an aircraft is not like releasing it to normal users. It takes much time and effort to be sure that it is as safe as possible (ALARP) and will not interfere with the safe conduct of the flight.

Mac-OS and Linux would struggle to conform, specialist OSs are needed for safety.

VnV

[alex_ledin]

Mike-Bracknell: I agree that you'd have to do input checking, but even this would probably have positive safety consequences. For example, if the pitot gets plugged and your airspeed goes from M.80 to M.27 in less than a second, you know that something's gone wrong, and it's overwhelmingly likely to be related to the pitot / static system if there's little or no recorded vertical acceleration. (Again, this presupposes that you're getting VS from a gyro or accelerometer and not your static source!) I don't know quite what you mean about loss of control of the senses: are you concerned that pilots would lose the ability to determine which systems had failed?

bob.arctor: A synthetic IAS / TAS system would require a purpose-built real-time OS that was far more reliable and rigorously tested than a general purpose operating system like Linux or most versions of BSD, including the Mac OS. Thankfully, such RTOSes already exist. Additionally, if a synthetic IAS system were to be implemented, it would have to function with the limited computational resources that are available on an aircraft. That's why I suggest that using a database with known-good data would be a better approach than some sort of real-time modeling. Any system that requires heavy analysis (say, finite element analysis) is almost certainly inappropriate for in-flight use, at least right now. I can imagine that in ten or twenty years we'll have the computational horsepower to alter airfoils in-flight to achieve whatever results are desired. :)

Given recent rumor and information about similar problems with A330s at TAM, NWA, Qantas, and Air Caraibes, it's tempting to focus on pitot/static system-specific problems. I would suggest that the bigger issue is how to keep a computer-controlled aircraft flying safely with persistent bad data from all of the sensors of a single type. Since we cannot reasonably build aircraft with three different designs of pitot tube (for example) from three different vendors, we have to prevent an unanticipated design flaw in that one pitot tube design from delivering fatally bad data. There’s no reason for that bad data to cause a loss of control, never mind a bunch of bent metal and lost lives. This goes for all sensor subsystems: accelerometers, AoA vanes, engine performance monitors (FADECs), etc., and not just pitot / static systems.

Cheers,
Alex

[Mike-Bracknell]

I don't know quite what you mean about loss of control of the senses: are you concerned that pilots would lose the ability to determine which systems had failed?

Got it in one...well almost - I was also including the computers, as with more than one failed component you would deviate from the known failure for one axis of the plotted database, thus leaving your computers scrabbling for knowledge of which system had failed and where exactly it was supposed to be in this database.

Trend analysis would have to be a feature of the database I think, and I also think you do computational power a disservice as nowadays there's a hell of a lot of number-crunching can be done with a lowly PC processor. Certainly enough to give a *rough* estimate of flight parameters i'd assume.

I do think there's mileage in trying to apply modern techniques as backups to firstly augment (and then maybe replace) the systems that have done well up to now on aircraft though. Having watched Wimbledon tonight, and seen how well Hawkeye does nowadays with line call predictions purely from camera data, there's an example of something that wouldn't have been possible 20 years ago, but works well enough now that the line judges are almost redundant.

[bob.arctor]

Goes to show there are a lot of complicated ways to measure airspeed. Using a couple of vertical accelerometer sensors connected over WIFI to an iPod running dedicated low frequency audio software could be sufficient to give a decent reading (calculated with the static pressure). The reading should be more detailed by also using horizontal accelerometer data from the wing tips.

A good high school project perhaps.

specialist OSs are needed for safety.And that would often times mean [hands on] by some version of Windows or Vista (referring to various EFIS devices here), right ;)

[VnV2178B]

Bob.a,

by specialist I mean something like Integrity from Greenhills or LynxOs (OK the latter is sort of based on Unix/Linux) but certainly a certified to 178b system (hence my moniker)

VnV

[bob.arctor]

...alright, foremost I meant an IAS synthesizer well could be using Apple hardware :)

[VnV2178B]

Ok, but at the core of that hardware these days is still some Intel chip.

VnV

[MPGiles]

Very intersting stuff about Airspeed. Two questions How is speed calculted on the stealth aircraft which I presume don't have poles sticking into the breeze and what exactly are the failures in the Airspeed sytems in the AirBi?
The way it is phrased leads to me wondering if it was not actually icing but some other unknown failure:ok:

[Mad (Flt) Scientist]

Stealth AC in general have flush mounted sensors, AFAIK.

Those on the B-2, handled incorrectly with regard to the anti-ice system, caused the loss of the B-2 in Guam (?) IIRC so those systems are by no means foolproof either.,

[Towhee]

January 9, 2004
The End of the Tube?

UK engineers are developing a new generation of air-speed sensors that use the latest laser technology to improve on traditional methods. Jon Excell explains.

The pace of technological change - from passenger jets to helicopters - has been staggering over the past 100 years, yet there are still a few critical components that have remained unchanged.

One is the Pitot tube - a simple mechanical device at the heart of the speedometer on most modern aircraft - designed almost 300 years ago by French inventor Henri Pitot, to measure fluid velocity. The basic instrument consists of two coaxial tubes: an interior tube that is open to the flow, and an exterior tube open at 90 degrees to the flow. By measuring the difference between these two pressures the flow rate of the fluid can be calculated.


But the tube's lack of accuracy at low speeds and poor aerodynamic performance has led some in the aerospace industry to question its validity in the 21st century.

When mounted on an aircraft, the central passage in the tube points in the direction of travel. Meanwhile, a number of openings in the outside wall of the main tube lead to a second set of smaller passageways. These two sets of tubes are typically connected to either side of a pressure transducer attached to the base of the unit. The transducer measures the difference in atmospheric pressure in the two groups of tubes, and this pressure difference can be used to calculate the aircraft's speed.

While the tube is accurate at high speeds, its ability to resolve differences in pressure at low speeds is limited which compromises its performance.
An additional problem is that on fixed wing aircraft its profile significantly increases the amount of drag and therefore has an impact on fuel efficiency.

With these problems in mind, engineers at BAE systems Advanced Technology Centre have launched The Laser Air Speed Sensor Instrument programme (LASSI) a two and half year project aimed at developing air speed sensors that are both accurate at all speeds and won't contribute towards drag.

With the programme still in its infancy, LASSI project leader Leslie Laycock would not go into technical details. He did however reveal that the system will use a compact, short-pulse UV laser and a fibreoptic system. By firing the laser into the atmosphere, the nature of the light reflected from air molecules will change according to speed. This variation is measured by LASSI and used to calculate airspeed.

Laycock said that while engineers have been aware of the Pitot tube's drawbacks for some time, there hasn't really been a viable alternative. Engineers have looked at using lasers before, but they've been too bulky. Laycock claimed that it is largely due to cutting edge work in the UK on compact high-power lasers that laser-based systems are finally becoming viable.

Although LASSI has yet to yield any solid test data, Laycock said that initial estimations indicate that over the operating life of a long- haul plane, significant fuel savings could be achieved by replacing the Pitot tube with something that's flush to the airframe.

Clearly, the manufacture and installation of a system that requires compact lasers and fibreoptic cables will initially be more expensive than the Pitot tube. Laycock is confident however, that LASSI's ultimate benefits will far outweigh the initial cost. Laycock's team is also looking into the application of the technology in other areas. It could, for instance, be used to model airflow around buildings.

There's still a long way to go and at the end of the project Laycock hopes to be able to unveil a demonstrator that will be deployed either on an aircraft, or in a wind tunnel to both prove the principle and show just how small the system could be. If all goes smoothly, he said that the system could be appearing on planes in about five years.

from: The Engineer


also:



May 13, 2003
Can MEMS point the way ahead?

Honeywell says micro electromechanical systems could lead breakthrough in sensor and weapon guidance technology

by Graham Warwick

Honeywell is pursuing development of micro electromechanical systems (MEMS) as a "potentially disruptive" technology in aircraft avionics and weapon guidance. The company is already developing an attitude and heading reference system (AHARS) and standby display for commercial aviation applications using MEMS sensors.

MEMS combine micron-scale electrical and mechanical features on the surface of a silicon chip, and are batch-produced using integrated-circuit fabrication techniques. "MEMS is an enabling technology across Honeywell," says Eric Doremus, vice-president precision sensors and components. Applications range from biomedical sensors to aerospace devices including attitude and pressure sensors and inertial measurement units.


Doremus says MEMS offer significant reductions in cost, size, weight, volume and power over conventional sensors. The company is already producing air-data systems using MEMS precision pressure sensors. The next step is a flush-orifice air data system, now in development, which uses distributed pressure sensors to eliminate the pitot probe, he says. The first developmental MEMS-based inertial measurement units have been delivered to customers.

Honeywell is applying MEMS gyros to inertial systems small and robust enough to guide gun-launched projectiles. In the longer term, the technology promises to provide navigation-grade performance, allowing MEMS gyros and accelerometers to replace ring-laser and fibre-optic gyros in aircraft inertial systems. Doremus expects the Lockheed Martin F-35 Joint Strike Fighter's inertial navigator to be a MEMS-based device just 50cm3 (3in3) in size, compared with the F-16's 7,900cm3 laser-gyro unit.
Honeywell's MEMS-based AHARS is scheduled for introduction in 2004-5. GPS aiding will reduce errors, to provide an attitude accuracy of better than 0.1deg., says Doremus.

Back-up true-airspeed aiding will provide an attitude accuracy of 1-2deg.. The AHARS will be part of Honeywell's new flat-panel standby display, which will combine the unit with MEMS-based air data sensors and magnetometer. GPS integration and flight-control output will be optional features.

Honeywell has MEMS fabrication facilities in Redmond, Washington, and Plymouth, Minnesota. Both are capable of producing 150mm (6in)-diameter wafers, each containing 700 micro-scale gyros. The Plymouth site is being upgraded to handle 200mm wafers for the production of pressure sensors with 1.5 micron-sized features.

from: Flight International

[Obi Offiah]

Two questions How is speed calculted on the stealth aircraft which I presume don't have poles sticking into the breeze

Michigan Aerospace:
"Pitot Probe/Static Ports/Alpha Sensors are used on the F-16, F-18, and the F-117. Flush Port Sensors are used on the X-15, Space Shuttle (SEADS Experiment), B-2, and the A-12. A hybrid system (Pitot Probe/Alpha Sensor/Flush Port) is used on the F-22. The B-2 has quad flush port sensors for the quad flight control systems. Flush Port Pressure Sensors are symmetrically placed on the airframe. The pressure measured at each port varies as the airframe maneuvers. Pressure differences between symmetrical ports correspond to changes in angle of attack and sideslip. Pressure data is derived from wind tunnel and flight tests. The B-2 air data system is made up of two gust load alleviation static ports, differential beta static ports, and left and right alpha/static ports. Each port contains its own 1553 terminal, which is tied to the Flight Control Computer (FCC) along with the total temperature (probe measured at engine inlet) and nose gear position. The air data computation routines in the FCC then determine angle of attack, angle of sideslip, static pressure, pressure altitude, mach number, dynamic pressure, and true air speed."

One of the advantages of laser based/optical air data systems, as well as the so called 'smart probes' is that they are very accurate (even down to very low speeds) and don't suffer from the pneumatic lag inherent in contemporary systems. This enables the FCS to have access to the most accurate current data, and aids flight stability.

Because OADS are capable of detecting air molecules in advance of the airframe, this gives the FCS the ability to act partially as a predictor 'crystal ball' system and issue corrective control surface commands before the airframe has encountered a particular body of air. I think this can be viewed as a flight safety as well as a flight comfort benefit. Turbulence detection and wind profiling (http://www.ophir.com/turbulence_detection.html)

Obi

[Hyperveloce]

Hey
MEMS are related to the IRU part. They probably have a potential to cut prices. IRUs and GPS are very complementary to each other: IRUs have a drift tendancy (summing of errors on acceleration increments) while GPS will not drift but will have larger instantaneous errors. They can correct each others. With 3 GPS antennas, the GPS can also estimate the attitude angles, enabling a deeper hybridation with the IRUs. A complete hybridation (we call it sensors hybridation in French but in English, it might be termed as sensor data fusion ?) may be possible with all the air references, including new sensors like laser anemometry (which can remotly sense the 3D airspeed field, hence also mesure angles of incidence, slide, etc...) or a Lidar (new weather radar for improved detection of turbulences/CAT, with doppler/interferometry capabilities).

I may be dreaming, but this full hybridation of IRUs, Air references, Laser Velocimeter or Lidar, barometric/radioaltimetry altitude, and possibly other air reference sensors (cinetic pressure transducers on the airframe or in the compression stages of the engines as suggested by others), implemented via a large extended Kalman model (mainly describing the airframe & flight/fluid mechanics, the sensors and their measures... the underlying physics) would provide an optimal error detection/isolation scheme (nothing to do with constant thresholds in magnitude/duration to detect faults as it is done) and the capability to estimate some failed air parameters via all the others and through the real time monitoring the aerodynamical authority of the airframe (and its control surfaces, via microorders added to the normal orders). This real time aerodynamical authority estimation (through slight modification of the "posture" of the aircraft) coupled with the direct sensing of the air stream and the resulting linear/rotational accelerations provided by the IRUs (and possibly laser/lidar imagery of the air stream) would be close to the proprioceptive sense used in bird flight. This extended Kalman model would "know" that spikes of a few degrees magnitude and of a few milliseconds duration on the AoA or pitch are not a real thing in flight mechanics it has to react to (fault detection & exclusion), all the more if other paramaters remain unchanged (vertical acceleration). It would also know that a CAS drop approaching 200 kts in amplitude in a blink of an eye is not possible if the control surface authority and other parameters remain constant. It would exclude corrupted measures in an optimal manner, relying on measures consistant with each others from an aerodynamical point of view. Otto would be a lot smarter ? :8 This sensor hybridation scheme is used by some military flying vehicles.
Nothing to do with the air reference improvement but in difficult conditions, the reading of the instruments may be a problem according to some pilots: the military have head up displays, why not for the civilians too ?
Jeff

[timpara]

Re Hyperveloce's idea of using control surfaces as part of a backup aurspeed system: I'd been thinking along similar lines. But a simpler way to calculate airspeed might be to measure the pressure required (in say a hydraulic system) to deflect a control surface (for example, rudder) by a known amount. This backup calculation could be done continuously.

[Mad (Flt) Scientist]

Requires a very good understanding of the hinge moments at small deflections. Not a trivial thing to do, since it also varies with other conditions. Doing something similar (but with known AS in order to estimate control effectiveness for FBW purposes) has been done before.

[rubik101]

Doesn't anyone like my idea of a bendy piece of plastic for airspeed and a lead ball on a piece of string instead a gyro? (or a blob of depleted uranium on a carbon fibre thread if you want expensive)

[Hyperveloce]

the real time estimation of the control surfaces authority would at least require a complete aerodynamical model of the airframe for all the flight enveloppe and control surfaces configurations, and a structural description of its deformations under load factors. This complete aerodynamical/structural model is of course available to the aircraft manufacturer.
Jeff

[Mr Optimistic]

problem with complex hybrid systems is that it would be a bit of a task to elucidate teh myriad failure modes and fault combinations. Sometimes the burden of providing such 'truth' makes the enterprise doubtful

[thomas8]

Airbus designs a new function for A380 & A330, it is called "BUSS", does anyone know this function ?

[Mad (Flt) Scientist]

It's an Angle-of-Attack based display; it basically has a green (target) range of AOA that you are supposed to fly in, with the low and high ends being red "no go" areas representing overspeed (low AOA) or stall (high AOA).

Fairly simple in concept - though of course it requires the AOA sensors be operational, and depending on the nature of the airspeed problem that might be in doubt (ADC hardware failure would be a good case for using BUSS, but something affecting the outside environment, such as extreme icing (SLD) or hail (physical gamage to probes and sensors) could disable BUSS as well as conventional airspeed)

Still, it's a fairly good solution in that it uses existing installed equipment, and has some significant dissimilarities to enhance the chance of it being available.

[rgbrock1]

re: the somewhat snide comments about Mac OS X and Linux.
Yes, both operating systems may, on occasion, crash or even hang. (I'm a systems engineer by profession) But they certainly do so far, far less than
Windoze. And let's not even discuss the security implications. (Windoze being akin to the proverbial swiss cheese as far as security goes.) Nor should we discuss the inherent bugginess of any Windows release.

[chase888]

I will have to take my chances on being shot down, but I simply have not had the time to go back over so many posts.
Just wondering if any consideration has been made on how icicles form?
Water from the pitot drain would I suspect be only just above freezing, so meeting 800k/hour sub zero air on the outside would probably induce a further blockage in the form of a lump of ice externally.
Maybe the pitot needs an external heater also?

[Graybeard]

It might be instructive to compare pitot tubes and heaters certified on Airbus to pitot tubes and heaters on Boeings, Douglas, Lockheed, et al. Multiple pitot icing appears to be unique to Airbus.

GB

[Will Fraser]

Graybeard I know it's generally accepted that the pitots and heaters on Airbus produce Unreliable Airspeed data, can you tell me where that is demonstrated?? With the Ua/s incidents, (NOT accidents, AF is far from known), what is the data derived that implicates this equipment??

Will

[jepprogs]

When real time streaming of position and aircraft parameters becomes the norm over black box recording, there might be enough collective data to derive wind velocity maps. i.e. comparing GPS with working pitot static data in the majority of working planes travelling known routes. Apart from general meteo' data gathering uses, might that be useful for an aircraft with trouble to at least have a stab at estimating local wind from GPS as secondary / backup - or could there be a use for comparing the two sirspeed data sets if pitot dynamic (and possibly static) information is lost ?

[stilton]

All good stuff but the 787 already has 'alternative' input airspeed based on corrected AOA input.


Seems like the answer, you're not going to block an AOA vane.

[Mad (Flt) Scientist]

Yes, you can. In fact there a whole bunch of ADs out thee regarding exactly that (at least one of the airbus ADs has an active thread I think)

What you do gain is having a (slightly) different means of sensing airspeed, but both are ultimately sensors, in the airflow, somewhere on the nose. That means the are still common cause failures that need to be addressed. The goal is to design those to be sufficiently improbable that a procedural solution is acceptable . i.e. pitch/power tables.

[Mr Optimistic]

Isn't there a role for the INS here, admittedly not for sensing AoA ? OK won't help avoid a stall but the initial developing downwards motion must be sensed by the INS and integrated up to give the speed? Could be that today's clever data fusion technology mean that currently the output is potentially contaminated by bad ADR data but the inertial calculations over a short period would be good wouldn't they?

[stilton]

Must have missed that MadScientist.


Do you know of an incident where both AOA vanes stopped working ?

[Mad (Flt) Scientist]

XL Airways A320, 2 of 3 AOA sensors froze.

The values of the n° 1 and 2 local angles of attack recorded by the SSFDR remained fixed from FL 320 until the impact with the surface of the sea. It was not possible to determine the evolution of the value of the n° 3 local angle of attack in the course of the flight, which is not recorded. The initiation of the stall warning, however, shows that angle of attack sensor n° 3 was working at the end of the flight.