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Technical Alternatives for Pitot Tubes?

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Technical Alternatives for Pitot Tubes?

Old 11th Jun 2009, 08:49
  #41 (permalink)  
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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.
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Old 11th Jun 2009, 10:34
  #42 (permalink)  
 
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may I throw in

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..
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Old 11th Jun 2009, 12:12
  #43 (permalink)  
 
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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.
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Old 12th Jun 2009, 02:19
  #44 (permalink)  
 
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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.
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Old 12th Jun 2009, 02:21
  #45 (permalink)  
 
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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.
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Old 12th Jun 2009, 08:36
  #46 (permalink)  
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It it ain't broke don't fix it.

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.
 
Old 12th Jun 2009, 16:57
  #47 (permalink)  
 
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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
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Old 12th Jun 2009, 19:41
  #48 (permalink)  
 
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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/goodr...SmartProbe.pdf

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Old 12th Jun 2009, 20:52
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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.
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Old 12th Jun 2009, 23:45
  #50 (permalink)  
 
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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
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Old 13th Jun 2009, 00:05
  #51 (permalink)  
 
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Originally Posted by Mad (Flt) Scientist
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.
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Old 13th Jun 2009, 18:34
  #52 (permalink)  
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reversed airflow direction?

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.
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Old 14th Jun 2009, 02:35
  #53 (permalink)  
 
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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.

Last edited by ampclamp; 14th Jun 2009 at 10:25. Reason: poor English
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Old 14th Jun 2009, 06:43
  #54 (permalink)  
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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/3759...ml#post4993312


The
computers can't interpret long periods of 'Rubbish in' so they gave 'Rubbish out'.
quote was well covered in a post earlier.
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Old 14th Jun 2009, 07:15
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Could heated Pitot tubes be subject to vapor locks, i.e. hot bubbles of air/steam trapped inside and yielding erratic pressure measurements ?
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Old 14th Jun 2009, 10:21
  #56 (permalink)  
 
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vapour lock?

No I dont believe this could happen.
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Old 16th Jun 2009, 05:13
  #57 (permalink)  
 
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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.
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Old 16th Jun 2009, 11:31
  #58 (permalink)  
 
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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.
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Old 16th Jun 2009, 20:46
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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...
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Old 16th Jun 2009, 23:18
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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

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
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