The Use of Pitot Tubes
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Joined: Jan 2009
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From: Aus
The Use of Pitot Tubes
Just been watching a TV program with the old man about AF447. He being an engineer thinks he knows everything about everything said it was ridiculous pitot tubes are still used on modern passenger jets for the measurement of airspeed. A 20 minute long debate ensued.
He stated there are other ways airspeed can be measured which would be less prone to icing. his examples were laser, ram air turbine type meter.
I basically said - it's simple, accurate, relatively aerodynamic and not prone to failure (besides blockage). he wouldn't have a bar of it.
There's obviously reasons why Mr Boeing and Airbus still use them over more sophisticated devices. He might listen to reasoning of more experienced pilots over myself; a 500h cpl bushie.
Damien
He stated there are other ways airspeed can be measured which would be less prone to icing. his examples were laser, ram air turbine type meter.
I basically said - it's simple, accurate, relatively aerodynamic and not prone to failure (besides blockage). he wouldn't have a bar of it.
There's obviously reasons why Mr Boeing and Airbus still use them over more sophisticated devices. He might listen to reasoning of more experienced pilots over myself; a 500h cpl bushie.
Damien
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I kinda agree with the old guy. At first I thought why a pitot tube on a massive airliner and not a computer or sensor that can calculate it all. Same with the altitude measuring device, why not having sensors to see its EXACT altitude. How reliable are pitot tubes? Do they show EXACTLY what the EXACT datas are? Say for the speed does it ever get things very very slightly off? Like if the very EXACT speed was 25.34586735689 knots and it shows 25.1729643956738.
Just an example
Just an example
Joined: Jan 2011
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From: Seattle
Conservative engineering
I suspect that the continued use of pitot-static airspeed indication is due largely to the conservative nature of aircraft engineers. The operation and failure modes of these systems is well understood. So unless there is some major advance in reliability or performance, there's no justification for incurring the high certification costs to bring out something new.
Joined: Dec 2007
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From: Montreal
Pitot tubes incorporating static pressure measurements
I have spent 60 years plus in aviation as a pilot and only had one
pitot tube fail to give me airspeed (a wasp had entered). Attitude
reference (real or artificial horizon) takes over to control the aircraft.
Pitot tubes when heated, as most of them are, are extremely reliable
and I suspect that Thales (in the case of AF447) may have made changes
to accomodate computer sensor pick-up etc.thus complicating it with more
chances of something failing.
Normally you burn yourself badly when touching a heated pitot tube and icing it would be next to impossible.
Accuracy of airspeed? If you ever flew in realy turbulent conditions, you
would not question a decimal difference, leave alone plus or minus
10-15 kn as can happen in mountain flying and rotors.
So my conclusion is : don't re-design the wheel, unless you are building
a sled !
pitot tube fail to give me airspeed (a wasp had entered). Attitude
reference (real or artificial horizon) takes over to control the aircraft.
Pitot tubes when heated, as most of them are, are extremely reliable
and I suspect that Thales (in the case of AF447) may have made changes
to accomodate computer sensor pick-up etc.thus complicating it with more
chances of something failing.
Normally you burn yourself badly when touching a heated pitot tube and icing it would be next to impossible.
Accuracy of airspeed? If you ever flew in realy turbulent conditions, you
would not question a decimal difference, leave alone plus or minus
10-15 kn as can happen in mountain flying and rotors.
So my conclusion is : don't re-design the wheel, unless you are building
a sled !
Fleet Manager
Joined: Apr 2001
Aviation Qualifications: ATPL
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From: various places .....
Like if the very EXACT speed was 25.34586735689 knots and it shows 25.1729643956738
Most of us Industry PEs understand the problem precisely.
The above would be approximated near-exactly as being somewhere between 20kt and 30kt ... near enough for Guv'mint business.
The really pedantic folks probably might opt for "around 25kt".
Most of us Industry PEs understand the problem precisely.
The above would be approximated near-exactly as being somewhere between 20kt and 30kt ... near enough for Guv'mint business.
The really pedantic folks probably might opt for "around 25kt".
Joined: Nov 2010
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From: Sweden
I can recall changing 1 or maybe 2 pitot probes in 20 years due to internal pressure leakage which we discovered doing leak checks during scheduled maintenance. No tech log entry had been made prior to the aircraft coming in for it's check. Therefore the leak whilst big enough to fail our test, was either not noticeable to the crew or it wasn't producing a big enough split between the 2 ASI's for the crew to deem it out of limits.
I have changed many probes due to erosion or other damage, nicks, dents, (melted pitot covers!) etc. and 1 where I wasn't happy with the heater element.
In short they are a proven and dependable system, and the AF447 accident as tragic as it was, is not going to change that.
I have changed many probes due to erosion or other damage, nicks, dents, (melted pitot covers!) etc. and 1 where I wasn't happy with the heater element.
In short they are a proven and dependable system, and the AF447 accident as tragic as it was, is not going to change that.
Joined: Aug 2010
Posts: 144
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From: Australia
Yep, saw part of very same show myself last nigh until I turned it off in disgust, all a bit dramatised for me. You'll learn more on here than what that nonsense said. All the glorified stuff and a lot of focussing on the pitots but nothing (as much as I saw) on the PF,PNF, crew resting etc. Notwithstanding it may have been produced before the FDR material was released. Don't put too much weight on it, is all I am saying.
When we were spraying our a/c water pumps were driven by a small fan in the propwash. Time and time again we were asked by farmers, "What's th' little propeller fer?" It worked, pure and simple. Even more so for a pitot perhaps?
When we were spraying our a/c water pumps were driven by a small fan in the propwash. Time and time again we were asked by farmers, "What's th' little propeller fer?" It worked, pure and simple. Even more so for a pitot perhaps?
Joined: Mar 2011
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From: Canada
@Damien1989,
Tell (remind) your father that the pitot tube doesn't - and isn't there to - measure airspeed. It's there to measure the dynamic pressure, (1/2.rho.v^2) which governs the aerodynamic behaviour of the wing and tailplane.
We're all living in a fantasy land where we "invent" a thing called the "Calibrated Airspeed" as if it's a real quantity and then pretend that the ASI measures it, or something close to it (i.e. IAS).
It would be much more honest to calibrate the ASI in Pa (or lbs/sq.in.) and note the stalling "pressure" of the aircraft, the never exceed pressure, etc. - then note (with only passing interest as it's really not that important) that the true airspeed at which each of these pressures is achieved depends on altitude, temperature and humidity.
That's too complicated though so we calibrate it in terms of the airspeed to achieve a given pressure at sea level.
But it's a fiction to call it an "airspeed" indicator.
Tell (remind) your father that the pitot tube doesn't - and isn't there to - measure airspeed. It's there to measure the dynamic pressure, (1/2.rho.v^2) which governs the aerodynamic behaviour of the wing and tailplane.
We're all living in a fantasy land where we "invent" a thing called the "Calibrated Airspeed" as if it's a real quantity and then pretend that the ASI measures it, or something close to it (i.e. IAS).
It would be much more honest to calibrate the ASI in Pa (or lbs/sq.in.) and note the stalling "pressure" of the aircraft, the never exceed pressure, etc. - then note (with only passing interest as it's really not that important) that the true airspeed at which each of these pressures is achieved depends on altitude, temperature and humidity.
That's too complicated though so we calibrate it in terms of the airspeed to achieve a given pressure at sea level.
But it's a fiction to call it an "airspeed" indicator.
Joined: Jun 2004
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From: Australia
photofly,
Interesting post with respect to replacing CAS with Dynamic Pressure. You must have been referring to some vintage aircraft where attempt was made to use Dynamic Pressure to convert it to CAS (Even 1950's text books describe it as archaic). These olden days aircraft used the following formula for instrument calibration -
V = SQR (2q / Rho)
Where-
q = DYNAMIC Pressure
Rho = Air Density
V = True Air Speed
Nice sentimental stuff which actually produced DAS (Density Air Speed), now THAT's an "invented" and meaningless speed. For the low and slow Piper Cubs and Gipsy Moths it was a good enough substitute for TAS.
Unfortunately for the well-meaning folk who thought they were measuring Dynamic Pressure, ALL pitot tubes 'capture' Impact Pressure, even that on the Piper Cub. They were assuming that air was incompressible, but it is.
Now, if we want to get serious about airspeed measurement (as aircraft have moved on somewhat since the Piper Cub), we must consider IMPACT PRESSURE.
The calibration formula for this is -
Vc = SQR ((Y/( Y-1) X p0/qc X [(qc/p0 +1)^((Y-1)/ Y) - 1] X 2qc/Rho0)
Where-
Vc = Calibrated Air speed (ft/sec)
qc = IMPACT Pressure (lb/ft²) ........ What we’re talking about!
Rho0 = Sea Level Air Density (0.0023769 Slugs/ft³)
p0 = Sea Level Air Pressure (2116.2 lb/ft²)
Y = A constant for Air = 1.4 (Ratio of specific heat of air at constant pressure to specific heat of air at constant volume) (Y=Gamma)
Now this is good for Sea Level, for which the ASI is calibrated. At Altitude we need to replace p0 with ps (the static pressure at the particular altitude).
Now, I'm not trying to blind anyone with science, and it aint rocket science, but you wanted Dynamic Pressure, and there it is embedded in the formula! qc is impact pressure, and SQR(2qc/Rho0) is the compressibility factor (known as f), which if applied in reverse yields ...... Dynamic Pressure
So go and do the algebra and the maths to exract to extract your preferred Dynamic Pressure, and produce a table or simple computer programme to give you the more useful figures that you seek
Here's a nice simple one for you to pracice on. An aircraft I recently flew had a dynamic pressure limit of 79.4 lb/ft² for full flap exension. What was the CAS limit for full flaps?
A piece of cake!
Regards,
Old Smokey
Interesting post with respect to replacing CAS with Dynamic Pressure. You must have been referring to some vintage aircraft where attempt was made to use Dynamic Pressure to convert it to CAS (Even 1950's text books describe it as archaic). These olden days aircraft used the following formula for instrument calibration -
V = SQR (2q / Rho)
Where-
q = DYNAMIC Pressure
Rho = Air Density
V = True Air Speed
Nice sentimental stuff which actually produced DAS (Density Air Speed), now THAT's an "invented" and meaningless speed. For the low and slow Piper Cubs and Gipsy Moths it was a good enough substitute for TAS.
Unfortunately for the well-meaning folk who thought they were measuring Dynamic Pressure, ALL pitot tubes 'capture' Impact Pressure, even that on the Piper Cub. They were assuming that air was incompressible, but it is.
Now, if we want to get serious about airspeed measurement (as aircraft have moved on somewhat since the Piper Cub), we must consider IMPACT PRESSURE.
The calibration formula for this is -
Vc = SQR ((Y/( Y-1) X p0/qc X [(qc/p0 +1)^((Y-1)/ Y) - 1] X 2qc/Rho0)
Where-
Vc = Calibrated Air speed (ft/sec)
qc = IMPACT Pressure (lb/ft²) ........ What we’re talking about!
Rho0 = Sea Level Air Density (0.0023769 Slugs/ft³)
p0 = Sea Level Air Pressure (2116.2 lb/ft²)
Y = A constant for Air = 1.4 (Ratio of specific heat of air at constant pressure to specific heat of air at constant volume) (Y=Gamma)
Now this is good for Sea Level, for which the ASI is calibrated. At Altitude we need to replace p0 with ps (the static pressure at the particular altitude).
Now, I'm not trying to blind anyone with science, and it aint rocket science, but you wanted Dynamic Pressure, and there it is embedded in the formula! qc is impact pressure, and SQR(2qc/Rho0) is the compressibility factor (known as f), which if applied in reverse yields ...... Dynamic Pressure
So go and do the algebra and the maths to exract to extract your preferred Dynamic Pressure, and produce a table or simple computer programme to give you the more useful figures that you seek
Here's a nice simple one for you to pracice on. An aircraft I recently flew had a dynamic pressure limit of 79.4 lb/ft² for full flap exension. What was the CAS limit for full flaps?
A piece of cake!
Regards,
Old Smokey
Joined: Jun 2009
Posts: 202
Likes: 3
From: France
Good post from Old Smokey. Just my two cents to follow.
There is ongoing research on new ways to measure airspeed using lasers BUT such systems would measure True Air Speed (TAS). Of course, TAS like GS are important data for navigation but a pilot would still need CAS or its close cousin EAS. This is because aerodynamic forces acting on an airplane are directly proportional to dynamic pressure hence CAS helps a pilot flying safely and accurately an airplane. BTW, all operational limitation and performance data that depend on airspeed are better and more simply expressed using CAS than TAS. Automatic systems also need to use CAS.
Now, to get CAS or EAS from TAS you will need temperature, static pressure and a few lines of code in a computer. This means that you need one more probe than with pitots: a TAT probe which is itself as prone to icing as the Pitot tube. Incidentally, it is not clear yet that a laser sensor would be insensitive to icing.
In the end, it’s seems hard to build a more reliable air data system without pitots.
DJ
There is ongoing research on new ways to measure airspeed using lasers BUT such systems would measure True Air Speed (TAS). Of course, TAS like GS are important data for navigation but a pilot would still need CAS or its close cousin EAS. This is because aerodynamic forces acting on an airplane are directly proportional to dynamic pressure hence CAS helps a pilot flying safely and accurately an airplane. BTW, all operational limitation and performance data that depend on airspeed are better and more simply expressed using CAS than TAS. Automatic systems also need to use CAS.
Now, to get CAS or EAS from TAS you will need temperature, static pressure and a few lines of code in a computer. This means that you need one more probe than with pitots: a TAT probe which is itself as prone to icing as the Pitot tube. Incidentally, it is not clear yet that a laser sensor would be insensitive to icing.
In the end, it’s seems hard to build a more reliable air data system without pitots.
DJ
Joined: Mar 2011
Posts: 47
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From: Canada
This is because aerodynamic forces acting on an airplane are directly proportional to dynamic pressure hence CAS helps a pilot flying safely and accurately an airplane. BTW, all operational limitation and performance data that depend on airspeed are better and more simply expressed using CAS than TAS.
Joined: Jul 2006
Posts: 71
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From: London
This is an interesting topic which has been discussed before, so the following link may help: http://www.pprune.org/tech-log/37688...tot-tubes.html
Also apparently OADSs are able to derive CAS and static pressure from the laser measurements made.
Also apparently OADSs are able to derive CAS and static pressure from the laser measurements made.
Joined: Jul 2006
Posts: 451
Likes: 2
From: USA
As usual, an excellent post from Old Smokey
As usual, an excellent post from Old Smokey.
Let me throw this out here....for discussion.... What do you think of having a nicely calibrated angle-of-attack indicator?
Thrust and angle of attack.....you can use that for all phases of flight, provided we have marks on the indicator for each phase....In cruise, a mark for high speed cruise, for long range cruise, for 'econo-cruise'....etc.
If nothing else, it'll provide a nice back up for loss of airspeed.
What does everyone think?
Fly safe,
PantLoad
Let me throw this out here....for discussion.... What do you think of having a nicely calibrated angle-of-attack indicator?
Thrust and angle of attack.....you can use that for all phases of flight, provided we have marks on the indicator for each phase....In cruise, a mark for high speed cruise, for long range cruise, for 'econo-cruise'....etc.
If nothing else, it'll provide a nice back up for loss of airspeed.
What does everyone think?
Fly safe,
PantLoad