Hi All,
I am not a pilot but an engineer. I find the pitot discussion very interesting. There have been many suggestions on how to make this speed detection devise better as it appeared to be the first thing to fail in terms of proper operation in the AF447 flight. I am going to confine my thought only to the pitot itself, not the system of the aircraft to which it is connected that interprets and responds to the signals from the pitot tube or multiple tubes.
In designing a devise of any sort, the design engineer has to know as much about the parameters the devise is going to experience and operate within. Given this information, he/she can then go about designing the devise based on this information and perhaps a data base built on prior experience. Once the devise is designed and manufactured it is then tested to parameters across the envelope it will experience in operation. If these tests are successful, then it can be placed in service with some degree of confidence that it will perform the intended function it was intended to perform. So what could go wrong? What could go wrong with a pitot tube where the basic physics and general design has been known dating back to the late 1700s? And why is it that one manufacturer's design seemingly has less problems than another manufacturer's product when both met mandatory testing and certification requirements? Why is it that when the devise is used on one aircraft, it is more susceptible to non-performance than on another aircraft?
I would postulate to you all, there is nothing wrong with the basic concept of the current day pitot tubes, they will and can work successfully across the total flight envelope without any new bells and whistles. The reason they may not is because the parameters used in the design and testing did not and for that matter, do not match what is experienced across the total operating envelope, particularly at high altitudes and high speed in icing conditions. I have thought about this for some time and wondered in the instance of Airbus aircraft, could there be a difference in pitot tube performance verses that of Boeing aircraft? Could it have something to do with installation or location or shape of the fuselage forward of the mounting point or how the air passes into it or the actual testing and certification requirements? Was Boeing lucky and Airbus unlucky?
I think it has a lot to do with everything mentioned above and some other thing not thought of. Here is some interesting information that has been developed out of the studies resulting from AF447 and commented on by Airbus regarding changes to testing and certification:
Icing Conditions
The certification icing requirements defined in CS 25 Appendix C include liquid water contents, temperatures and droplet diameters in excess of those specified in the TSO. In addition the AMC to CS 25.1419 defines mixed phase and ice crystal conditions. Whilst it is recognized that the TSO tests are not intended as a means of compliance for the certification regulations Airbus believes the ETSO should include icing conditions that are more comprehensive than those defined in the TSO.
There would appear to be little benefit in designing and testing a probe to the TSO requirements if it is necessary to repeat the tests to more conservative conditions to support the aircraft certification.
Pitot and pitot static probes are known to be sensitive to ice crystal and mixed phase conditions and therefore Airbus always tests its probes in these conditions. The AMCs to CS 25.1323 and 25.1325 states:
“Airspeed Indicating System
1 Tests should be conducted to the same standard as recommended for turbine engine air intakes (see AMC 25.1093(b)(1)) unless it can be shown that the items are so designed and located as not to be susceptible to icing conditions. Ice crystal and mixed ice and water cloud will need to be considered where the system is likely to be susceptible to such conditions.
2 However, in conducting these tests due regard should be given to the presence of the aeroplane and its effect on the local concentration of the cloud”
In addition the AMC to CS 25.1419 paragraph 4 states that an assessment of the vulnerability of pitot heads to ice crystal conditions must be made. Conversely TSO C16a does not require tests to be performed in mixed phase or ice crystal conditions. In Airbus view such an omission is contrary to the objective of setting a minimum level of performance particularly as most aircraft fly in such conditions. Furthermore a probe designed and tested in liquid icing conditions only may require a significant redesign to meet the ice crystal and mixed phase requirements.
It should be noted that recent evidence indicates that the ice crystal and mixed phase conditions defined in AMC 25.1419 may not be adequate for pitot and pitot-static probes.
Airbus' comments go on to observe:
Probe Installation Effects
The TSO requires probes to be tested to the liquid water icing requirements of BS2G135 amendment 1 to asses anti-icing performance and modified ISO 8006 icing conditions for de-icing performance.
Test N°2 specifies Max intermittent icing conditions that are considered below JAR25/CS-25 Appendix C requirements. Accounting for installation effects on A330/A340, local LWC at –30°C should be 1.5g/m3 for maximum intermittent icing (without safety factors). The TSO C16A recommendation is 1.25g/m3, which therefore does not cover installation effect on Airbus A330/A340.
These conditions are free-stream conditions and do not consider the effect of the potential installation effects. Depending on the probe design and aircraft installation these installation effects can lead to the Liquid Water Content (LWC) at the probe location several times greater than the free-stream conditions. The TSO should at least highlight the potential installation affects to applicants.
The TSO requires probes to be tested at 0° angle of attack only whereas angles of attack up to 15° are not uncommon in service. Airbus believes that tests at angles of attack up to at least 15° should be included in the ETSO.
And,
Scaling of Icing Conditions During Icing Tunnel Testing
During recent icing tunnel testing it was found that the electrical current drawn by air data probe heaters varied with the mach number of the airstream such that at lower mach numbers the probe current reduced due to a change in the heater element resistance. This effect needs to be considered when scaling icing conditions as for some heater designs increasing the LWC to offset lower attainable icing tunnel speeds and vice versa may not be representative. Airbus recommends that the ETSO highlights this phenomenon.
So I think with proper design parameters, proper mating with the aircraft and proper testing requirements pitot probes from any manufacturer can be designed that work throughout the flight envelope. We are not there yet in full understanding, but it is not hard to imagine the icing problems can and will be solved very soon.