Hi,
Originally Posted by Graybeard
Agreed, Jig Peter. There are thousands of planes relying on pitot at this instant. Icing seems to be a rare event, and even more rare in non-Airbus.
Excepted very few experimental aircraft, all others, civil or military, be there small or big, are still relying on pitot tubes today because it is the most robust piece (of very simple hardware) available for doing quite well the job required in most circumstances. Nonetheless, it is also known as one of the major cause for aircraft accidents (mostly in light aviation).
Icing/obstructions by rain, foreign objects, poor maintenance, is not rare. What is a very rare is the fact that long hauls airliners could be affected at cruise levels and lose more than one probe, or all of them, in a very short window of time (as generaly, it doesn't last more than few dozens of seconds, it is erratic for a short while, then return to normal).
I'm not sure that Airbus airframes are more affected than competitors as no statistics about it is available anywhere (and most of the time, plenty of frequent events, like single failures, are not even reported at all). Only companies operating both Boeing and Airbus on the same lines (and for a long time) could really make an informed comparison about those failure rates related to manufacturers/models. Sadly, they really don't like to publish those informations about their fleet "failure rates": classified matter.
Beside, those probe makers are working for any manufacturer and their specs are based on the same regulation. Hence, such consideration about Airbus probe failure rate could be only based on the fact that AF447 is attracting a wide attention on probes issues related to certain flight conditions. This caused some data to be collected around by digging in A340/330 fleet archives (many such cases were unknown by operator/regulators and Airbus before June 1st 2009).
For example, an internal Delta Air Lines memo is quoted here
Incident: Northwest A333 over East China Sea on Jun 23rd 2009, unreliable airspeed and is showing that there was an internal survey of such kind of issues dating from 2006, which is giving some intersting figures about this failure/rate and issue analysis on Northwest A330s fleet:
A memo dated June 17th 2009 circulating within Delta Airlines (parent company of Northwest Airlines) states, that the airline's data support the theory of blockages rather than icing of pitot tubes stating, that constantly heated items don't just instantly freeze up and thaw again. The manufacturer has indicated, that the drain size of the pitot tubes is insufficient in the original probe. 0.0012 percent of the flights (approx. 1 in 83.300) are affected according to data collected by Delta Airlines since 2006, incidents have only occured on flight levels 330 to 400 in moderate and stronger turbulence during months of May to December between latitudes North 3 and North 37 degrees.
The memo also describes, that usually the airspeed drops to between 80 and 60 KIAS, the airspeed returning to normal within 4 to 32 seconds. The encounter may be single or repeat over a couple of minutes, followed by normal airspeed operation during the remainder of the flight. Master Caution and Master Warning will activate, the autopilot and autothrust systems disengage, the autopilot not accepting to be reengaged until airspeed is back to normal. Autothrottle will enter thrust lock until throttle levers are moved or autothrust is reengaged. If two systems are involved, an ADR DISAGREE message occurs, the fly by wire will drop to alternate law if the disagree lasts more than 10 seconds and will remain in alternate law for the remainder of the flight. Normal autopilot and autothrust operation can be expected once the event is over.
Delta/Northwest Airlines are replacing the pitot tubes on all their A330 aircraft as quickly as possible.
One should note also the fact that Northwest A330s were equiped with the same Thales/Sextant "C16195AA" probes as Air France A330s and that they did not change them, neither before nor right after AF447 (despite being aware of some issues as it was monitored from 2006): in fact, there is another incident on their fleet on the 23 June 2009, involving their A330-323, #552, registered N805NW using the same probes (see BEA 2nd report), following the one of the 11 June 2009 involving their A330-223 #620, reg. N854NW, following another one on 10 August 2008 to N809NW (A330-323 #663) after the same thing happening again on 6 August 2007 to N854NW. This frequence of "0.0012 percent of the flights (approx. 1 in 83.300)" should have been considered quite low for an event lasting only "4 to 32 seconds" for such flights cruising at FL330 to 400.