Originally Posted by
slats11
There are two problems with unreliable airspeed:
1. The unreliable airpeed per se.
2. The bigger problem is when the system (and the pilot) does not recognise that the airspeed is unreliable - and automation then insidiously takes the aircraft closer to the edge of the flight envelope.
Although the system obviously works most of the time, there is a weakness if the system uses 3 identical tubes to detect a problem. If the problem effects all 3 equally, then it can be missed. It will be unreliable, but not recognised as unreliable. Just as three 2nd graders can't together do 6th grade math, 3 problem tubes can't reliably determine airspeed (nor even the state of unreliable airspeed).
With pitot tubes, you are using air flow through the tube to create a pressure, which you then compensate for altitude by using static sensors, and then converting this compensated pressure back into flow (or airspeed). There are a lot of links in this chain. Even worse, a problem could generate either an over-speed (eg blocked pitot drain hole) or an under-speed (blocked tube).
When it is difficult to measure something important, an alternative that is easy to measure becomes important. And so it was 200-300 years ago - flow was difficult to measure, pressure was easy, and so we calculated flow from pressure.
Maybe the time has come for us to measure flow, and thus airspeed directly. Presumably that is the principle behind the laser systems currently being developed.
It may be that pitot tubes today are dinosaurs looking for a tar pit.
Pitot tube measure airspeed via dynamic pressure, this is also what the wings use to generate lift. Measuring air molecule approach speed (laser?) will still need air density from static pressure and temperature from temperature probe. Pitot tubes are simple and have well known failure modes - how long will it take to develop alternatives to the same level of maturity?
What is not clear is why these simple devices seem to have increased number of incidents in cruise - in conditions that are uncertain and do not form part of any current certification requirements. Is it subtle changes to planned flightpaths or "Climate Change". I have no doubt if a reliable test of the "alleged supercooled liquid" or my preferred explanation of "particle size distribution of micro-fine ice crystals" can be generated - pitot tube heating can be redesigned to prevent blockage - but don't hold your breath waiting to get them introduced any time soon.
The now preferred Goodrich probe has also already suffered icing, but my understanding is that it is less likely for the drain hole to block, so blockage of the ram port should give a more obviously failed low reading, for example:-
Investigation: AO-2009-065 - Unreliable airspeed indication - 710 km south of Guam, 28 October 2009, VH-EBA, Airbus A330 202
Originally Posted by
keitaidenwa
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To get back to AF447, the Air France beancounters were reluctant to upgrade pitot tubes to safer ones. I have no idea what the price of the pitot upgrade was, but probably it was not cheap, since it got beancounters on their toes. Blame beancounters? Well, the other side of the coin is that Airbus/Thales was making a profit on what was essentially a safety fix...
To be fair there was (and still is) a technical debate as to whether the alternate probes are significantly better for high altitude icing. The change was not a mandatory safety requirement, there was just a logistical supply issue and biggest cost would have been taking planes out of revenue generating service for unscheduled service.