Originally Posted by
airtren
With the current design, the 3 pitot tubes are not only of the same type and same manufacturer, but they are also located pretty much in the same place - under the nose...
Not quite true. Since the problems, it seems airlines have either switched to a combination of Thales AB and Goodrich, or all-Goodrich (which are deemed more reliable). The tubes themselves are located on opposite sides of the airframe (with the standby just forward and below the Captain's primary pitot tube).
Originally Posted by
Sriajuda
Since airspeed information is probably THE most important data in an AC (pilots, please correct me), this neglience to provide both true sensor redundance and obviously not implementing secondary, heuristic means to verify and/or determine air speed is incomprehensible to me.
Welcome to the board!
Right - to answer some of your queries, the issue is one of complexity. The BUSS (Backup Speed Scale) device now offered as an option by Airbus works along the principles you describe - but the fact is that pitot tubes remain the most reliable, accurate and elegant engineering solution to the problem of airspeed determination.
Previous accidents attributed to loss of airspeed information involved either a lack of simplistic redundancy of the kind provided by the Airbus design, or (as in the case of Birgenair) failure to diagnose a fault within a single pitot tube and correctly use the redundancy available. The point behind the "quorum" design that existed prior to AF447 was that the systems themselves would be able to diagnose the problem and switch to the working sensors automatically - cases where all sensors failed within a tight timeframe were practically unheard of up until that point and as such, a triple failure was considered out-of-scope. As is so often the case, reality can intrude and make fools of us engineers in unexpected ways (in this case switching to the Thales AA pitot tube model without exhaustive testing to check whether the replacement was like-for-like in all conditions).
This "failure of imagination" has affected every complex engineering endeavour human beings have embarked upon, from bridge-building through seafaring and aviation right up to spaceflight. The only way to avoid that risk completely is to never embark on such endeavours in the first place.
Originally Posted by
Sriajuda
If I remember correctly, in the Airbus the trim wheel does not make the 'clack-clack-clack' sound that other aircraft do. Another example of technology that seems to be designed to withhold information from the pilots? As in the sidestick configuration?
You could look at it that way, but (IMO) only if you've come at it from a pre-determined conclusion that the Airbus FBW design was intended to withhold information from pilots (which, while a stubborn notion to disprove, is in fact not borne out by the evidence). The sidesticks are not interconnected and do not provide force-feedback because of the additional engineering complexity required to do so, and in jet airliners from the late '40s onwards, any feedback through the control column is in fact artificial and as such can be prone to failure.
The trim wheels are quieter on the FBW Airbus because unlike other airliners of the vintage, trim is an automatic function in every law except Direct and MAN PITCH TRIM ONLY. Even in aircraft of a similar vintage (e.g. 757, 767), pilots will be trimming manually whenever autoflight is disengaged, and the trim wheels are an integral part of manual flight. Because this is not the case on the FBW Airbus in over 99% of cases, the trim wheels are not as intrusive.
Now - obviously in this case, having the trim wheels announce movement more forcefully might have drawn attention to the way the aircraft was being mishandled - but given the fact that neither of the two F/Os had high-altitude manual flight training on type this can't be taken as read.
It may interest you to know that when we ran the scenario in an A320 sim, it turns out that in Alternate [no speed stability] (the A320 equivalent of Alternate 2), autotrim had a hard limit of approximately 5 degrees nose-up, even when holding the sidestick against the back stop. As a result of this we managed to recover with stick full forward after losing barely 8,000ft. In order to test the exact circumstances, our TRE had to manually wind the trim up full once the limit had been reached on the following experiment - this time we lost closer to 18,000ft before recovering, but importantly we found that recovery was theoretically possible using sidestick alone - i.e. without having to manually adjust trim, *provided* that the problem was diagnosed relatively quickly.
Why there's a difference in autotrim behaviour between the A320 and her larger sisters I don't know, but it's definitely an interesting fact to take into account.