Originally Posted by
barrymung
As I understand it, the plane was fitted with three identical pitot tubes. Presumably, the reason 3 are fitted is in case one or more fails.
Who, in their right mind therefore, decided it was a good idea to fit three
identical tubes, all from the same manufacturer, all of the same type and presumably the same date of manufacture??
If one tube had a stock fault, all three would have it.
Surely it'd be far more sensible to use three different tubes from three different manufacturers?
That way, if one failed, chances are the remaining two would be unaffected.
Well, there are various practical problems with dissimilar designs, especially for systems that need to have data in agreement from multiple sources in order to assure safe operation. With three different probe designs you'd get far more ADR DISAGREE type situations with the increased hazard that represents.
You also need to spec the dissimilar designs, and the performance specs would presumably be identical. Since the state of the art for pitot probe design is mature, chances are you'd get very similar designs even from dissimilar vendors.
You'd also end up with the same environmental spec, so the same conditions which would overwhelm one probe's heater capability (if that happens) might very well overwhelm all the dissimilar probes.
And, finally, they didn't put all their eggs in one basket. The ISIS/standby is using a different means in that the standard probes have ADMs at/near the probe, while the ISIS is (I understand) plumbed traditionally with the sensing as part of the ISIS unit. So if you were to have some kind of catastrophic design flaw that took out every ADM the ISIS should still be running.
I don't think the air data system designers are as clueless as you seem to think.