from a PM
Why wouldn't this problem afflict every airplane model? What is it about the A300-600 that makes it
different? Good question. Let’s see now:
1. Static Port placement? (and mayhap one that sucks in water flow-by when the atmospheric pressure is varying –
see *more below on this)
2. Early design that didn’t accommodate any real safeguards against a feedback loop in the FCS
3. Deficient safeguards for aircraft washing?
4. Airborne airflow patterns that allow rainwater to be entrained into static ports (or melting ice on the airframe to flow down/along and to be sucked in)
5. A CADC design that has sampling rates so high that you could say it was “hair-triggered” and over-responsive)
6. Designed-in low points in the static lines that allow water to pool and completely (rather than partially) obstruct the static lines (and so maximizing the hydro-pneumatic damping of air pressure sensing under rapid yaw.)
7. Insufficiently frequent specified water-trap draining intervals – i.e. allowing the water to accumulate.
8. Servicing manual deficiencies that don’t specify the water traps to be checked/drained after aircraft washes.
* My first experience with the ability of static ports to take in water was a long time ago. Imagine a rubber
bung that’s concentrically hollow to allow the static system to accommodate atmospheric pressure changes while
parked (and so allow air-fed flight instruments to “breathe” properly). Even though the bung is designedly
inserted upwards so that the hole faces downwards, water flowing down over it (in torrential rain) whilst parked
can still get sucked up that hollow rubber tube by capillary action. We proved that after a nasty mass incident
back in 1974 (or thereabouts). So you can get trapped water in the lines. In the case referred to here there was
no static port or line heating and that trapped water froze causing all pneumatic flight instruments to be lost
during a climb above freezing level. Not really applicable to AA587 but water in the lines can still have an
effect as outlined in the theory.
Explanation: “and so allow flight instruments to “breathe” properly” Pneumatic instruments can be destroyed (or
at least require removal and recalibration) if subjected to large pressure transients (like blowing orally into
a static port or obstructing a tube and removing a solid bung in a much changed atmospheric pressure days later
etc.) They are best left not 100% sealed in other words).