This is our open thread. Please feel free to offer your own comments.
The pitot tubes used on some Airbus products have entered the news again. This time it is affecting probes made by Goodrich Corporation, based in Charlotte, North Carolina. These were the preferred replacement for the pitot tubes made by France’s Thales that were suspect in the crash of Air France flight 447 into the Atlantic Ocean. Here is a basic diagram of a pitot tube:
Ps, the static pressure, is derived from a series os small holes along the circumference of the pitot tube. The small holes reduce turbulence and provide a near perfect representation of the average air pressure of the environment.
Pt, the dynamic pressure, is derived from the head-on airflow and represents the air pressure felt from the forward motion of the airframe.
For those interested in the exact technical details of how this works, please go to the reference link from “pitot tubes” at the top of this thread. I will try to describe the issue at hand in layman’s terms. Otherwise, this thread would become a potential doctoral dissertation.
The ratio of the static versus the dynamic pressure along with a few algorithms (Engineers call these “Rules of thumb” which mean “I don’t know why this works, but it does.”) are used to determine the forward airspeed of the airframe. The automatic pilot is privy to the calculated airspeed. A problem arises when either or both of the input port systems are blocked by ice formation. If ice clogs both port systems, then the plane is assumed to be at a standstill. If the static ports are blocked, then it is assumed to be going forward a speed limited only by the degree of maximum deflection of the diaphragm. If the dynamic port is clogged, it is seemingly flying in reverse at a rapid pace. There are three pitot tubes per plane in order to allow for the failure of any one. If two or more fail, the autopilot is confused and relinquishes control of the plane. If the pilot and/or copilot fail to rectify the situation all on the airframe will buy the farm.