Protection against ice formation is another challenge faced by designers of air data sensors. Generally this is accomplished in the pitot tube by using electrical heating systems. As many aircraft have this probe mounted using a mast or pylon, multiple heaters may be required to not only keep ice from obstructing the air port but also from getting a significant accumulation on the mast. If uncontrolled ice were allowed to form here, it may break free and contact some other area of the aircraft causing impact damage. In fact there is a requirement on aircraft that operate in icing conditions that an anti-icing system has to exist, and operation of the pitot heat system has to be monitored from the flight deck by a light that will illuminate either when the system is off or the heater is not functioning.
Caution should always be observed when working with these heated sensors as burns can be obtained easily and the noticeable electric current flow needed to maintain the heater could also cause arcing in the event proper precautions are not in place. Another feature found in most pitot tubes is an orifice strategically located for the purpose of draining trapped moisture from within the probe. Lack of awareness of this component has cost many technicians hours of needless troubleshooting looking for a pitot pressure leak. This calibrated opening is most often blocked when conducting system leak tests.
External covers are routinely placed over the pitot tubes at the conclusion of a flight for the purpose of preventing foreign object ingress. It has been noted that either improper use or inappropriate covers have caused probe damage. This can be as serious as changing the angle of the probe or as simple as melting a plastic cover on a still hot heating element. Other serious conditions can occur where sharp edges from the cover have gouged the probe or even removed protective coatings. It is always advisable to use the correct air data covers when securing and parking the aircraft.
Checking for Leaks
Modern day pitot tubes serve as a pressure sensor and contain sophisticated internal circuitry that has the ability to convert the pressure values into a digital format and rather than using mechanical plumbing for data delivery, digital buses become the media of choice. The majority of aircraft still depend on a wide array of plumbing to deliver pressure variations to the various consumers. Airframe manufacturers have come up with numerous methods and materials to fulfill this function. Few are without problems. Leaks are a troublesome reality. Lines made of metal although durable are susceptible to fatigue and cracks, and some flexible lines are prone to age-related breakdown. There are situations where chafing or seal degradation can almost be anticipated and many technicians find it prudent to renew such components during routine maintenance access.
It is also common to find moisture drains installed at strategic low points in the system. Airframe manufacturers will often provide a recommended frequency for inspecting these moisture traps.
Air data plumbing can provide challenges in its own right. A small accumulation of water at a low point either outside the pressure vessel or next to the skin may result in the water temperature dipping below the freezing point. When the water freezes it also expands and may result in a total blockage of the line. Of course by the time the aircraft gets on the ground and testing equipment is connected, the ice has melted and the problem can not be duplicated. Moisture traps are often located at low points in the system and they do require periodic inspection. Often these drains utilize clear bowls, allowing quick visual checks for the presence of water. Unfortunately these bowls often prove to be the weak point in the system which results in air leaks.
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