Never has a truer contradiction been stated. First of all, we all know that “air” is in fact a gaseous mixture in the scientific context. And “primordial,” wasn’t that a movie from the 1970s starring Raquel Welch? In fact the true definition of primordial is “primary or fundamental component”. It just so happens that air is the one fundamental component that makes our business a reality.
So just what is air? According to the American Heritage Dictionary, air is a colorless, odorless, tasteless, gaseous mixture consisting of mainly nitrogen (78 percent) with a bit of oxygen (21 percent) and a sprinkling of argon, carbon dioxide, hydrogen, neon, helium, and a few others.
The troposphere is the portion of the atmosphere that extends outward from the earth’s surface around seven to 10 miles.
This is the region in which most aircraft operate. It is also in this area where varying amounts of particulate matter and moisture are prevalent. Air density is also greatest closest to the ground. This is due to the earth’s gravity and compressibility of the air and can be observed by noting the rapid pressure decrease with altitude increase. Atmospheric pressure will reach about one half its sea level value at about 18,000 feet. The air in the troposphere is in constant motion with both horizontal and vertical wind currents and in this region temperature changes with altitude at an average rate of 3.6 F per 1,000 feet. This means, at the outer reaches of the troposphere, -70 F can be expected.
So why the review of elementary school science? Well, our industry is all about air pressure. If the pressure on the top of a wing increases and begins to equalize with lower wing pressure then the aircraft assumes the glide path of an aerodynamic crowbar.
The challenge here is to measure variations in air, provide correction factors where needed, and then deliver the information in a way that can be interpreted by the flight crew. In fact interpretation is a big part of the issue; many of the terms may have different implications based on context. For example, airspeed is only a partial term, indicated airspeed (IAS) is considered to be uncorrected for instrument error and the effects of air density; calibrated airspeed (CAS) which compensates for instrument error; and true airspeed (TAS) which is CAS plus compensation for air density and temperature variation. They do all relate to the measured forward movement of the aircraft through the air. Temperature is another area that includes words like true air temperature (TAT) and static air temperature (SAT). Do they mean the same thing? Probably not.
So what information is it exactly that needs to be gathered and how do we accumulate it?
The History of the Pitot
In the mid-1700s a French physicist named Henri Pitot concluded that by placing a hollow tube directly in the path of flowing water, a certain pressure would be present. By then factoring the reference pressure of the water with the sensing pressure on either side of his tube, a fairly exact calculation could be made of the water’s velocity as it flowed. As a result of his efforts Pitot has earned himself a place in aviation.
The pitot tube as we know it today varies little from the original application. Of course refinements and adaptations had to be made to accommodate aircraft installation. Location and alignment on the airframe is most important. It is imperative that the tube have an unobstructed sampling of relative air. As these devices are frequently in prominent positions, they often become the recipient of hangar rash. Remediation of mechanical damage to a probe may involve sophisticated measurement and alignment procedures using in some situations laser levels or specially manufactured alignment fixtures. Routine inspection of pitot tubes should include looking for deformities in the probe or supporting mast as well as obstructions and anomalies at the sensing port. Many of these devices are designed with a great deal of sensitivity and in some cases a minor amount of corrosion or even erosion in the wrong place could have an undesirable result on the pressure being measured. Manufacturers’ references regarding inspection criteria should always be consulted during probe evaluation.
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.
Impact of Static Ports
Sensing current atmospheric pressure has its own unique challenges. Federal guidelines for aircraft certification stipulate that static ports be located on an aircraft where they are impervious to ice formation along with other contaminants plus be free of disruptions in airflow. Maintaining this situation can be a significant challenge as repairs or modifications to aircraft skin in the area of static ports may alter the air path over the sensing opening. Something as simple as a protruding rivet can cause a static system error. In fact on some high-speed aircraft the inspection criteria for the area around static ports will list requirements for the edges of paint stripes.
It is safe to say that any surface anomalies that could produce a ripple in airflow could be a detriment to the proper operation of the static pressure system. One consideration that is often overlooked is a pressurization leak. Once the aircraft is operating at cruise speed in the upper atmosphere, a differential cabin pressure can result in a high velocity air leak, which if in the airflow path across the static port will result in an altered air-path and a resulting static error.
Reported static system discrepancies that are not replicated during ground tests may well be the result of structurally induced errors. Frequently these errors will vary with airspeed changes. In this case a detailed visual inspection of the area forward of the static sensors would be a good starting point in the troubleshooting process. Like with the pitot system, static pressure is most frequently conveyed by plumbing to the end consumers, being either instruments or an air data computer (ADC).
Sensing air temperature is most frequently accomplished electrically. Devices with air inlets that draw a sampling of air and eliminate visible moisture and compensate for the effects of compressibility will allow a metered airflow to circulate across a platinum wire element. This wire is in fact a thermistor with specific and predictable responses to changes in temperature by varying a resistance value. Like most other air data probes the temp sensor may be electrically heated. This of course can be a problem. If the heat is selected on before the aircraft has adequate forward speed, the sensor may quickly exceed its high temperature limit and cause erroneous indications from the devices connected to the sensor. Many of the air temperature sensors contain dual thermal elements. This can be a plus when troubleshooting as the resistance value of each element should be the same and have the same coefficient to temperature change. Often filling a plastic bag with ice and securing it around the probe will provide a means of checking the resistance elements while simulating conditions at higher altitudes.
Testing and Certification Procedures
U.S. Federal Air Regulations (FAR) dictate testing and certification stipulations. FAR 91.411 states that static systems shall be tested for leaks as well as altitude accuracy once every 24 months. It goes on to say that leak tests should be performed anytime the static system is opened. The exception is the removal of static system moisture drains. The procedure for conducting the tests can be found in FAR Part 43 Appendix E. Airframe manufacturers’ tests will take precedent as long as all the basic elements of the Appendix E checks are carried out.
In recent years changes in navigation performance have resulted in a means to allow more aircraft to occupy existing airspace at the same time. Reduced vertical separation minimums (RVSM) will allow aircraft flying between 29,000 and 41,000 feet to fly with 1,000 feet of vertical separation. Prior to this initiative vertical spacing was maintained at 2,000 feet.
Any aircraft having planned flights in RVSM airspace will first have to qualify to be there. Part of the qualification is to demonstrate the accuracy of the altitude indicating system along with implementing a very accurate static correction. In studies conducted by the FAA, the vast majority of static errors are the fault of disrupted airflow over the static ports. A big part of the qualification process is verifying the fuselage skins and structure forward of where ambient pressure is monitored. In some cases, this involves complex skin mapping procedures and techniques.
Airframe manufacturers or design agencies have created criteria to allow different aircraft models to qualify and in all cases there are stipulations for continued airworthiness. It does become the responsibility of each maintenance technician to learn the criteria for the equipment being maintained as routine maintenance may have a bearing on the safe operation of the aircraft in the tighter confines of RVSM airspace. This could be something as simple as verifying proper closing of a nose compartment to something as complex as running an integration check of circuit cards during certain maintenance operations. It does require a visual inspection of RVSM critical areas during a daily pre-flight.
Just think, all this effort over a simple gaseous mixture. Seems like life was a lot simpler when air was used exclusively for breathing. I guess if that were still the case today all of us would be involved in another line of work.