One thing for certain; if it weren’t for air, none of us would be employed (much less alive). The “air” in aircraft is, for the most part, what makes it all work. Without just the right concentration of various gases making up our atmosphere, engines would not run and there would most probably be no wind beneath our wings. In recent times there has been some publicity regarding devices used to monitor the air surrounding the aircraft.
A 2009 crash of Air France Flight 447 involving an Airbus over the Atlantic Ocean has cast some skepticism on what we all considered to be a reliable means of operating aircraft.
Overreliance on computers
A representative of the Flight Safety Foundation said “the crash highlights a creeping overreliance on computers to fly today's jetliners. Crews blindly follow the computers, but they're still supposed to be able to look through the data and see what their plane is doing.”
The crew of the Air France Airbus never realized that the plane had stalled, was the conclusion of the investigating team and the report's conclusions were more pointed in their criticism of pilot missteps and confusion than preliminary findings.
The causes of the accident were well-documented in a previous report by the French Bureau d'Enquêtes et d'Analyse a year ago, after recovery of the aircraft's flight recorders from the ocean bed following a two-year long search. The crash already has prompted sweeping changes in pilot training.
So just what kind of information should flight crews of today be concerned with regarding air data? Quite frankly, the “need to know” regarding air data is still very relative to the basic theory of flight. An airfoil will produce lift when a differential pressure occurs between the upper and lower surfaces. Most often this is achieved by the aircraft reaching a specific forward velocity and applying a nose up attitude. The factors include static pressure, ram air pressure, and angle of attack (AOA). Temperature is another key player as it will impact air density.
Back to basics
Even with all the sophistication of today’s fly-by-wire aircraft, the basic factors are as important now as they were in Kitty Hawk back on that fateful day in 1903. Unfortunately, as is evident in the demise of AF447, the basic parameters are perhaps not as pronounced in modern aircraft where automation controls are based on inputs from electronic sensors. Just within the last few months directives have been issued by several airworthiness agencies pertaining to air data gathering devices.
Pitot tubes are externally fit on aircraft to sense ram pressure and proper positioning is critical. Although an inherently simple design, the tube is a precision device. Any deformation or blockage in the air inlet will produce some degree of error. Should the angle of the device be varied from the calculated position, a variance in airspeed will be perceived and often minor alignment anomolies are difficult to detect at least visually. The symptom most often spotted is on aircraft with multiple pitot pressure systems experiencing a split in airspeeds where the amount of the split will vary as speed changes. An effective test is to run all pressure-sensing systems with a common test box and if there is no perceivable deviations on the ground, suspect an airflow issue.
Several aircraft manufacturers do provide criteria to verify pitot tube alignment as part of their continued airworthiness programs.
Air inlet blockage is another common occurrence. It is advantageous to remove any protective cover (frequently addressed in pre-flight checklists) prior to attempting flight but the most common culprit is bug infestation. These insects often deposit debris including mud well inside the tube making discovery and removal prior to flight very challenging.
Most operators in regions of known bug populations will allow protective covers to remain on the aircraft until just prior to departure and/or will routinely and carefully clean the inside of the tube. A campaign to verify the free passage of air may also be implemented and can include utilizing a borescope.
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