While not a new subject, the Air France 447 accident may have generated new focus on dealing with Unreliable Airspeed (URA) indication often attributed to the blockage, loss or failure of a pitot and static system or air data computers. Many of the modern aircraft are flown through a Flight Control System (FCS) with many components. The autopilot, which is at the hub of the FCS, takes inputs from the air-data system, which is fed from sensors that provide airspeed and static pressure. If these otherwise very reliable and accurate sources generate false information, they can seriously impact the autopilot’s performance.
This is a critical malfunction, because commercial air carriers and other jet operators rely on the auto pilot the majority of the flight time. However, it is the pilot’s responsibility to maintain control of the aircraft – fly a safe flight path. When an autopilot receives erroneous information from the air data system, the system does not necessarily warn the pilot of that problem. The FCS may continue to appear to be operating normally, thus adding to the potential confusion facing the pilot and aircrew.
Deviations from normal, controlled flight can be insidious – the system continues to “function” normally, thus reducing the time in which the pilot(s) can try to react and take corrective action. The problem(s), airspeed rising or falling just outside the normal rate, may be hidden from the cockpit crew, adding a progressively increasing increment of what is usually expected. Unfortunately, the autopilot responds in a way it was designed – a normal and logical way based on the inputs it is getting.
The URA guidelines, developed through the Flight Safety Foundation URA team (Airbus, Boeing, Bombardier, Embraer, Airlines for America, ALPA and IATA), establish essential considerations for manufacturers, operators and individual pilots alike. The guidelines re-enforce what should already be second nature to all pilots along with what has been taught to them for the aircraft type being flown.
When diversions from normal flight/unreliable airspeed indications are evident, the essential pilot training is for the flight crew to intervene and follow the memorized emergency items. Such a reactive procedure would likely include disengaging the autopilot, flight director and auto-throttle, and then the pilots resume manually flying the aircraft to a stable state.
Sounds simple doesn’t it? Every operator, whether commercial, corporate or GA who operates a modern, sophisticated airplane should use the FSF guidelines and apply them in their simulator training and make them a standardized approach to the problem. It will lead to a successful outcome in the event you face the problem during a flight.
If assistance is needed to revise your training syllabus to assure that your cockpit crew is aware of the new URA guidelines and that your pilots are prepared to react to malfunctioning FCS, there are professionals available to incorporate this new information on a fully integrated basis.
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