April 2019

Why is the table of an Airbus also highly complex

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SATIRE: “Airbuses are extremely complex aircraft, regardless of type or series. Therefore, pilots are familiarized only with the most important systems. Many technical facilities are not included in the manual. We want to present one of these unknown systems here.

This is the technique behind the folding tables in the cockpit. Pilots of conventional aircraft have something between their legs, namely a steering horn. Airbus pilots, on the other hand, do not have to do their work with their legs spread. You have in front of you a table where you can work and eat comfortably.

These tables are pulled out by hand. Believe the pilots! But they also think they're driving the plane when they move around the sidestick. The sidestick is like a joystick and replaces the steering horn. And just as the plane is actually controlled by computers, the tables are pulled out and retractable.

When a pilot pulls at the table, he activates the “Table Movement Sensing Computer” (TMSC). This sends a digital pilot pulls signal to the Table Extension and Retraction System (TEARS). The captain's table is the TEARS 1 system, the table of the co-pilot TEARS 2. The systems TEARS 3 to TEARS 7 serve as a backup if one of the primary systems fails.

Each TEARS consists of the “Table Extension Computer” (TEC) and the “Table Retraction Computer” (TRC), as well as the “Table Computer Monitoring, Operating, Supervising, Analysing and Fault Preventing Computer” (TCMOSAAFPC). A “Table Data Acquisition Unit” (TDAU) stores thousands of data that provides important troubleshooting information in the event of a malfunction in the maintenance table.

If a table is to be extended, the responsible TEARS will send a corresponding signal to the Hydraulic Pilots Table Extension and Retraction Unit (HPTEARU). This uses the green hydraulic system to extend the table. The “Pilots Deception Computer” (PDC) ensures that the pilots do not notice that the computer is running out the table rather than themselves. (The PDC is also active on 741 other systems.)

The operation of the table is therefore very simple: Upon request by the TMSC, the TEARS activates the TEC under monitoring by the TCMOSAAFPC, which expels the table with the help of the HPTEARU. So far, so simple.

Of course, the TEARS is not nearly as simple as described above. As professionals, we naturally want to know more about the individual operating modes.

The TEARS has three modes of operation (“Laws”): If the Purserin comes into the cockpit with two steaks, then the “Meal-Law” is activated immediately. Both tables exit and Starvation Protection is activated. When the Purserin enters the cockpit with two starters, the system switches from “Meal-Law” to “Snack-Law”. Starvation Protection remains active and at the same time the signal “And there's no more?” is enabled. Finally: When a flight attendant comes into the cockpit with dry rolls and mineral water, the “Fuck-off Law” is automatically activated. In this mode of operation, the responses of the pilots are very direct, so this is also referred to as “direct law”.

So far the basics. But what happens in the event of a failure in the hydraulic system? We remember: the tables are driven out and retracted by the green hydraulic system. But if the green system fails, an emergency threatens. Of course not, because Airbus has thought of everything. The systems are extremely redundant.

If the green system fails, the yellow and blue hydraulic systems are activated fully automatically. A “Hydraulic Color Mixing Unit” (HCMU) mixes blue and yellow hydraulic oil so that green oil comes out. The whole thing is controlled by the “Hydraulic Color Management Computer (HCMC)”. As a result, green hydraulic oil comes back to the tables, and everything works perfectly.

As far as the basic functions. Of course, Airbus's philosophy is to make all systems as complex as possible. So let's take a look at the “Table Priority System” (TPS) next.

To understand this very complicated system, first an example. Suppose the Purserin comes into the cockpit and shouts: “There's a business dinner left!” Let's say the copilot pulls out his table. Then the TEARS 2 is activated, and the First Officer is ready for food intake.

Now, however, the captain has the option to press the “Instinctive Meal Take Over Pushbutton” (IMTOP). If it does, the Table Priority System (TPS) ensures that the co-pilot's table is retracted at maximum speed and the captain's table goes out. At the same time, the computer voice “Meal Left” sounds and the Flight Director points to the left on the Primary Flight Display. If the captain presses his Meal Take Over Pushbutton for a longer period of time (more than 0.4 seconds), the co-pilot's table will remain locked for the rest of the flight.

Now on an unpleasant issue: emergencies. In extremely rare situations, despite all redundancy, an emergency situation can actually occur. In the event of the simultaneous failure of all hydraulic systems, the tables of both pilots may not be able to extend. In this case, the aircraft manual (OM-B) must be consulted. Here we find the appropriate emergency procedure.

In the event of failure of all hydraulic systems, the food must be placed on the knees of the pilots. It is therefore necessary to carry out the “Meal on Knees” procedure. It is advisable to inform air traffic control first. (“Mayday, mayday, mayday, mayday. We have a double table failure. We have to eat from knees. Upon landing we request cleaning personnel at the runway!”) The “Meal on Knees” procedure will be a focus of the next emergency training. In cooperation with the catering service, Airbus pilots will be prepared for this emergency.”

The original text was written by Peter Gugerell/ OS and adapted to all airlines.

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