The Autopilot in planes

Résumé du document

Autopilot is a complex but very interesting system. We're going to tell you about the Autopilot in planes. The Adiru is one of the two parts of an Autopilot, the one that gets information from the outside, such as speed, altitude, etc. and transmits it to the computers that calculates and makes the plane follow the right trajectory according to the pilot's intention. The Adiru is of two parts, the Air Data Module (ADM) and the Inertial Reference Unit (IRU). The ADM is to give to the AP all the data concerning pressure, temperature, and different speeds relative to them. The IRU calculates the exact position of the plane using an Inertial Navigation System (INS), and a GPS, to get an accurate position. In most of the commercial planes today, we can find three different Adiru's. The first one goes to the Captain, the second one to the First Officer, and the last one is for standing by. The three always compare their values, and if one of them gives wrong results coming from wrong calculation or wrong measures, it is directly eliminated for the rest of the flight. Moreover, the three Adiru's have their own set of sensors, so that if a sensor is dead, the unit it belongs to is disconnected.

Extraits

[...] This is the control segment. Finally, the user segment is the GPS receiver that measures the time the signal takes to come from the satellite to the antenna. But it is easy to understand that one single satellite is not enough, and that's why the user needs at least four ones to get a satisfying localisation. Of course, the more you see different satellites, the more accurate is your GPS position. Indeed, the precision of this device is always changing, because of the accessible satellites. [...]

[...] First, the elevators allow the plane to go up and down. These controllers placed along the back of horizontal stabilizers (in green on the schematics) are moving together. When they are turning up, the air pushes on top on the wing, so the rear is going down, so the plane swings up. To go down, you just need to turn the elevators down using the pressure of the oil. That's the opposite. Thus, it's a rotation around the transversal axis called the pitch. [...]

[...] So when some force tries to make it deviate from its original axis, it tends not to follow that force and to remain in the same position. But the force the device exerts to balance the other one acting upon it is transmitted to a sensor, then to a computer, which is the inertial navigation system. The accelerometer The accelerometer is a complicated object also. Fortunately the basic principle is easily understandable. The mass M has got inertia, so when the plan P goes up or down, M tends to keep its position because of its weight. [...]

[...] It could give an altitude indication but with a very bad precision, so the ADIRU takes that information from other sources. Now let's explain the operating of the GPS. It is composed by three parts (called segments) : - Spatial segment - User segment - Control segment The spatial segment is a constellation of 24 satellites turning around the Earth on different orbits in exactly 12 hours. They are about 20.200 km above sea level. Those satellites are continually sending data to users and Control stations. [...]

[...] This displacement is called the roll. Conclusion So we've seen how the Autopilot in planes works, from the acquisition of data down to the action on the control surfaces. We can say that such a system is able to pilot a plane by itself from the take off to the landing. so air goes that way. Plane goes that way Electronic part P The three rotation axis around which the plane turns. With this scheme corresponding to the situation described above, we can easily see that an error in the IRU, how small it is, can lead to disaster. [...]