It is known that, since the start of civil aviation, the take-off of an airplane is handled manually by the airplane's pilot by means of piloting units (central control column or side-stick controller, rudder bar) that are specific to the airplane concerned. During this take-off phase, the crew is responsible for controlling the path of the airplane, both in the vertical plane and in the lateral plane (or horizontal plane). The crew has to follow theoretical guidance objectives (recommendation of the airplane's flight manual), visual guidance objectives (axis of the runway) and/or physical guidance objectives (for example, reference of an LOC-type antenna, specified below).
In such a standard manual take-off, there arises the problem of the quality with which the guidance objectives are followed during the take-off.
If we look firstly at the elevation guidance (that is, in the vertical plane), the fact that the crew is responsible for the take-off renders the quality of the rotation directly dependent on the behavior of the pilot.
Assessing the rotation speed, the speed at which the pilot must pull on the control column (central control column or side-stick controller) to initiate the rotation of the airplane, is done visually.
Because of this, a greater or lesser time delay may occur between the moment when the pilot realizes that he has reached the rotation speed and the moment when he pulls on the control column.
At this instant in the take-off, the airplane is at high speed, and this time delay is reflected in a dispersion over the take-off distance.
In the opposite case, it may be that the pilot pulls too early on the control column. In this case, the speed of the airplane does not allow it to take off, and the airplane continues to accelerate with the nose lifted. This is reflected in a very significant drag, and also, a direct impact on the take-off distance.
Furthermore, with large airplanes, a risk of tailstrike on take-off must be taken into account. It is, in fact, not rare to observe this type of incident in service, which normally results in a lengthy lay-up of the airplane. Such an incident is normally the result of a nonconforming action on the part of the pilot (excessive pull-up command) or an unforeseen external phenomenon (violent gust of wind, for example).
If we now consider, generally, the lateral guidance of the airplane, there is today just one source of help in the lateral guidance, namely a lateral alignment beam of LOC type which is emitted from the ground and which is detected on the airplane using a detection system. Such a lateral alignment beam or LOC beam is normally emitted by a directional VHF radiotransmitter, which is placed on the axis of the runway, at the end opposite from the threshold. This LOC beam is intended for landing and, in principle, ensures azimuth guidance along the approach axis, and this according to an ideal lateral alignment profile in a precision instrument approach of the ILS (Instrument Landing System) type. This radiotransmitter normally emits two signals with different modulation which overlap in the approach axis where the two-signals are received with equal intensity. Although this LOC beam can also be used for the take-off, it was created, and is characteristics have therefore been more particularly adapted, for use on a landing.
If a sight guidance mode (as mentioned above) remains acceptable in clear weather, it becomes very difficult in reduced visibility conditions. Similarly, the workload of the pilot greatly increases if an engine fails. The pilot must then trim the thrust difference while ensuring a good rotation.
Thus, the performance levels of the airplane on take-off directly depend on the behavior and the feelings of the pilot (stress, fatigue). This phenomenon is amplified when the weather conditions worsen. The wind is in particular the most disturbing factor in this phase.