EMERGENCY PROPULSION-BASED AUTOLAND SYSTEM Nicolas Fezans∗ , Maxence Gamaleri∗ ∗DLR (German Aerospace Center) - Institute of Flight Systems [email protected] Keywords: Autoland, Propulsion-Controlled Aircraft, Emergency System, Monte Carlo Simulations Abstract still happen and has happened several times in the past (e.g. Japan Airlines flight 123 near Tokyo, In this paper, an emergency autoland system ca- United-Airlines flight 232 in Sioux-City or DHL pable of landing automatically with the ATTAS in Bagdad). As shown in the three aforemen- research aircraft only using engine thrust vari- tioned accidents it might still be possible to con- ations is presented. This autoland is intention- trol the aircraft by means of thrust variations. Of ally kept as simple as possible, but demonstrates course the maneuverability of the aircraft is then good performance even in the presence of uncer- very restricted, but this possibility has been in- tainties and external disturbances. The perfor- vestigated since the early 90’s when a research mance in the presence of wind shears is shown program on propulsion controlled aircraft (PCA) in the paper. It is based on a previously published took place at NASA. In this program, a pilot as- propulsion-based control law for the inner loops. sistance system was developed and demonstrated Implementing this simple emergency autoland in in simulator as well as in flight tests with several modern aircraft is not challenging. Aircraft al- aircraft types [1–3]. More recently developments ready in service could also be retrofitted. have been made on PCA technologies at the DLR and a propulsion-based control law was even suc- 1 Introduction cessfully demonstrated in flight test [4, 5]. The developed system was able to assist pilots and Jet airplanes are usually designed to be con- provide them good chances to land successfully. trolled by means of both engines (mainly act- During the first part of our simulator studies, pi- ing on speed/energy) and control surfaces which lots did not receive explanations on the way they are deflected to generate aerodynamic forces and should use the system. The goal was to check moments at several places on the airplane frame. experimentally the affordance of the developed These control surfaces are usually actuated by system and how fast pilots were able to figure means of hydraulic actuators. Quite recently, out how to use it. Results were very encourag- electrical actuators have also been introduced in ing but it appeared that some of the airline pi- the most modern jet airplanes. As these sys- lots taking part to the studies misunderstood the tems are crucial for the control of the aircraft, si- basic flight dynamics principles the control law multaneous failures affecting them must be pre- relies on, leading sometimes to dangerous reac- vented. Therefore, aircraft control surface actu- tions. After short explanations, all pilots were ation systems are implemented using highly re- able to land with acceptable touchdown condi- dundant architectures: multiple actuators, control tions in almost all following trials. signal transmission chains and power sources. In our opinion, this good result reproduces Even though aircraft are designed such that the results obtained by NASA in the 90’s but a complete failure of the primary control effec- still is not fully satisfying. Indeed, both airline tors is extremely improbable, this situation can 1 NICOLAS FEZANS, MAXENCE GAMALERI and military pilot trainings should not be used level obtained is sufficient, which validates this to train intensively for this very remote situation. initial choice. Without specific training and without making pi- The global structure can be represented by the lot aware of the way aircraft can be controlled block diagram in Fig. 1 in which the way the au- by means of thrust variations, the added value for topilot uses the already existing propulsion-based the system is clear but much lower as it seems it control law is explicitly shown. In this figure only could be. Apart from that, one of the results of the components of the autopilot that are activated the simulator tests was that the performance of during autoland operations are represented inside pilots seemed to be mainly limited by the men- the “autopilot” block. tal workload. This was not a surprise as the air- craft reaction is very slow and pilots must be ex- Glideslope γREF PLA L tremely attentive to predict the consequences of controller Control t law Airplane o Ground their actions (even with the the assistance of the l i ΦREF from [5] p track o control law from [5]). This suggests a completely t controller PLA R u different solution: the design of an autopilot with A χREF an autoland function. This autoland permits to Localizer γ, Φ, p, q, r, nz, N1 L, N1 R get results independently of pilot understandings controller of the way the system is working internally. Ad- Inertial position and speed ditionally, this will permit to get a performance Fig. 1 Control structure for automatic landing level in the presence of disturbances that a human pilot would never obtain. The autoland function has to intercept the The autoland function of the autopilot will be runway centerline and the glidepath, to track presented in details in section 2. In section 3, the them up to the ground and to land possibly with behaviour of the autoland is demonstrated using a flare. In a previous version of this autoland [6], Monte Carlo simulations. Finally, conclusions the deviations with respect to the runway cen- and outlook are provided in section 4. terline and the glidepath were provided by the glideslope and localizer indicators. When the air- 2 Propulsion-based autoland craft is quite far from the localizer and glideslope emitters, these indicators can be interpreted as an For the design of the propulsion-based autopilot, angular measurement of the lateral and vertical two main options were considered: deviations. As shown in Fig. 1 and with the aim of giving more flexibility to the pilot to choose • the design of a standalone autopilot, the approach slope and later to ease the defini- which would directly command the en- tion and realisation of a flare maneuver, the clas- gines through the power lever angles, sical ILS measurements were replaced by the in- or: ertial position and speed. Internally the reference • the design of an autopilot as an outer loop system used is the WGS-84 system. Of course for the pre-existing propulsion-based con- a relative positioning cannot be replaced by an trol law (see. [5]). absolute one without some other changes: in the present case it must be additionally assumed that As it was estimated that the second option would the position and orientation of the runway are ease significantly the development of the autopi- known with precision and that the absolute po- lot, this option was chosen. Of course, when im- sitioning is also precise enough. Note that “ab- posing a structure to a controller the reachable solute positioning” does not mean GPS and only performance and robustness might be reduced GPS, but could be obtained by coupling inertial (compared to the unstructured case). It is later platforms with GPS and barometric+radar alti- shown by simulation results that the performance tude or any other useful source available. 2 Emergency Propulsion-based Autoland System 2.1 The underlying control law of [5] the flight path angle. Such a control law will ba- sically consist of controlling the phugoid (accept- This autopilot is designed as an outer loop using able response time, good damping, and no static the control law of [5], which is already follow- error on the flight path) while avoiding unneces- ing the references on the flight path angle and on sary excitation of the short-period mode. the roll angle. In the current paper, this control Note that the flight path angle cannot be con- law, how it works and how to tune it will not be trolled independently from the speed. In order to re-explained in details but a short overview is pre- reach the runway the angle of descent (i.e. the sented hereafter. flight path angle) must be controlled. Once the flight path angle is determined, there is no degree 2.1.1 Control law requirements of freedom left to select the speed. In this section, the main requirements for suc- Lateral Control cessful approach and landing by means of a PCA system are discussed with focus on how desirable The lateral dynamics of an aircraft are com- they are, how difficult it will be to reach them, posed of: and which trade-off between the performance cri- • the Dutch roll mode exhibiting a pair of teria should be made. Obviously, classical han- complex conjugate and stable poles with dling qualities criteria are not applicable for an very low damping, aircraft having a propulsion-based control law. • the roll mode which is aperiodic and stable, Longitudinal Control • the spiral mode which is slow and quite of- The longitudinal motion is mainly composed ten slightly unstable. of the phugoid mode and the short-period mode. As for the short-period mode, the Dutch roll The period of the phugoid is generally between mode and the roll mode are generally too fast 30 and 60 seconds for transport airplanes. The to be significantly modified by means of the en- frequency of the short-period mode depends on gines, in particular in the low-thrust domain that the aircraft and its center of gravity location, but will generally be required for descent and ap- would typically lie between 1:5 and 3 rad=s. proach. However, a control law based on thrust Increasing the total thrust of the engines leads can easily modify the spiral mode in order to to an increase of the energy rate of the aircraft.