An analysis of Navy approach power compensator problems

Abstract

The current design concept of Naval aircraft Approach Power Compensator Systems (APCS) is briefly described. Problems with this concept are identified from flight test reports as being associated with flight path control and inappropriate thrust responses. Aircraft responses with an APCS are contrasted to those without, thereby demonstrating certain fundamental, and not always beneficial, changes in the response characteristics in the low and intermediate frequency range. Parameters descriptive of those responses are shown to depend upon not only the APCS parameters (e.g., gains) but also certain unalterable (by the APCS designer) aircraft characteristics. Among these are the rapidity of the engine's thrust response, the thrust vector inclination, and the heave damping (Zw) of the aircraft. HE use of Approach Power Compensator Systems (APCS) in the Navy dates back at least ten years. During this interval the state of the art has evolved to increasing levels of sophistication in the choice of basic feedbacks, their mixing, and the equalization (often nonlinear) used. These systems have the purpose of relieving the pilot of the throttle management task during all phases of carrier approach. This is particularly important on modern jet aircraft whose characteristics at approach speed are relatively unforgiving, thereby making the pilot's task quite demanding. The throttle (or thrust) management function is also essential to approaches with the Automatic Carrier Landing Systems (ACLS). This paper presents some of the key results of a study of Navy Approach Power Compensator Systems, § fully docu- mented in Ref. 1. In particular, certain deficiencies in these systems' performance (as evidenced by both pilot commentary and objective performance measures) are correlated with changes in the dynamics of the APCS-equipped aircraft from that of the aircraft without an APCS. In brief, the APCS acts not only as a regulator of airspeed (as intended), but also as an augmentor of the aircraft's flight path responses. Not all aspects of such augmentation are desirable. Description of Current APCS The general features of the control laws for the current Navy APCS concept are defined in the functional block diagram of Fig. 1. The basic relations expressed by this diagram involve feedbacks (and associated equalization) of sensed angle of attack (a), normal acceleration (nz), and (sometimes) feedforward of elevator deflection (Se) to the throttle (ST). The engine thrust response dynamics are a part of the airframe characteristics. The airframe may include a stability augmentation system (SAS). The APCS is required2 to control both airspeeed and angle of attack for pilot (or ACLS) control inputs and external (gust) disturb- ances.