By Ronald A. Keith

FLIGHT PATH Control, otherwise described as "brains for the automatic pilot," has been adopted by Trans-Canada Air Lines and is expected to provide the airline's entire North Star fleet with a fully automatic approach system within a few months.

        Briefly, FPC is a supplement to the well-known PB-10 Automatic Pilot. It is fundamentally an electronic computer which, through the PB-10, flies the aircraft automatically on radio beams and down the ILS approach beam to the runway. It interprets the indications of the ILS cross-pointer meter and applies control through the Automatic Pilot to operate the airplane's control surfaces and throttles.

        First commercial carrier to receive this automatic approach equipment, Trans-Canada Air Lines has collaborated with the manufacturer (Eclipse-Pioneer Division of Bendix Aviation Corporation) in development and flight testing of the FPC for commercial use.

        Research and testing now are complete. About 40 approaches have been made to date with the FPC in North Stars. The system was demonstrated recently at Dorval Airport, Montreal. Further flights will be made during the next few months but these will be in the nature of shakedown trials. It is anticipated that the automatic approach equipment will be in general familiarization use on the airline next summer and will be ready to cope with the bad-weather period next fall.

        Incidentally, the North Star was the first four-engined aircraft to be equipped with the Pioneer Flight Path Control.

        The Instrument Landing System has been accepted by the International Civil Aviation Organization and is in use at the main air terminals in Canada as well as at 70 U. S. airports. It consists of a transmitting unit on the ground which sends a set of VHF (very high frequency) beams into the sky.

        One beam, the "localizer," defines the aircraft's position laterally in relation to the centre line of the runway. The other beam, the "glide path," establishes the aircraft's position in reference to a sloping approach line. The two beams are projected so that they intersect each other.

        The crosspointer meter on the instrument panel has two needles which intersect each other at right angles in the centre of the dial when the aircraft is on the centre of the localizer and glide path beams.

        Now let us assume we are air-borne in a North Star equipped with the ILS, the PB-10 Automatic Pilot, and the Pioneer Flight Path Control.

        Following the TCA procedure with this equipment , the pilot has flown outward on the ILS localizer beam for the specified time. This was followed by a procedure turn (45 degrees) off the heading, then a 180-degree turn to intercept the beam at a 45-degree angle. On the outbound leg the automatic pilot has been tested and is now engaged.

        The vertical needle on the cross-pointer meter has been at full deflection. As the localizer beam is approached this needle starts to flick toward the centre. At this point the pilot turns the sequencing switch

    "LOCALIZER." The procedure now is entirely automatic.

Precision is Remarkable

        It is uncanny to witness the process as Flight Path Control takes over. With remarkable precision the aircraft banks, turns and levels without a hand touching the controls. The beam bracketing manoeuvres are smooth and perfectly co-ordinated. The airspeed indicator stays right the mark with hardly a flicker, even in rough air.

        As the plane tracks the localizer beam at the altitude specified for proper interception of the glide path, the pilot watches the horizontal needle of the crosspointer meter. When the needle nears the centre of the meter, indicating that the plane is close to the glide path beam, he turns the switch to "LOCALIZER AND GLIDE PATH."

        The Flight Path Control takes over to provide the control of elevator and throttles, as well as rudder and ailerons to track the glide path as well as the localizer path automatically.

        As the aircraft descends along the glide path, any deviation from it automatically applies a given amount of elevator and throttle adjustment to correct the error, at the same time maintaining constant airspeed.

        A noteworthy feature of the system is the fact that the pilot can increase or decrease this constant airspeed by merely by rotating the pitch knob on the automatic pilot. At any time the throttles may be operated manually.

        When the FPC has brought the aircraft down close to the end of the runway, the pilot simply presses a button on the control wheel to disengage the automatic pilot and take over manual control for the landing.

        To get a clear understanding of Flight Path Control, it is essential to bear in mind that we are in reality dealing with three systems: The PB-10 Automatic Pilot, the Flight Path Control and the Instrument landing system. Both the PB-10 and the Flight Path Control are essentially "error" sensing systems but the fundamental object of each system is different.

        The PB-1O establishes flight references for the heading and attitude plane, senses any "errors" or deviations in flight from these references, and applies control to maintain the plane at the required head and attitude.

"George" Applies Correction

        In other words, the PB-10 senses any movement of the plane about its centre of gravity and applies corrective control; the PB-10 does not sense any change of position in space of the centre of gravity of the plane. That is, the PB-10 will not constrain the flight of the plane to a pre-determined ground track or flight path. This is the objective of the Flight Path control. However, Flight Path Control does not define the flight path; this is accomplished by the directional radio beams which are detected by the Instrument Landing System receivers.

        Therefore, to fly an airplane auto on radio beam references, three basic requirements must be fulfilled; first there must be a means of detecting and sensing the position of the radio beam which defines the flight path; secondly, the sensing signals must be modified to include proper control characteristics for application of automatic control; and finally, there must be a means of applying these signals to produce control for tracking the radio beams.

        The first of these requirements is met by the Instrument Landing System, the last by the PB-10 automatic pilot, and the intermediate requirements by the Flight Path Control.

        The Instrument Landing System to which the Flight path control is connected, consists of two separate receivers for detecting localizer and glide path beams, a control box, and a cross-pointer meter. Each of these receivers can be tuned by means of the control box to the frequencies of the various beams that the pilot may care to track.

        The cross-pointer meter is an indicator with two needles which are operated by signals from the localizer and glide path receivers. The meter, in reality, is composed of. two sensitive centres-reading dc. volt-meters, the needles of which are crossed on the face of the indicator. The vertical needle is actuated by signals from the localizer receiver and the horizontal by signals from the glide-path receiver.

        When the pilot tunes in the carrier frequency of the particular range, he is, in effect, simultaneously tuning in two signals from two carrier waves of the same frequency but which are modulated at different frequencies. These carriers are transmitted as directional radio fields which overlap.

        At the centre of the overlapping section, between the pair of fields, the intensity of one field equals that of the other. This line of equal intensity is the beam. To either side of this beam, one field or the other will be stronger.

        It is this difference in field intensities that is used to determine the angular displacement of the airplane relative to the beam.

        In order to separate and identify these two fields, the receiver includes two filters which operate simultaneously. One filter passes the modulation frequency of one field and the other filter passes the modulation frequency of the other field. The output of each filter is rectified and, emerges as a d-c signal. These two outputs are combined to produce a single differential d-c signal which applied to the cross-pointer meter to actuate the needle of the indicator.

        Therefore, when the intensity of the radio fields balance each other, the output of one filter exactly balances that of the other, resulting in a zero differential, no current flow, and no deflection of the needle. But when the lane is to one side of the beam the received radio signal from one field is stronger than the other. This in turn produces current flow and corresponding deflection of the needle. At the same time, the magnitude of the differential signal controls the amplitude of the needle deflection, thereby providing a direct indication of the angular displacement of the plane from the beam.

        In this manner, the localizer and glide-path receivers of the Instrument Landing System supplies signals to operate the cross-pointer meter providing a visual indication for tracking the beams manually.

        Although the signals from the localizer and glide-path receivers are adequate to operate the cross-pointer meter to provide satisfactory visual indication for tracking the beam by manual flight, they do not possess all the necessary control characteristics far automatic tracking of the beams. Therefore to produce these control characteristics, an intermediate step must be added between the receivers and the PB-10 Automatic Pilot. This is the function of the Flight Path Control.

Details of FPC

        The Flight Path Control system consists of three major components: The Flight Path Computer, the Throttle Servo Amplifier, and the Throttle Servo. The Flight computer is the heart of the generates signals having the proper characteristics for automatic operation of the airplane’s controls and throttles.

        The Flight Path Computer consists of two main channels, one for the localizer signals and the other for the glide-path signals. They are identical in design and operation. The output of the localizer channel is super-imposed upon the rudder and aileron signal systems, of the PB-10 to produce co-ordinated automatic control for tracking range and localizer beams.

        The output of the glide path channel is super-imposed on the elevator signal system of the PB-10 and simultaneously is applied to a throttle servo control system to produce controlled rate of descent for tracking glide-path beams.

        Once the plane is tracking the beam, there is no output from the computer and the PB-10 system alone flies the plane. This means that a tendency of the airplane to change heading or attitude so long as it does not change its position relative to the beam, will be corrected by the PB-10. However, any disturbance which shifts the plane so that it is tracking the beam, again results in FPC signals to put the plane on the beam.

        Regardless of the drift conditions the FPC will compute the proper signals for getting the plane on the beam and holding it there, automatically establishing the proper crab angle.

        Now let us see what happens when the pilot makes a landing approach on the localizer and glide-path beams.

        In making landing approaches on FPC, the pilot starts his approach with the Sequencing Switch on the "LOCALIZER" position and with the plane at an altitude that will make it possible to contact the glide-path from below.

        When the plane enters the lower field of the glide-path range, the horizontal needle of the cross-pointer meter will move off centre against the upper stop. Then, as the plane nears the beam, the needle will start to move away from the upper stop. When the needle nears the centre position, the pilot changes the sequencing switch from "LOCALIZER" to "LOCALIZER AND GLIDE PAM" This connects the glide path receiver as well as the localizer receiver to the computer.

        The differential signals from the glide path receiver now operate the glide path channel of the computer to generate signals for operating the elevator and throttles of the airplane. If the needle was not at the centre when the sequencing switch was turned to "LOCALIZER AND GLIDE PATH" position, the computer would generate signals to apply up elevator through the PB-10 and, at the same time, apply signals to the throttle servos for advancing the throttles proportionately for the amount of up elevator control applied. Therefore, the plane would climb to intercept the glide path beam. Climb, however, is limited to prevent any unsafe pitch attitude.

        If the plane approaches the glide path from below, the differential displacement signals from the glide path receiver gradually diminish. This means that the up elevator control would gradually be diminished and throttles would be gradually retarded the closer the plane came to the glide path.

        In a manner similar to that used for automatically establishing the proper crab -angle when flying range or localizer beams, the integration circuit of the glide-path channel generates the signal for automatic tracking of the glide-path under varying conditions of loading or head winds.