Patent Publication Number: US-7218057-B1

Title: PAPI 1 style B combination lamp bypass and tilt switch and control system

Description:
BACKGROUND OF THE INVENTION AND PRIOR ART 
   The present invention relates generally to Precision Approach Path Indicator (PAPI) visual guidance systems for aiding pilots in landing an aircraft. Specifically, the PAPI system defines the vertical approach angle to the runway and indicates to the pilot, via colored lights, whether the angle of approach of the aircraft to the runway is correct. The colored lights are produced in a number of Lamp Housing Assemblies (LHAs), as will be described below. The Federal Aviation Administration (FAA) establishes the standards for PAPI systems in the United States, whereas the standards for foreign PAPI systems may differ. It should be understood that, while the present invention is described with respect to the FAA endorsed systems, its application isn&#39;t limited to FAA endorsed systems. 
   The components in an FAA Style B PAPI system are powered by the well-known and widely used constant alternating current (AC) loop used in most of the world&#39;s airport lighting systems, whereas the components in an FAA Style A PAPI system are powered in parallel directly from utility line power. In any PAPI system their are a number of important considerations, among them being: power consumption; number and type of lamps; size of the LHA; system reliability; ease of installation and service; safety with respect to exposed wiring and high voltages; ease of detection and identification of lamp or housing problems; environmental impact of components used; and minimization of the number of wires and interconnections. 
   The PAPI system generally comprises an array of two or four Lamp Housing Assemblies (LHAs), each of which may contain two or three individual lamps. The LHAs are located adjacent the side of, and perpendicular to, a runway and precisely aimed to define a correct vertical approach angle for guiding an incoming aircraft. Each LHA is usually fitted with an optical filter to present a white light when the aircraft is too high, i.e., above the correct approach angle, and a red light when the aircraft is too low or below the correct approach angle. When the aircraft is too high, all of the LHAs will be seen as white lights, when the aircraft is too low, all of the LHAs will be seen as red lights and when the aircraft is within the correct approach angle, one-half of the LHAs in the array will present a white light and one-half will present a red light. Generally the PAPI system comprises either two LHAs or four LHAs, with each LHA having either a set of two lamps or a set of three lamps. The two or three lamp sets appear as a single light when viewed at a far distance. A two LHA system will therefore show: two sets of red lights when the aircraft is too low; one set of white lights and one set of red lights for a correct approach; and two sets of white lights when the aircraft is too high. A four LHA system will also indicate intermediate positions within the correct approach angle. Thus the light indications will be: four sets of red for too low; one set of white and three sets of red for slightly low; two sets of white and two sets of red for correct approach angle; three sets of white and one set of red for slightly high; and four sets of white for too high. Each LHA also includes a tilt detection system and tilt switch control circuitry for disabling the entire LHA array should the physical attitude or positioning of any of the LHAs be disturbed a predetermined amount. This is necessary since the color of the light seen by the pilot could be erroneous and create a potentially hazardous situation should the LHA position be disturbed sufficiently to change its aiming. The choice of PAPI system selected is determined by a number of factors, such as airport size, aircraft size and location, traffic density, economics and the like. For example, some airport installations use multiple PAPI systems located at differing distances (touch down points) along the runway to accommodate aircraft having different landing requirements. The present invention is useful in all PAPI Style B systems. 
   The current state-of-the-art FAA Style B PAPIs include one or more series isolation transformers for the lamps in each LHA and a separate transformer at a designated master LHA to power tilt switch circuitry. A master LHA also contains the control and timing circuits for the tilt system, which includes a tilt status monitoring loop and a tilt control loop. Each LHA has a tilt switch and some mechanism to cause the master LHA to disable the entire LHA array should a tilt condition occur in any of the LHAs. In practice a tilt status signal from each LHA is sent to the master LHA which, in turn, directs each of the LHAs in the array to disable its light output, via a shorting device such as a relay, should a tilt condition occur at any of the LHAs. In order to maintain the constant current series circuit, it is also common practice to have either; (a) one isolation transformer per lamp in each LHA or; (b) a single transformer and a lamp bypass circuit for each lamp in the LHA. Therefore, with each LHA having either two or three lamps, two or three isolation transformers are required, or one isolation transformer and two or three lamp bypass circuits are required. Such systems are relatively costly and consume a substantial amount of electrical power. 
   The LHA is mounted above ground adjacent to the runway and connected to a small container, colloquially referred to as a “handhole” that is buried behind the LHA. Each container includes a current transformer and is connected to a main constant current source and to the other hand-holes via an underground conduit or directly buried cabling. It will be appreciated that a minimum number of wires and connection points in the PAPI system is a desirable objective with respect to cost, installation and maintenance. Also, it is desirable to minimize the amount of above-ground equipment to avoid damage to or from vehicles and aircraft. In general, the handholes and LHAs are interconnected through break-away type connectors that are designed to readily separate in the event of contact with a moving vehicle. The connectors are also arranged to minimize exposure of high voltages in the event of separation. In these aspects, the invention will be seen to provide major improvements. 
   Multi Electric Mfg., Inc., the assignee of the present invention, has developed and received FAA approval for a PAPI Style B system utilizing three 105 watt pre-focused lamps per LHA as opposed to the previous standard of two or three 200 watt lamps. The system includes bypass circuitry to permit continued LHA operation with one or more lamp failures. The total power consumption for each LHA is reduced from 400/600 watts to 315 watts. The current system uses standard tilt switches including pendulum types where contact is made via a ball of mercury. These mercury vial switches are not only environmentally objectionable, they are becoming more difficult to obtain. In Multi&#39;s view, an optical pendulum switch is preferred except that it also requires a power connection for a light-emitting diode (LED) and a signal connection for a photo transistor detector, which undesirably adds to power consumption and the number of interconnections. In the Multi system, the lamps are powered through a single isolation transformer requiring two wires entering the LHA. A single 300 watt transformer of minimum 90% efficiency is used, as opposed to the three-100 watt transformers of minimum 85% efficiency in the prior art. 
   The present invention utilizes the above-described FAA approved Multi Electric PAPI Style B system, with the power for a microcontroller controlled bypass system and an opto-coupler pendulum tilt system being developed from a small power supply energized by the series constant current loop in each LHA. The small power supply in each LHA may be of standard design and still produce an energy savings over the 30 watt transformer used in the master LHA of the prior art for the control circuitry. However, also disclosed herein is a novel power supply that requires only a 2 watt transformer in each LHA. 
   In prior art systems, the interconnect wiring for the power leads and tilt switch control circuits could have as many as ten connections at the master LHA and eight connections at each of the other (slave) LHAs. In contrast, in the inventive system all of the LHAs are identical, each having only four connections—two power connections and two control connections. There is no master LHA. Further in the inventive system each LHA includes a microcontroller, the power for which (as well as that required to operate the bypass circuits and opto-coupler tilt system) is derived from the constant current lamp circuit. In this context, the term microcontroller connotes an internal memory, as opposed to the usual microprocessor that uses an external memory. The arrangement reduces system cost and complexity and enhances its manufacture, installation and service. 
   SUMMARY OF THE INVENTION 
   The present invention energy efficient PAPI Style B system is characterized by a number of novel aspects, among them being: a microcontroller combination tilt and electronic bypass system that is powered from the series constant current circuit for the lamps; a readily installed optical pendulum tilt switch arrangement of improved accuracy; identical LHAs without a master LHA; and a reduced number of above-ground components with a minimal number of interconnections. 
   OBJECTS OF THE INVENTION 
   A principal object of the invention is to provide a novel PAPI Style B system. 
   Another object of the invention is to provide a PAPI Style B system of lower cost and higher efficiency than the prior art. 
   A feature of the invention is the provision of a combination tilt and bypass arrangement in a PAPI Style B system. 
   Another feature of the invention resides in the use of microcontrollers in a control loop for tilt control in a PAPI Style B system. 
   A further feature of the invention resides in the use of a readily installed optical tilt switch arrangement of greater accuracy. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects features and advantages of the invention will be apparent upon reading the following description in conjunction with the drawings in which: 
       FIG. 1  is a block diagram of a PAPI Style B system incorporating the invention; and 
       FIG. 2  is a circuit diagram of portions of  FIG. 1 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , a PAPI Style B system, constructed in accordance with the invention, includes four LHAs  12 ,  14 ,  16  and  18 , with only the components in LHA  12  (within a dashed line block) being generally disclosed. It will be understood that all of the LHAs  12 – 18  are identical and that the discussion of LHA  12  is applicable to the other LHAs  14 ,  16  and  18 . LHA  12  includes three lamps  20 ,  22  and  24  and a suitable lens (or lenses)  26  for displaying either a white light or a red light, depending upon the vertical viewing angle, to an approaching aircraft, as discussed above. In practice, a split beam lens is used, i.e., the lower half of the lens is colored red while the upper half is clear, thus producing a vertical discrimination in the perceived light beam color. Each of the lamps  20 ,  22  and  24  is coupled to a respective one of electronic bypass circuits  28 ,  30  and  32  that, in addition to being individually activated in the event of a lamp failure, are jointly controllable by a common lamp control circuit  48 . The LHAs are powered via two power terminals, labeled P, from a constant current source  34  that supplies the series-connected primary windings of a plurality of current isolation transformers  36 ,  38 ,  40  and  42 , each of which is individually located in a corresponding one of handholes  37 ,  39 ,  41 , and  43 . As discussed the handholes are buried adjacent the runway and have four power terminals (P) and four control loop terminals (C) extending above ground, whereat connections are made via break-away connectors. The actual terminals and connectors employed throughout are not part of the invention and are therefore not illustrated in detail. 
   The secondary winding of isolation transformer  36  is part of another constant current series circuit that includes the three lamps  20 ,  22  and  24  (and their respective electronic bypass circuits  28 ,  30  and  32 ) and a DC power supply  44  that provides power to the tilt and bypass circuits. A microcontroller  46  establishes timing for monitoring and control of a common lamp control circuit  48 , a local tilt detector circuit  58 , and a remote tilt drive circuit  90 . The remote tilt drive circuit  90  and a remote tilt detector circuit  80  are connected in a control loop via the C terminals labeled OUT and IN on handhole  37 . The control loop comprises a series circuit among all of the LHAs, as will be discussed in connection with  FIG. 2 . 
     FIG. 2  illustrates details of LHA  12 , it being understood that the other LHAs  14 ,  16  and  18  are identical. Similarly, only bypass circuit  28  is shown in detail, the other bypass circuits  30  and  32 , being the same. As indicated by the dashed line box, common lamp control circuit  48  includes a field-effect transistor (FET)  49  and a transistor  50 . The common lamp control circuit  48  is coupled to pin  6  of microcontroller  46  and controlled by the logic voltage thereon. The collector-emitter output of transistor  50  is connected in series with the bypass circuits, specifically with three opto-couplers  29 ,  31  and  33  in bypass circuits  28 ,  30  and  32 , respectively. Transistor  50  and the three resistors connected to it form a constant current source for opto-couplers  29 ,  31  and  33 . Lamp  20 , which is connected in the constant current series circuit with lamps  22  and  24  and power supply  44 , has oppositely poled SCRs  64  and  65  connected across its terminals. The gate of SCR  64  is connected to a corner “a” of a diode bridge  62  and to a bias arrangement of a diode  66  and a resistor  67 , whereas the gate of SCR  65  is connected to an opposite corner “b” of diode bridge  62  and to a bias arrangement of a diode  68  and a resistor  69 . A zener diode  63  and a series resistor are connected across the other opposite corners “c” and “d” of diode bridge  62  and across the output of an SCR  61  of a lamp control circuit  60  that is activated by opto-coupler  29 . 
   Power Supply Discussion 
   As mentioned above, power supply  44  is a novel arrangement that enables very high efficiency. A conventional power supply comprises a voltage driven transformer followed by a half-wave or full-wave rectifier connected to a filter capacitor and load (and often a voltage regulator). The transformer delivers current in periodic, discontinuous parcels at the peaks of the AC line power sinusoid for replenishing the charge stored in the filter capacitor (and discharged by the load and regulator, if any). Thus the transformer delivers little to no current during a significant portion of the AC line cycle. The preferred power supply  44  used with the PAPI Style B system is the dual of such conventional power supply. In it a current transformer  101  delivers discontinuous parcels of current, as in a conventional power supply. However, during non conductive portions of the AC current waveform, current transformer  101  is connected to a low impedance or short circuit rather than to a high impedance. This low impedance results in low power losses being reflected back to the current circuit that is coupled to the current transformer  101 . The primary winding of transformer  101  is connected to the X and Y terminals in the constant current lighting loop ( FIG. 1 ). The secondary winding of transformer  101  supplies a full-wave bridge rectifier  102 , the anode of which is connected to a node A that comprises the junction of the anodes of diodes  104  and  106  and the anode of an SCR  103 . The cathode of diode  104  is connected to a filter capacitor  105 , where the voltage V 1  is produced, and to the input of a voltage regulator  110 . The output of voltage regulator  110  provides a 5 volt power supply V 2  for use by the microcontroller  46  and the components connected thereto. The cathode of diode  106  is connected to the cathode of a zener  107  that is returned to ground through the series connection of resistors  108  and  109 . The junction of resistors  108  and  109  is connected to the gate of SCR  103 , the cathode of which is connected to ground. 
   In operation of power supply  44 , DC charging current is supplied to capacitor  105  from diode bridge  102  each half-cycle of the AC input wave. When the instantaneous voltage at V 1  across capacitor  105  reaches approximately 15 volts, diode  106  conducts, zener  107  goes into avalanche and the current to the gate of SCR  103  triggers it into conduction and reduces the voltage at node A to approximately zero volts. Diode  104  is therefore reverse biased and a low impedance is coupled to the secondary winding of transformer  101  (and hence reflected to the primary of the transformer) via bridge rectifier  102  and conducting SCR  103 . The low impedance at the secondary of the transformer results in a low voltage burden to the primary of the transformer and reduces the voltage drop in the constant current lighting loop. SCR  103  remains in conduction until the end of the half cycle of the constant current lighting loop when the polarity of the current reverses. At the beginning of the next power half-cycle, transformer  101  again provides current to recharge capacitor  105  for a period of time until the voltage across capacitor  105  again reaches about 15 volts, at which time zener  107  conducts and the cycle repeats. Thus, transformer  101  provides an average current for the 15 volt power supply V 1  for a small interval of time during the constant current lighting loop cycle and a nearly short circuit during the remainder of the power cycle. 
   Lamp Failure System Discussion 
   The constant current series circuit preferably operates at 6.6 amperes. At this current level, the resultant voltage developed across 105 watt lamp  20  is about 16 volts. Under normal operating conditions with lamp  20  conducting, zener diode  63  is not in avalanche. When the upper terminal of lamp  20  is positive, current flow is as follows: through diode  68  to corner “b” of diode bridge  62 ; from corner “d” of diode bridge  62  to the cathode of zener diode  63 ; from the anode of zener diode  63  to corner “c” of diode bridge  62 ; from corner “a” of diode bridge  62  through resistor  67  to the lower terminal of lamp  20 . When the lower terminal of lamp  20  is positive, current flow is as follows: through diode  66  to corner “a” of diode bridge  62 ; from corner “d” of diode bridge  62  to the cathode of zener diode  63 ; from the anode of zener diode  63  to corner “c” of diode bridge  62 ; from corner “b” if diode bridge  62  through resistor  69  to the upper terminal of lamp  20 . As mentioned, the approximately 16 volt potential developed across corners “c” and “d” of diode bridge  62  is not sufficient to drive zener diode  63  into avalanche. Therefore the voltages across corners “a” and “b” of diode bridge  62  (the voltages supplied to the SCR gates) are insufficient to initiate conduction in SCRs  64  and  65 . 
   By the very nature of a constant current source, its load voltage is proportional to load resistance or impedance. Therefore, should lamp  20  fail, the voltage across its terminals increases substantially (to about 47 volts) causing zener diode  63  to avalanche and apply a first-trigger voltage to the gates of SCRs  64  and  65  causing them to alternately conduct as the polarity of the AC input voltage changes. When SCR  64  or  65  conducts the voltage appearing across the terminals of lamp  20  is reduced substantially effectively shorting out the lamp whenever zener diode  63  is in avalanche. Should another lamp in the LHA experience a failure, a similar sequence of events will cause its associated zener diode to avalanche and apply the first-trigger voltage to its associated SCRs, driving them into alternating conduction to effectively bypass its terminals. 
   It will be noted that should a lamp failure be intermittent or a lamp experience a temporary fault condition, normal lamp operation will resume when the abnormal condition is removed since the increase in voltage across the lamp will not occur to drive the zener diode into avalanche. The time for the system to return to normal is dependent upon the time constant of the bias networks coupled to the SCR gates. The operation of the bypass circuitry is thus seen to be automatic, which translates into a significant benefit in system reliability. More importantly, however, the operation of the bypass circuitry is necessary in order to maintain continuity within a series-connected constant current circuit in the event of the failure of a load in the current circuit. 
   Tilt System Discussion 
   In accordance with the invention, the electronic bypass circuits also disable all of the lamps in response to a tilt condition occurring in any of the LHAs in the system. A tilt condition results (and a tilt signal is generated) when any of the LHAs in the system experiences a predetermined magnitude of change in its physical attitude. Tilt system operation is controlled by microcontroller  46 . When a tilt condition exists, microcontroller  46  not only causes the disablement of all of the lamps in its associated LHA, but signals the occurrence of a tilt condition, via the control loop that is monitored by the other microcontrollers in the corresponding LHAs. These microcontrollers sense the tilt signal in the control loop and deactivate their corresponding lamps. The criteria for generating a tilt signal may be established by the airport operator or other controlling authority so that temporary disruptions of orientation due, for example to strong wind gusts or the like, do not result in disablement of the PAPI system. The inventive arrangement will also be seen to be automatic and when a tilt condition is removed, operation of the LHA system returns to normal. 
   A tilt switch  54  (an optical pendulum in the preferred embodiment) develops an indication whenever the physical attitude of the LHA has been compromised by a predetermined amount, i.e., to the extent that it no longer can be relied upon to accurately establish the correct vertical approach angle. While the optical pendulum tilt switch shown is preferred, the present invention may be used with any prior art tilt switches to good advantage. The local tilt drive  70  comprises an LED  71  that is energized through a resistor  72  from V 2  to transmit a beam of light through a slot in the freely movable pendulum of tilt switch  54 . Local tilt detector  58 , which develops a tilt signal, includes a photo detector transistor  73  that receives the light beam from LED  71  as long as the physical attitude of the LHA is not disrupted beyond the tolerance of the slot in the pendulum of tilt switch  54 . The collector of photo detector transistor  73  is connected to V 2  through a resistor  57 . The junction of resistor  57  and transistor  73  is coupled, through a resistor  59 , to the gate of a buffer-inverter FET  55  that has its drain connected to V 2  through a resistor  56  and to pin  3  of microcontroller  46 . 
   With photo detector  73  conducting in response to the light beam from LED  71 , the potential at the gate of FET  55  keeps it nonconductive. Should the LHA be physically moved, the slot in the pendulum of tilt switch  54  will no longer be in alignment with the light beam between LED  71  and photo detector  73  and the light beam will be interrupted. Therefore photo detector  73  ceases conduction, whereupon the gate voltage of FET  55  rises and drives FET  55  into conduction. This change, signifying the possible existence of a tilt condition, is sensed by microcontroller  46 , which after a predetermined time interval selected to avoid spurious operation, verifies that a tilt condition does exist and generates a signal to simultaneously activate common lamp control  48  and remote tilt driver  90 . 
   Remote tilt driver  90  comprises a transistor  91  having its emitter-collector circuit connected between V 1  and the input of a voltage regulator  96 , and its base coupled to the drain of an FET  92 , the gate of which is coupled to pin  2  of microcontroller  46 . The output of regulator  96  is connected to a protection diode  93  and is connected to the OUT terminal through a resistor  97 . The OUT terminal is also connected through a resistor  81  to: the cathode of a diode  94 , the anode of which is connected to ground; the anode of a diode  95 , the cathode of which is connected to V 2 ; and to the junction of pin  7  of microcontroller  46  and a capacitor  82 , the other terminal of which is returned to ground. Diodes  94  and  95  act as protective diodes at pin  7  of microcontroller  46 . The IN terminal is connected to ground. It should be noted that the ground points in each LHA are isolated and distinct from earth ground. The control loop circuit is formed by connecting the OUT terminal of LHA  12  to the IN terminal of LHA  14  and the OUT terminal of LHA  14  to the IN terminal of LHA  16  in a daisy chain configuration that ends with the OUT terminal of LHA  18  being connected to the IN terminal of LHA  12 . 
   FET  49 , in common lamp control  48 , is driven from pin  6  of microcontroller  46  and controls operation of transistor  50 , whose output is in series with opto-couplers  29 ,  31  and  33  in the bypass circuits. Normally microcontroller  46  places a logic 1 voltage on pin  6  which drives FET  49  into conduction causing conduction of transistor  50  and energization of the opto-couplers. When opto-coupler  29  is energized, the transistor  100  within opto-coupler  29  is conductive. Gate drive current to SCR  61 , via resistor  99 , is bypassed to the cathode of SCR  61  by conducting transistor  100 . This keeps SCR  61  nonconductive, placing bypass circuit  12  in a non conductive state, allowing lamp  20  to be illuminated. Responsive to a tilt condition, microcontroller  46  places a logic 0 voltage on pin  6  which keeps FET  49  non conductive, causing non conduction of transistor  50  and deenergization of the opto-couplers. Opto-coupler  29  allows resistor  99  to trigger SCR  61  into conduction which places a low impedance path across corners “c” and “d” of diode bridge  62 , simulating an avalanche condition in zener diode  63 . This gives rise to a second-trigger voltage at the corners “a” and “b” of diode bridge  62  which drives SCRs  64  and  65  alternately conductive, as before. Thus the voltage across the terminals of lamp  20  is reduced (effectively extinguishing the lamp) as previously discussed. This action simultaneously occurs in each of the other bypass circuits  30  and  32 . Here again, the lamps will remain extinguished as long as the tilt condition persists. It will be noted that when lamp  20  fails, the trigger voltage is determined by zener  63  and is about 47 volts. SCR  61  conducts at a much lower voltage. 
   The programming of microcontroller  46  includes a timing arrangement for assuring that a tilt condition exists for a predetermined time before developing a tilt signal. This reduces spurious operation due to transient disturbances of physical positioning, such as from strong wind gusts and the like. Under normal operating conditions, when the LHA is not in a tilt condition, transistor  91  and FET  92  are conducting. The voltage V 1  is about 15 volts and the action of the voltage regulator  96  results in a voltage at the output of voltage regulator  96  of about 12 volts. With all of the OUT and IN terminals of all four isolated LHAs  12 , 14 , 16  and  18  connected in series, the sum of the voltage rise from voltage regulator  96  (and the corresponding regulators) and the voltage drop across resistor  97  (and the corresponding resistors) is zero. The resulting voltage at terminal OUT is zero volts. When a tilt signal is generated, due to a tilt condition existing for the requisite amount of time, the voltage at pin  2  of microcontroller  46  turns FET  92  and transistor  91  off which results in an output of 0 volts from voltage regulator  96 . This represents a loss of 25% of the total voltage produced by all of the voltage regulators in the loop. The net effect is to reduce the voltage across resistor  97  and its counterparts in all of the LHAs by 25% (from 12 to 9 volts), resulting in a net voltage of 3 volts at terminal OUT of all of the LHAs. This voltage is recognized as a logic 1 (at pin  7 , via resistor  81 ) of all of the microcontrollers in the array. The microcontrollers in the LHAs sense the logic 1 at pin  7  and provide a logic 0 at pin  6  which deenergizes all of the opto-couplers. This allows resistor  99  (and its corresponding elements in the other bypass circuits) to provide gate trigger current to SCR  61 , causing all bypass circuits to conduct and effectively extinguishing all of the lamps in the array. It will be noted that if a two LHA array is used in the PAPI system, the total voltage from all of the regulators is reduced by 50%. 
   Remote tilt detector  80 , consisting of a resistor  81  and a noise filter capacitor  82 , is coupled to pin  7  of microcontroller  46  which senses a drop of about 25% (or 50% in the case of a two LHA array) in the voltage at the OUT and IN terminals as constituting a tilt signal. This voltage drop appears across all of the OUT and IN terminals of the LHAs and is sensed by the corresponding remote tilt detectors and microcontrollers in the other LHAs, which respond by disabling all of their lamps. Thus, a simple two-wire control loop among the LHAs in the array is used for monitoring and disabling all of the LHAs in response to a tilt signal generated by any of the LHAs in the array. 
   Each LHA also includes a tilt indicator  85  for providing a visual indication of a tilt condition. In accordance with this aspect of the invention, a red LED  86  is illuminated when the physical attitude of the LHA is disrupted to cause a tilt condition and a green LED  87  is illuminated when the LHA is in vertical alignment. The housing of prior art pendulum type tilt switch is affixed to a properly aimed LHA and aligned with a spirit level. After the housing is leveled, the LHA is tested to verify that the tilt switch is properly installed. Not only is this procedure time-consuming, the use of a spirit level adds a variable degree of installer skill which results in an undesirable range of tilt switch tolerances. With the invention, the tilt switch housing may be easily and accurately installed on the LHA by the installer simply observing an LED. When the green LED is illuminated, the switch is properly installed. In the preferred embodiment of this invention the tilt switch pendulum has a very narrow, laser formed slot, which assures close tolerances on the tilt switch. 
   The local tilt indicator  85 , as mentioned above, is coupled to pin  5  of microcontroller  46 . Pin  5  of the microcontroller is connected to the junction of green LED  86  and red LED  87 . Assuming the LEDs to illuminate at 5 volts, it will be seen that when pin  5  is at logic 1, green LED  86  is on (illuminated) and red LED  87  is off. For a logic 0 at pin  5 , green LED  86  is off and red LED  87  is on. Thus the interruption of the light beam in tilt switch  54  translates into an illuminated red LED  87 , whereas an uninterrupted light beam, corresponding to a correct positioning of the LHA, translates into an illuminated green LED  86 . 
   What has been described is a novel PAPI Style B system of improved cost and efficiency and that is more readily and accurately installed and maintained. It is recognized that numerous changes in the described embodiment of the invention will occur to those skilled in the art, without the need for inventive skill. Therefore, the scope of the invention is to be limited only as defined in the claims.