MONITORING TECHNIQUES

Performance and Failure Assessment Monitor (PAFAM) System

This system is also one which uses a digital computer and a colour CRT display, its purpose being to operate in conjunction with an automatic flight guidance system (AFGS) to provide a flight crew with a prediction of the quality of an automatic approach and landing manoeuvre being carried out in low visibility. It monitors aircraft attitude, heading, and performance of the AFGS and makes a continual assessment of whether or not a successful automatic landing will result. In the event that the progress of the manoeuvre is unsuccessful, a `TAKEOVER' command is displayed; if the aircraft is being flown manually with commands from the flight director system, and the approach path is unacceptable, the legend NO TRACK is displayed. A block diagram of the system  necessary for proper operation of the AFGS and auto throttle/speed control system. Electrical power is applied when the AFGS LAND ARM mode or flight director ILS modes of operation are selected, and the system is automatically switched to its operational condition when the ILS localizer and glideslope are being tracked.

The signal inputs to the computer are a.c. and d.c. analog and are multiplexed into an A/D converter which is under programmed  memory control by one of two control processors in the computer; this processor performs most of the landing performance and prediction computations. Discrete signal inputs are multiplexed directly into the second processor, which provides display drive commands, landing system failure assessment, and controls signals for discrete outputs. Interconnection between the two processors is through two 18-bit storage registers.

 Analog signals from the computer are applied to the display electronics unit, and they provide commands for blanking out a portion of two raster-scanned CRT display units (one for each pilot) as well as commands which determine the location of desired characters in the display. The location of a display unit is shown in  the viewing area of the CRT is 38 mm x 76 mm. Discrete signal outputs are supplied to the AFGS and auto throttle/speed control system.

The digital signal outputs from the computer are applied to timing and logic circuits in the display electronics unit for the development of analog character signals via fixed memory circuits in a symbol generator. The character signals are amplified by horizontal and vertical summing amplifiers, and then fed to deflection amplifier and blanking circuits so that desired symbols and words are `painted' on the CRT screen. A colour control logic circuit supplies the CRT with a command signal which varies the level of a high - voltage supply so as to vary the colour. As the voltage is increased in selected steps, the colour of the character or raster being generated changes from red to red-orange, to amber, to yellow, and finally to green.

 Operation

When the LAND or ILS mode logic is available from the AFGS computers, a TEST mode display first comes into view

 . Then, after a very short time period, a raster pattern is displayed on the CRTs representing the airport runway over which is superimposed a cross depicting the predicted touchdown point, . The runway symbol has a yellow border within which there is a green area representing the center touchdown zone. The area corresponds approximately to a zone ±18 m (60 ft) laterally and ±300 m (1000 ft) longitudinally about the nominal or ideal touchdown point, which is on the centreline and approximately 150 m (500 ft) beyond the glideslope transmitter location. The horizontal line closest to the bottom of the display corresponds to the runway threshold.

The symbol `expands' by moving downwards as a function of computed range in a manner corresponding to the same rate of expansion that would be apparent if the real runway were visible out the flight deck windscreens. As the aircraft crosses the runway threshold, the bottom line of the display moves out of view, and on the basis of range computation, the lower edge of the green area reaches the bottom of the display when the aircraft passes over the nominal touchdown point. The green area continues to move down until touchdown. At touchdown, a green downward-pointed triangle is displayed on the CRT screen (Figure 2.3(c)) and the display then remains static for about three seconds, after which the whole system is de - energized.

The orange-coloured cross symbol is positioned to show the predicted touchdown point on the runway as determined by simplified dynamic models of the aircraft/AFGS combination operated in an accelerated time scale. In an ideal landing situation, the cross will be superimposed over the touchdown area, with the intersection of the arms coincident with the centre of the area. The arms of the cross represent the uncertainty of the basic touchdown prediction (e.g., the effect of winds, ILS beam anomalies, and acceptability of AFGS responses), the uncertainty being reflected by variations in the lengths of the arms, and deflection of the cross from the centre of the displayed runway area. All parts of the cross should remain in this area for an acceptable landing.

If the system detects such uncertainty of performance that an approach could be seriously impaired, then the message TAKEOVER or NO TRACK is displayed in red letters on each pilot's display unit, together with a yellow arrow symbol in one of the four corners to indicate which of the two pilots should perform a manual go-around procedure, or a continued manual landing. The arrow appears in the left or right corners depending on which pilot the more valid information (Figure 2.3(d)) has.

In the event of failure of either of the ILS ground transmitters (localizer or glideslope), the loss of the desired guidance signal information is detected by the PAFAM system and the message NO ILS is displayed in red (and flashing) on the indicator units (Figure 2.3(f)). Normally under these circumstances the AFGS would automatically disengage, but as the PAFAM system uses other references to supply equivalent ILS deviation for assessing performance, then if no deterioration in performance is detected, disengagement of the AFGS is inhibited for up to 5s. If, during this interval, ILS signals are restored, the PAFAM system reverts to the normal display mode and the landing continues uninterrupted. If the signals are not restored, or if the performance is affected, the AFGS would then disengage and the NO ILS message in the display units would automatically change to TAKEOVER.

 An automatic self-testing function is built into the system and checks the whole of its operation twice each second. Any malfunction causes latching-type annunciators to trip, and gross distortions or blanking out of symbols on the display units.