Try our ILS tutorial that illustrates and describes how to display ILS navigation signals on board aircraft in various positions, which may occur during a safe approach for a landing. The whole program, as you can see, consists of four parts. The section in which you're reading this text is called the status window. In this window, you'll be able to read notes about the flight conditions in which the aircraft got itself into during the course of the animation. Left of the status window, you can watch the landing approach as seen from above. Under the status window is the landing approach displayed from a side view. Both planes (the above and side view) are interconnected, meaning that the aircraft's position is identical in both views. The last part represents the onboard instrument panel equipment, which is situated under the status window, to the left. Even if it's in the view from above, it forms an independent module. The displayed panel equipment serves for the indication of the aircraft's position towards the horizontal and vertical plane and also for a discontinuous distance measurement with the assistance of marker beacons, as seen with the O-M-I indicator.
situation #1 - At this moment, the aircraft is out of range of the localizer's signal and also beyond reach of the glide slope's signal. We can verify the given situation on both views, whether from above, or the side perspective. As seen, the aircraft's located off coverage. The absence of the navigational signal's reception is confirmed by the onboard indicator which displays red NAV and GS bats as a warning for the pilot. Relative to the aircraft's momentary position, meaning more than 13 km away from the runway's threshold, we can't yet talk about a landing approach with the use of the ILS system. So far, it applies as a standard flight according to a beforehand set flight plan, however descent to an approach altitude has been initiated.
situation #2 - The aircraft got into reach of both navigational signals, i.e. the localizer's and glide slope's signal. Please notice and check that the plane is already in touch with the depicted navigational signals in the view from above, as from side. The onboard indicator interprets the presence, respectively interception of the navigational signals by switching off the NAV and GS bats. Next, the onboard indicator also evaluated the plane's position in the landing approach space. By evaluation we mean deflection of the indicator's gauges into positions of the exact glide slope. It's necessary to realize that it is a matter of the so-called command indicator, meaning that if the vertical gauge (evaluating the localizer's signal) is deflected to the right, it indicates a necessity for a correction to the right and vice versa. The same goes for the horizontally set gauge (evaluating the glide slope's signal), to the intent that if the aircraft is located under the glide slope, the gauge is deflected upwards - therefore indicating a necessity for correction upwards and vice versa. In the current position, the plane's located partially left of the approach path and slightly over the glide slope. The onboard indicator evaluated the situation by deflecting the horizontal gauge downwards and the vertical gauge to the right.
situation #3 - As you could notice, the aircraft has changed it's position since the last situation by considerably deflecting leftward towards the approach axis as can be seen from above, but it hasn't changed the flight altitude, as proved by the side view. Looking at the onboard indicator you could notice that only a movement of the vertical gauge took place, even more to the right, meaning that the aircraft put even more distance between the approach axis, to the left. Let me remind you that if the vertical gauge would move rightward, it would signify that the pilot is advised about the size of a necessary correction to the right, since we're dealing with the command indicator.
situation #4 - The aircraft hasn't changed its course since the previous situation (in our case its position towards the approach axis), neither its altitude (it didn't climb nor descent), thus it flew a steady, stable, straight flight. The plane has reached a distance of 11 km, which designates the distance between the outer marker that is from the bird's eye view marked with the letters OM and depicted by a blue beacon. The outer marker beacon is always placed in the overlapping point of the navigational signals. As you can see, a blue "O" bulb has lit up on the onboard indicator O-M-I that briefs the pilot about the distance towards the runway. You could also hear a sound, typical for the outer marker beacon. This sound is identical with the one that the pilot hears in his headphones. For further detailed information concerned about the outer marker beacon, please point and hold your mouse over the blue beacon marked as OM.
situation #5 - The pilot has carried out a flyover over the outer marker, consequently the aircraft flew over the approach axis while changing its flight altitude, specifically it passed to the right and dropped under the glide slope's axis, which could be seen in both views. The onboard computer evaluated the situation by moving the vertically oriented gauge to the left, since the plane's positioned right of the horizontal approach axis, while the size of deflection shows the size of a necessary correction leftward. The same applies also for the horizontally oriented indicator's gauge that shifted upwards. Next you could notice that the indicative blue "O" bulb switched off on the onboard O-M-I indicator and the tone in the pilot's headphones ceased as well, on account that the aircraft got out of reach of the outer marker's signal.
situation #6 - The pilot has continued to fly rightward, simultaniously descending as in the previous situation, however he departed from the horizontal approach axis to the extent that he got out of the localizer's range and therefore a warning NAV bat lit up on the onboard indicator. Please take notice that in case the warning NAV bat lights up, the onboard indicator's vertically oriented gauge returns to a neutral position, thus in the middle. In this case, only a deviation from the localizer's signal took place, ergo from the horizontal aprroach axis, and as you can observe, the horizontally oriented gauge indicating a deflection towards the glide slope is still operating, therefore indicating a correction upward. Given that the onboard system respectively the indicator is receiving the glide slope's signal, the warning GS bat is still inactive, thus turned off.
situation #7 - The pilot has corrected the flight to the left, consequently returning to the localizer's signal range. After receiving the horizontal navigational signal by the onboard system, the onboard indicator has switched off the warning NAV bat, shifting in the process the vertically oriented gauge to a corresponding position, which dictates the size of a correction. On both views you could observe that the pilot has climbed simultaniously during the correction leftward, almost to the glide slope's axis and by doing so the deflection of the horizontally oriented gauge has noticeably decreased. Since the aircraft is located almost on the glide slope's axis, the gauge has moved closer to zero.
situation #8 - The pilot has continued correcting to the left and to climb at the same time, whereat he flew over the horizontal approach axis and he's also changed the flight altitude, all of which you can watch in both views. Please take notice that the pilot has only partially departed left from the horizontal plane, which accounts for a slight deflection of the vertically oriented gauge rightward. You can observe in both views that the aircraft has considerably departed upward from the glide slope, which caused a substantial deflection of the horizontally oriented gauge downward.
situation #9 - We can notice on the onboard computer of the ILS system that the pilot has managed to guide the aircraft directly both on the horizontal and vertical plane of the navigational signals, which was confirmed by the animation in both views and thereby the onboard indicator assessed the situation by moving both horizontally and vertically oriented gauges to the neutral position, i.e. the middle of the onboard indicator. This information represents an assurence for the pilot that if he keeps each gauge in the neutral position, i.e. in the middle, he'll lead the plane safely up to the point of touch down. The aircraft has reached the distance of 1200 m that represents the distance to the middle marker, which is from the bird's eye view marked with the letters MM and depicted by an orange beacon. As you can see, a yellow "M" bulb has lit up on the onboard O-M-I indicator, which briefs the pilot about the distance to the runway. You could also hear a typical sound for the middle marker. This tone is identical with the one the pilot hears in his headphones. For further detailed information about the middle marker, please move and hold your cursor over the orange beacon marked as MM.
situation #10 - The aircraft maintained itself right in the middle of the horizontal and vertical approach axis, which has been proved by the animation in both views as well as by the onboard ILS indicator that is displaying both the horizontally and vertically oriented gauges in the neutral position. It can be seen from both views, that the plane has reached the distance of 60 m that represents the distance to the inner marker, which is from above marked with the letters IM and depicted by a white beacon. As you could notice, a white "I" bulb has lit up on the onboard O-M-I indicator that notifies the pilot about the distance to the runway. You could also hear a typical sound for the inner marker. This tone is identical with the one that the pilot hears in his headphones. For further information regarding the inner marker please move and hold your cursor over the white beacon marked as IM. It's a matter of the final approach for landing.
situation #11 - The aircraft did descend up to the point of touch down and has securely landed.