Lining Up with the Correct Glide Slope

The first case happens during an ILS glide slope interception from above. The aircraft descends in —OP–DES_ guidance mode (fig.1). The air traffic controller clears the flight crew for approach. ① The flight crew consequently presses the _APPR_ pushbutton. ② The PFD indicates a glide slope below the aircraft, as expected by the flight crew. ③ A few seconds later, the —G/S*– mode engages and the autopilot orders a pitch down command toward the glide slope. The pitch down command continues until it reaches the 13° pitch down limit for autopilot disconnection ④. The flight crew must take over to recover the situation and perform a go-around.

A secondary glide slope capture was the cause of this event

Analysis of the data from the flight recorders enabled us to identify that this excessive pitch down order was caused by an initial capture of a secondary glide slope that caused undue early engagement of the _-G/S*– guidance mode. Before explaining what happened, we need to know what the secondary glide slope phenomenon is.

Secondary glide slopes are an inevitable characteristic due to the ILS antenna design. When an aircraft flies well above the main glide slope, the glide slope deviations displayed on the PFD will refer to the nearest glide slope, which may be a secondary glide slope instead of the primary one. This can lead both the flight crew and the autopilot to erroneously consider the secondary glide slope as the reference for the final descent.

There are several types of ILS glide slope antennas that use different technologies. They can be classified into two theoretical categories: “Inverted” glide slope and “repeated” glide slope. This considers the associated impact on the autopilot behavior and the indications observed by the flight crew.

ILS with “inverted” secondary glide slopes

This category of glide slope antennas inverts the orientation of the glide slope at every other glide slope. For example, in the case of a -3° glide slope, secondary glide slopes exist at -9°, -15°, -21° and every other 6°, but the glide slopes at -9°, -21° and every other 12° are inverted. The PFD glide slope deviations are inverted for these glide slopes, i.e. the aircraft is seen above the glide slope when it is below and vice versa (fig.2).

ILS with only “repeated” secondary glide slopes

This category of antennas has only repeated glide slopes above the main glide slope (fig.3). For example, in the case of a -3° glide slope, repeated secondary glide slopes exist at -9°, -15°, -21° and every other 6°.

Variable measured signal characteristics at the boundary between two glide slopes

Test flights performed to analyze the secondary glide slope structures showed that the real characteristics of the glide slopes may differ from the above theory, in particular in the boundary region between two glide slopes (shown as amber lines in fig.2 and fig.3). Therefore, it is difficult to predict the behavior of the autopilot in these zones.

Possible inappropriate engagement of —G/S*– when crossing the boundary between two glide slopes

When an aircraft crosses the boundary between two glide slopes (at approximately -6°, -12°, -18°, etc…), temporary deviation “jumps” and/or a false deviation value of zero can trigger inappropriate engagement of the —G/S*– capture mode. This can happen when the approach guidance modes are armed, meaning that the _APPR_ (LAND for A300-600/A310) pushbutton was previously pressed. The physical characteristics of the glide slope or the speed and angle at which the aircraft crosses the boundary, will influence if engagement occurs. Note that this phenomenon is possible for both the “repeated” and “inverted” types of secondary glide slope. Therefore, it is difficult to anticipate the autopilot behavior when crossing the boundary between two glide slopes.

With the secondary glide slope theory in mind, we can review the scenario of the event to better understand what happened.

The aircraft is in —OP–DES – (LVL/CH for A300-600/A310) mode (fig.5). ① The flight crew presses the _APPR_ (LAND for A300-600/A310) pushbutton well above the -3° glide slope, within the -9° zone of influence. The aircraft is nearer to the -9° secondary glide slope but far enough from it so that the —G/S – mode is armed but not engaged. ②The inverted glide slope deviations mean the flight crew cannot detect that their aircraft is in the zone of influence of an inverted secondary glide slope. The glide slope indicated below the aircraft is as expected by the flight crew. ③ When the aircraft crosses the -6° boundary between the -3° and -9° glide slopes, a temporary false deviation value of zero received by the MMR triggers the undue engagement of the —G/S*– mode. Therefore, the autopilot orders a pitch down command toward the -3° glide slope. As the aircraft is flying high above the -3°, there is sufficient time for the pitch to reach the 13° pitch down limit for autopilot disconnection ④. In manual flight, the flight crew must take over to recover the situation and perform a go-around.

Prevention: Quick check of the aircraft position before pressing the APPR pushbutton when intercepting a glide slope from above

When intercepting the glide slope from above, the flight crew should ensure that the aircraft is below the upper boundary of the main glide slope before they press the _APPR_ (LAND for A300-600/A310) pushbutton. This boundary is located at approximately twice the value of the primary glide slope angle (approximately -6° in our example). This ensures that the capture will be done on the correct glide slope. As a rule of thumb, a quick altitude vs. distance check can be done to ensure that the aircraft is below the upper boundary of the main glide slope. The aircraft altitude above airport elevation (in ft) should be less than 6 times the distance to runway (in NM) multiplied by 100 (fig.6):

h(ft AAL) < 6 x d (NM) x 100

FCOM procedure: guidance mode for glide interception from above

In the described event and in the next one, the flight crew uses the —OP–DES – guidance mode to intercept the glide slope from above. This is not recommended in the FCOM. As per the FCOM “Glide interception from above” procedure and the FCTM, after the aircraft is established on the localizer, the flight crew should press the _APPR_ pushbutton, set the FCU altitude above the aircraft altitude, and then select the –V/S– mode to intercept the glide slope.

In this second case, an aircraft also intercepts the glide slope from above (fig.7). ① The flight crew presses the _APPR_ (LAND for A300-600/A310) pushbutton but does not set the FCU altitude target above the aircraft altitude, because it is usually requested by the SOP for an ILS approach. The aircraft converges toward the -3° glide slope, but reaches the target altitude before the —G/S*– can engage. ② The aircraft levels off and starts to diverge from the -3° glide slope and to converge with the -9° secondary glide slope. ③ When crossing the -6° boundary between the -3° and -9° glide slopes, the MMR receives a temporary false deviation value of zero, but it is not sufficient to engage the —G/S*– mode. ④ The -9° glide slope is inverted, and as a result, the glide slope deviation indications show the glide slope below the aircraft. ⑤ When the aircraft crosses the -9° secondary glide slope, the —G/S*– guidance mode engages. The autopilot then commands a pitch up when the aircraft crosses the -9° glide slope, due to its inversion. In this case, the flight crew has no choice but to perform a go-around ⑥.

Prevention: Correct FCU altitude setting during glide interception from above

The “glide interception from above” FCOM procedure requests the flight crew to select the FCU altitude above aircraft altitude. This should be done after the flight crew presses the _APPR_ (LAND for A300-600/A310) pushbutton to prevent unwanted —ALT*– engagement and possible level-off that can lead to the capture of a secondary glide slope as shown in this example. This important step of the procedure can prevent such an occurrence.

① The flight crew needs to interrupt their glide slope interception from above at the request of ATC, due to unavailability of the runway (fig.8). ② The flight crew presses the _VS/FPA_ knob-selector to level off but does not press the _APPR_ pushbutton to disarm the —G/S – guidance mode, which is usually expected during a discontinued approach. The aircraft levels off as expected. This now follows the the same scenario as described in case 2: ③ When the aircraft crosses the -6° boundary between the -3° and -9° glide slope, the duration of the temporary false deviation value of zero is short enough not to engage the —G/S*– mode. ④ When the aircraft crosses the -9° glide slope, the —G/S*– mode engages and commands a pitch up due to the inversion of the secondary glide slope. ⑤ The flight crew must take over and perform a go-around.

Prevention: Disarming of the approach guidance mode during discontinued approach procedure

After the flight crew announces “CANCEL APPROACH”, they must remember to press the _APPR_ pushbutton to disarm the —G/S – guidance mode as per the “discontinued approach” SOP. This will prevent engagement of the —G/S*– and —G/S— modes on a secondary ILS glide slope.

Airbus developed some protections to limit the Flight Path Angle (FPA) in the case of a secondary glide slope capture by doing some modification on the autopilot flight guidance laws. These are available on A330, A340, A350, and A380 aircraft and limit the maximum Flight Path Angle (FPA) between 0° and -6° in —G/S*– mode. A320neo family aircraft have FPA protection in both —G/S*– and —G/S— modes.

The protections referred to above are not available on A220, A300, A310, and A320ceo aircraft at the time of publishing. The same protections will be introduced on A320ceo, and the protections that are already available on A330, A350, and A380 aircraft will be updated with the next Flight Guidance computer standard update to make these protections available in both —G/S*– and —G/S— modes. A similar protection will be added in the —GS— guidance mode of A220 aircraft at the opportunity of its Avionics build 8B.