ILS SECONDARY GLIDE SLOPE

People from all over the world can agree that the instrument landings system (ILS) is the most accurate and reliable system ,but there is inevitably a characteristic phenomena called ILS SECONDARY GLIDE SLOPE.

This phenomenon is caused due to the current ILS ANTENNA design. And it results in a secondary glide slope’s appearance above the primary glide slope.

There were several cases reporting that while intercepting ILS ( from above ), they observed unwanted Auto pilot behavior and the main reason behind this was ILS Secondary glide slope .

As pilots we need to figure out and understand this hidden phenomena , to avoid any unwanted behavior of the aircraft during an ILS glide slope capture.

Background:

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.

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 pilots and the autopilot to erroneously consider the secondary glide slope as the reference for the final descent.

NOTE

There is no way pilots can know which category of ILS antenna (either with “inverted” or “only repeated”) is used at their destination airport.

Repeated antenna category

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

INVERTED ANTENNA CATEGORY

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.

Prevention

ICAO provides a guideline envelope as a recommendation for ensuring the quality of the ILS glide slope signal. Capturing the ILS glide slope within this envelope ensures that the aircraft is within the area of influence for the primary glide slope. It also ensures that the ILS signal is of sufficient quality to ensure a normal ILS glide slope capture.

• 10 NM from the runway threshold
• +/-8 ° laterally from the runway centerline
• 0.3 x Θ up to 1.75 x Θ (Θ, being the nominal glide path angle).

To prevent this secondary ILS glide slope capturing, the pilot should make sure before pressing the APPR that the aircraft is below the upper boundary of the glide slope

Case 1 : INTERCEPTING THE GS FROM BELOW

As rule of thumb, 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 3 times the distance to runway (in NM) multiplied by 100.

interception from below using this formula: h(ft AAL) = 3 x d (NM) x 100

Case 2: INTERCEPTING THE GS FROM ABOVE

As a rule of thumb, 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 .

Interception from above using this formula: h(ft AAL) < 6 x d (NM) x 100