Instrument Landing System

Last Friday (9th) I had my last lesson in the Seneca.  I flew to Birmingham for a radar vectored ILS approach.  Thanks to my failure of keeping this blog up to date, I haven’t described what an ILS is.  As a simple and brief introduction, here is a paragraph from Wikipedia:

An instrument landing system (ILS) is a ground-based instrument approach system that provides precision guidance to an aircraft approaching and landing on a runway, using a combination of radio signals and, in many cases, high-intensity lighting arrays to enable a safe landing during instrument meteorological conditions (IMC), such as low ceilings or reduced visibility due to fog, rain, or blowing snow. For further reading click here.

The information provided by this system is presented to the pilot on an instrument called a HSI (Horizontal Situation Indicator).

Above is a typical HSI.  Very similar to the one we have in the Seneca.  When the ILS is tuned in on Navigation Radio 1 (NAV 1) and the aircraft is within range, the yellow indicators will show where you are in relation to the centreline of the runway and the correct descent path.  When setting up for the approach you put the course select pointer on the runway heading.  Both the course deviation bar and the dual glide-slope pointers are ‘fly to’ indications meaning that if the glide-slope pointers are above the middle point, then you need to decrease your descent rate until they are back in the middle.  Don’t climb on an ILS, it would just make it rather more difficult!  Similarly, if they were below the middle point, you would need to increase your rate of descent to get back on to the correct approach profile.

If the course deviation bar is to the left, it means that you are to the right of the runway centreline and that you need to fly left to correct it.  As you get closer and closer to the runway, these indications get ever more sensitive.  Throughout the approach we are to maintain the localiser (centreline) and the glide-slope to within half scale deflection.  Going outside these limits would result in a fail for the precision approach section.  On the CDI, half scale deflection is 2.5 dots left/right and on the glide-slope indicators half scale deflection is the next marker above/below.

There are two ways of doing such an approach.  One way is procedural and the other is radar vectored.  Most of the time we can get a radar vectored ILS which means the approach controller directs you on to the runway centreline using radar vectors.  Radar vectors are heading instructions given to the pilot by the approach/radar controller.  The final heading they give you is usually a 30 degree intercept to the localiser.  It is then the pilots responsibility to intercept and maintain the localiser and glide-slope.

The approach is flown with reference to an approach plate.  Please note: plate use with permission; neither the CAA or NATS accept any liability or responsibility for the content of the information; plate is for information purposes only and not intended for operational use.

The plate above details all the information to carry out either a procedural or a radar vectored ILS approach into Birmingham using runway 33.  As stated earlier, when being radar vectored, the controller will direct you towards the inbound track of 328 degrees.  The pilot then follows the recommended profile glide-path which is detailed on the plate just below the area diagram.  DME I-BM is your DME distance from Birmingham and below that are the recommended altitudes for the relevant distances.  Checking these as you go down the approach verifies that you are on the correct glide-path and that your altimeters are set correctly.

When carrying out a procedural approach you would navigate to the BHX (an NDB on Birmingham’s airfield) when cleared to do so and then hold over that beacon until cleared for the approach.  The hold is shown on the chart as right hand and with an outbound track of 328 degrees and an inbound track of 148 degrees.  When cleared for the approach, category A aircraft (the Seneca is category A) fly outbound from the BHX on a track of 160 degrees to D7 (7 miles DME) and then turn back inbound to intercept the ILS.  This way the pilot it entirely responsible for positioning for and intercepting the ILS.  Being radar vectored is much more convenient!

During lessons this approach is flown with the screens covering the windows so we cannot use any outside references.  We use plates that are provided by another company and different minima are specified.  The approach is essentially the same.  The plate is used states that I can descend to 530ft (DA – Decision Altitude) on the approach which would put me at 204ft above the runway surface.  Oxford Operating Procedures state that 20ft must be added to ILS minima to compensate for errors that could be shown by the instruments with the aircraft in it’s landing configuration making the DA 550ft for Birmingham.  At decision altitude there are two options, land or go around.  Land if you’re visual with the runway, go around if you’re not.  We always leave the screens up to simulate still being in cloud at this point and so our ILS approaches always result in a go around.  There are go around instructions on the plate which are followed unless otherwise directed by ATC.