They’re on Instruments

Apart from being a classic one-liner from the 1980 comedy film ‘Airplane‘, being on instruments is related to flight so I thought I’d have a play with some of them in my new plane – the Cessna 172 Skyhawk.

The 172 Skyhawk is equipped with the Garmin G1000 flight computer, this brings a basic autopilot and instrument approach ability to the plane. I have not reached IFR or ‘Instrument Flight Rules’ flying properly yet as I’m still learning VFR (visual flight rules) flight, but in a sim you can dive from one thing to another easily so I did 🙂

To be able to land with instrument guidance, the plane systems need to pick up an ‘ILS’ (Instrument Landing System) signal from the chosen airport/runway. In basic form, the ILS signal consists of two radio beams or transmissions, picture below. A good explanation of ILS can be found here.

In the picture, the purple area is the ground/runway, the pink is the glide slope – this guides the plane down towards the runway and the green is the localiser – this keeps the plane aligned with the runway. By using the two signals, localiser and glide slope, the flight computer can automatically guide the plane close to the runway at which point the pilot turns off the autopilot and lands manually as normal. Small planes do not normally have the ability in their systems to fully land.

An ILS approach is quite nerve-racking – you have to have blind faith in your instruments and trust your ability to set them up correctly, an example would be landing in bad weather, below is the view from my seat on an ILS approach – you can see nothing at all. If the weather was not poor, you would likely just fly a normal VFR approach.

The outside view was little better: low clouds and heavy rain…

Just about all the pertinent information is displayed on the main screen of the flight computer…

On the main screen, bottom centre is the heading indicator, this shows the current heading 220 deg, the green arrow in its centre is the ILS localiser indicator. Upper left is the airspeed ribbon showing 75 knots, on the right is the altitude ribbon showing 1135 feet above sea level; the runway I chose is 358 feet above sea level so my height above the runway is currently 777 feet.

Just left of the altitude ribbon is the narrow ILS glide slope ribbon, and the tiny green diamond is about midway on its scale showing me that the computer has locked on to the slope and is controlling my descent at -350 feet per minute as seen in the tiny box on the right of the altitude ribbon, I am about 2 minutes to touchdown.

Being only a basic system it has no control over the throttle or rudder. You have to adjust the throttle as required to control speed: too fast and you will not descend and too slow can make the computer stall the plane and crash – it has very few safeguards built in so you do still have to ‘fly’ the plane even though the computer is doing most of the work. Naturally you also need to look out for other planes, talk to air traffic control and decide if you need to go around or divert if the visibility is not good enough when you reach your minimum altitude for decisions.

To be able to plan an ILS approach, various important bits of information are needed, these are found on airport approach charts or plates, the chart for London City ILS approach on runway 27 is shown below.

The ILS approach chart gives data on approach altitudes, frequencies for ILS, minimum decision altitudes, the procedure for a missed approach or go-around as well as local heights for obstacles that could cause a crash etc. These restrictions would need to be observed and used, you can’t simply program in an approach and fly it, you need to be at certain points at a specific minimum or maximum altitude and so on.

Not all airports will have ILS systems; it’s expensive, takes up a fair bit of land and needs maintenance. Along with GPS, I believe, came RNAV or Area Navigation – this uses the GPS system to create an approach that is very similar to ILS but needs no localiser or glide slope beams. Instead, there is a fixed NDB or non-directional-beacon and the the GPS system uses this along with various waypoints to create an approach plan which can be flown to bring the plane successfully to the runway threshold just like ILS can.

In use, RNAV is very similar to ILS, the plane is flown at specified altitudes towards a waypoint at which the system takes over control of the descent and the alignment to the runway. The slope is now called a glide path instead of the glide slope.

Of course, more informations is needed, just like ILS the RNAV approach has a chart which shows various specific information points, the chart for Cranfield airfield is shown below.

I’m still playing with RNAV so there’s plenty to learn 🙂

21st May 2022

I thought I would try and see if a trip can be made without looking outside once, just for fun. I did need to see out for the taxi to the runway but the next view was of the runway after landing 🙂

It worked very well, I used GPS for the route and ILS for the approach, turning off autopilot when I was about 20 feet from the tarmac, a fair bit late but was good fun. I might try again but do the whole trip manually and see how it goes when you only have the artificial horizon.