Next stop - instrument rating

Happy new year everyone.

"London" Oxford Airport

We are back home in the UK, Christmas binge over and the shock of the British weather has subsided. Some of us have got off to a 'flying' start with our new instructors on the next section of training — the instrument rating.

I've talked before about instrument flying (controlling the plane with no outside references). I've talked about asymmetric flight, check-lists and a little about radio navigation, holds and approaches.

The instrument rating is really about putting all this together at the same time.

By the end of this 10-week 50-flying-hour block we need to be able to convince a CAA examiner that we can plan and fly a twin engine aircraft on a complex route to public transport rules and standards, flying and navigating purely on instruments.

CAP 413; how to use the radio in
just 350 fun-filled pages.

We need to negotiate the complex UK airspace and deal with the UK's excellent but very formal air traffic controllers. We need to log our progress and write down lots of information. We need to quickly read and understand complicated approach plates. We need to do this on old fashioned analogue equipment without the help of an autopilot or GPS, and quite often we will need to do it with one engine 'failed'.

But most daunting of all we will need to do it completely unaided; the twin-engine single-pilot instrument rating test is quite possibly the most difficult flight we will ever undertake in our careers.

So far, we have been working in the simulators practising Oxford's departure, hold and arrival procedures, desperately trying to get everything together. There is never enough time, and as soon as some vital task diverts the attention, the plane will wander off course and off altitude almost like it is doing it on purpose.

The instructor assures us that with enough repetition things will become... not exactly easy, but possible.

Describing our flying in this section of the course is going to get unavoidably technical, but I will do my best to explain it. You can't understand radio instrument navigation without an idea of what the radio beacons and systems are and what they do, so here goes my attempt to explain.

The instrument approach

The approach is simply the phase of flight between arriving in the vicinity of the destination airport, and the point where you can see the runway in front of you in order to land visually.

The microwave landing system at Heathrow. That's an
A380 landing, so it's a fair bit higher than it looks!

This may be just a few hundred feet above the ground so clearly there has to be a way to arrive at the right place, at the right height and speed ready to land without being able to see outside. This is what instrument approach systems are for.

Almost all instrument approach systems use ground-based equipment to guide you in. GPS approaches are starting to appear but are still fairly new... 'new' in aviation being anything invented less than 25 years ago.

The cleverest approach system is the microwave landing system (MLS). First appearing in 1979, it also qualifies as a pretty new technology. It can guide the plane in via a whole range of different glide slopes and approach paths, is immune from interference and reflection problems, is reliable and accurate and is generally the bees knees.

Microwave systems were planned to replace earlier instrument landing systems at all major airports by 2010. In fact, only about three airports actually have one, and few planes have the equipment to use them.

So we don't get tested on MLS approaches.

The ILS is basically an invisible funnel of radio waves that
guide the pilot down the glide path to the runway

The next best system is the instrument landing system, or ILS, which has been with us since about 1938 so counts as a reasonably mature technology. You tune it in to a navigation radio just as you would with an FM radio station, and your instruments will indicate whether you need to fly left, right, up or down to get on the ideal three degree approach slope and centre line.

ILS has quite a few problems which I will leave you to read about on your own if you are interested, but it works well enough and has been installed at almost all large airports and many smaller ones worldwide. From the pilot's point of view it is reasonably easy to use, and we will be looking at ILS approaches next week

But ILS approaches are not considered difficult enough to be a 'real' test.

The microwave and instrument landing systems are known as precision approach systems, because they give guidance on height as well as direction. However it's perfectly possible to use simple radio beacon for directional guidance, the pilot taking care of the altitude. This is a non-precision approach and it's a bit more involved.

VOR stations like this are dotted around the country
at strategic points.

Of the two types of radio beacons that can be used for non-precision approaches, the newer and better one is the VOR. I really do my best to avoid acronyms in this blog, but I can't this time. VOR actually stands for VHF Omni Range — yes it is an acronym inside another acronym. (VHF stands for very high frequency, which is the type of radio waves that the FM radio in your car picks up).

VORs have been around since the forties, and there are about 3000 of them beeping away world wide. They send out two signals. One just splurges out equally in all directions, but the second is highly directional. By comparing the two signals, the equipment on board can work out the bearing from the beacon to your aircraft. You then know you are somewhere on an imaginary line that starts at the VOR and heads off in a specific compass direction, known as a radial.

The advantage of the VOR over the simple non-directional beacon (below) is that it works accurately irrespective of what the aircraft is doing. It can be heading in any direction, travelling in any direction or even be upside down and the reading will be steady and accurate, a small but useful advantage over the simple non-directional beacon.

But we don't get tested on VOR approaches either — still too easy!

Because there is one even more ancient and troublesome technology — the non-directional beacon (NDB). This is exactly what it sounds — a simple aerial sending out a blank carrier wave indiscriminately in all directions. They nothing more than an medium-wave radio station without the music. In fact medium-wave radio stations will work as non-direction beacons.

The Radio Magnetic Compass.
The yellow needle will point
towards an NDB, the green
one towards a VOR. We spend
hours staring at these needles.

There is no information contained in the signal from the non-directional beacon, but with a cunning arrangement of aerials on the aircraft it is possible to calculate where the signal is coming from. This drives a pointer on an instrument called an radio magnetic compass. All this pointer does is point to the beacon but this is enough, just, to enable entire holding and approach procedures to be flown.

First introduced in the thirties, non-directional beacons were supposed to be phased out decades ago, but are still with us. Mostly they are used for navigation but a handful of airports — including ours — use them for instrument approaches.

Compared to VORs, they are pretty tricky to use. Instead of a simple deviation bar telling you to fly left or right, you have a pointer on a compass face. You have to squint at it very closely to keep it within a degree or two, remember which end of the needle to use (the head when going towards the station, the tail going away) and work out which way and how much to turn when you are off course. You have to account for wind by flying just the right about of into-wind heading to keep the needle still.

They suffer from a multitude of errors — even nearby railway tracks can send them screwy. Most annoying of these is dip error. As soon as you bank the plane the needle veers off course, making it unreliable outside of straight and level flight. All in all, NDB approaches are a pain in the backside and most professional pilots would run a mile before they would attempt a NDB approach.

So guess which one we get tested on!

Here is part of the approach plate for the "Oxford NDB DME 19" approach, the one we use most often. DME stands for distance measuring equipment which is yet another type of radio navigation device which tells you how far away you are from something, in this case from the end of runway 19.

There are masses of information squeezed onto approach plates and they look a bit scary, but the basics are actually quite easy to decipher.

The airport is the little cross in the middle, and the round blob indicates the position of the NDB. The boxes show you what radio navigation facilities are available, what frequency they are and their Morse code identifiers (yes really, we do still use Morse code). In this case you can see that Oxford has both an NDB called 'OX' and a combined ILS/DME called 'IOXF'.

Extending down and right of the airport is a racetrack shape, this is the holding pattern. Normally the hold lines up with the approach and runway, but at Oxford the it is sort of bent off to one side to keep pilots well away from RAF Brize Norton.

Early attempt in the sim; room
for improvement!

To fly the approach, we arrive overhead the NDB from any direction and join the holding pattern, if necessary shuttling down to 3500'. Once we ready, we leave the pattern and fly off to the north on the outbound leg (the one marked CAT A & B), descending to 1800'. On reaching 6.5 miles away from the NDB, we hang a right and head inbound on a course of 194 degrees magnetic.

This course has to be held accurately accounting for wind and we have a few moments to get everything sorted before the final descent starts at 4.7 miles. Because this is a non-precision approach, we have to check our height every mile to make sure we are descending on the correct slope.

Eventually we will reach our minimum decision altitude. If we can see the runway, then we can go ahead and land. If not we have to go around and try again. This is called a missed approach and is indicated by the dotted line on the chart. Back to the hold and start all over again!

Well that's the idea anyway. There are a lot of checks and radio calls to squeeze into these very busy few minutes that make it a lot more difficult than it sounds... not that it sounds all that easy.

We should be flying some of these approaches for real this week. It's been over a month since I last flew a real aircraft so fingers crossed I can still remember how!