Sunday, 10 November 2013

Hold it!

Have you ever wondered how your holiday jet is able to fly thousands of miles with no sight of the ground, descend into the clouds, manoeuvre around, hold, approach the airport and pop out of the bottom of the clouds at exactly the right place, speed and height to land?

The Buckeye VOR DME C hold and approach
pattern used in our training and tests
You might think GPS has something to do with it, and computers and of course autopilots. But no.

Instrument approaches pre-date all of those, and the essentials are simply the basic flight instruments, a nearby radio beacon, a radio instrument to pick it up and a printed chart of the approach procedure known as a plate.

And a lot of practice.

On the right you can see an approach plate, in this case to Buckeye Municipal Airport which is a tiny little public strip near our old base in Goodyear. This is a reasonably complicated plate, and there is a lot of jargon, but it is not as bad as it first looks.

The main part is simply a drawing of the procedure seen from above. It's not drawn to scale, but just gives the general picture. You arrive from wherever you like and fly to the little round symbol in the middle, which is a radio beacon. Then, you fly round and round the racetrack pattern until you are ready to approach and land. Next, you fly off five miles to the north west, descend a bit, turn around, descend a bit more, go back over the beacon and finally descend and head in the general direction of the runway on the right.

Underneath is a side view of the same approach, you can see that each part has a specific altitude. There is also a table that give the lowest altitude you are allowed to fly to as you approach the airport. If you still can't see the runway at this point, you must do the missed approach procedure. In this case, turn around to the left, climb, go back to the beacon and start flying round and round again.

A radio magnetic compass. This model has two
needles, which track two different kinds of
radio beacon. Here, the plane is heading north. There
is a beacon off to the left and slightly behind, and another
ahead and just to the right.
The clever part is this whole procedure is defined and flown using just one radio beacon. In the aircraft the we have an instrument called a radio magnetic compass (RMI). It has a needle that simply points to the beacon, and around the outside is a compass card that rotates to show which way the plane is heading. Add in a stopwatch, and provided you remember to reset it every time you cross the beacon you can now pinpoint your position.

Sounds easy enough, but regular readers will have already guessed the next part... it's not that simple. Firstly, we are flying a plane with sole reference to the instruments, taking care of radio communications and performing various check-lists at the same time. Secondly, interpreting these basic instruments is not easy. And thirdly there is the wind to make the whole thing even more confusing.

Probably the most difficult part of the procedure is entering and flying the hold — the little race track pattern in the middle. A hold is simply a way to get an aircraft to stay in the same place for a while (aircraft, unlike every other vehicle ever invented, are physically unable to stand still.) It's something we are practising a lot at the moment.

Three ways to enter a hold, depending on
the direction you approach the beacon.
The hold always starts at a radio beacon, and it is defined by the compass direction (track) of the straight part heading towards the beacon. This has to be flown for a specific amount of time, usually one minute. Then the pilot turns — usually right — using a standard rate turn (three degrees per second), flies on the opposite heading for a minute. Another turn to the right and in an ideal world they are back on the inbound track one minute from the beacon.

The wind can make a right mess of this. If there is a cross wind component, you can correct for it on the way inbound by flying whatever heading keeps the needle pointing to the required ground track (the one on the plate). But to do this, you do need to get onto the correct track pretty early so you have time for the trial-and-error procedure required to find the heading. Confused? See blowing in the wind.

On the other three parts of the hold — the outbound leg and the two turns — you have no beacon to track to and hence no point of reference. In theory, if you needed a certain amount of correction on the inbound leg, say 5 degrees left, then you should need three times this amount on the outbound leg but in the other direction — 15 degrees right. This is because the wind is blowing you off course during both turns as well as the outbound leg, a total of three minutes of wind needs to be accounted for.

An early attempt at the Buckeye hold on a windy day.
You can see the strange shape of the turns caused
by the cross wind, and also where I got confused
and applied the wind correction the wrong way on one
of the circuits. The red blob is the beacon. The
wind appears to be from the south east here.
Head or tail wind components muck up your timing. In order to get the required one minute inbound, you have to adjust your time outbound. Again, it is trial and error though you do have some idea what to expect before you start from the winds aloft forecast. These are the purple numbers I have scrawled on the plate.

What you should end up with, after a few circuits, is a kind of egg-shaped pattern with the into-wind turn being tighter than the down-wind turn, and the outbound leg skewed to connect them together. What we tend to end up with is a right mess that looks like someone just dropped their knitting.

There is a lot more to it than this... there is the entry to the hold to get right, there are various 'gates' that are used to monitor your progress around the pattern and so on. I won't bore you with the whole lot, if you would like to find out more, here is a good explanation.

I have a couple more lessons to practice this on Monday before yet another progress test. Following this I have one final solo. That will be the last time I fly a single engine aeroplane on the course, as the following week we start on the Seneca. Apart from having two engines, it is fitted with retractable gear, variable pitch propellers, superchargers and flies about 50% faster than the Warrior. Twice as many knobs and levers and a lot less time to think!

I am also doing the APS upset recovery training course next week which should be an eye-opener. They use aerobatic Extra 300 stunt planes to explore all sorts of crazy flight attitudes and conditions from inverted to spins and of course how to get out of them.

It's going to be quite a week.

The next challenge; getting to grips with the Piper Seneca

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