Circuits Theory

In which I convince some people that I know what I’m talking about, discover that my Instructor is a mad supporter of Geelong Football Club and learn the theory behind flying a circuit.

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Today’s mission was to begin doing circuits. Unfortunately due to crosswinds and general turbulence we didn’t get to actually fly, but we did go through the (large amount) of theory.

When I arrived and was waiting, I got talking to a boy and his mother who were there for his TIF. His mother was asking me lots of questions, it felt rather strange to be the one with the answers for a change! She asked me who I learnt from and I pointed out J (who had just walked past) and said “J there”. She looked slightly unimpressed (maybe because of his age? who knows…) and was like “I think he was the one we spoke to on the phone” and her son just looked at her and went “You can tell that just by looking at him?”. I was trying not to laugh 😛

Anyway, then it was time for the briefing.

A circuit is the pattern planes fly around the airport. The main purpose of the circuit is to control traffic entering, leaving and around the airport. Without the circuit, as J put it, there was be absolute chaos.

Circuits consist of take off, flight around the circuit path and landing.

TAKE OFF

Take off is obviously where the plane leaves the ground.

The steps of a take off:

  1. After the runup checks and checking for traffic (on base, final and approach), the plane is lined up on the centreline of the runway
  2. Power is increased to full
  3. The plane is kept on the centreline using the rudder pedals
  4. When take off speed is reached, pull back on the stick and take off
  5. Climb out

Take off speed for a Jabiru is about 55-60kts.

Take off is generally done into the wind as it provides the shortest ground run and the lowers the groundspeed required for take off.

When taking off, flaps are put half down which increases the amount of lift available at the low airspeeds.

CIRCUIT

Circuit0The circuit has 5 legs:

  1. Upwind
  2. Crosswind
  3. Downwind
  4. Base
  5. Final

The downwind leg is parallel to the runway and the crosswind and base legs are perpendicular to the runway.

550px-Airfield_traffic_pattern.svg

Circuits are generally flown to the left, unless terrain or towns etc dictate that it must be flown to the right. This is because, if the circuit is flown to the left, then the pilot (LHS) will be closest to the airport at all times in the circuit.

The first leg of the circuit after take off is the upwind leg.

UpwindThis leg is generally thought of as an extension of the centre line of the runway. After take off it is generally advised to fly fairly flat to gain airspeed before moving to the climb attitude – the nose should be on the horizon until the plane reaches about 70kts (climb speed) before it is raised to the climb attitude.

The entire upwind leg is a climb. This climb is maintained until 500ft AGL (above ground level) after which the turn onto the crosswind leg can be made. It is both against convention and against the law to turn onto crosswind below 500ft AGL.

During this leg:

  • Power – full
  • Attitude – climb
  • Speed – around 70kts

The next leg of the circuit is the crosswind leg.

CrosswindThis leg is flown perpendicular to the runway. The plane also climbs during this leg to a height of 1000ft AGL. Depending on the type of plane and the circumstances (wind etc) the plane may reach 1000ft AGL on the crosswind leg, during the turn from crosswind onto downwind or on the downwind leg.

The turn from upwind onto crosswind is a climbing turn.

J asked me “How do you tell if you’re 90 degrees to the runway?”. I was like “Compass?” He was like “yessssss…but what if your compass is broken?” I thought about it and suggested the only other thing I could think of – “Look out the window?” J was like “Yes! The window!” It seems there are lots of pilots flying around out there who don’t like looking out the window…

During this leg:

  • Power – full
  • Attitude – climb
  • Speed – 70kts

The next leg is the downwind leg.

DownwindA radio call is made during the turn from crosswind to downwind. This turn is made when the plane is at a 45 degree angle to the threshold (end) of the runway. As a general rule, if you look out the window and see the threshold of the runway halfway between the wing and the tail, that’s about 45 degrees.

If it hasn’t already done so, the plane should reach 1000ft AGL during the dowind leg.

The plane will no longer be climbing during this leg, this leg is simply straight and level flying.

During this leg:

  • Power – 3000rpm
  • Attitude – horizon 1/3 up the windscreen
  • Speed – 95-100kts

About halfway down this leg, you do the pre-landing checks.

During this leg the plane should be flying parallel to the runway. To determine if you are in the right place, look out of the left window and if the runway (or the imagined extended centreline) is about halfway down the wing strut (between the wing and the bottom of the strut) that is about right.

The next leg is the base leg.

BaseThe turn from downwind onto base is one of the most complicated parts of the circuit, as it requires the pilot to do a lot of things at once. As well as beginning a descending turn onto base, the pilot also needs to reduce to power (to 1800rpm), keep the nose raised and make a radio call.

This turn is made when the plane is at a 45 degree angle to the base threshold of the runway. As before, to determine this the runway should be about halfway between the wing and the tail.

When the power is reduced to under 2000rpm, carby heat needs to be put on. This will generally be done as part of the pre-landing checks during the downwind leg.

After rolling out of this turn, the pilot lowers the flaps to 1/2, holding the nose down (it will try to rise when the flaps are lowered) and then further lowering the nose for descent. Many planes land with full flap but in the Jabiru it is generally easier to land with 1/2 flap (the plane is more responsive). Then the plane needs to be retrimmed.

This leg is a descending leg. It is also important to lower the airspeed during this leg.

During this leg:

  • Carby heat on
  • Power – 1800rpm
  • Attitude – horizon 1/2 up the windscreen
  • Speed – 65-70kts

The next leg is called final approach (or just final).

FinalThis leg is the approach to the runway.

The turn onto final should be done so that the plane rolls out onto the (imaginary) extended centreline of the runway. This turn should aim to be done around 600-700ft AGL.

During this leg (and when flying generally really) it is important to remember two things:

  • Airspeed is controlled by power
  • Flightpath (where the plane goes) is controlled by the elevator and the ailerons

Apparantly there are lots of people who don’t realise that the plane goes where you point it. They also apparantly don’t seem to realise that if you increase your power your speed increases (and vice versa). Assuming these people have drivers licences, why don’t they realise that it’s generally the same for both the plane and the car? 😐

The aim is to fly a nice stabilised approach (a straight line down to the runway) rather than being all over the place.

The pilot aims towards an ‘aim point’ on the runway. This point should remain in the same place on the windscreen, if it doesn’t you are either undershooting or overshooting. When heading towards the aim point, J said he imagines crosshairs in front of him, like on WWII planes. If the aim point is in those imaginary crosshairs, then you’re heading the right way.

To determine whether they are too high (overshooting), too low (undershooting) or correct, the pilot looks at the shape of the runway. If correct, the runway should look something like a trapezium (I think), with the two sides sloping in towards the middile. If overshooting, the runway will appear longer and narrower than usual and if undershooting the runway will appear very wide and very short.

The airspeed is also very important during approach. It is important to use power to make corrections if the airspeed increases or decreases.

Carby heat should be returned to cold during approach in case of a go around (where the landing is aborted for some reason or another).

If overshooting, the pilot should lower the nose to descend to join the planned approach path. If doing so, power should be reduced to compensate for the increase in airspeed.

If undershooting, the pilot should fly straight to join the planned approach path.

During this leg:

  • Power – 1800rpm
  • Attitude – horizon 1/2 up the windscreen
  • Speed –
  • Head for the aim point

LANDING

There are four parts to the landing:

  • Approach (discussed above)
  • Flying straight (holding off)
  • Touchdown
  • Ground roll

landing_profile

When about 5-10ft above the runway, the pilot stops looking at the aim point and starts looking towards a point at the end of the runway. Apparantly it’s easy to tell if the pilot isn’t looking at this point, because the point they are looking it is the point where the impact is going to occur (ouch).

When just above the runway, the pilot raises the nose and starts basically flying straight just above the runway. Power is reduced to idle and airspeed gradually decreases. As airspeed decreases the nose is raised to maintain height. Eventually airspeed will decrease to the point where no more lift is generated and the plane touches down.

The plane should touch down on the main wheels, with the nosewheel still in the air. This is because the main wheels are designed to take the impact of landing but the nosewheel isn’t. It is possible to cause damage to the nosewheel if the landing is incorrect.

The final part of landing is the ground roll, where the plane rolls down the runway, gradually losing speed. This roll is controlled by the rudder pedals.

I’m sure I’ve forgotten some of the things we went over today, but I think those are the main points. Circuits are basically putting lots of things we’ve already done (S+L, turns etc) together.  Unfortunately we couldn’t fly today because of a crosswind but I did get to book another 5 lessons or so. I now have lessons booked up to the end of my uni exams (which is scarily close really).

Random things I learnt today:

  • J is a mad Geelong supporter (oh dear) – he’s taking the day off on the 26th to watch the Grand Final
  • I appear confident enough that people actually ask me questions and think I know what I’m doing
  • Most of the instructors at Lilydale wear white pilot shirts, no tie and a blue jumper or jacket. However I discovered today that there is one instructor who wears a white pilot shirt with epaulettes (two bars), a tie and gold wings pinned to his shirt. I seriously thought he was going to come in and say “I just parked my 747 around the corner, do you mind if I use your bathroom?”
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2 Responses to Circuits Theory

  1. Zurtri says:

    Heh – reminds me of my what my instructor said last week:

    “Your circuits are good, you just have to work on the last 7 seconds”

    Ahahahaha!

    • darksarcasm503 says:

      pahahaha, good advice. At the airport where I fly, there’s a road just before the threshold of 36, I’m waiting to see how many cars I take out on approach 😀

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