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The Helicopter Circuit

(Reviewed on 5th April 2021)

Why Fly a Helicopter Circuit?

Airplanes fly circuits. They fly them a lot and we can understand how this helps the pilots improve their handling of the aircraft.

But most helicopters do not normally do their approaches to a runway. We normally approach directly to where we want to land. So why do we need to do circuits? There are a few very good reasons:

1. Circuits cover a lot of the flight exercises. E.g. hovering, spot turns, transitions, climbing, climbing turns, straight and level; to name just a few. By flying a circuit, your instructor or examiner can tell just how well you are flying the helicopter and what your weak points are.
2. By flying circuits, you gain an understanding of the operation of the airport and the circuit and you develop a situational awareness of other aircraft in the vicinity. When you arrive at an airfield for the first time, you will know the procedure to follow and what is expected of you in the circuit.
3. When you fly circuits, you improve your handling of the aircraft and the accuracy of your flying will improve.
4. Any confined area you land at will require you to fly a circuit.

How to Fly Helicopter Circuits

Prepare for the circuit. If you are going to be landing at an airport for the first time, then you will need to do a little bit of research.

• What is the layout of the runways?
• What radio frequencies will you need?
• What is the helicopter circuit height?
• What direction is the circuit?
• What is the altitude of the airfield?
• Are there any obstacles that you should be aware of?

All of this information may be found in the Aeronautical Information Publication (AIP) (in Ireland).

Let us assume that you have done all of this and you are going to fly left circuits at your local airfield.

Upwind Leg

Before commencing a transition for the circuit, make sure you complete a clearing turn to ensure that you do not cut in front of another aircraft or climb into the belly of an aircraft. Build up a mental picture of any other aircraft in the area. This includes taxiing aircraft and other aircraft in the circuit. Transition into forward flight. Overcome flapback and translational lift. Establish a steady climb at 60 knots. Check the engine temperatures and pressures during the climb.

Crosswind Leg

When you approach 500 feet above ground level (AGL), check that the aircraft is clear right, clear ahead and clear left. Pick a feature on the ground that is 90 degrees on your left and turn towards it. Continue climbing on the crosswind leg.

When you approach 1000 feet AGL, check that the aircraft is clear right, clear ahead and clear left. Level the aircraft at 1000 feet using the Attitude, Power, Trim method. 70 to 80 knots is a good speed for circuits (depending on the type of helicopter you fly). Pick a feature on the ground that is 90 degrees on your left and turn towards it. You are now on the Downwind leg.

Downwind Leg

You should now be in straight and level flight at 1000 feet AGL and 70 to 80 knots. Perform a “FREDA” check (Fuel, Radio, Engine temperatures and pressures, Direction and Altitude). When you are abeam your landing point, imagine a line projecting from it at 45 degrees. Where this imaginary line intersects your track, this will be your turning point. Check that you are clear right, clear ahead and clear left. Pick a feature on the ground that is 90 degrees on your left and turn towards it. You have now turned onto the base leg.

Base Leg

Once on base leg, lower the collective and make sure you have a positive rate of descent by checking the Vertical Speed Indicator (VSI). The idea is to descend to 500 feet AGL. Don’t worry of you have not reached 500 feet before turning on final. You will have plenty of time to lose the height. Check that you are clear right, clear ahead and clear left. Turn towards the threshold of the runway. You have now turned onto the final leg.

Final Leg

Perform another FREDA check. Slow the aircraft down to 60 knots. If there is a strong wind blowing, increase your speed by the amount of the wind velocity until you are closer to the threshold otherwise it will take a long time to get there. Fly straight and level at 500 feet and 60 knots until you have a good site picture (ask your instructor if you are unsure about what a site picture is).

When the site picture is in the correct position on the windscreen, lower the collective and ensure that you have a positive rate of descent. Use the collective. Be firm with it and make sure that the helicopter follows a constant angle to the approach point.

At approximately 300 feet, use a little aft cyclic to bring the nose up slightly and start reducing the speed. The idea is that you have a gradual reduction in speed from this point until you come to a hover at the threshold.

At 200 feet decide if everything is looking and feeling good on the approach. 200 feet is your commit height. If everything is not right at this point – go around and fly another circuit.

At 100 feet, let the site picture begin to move down the windscreen. Keep the helicopter moving forward and aim to come to the hover over your landing point. Anticipate the loss of translational lift and be ready to compensate by raising the collective. As you raise the collective you will need to adjust the pedals to compensate for the change in torque. Come to a steady hover or land.

You have now completed a helicopter circuit. As you already know by now, it requires a lot of concentration to do this properly. I always tell my students to never let the helicopter take them where their brain has not been 2 minutes earlier. Plan your helicopter circuits. Think ahead. Anticipate your turning points, your altitude level offs, FREDA checks, radio calls etc.

Strive for perfection in the circuit. You will never achieve it but this will certainly improve your confidence and precision in flying the helicopter. I hope this post has been of some value to you and any comments would be appreciated.

The Helicopter Engine (Turboshaft)

(Reviewed on 12th March 2021)

Helicopter Engines

One of the most common helicopter engines used today is the turboshaft engine. A turboshaft engine is effectively a jet engine and normally runs on a kerosene based fuel. There are many variants of turboshaft engines but they all follow basic concepts and principles. To keep things simple, I will be referring to the engine used by the Bell 206 JetRanger. The Rolls Royce Allison 250/c20J.

The working cycle of the turbine engine is similar to that of the piston engine. There is induction, compression, combustion and exhaust. The major difference is that unlike the piston engine, the turbine engines cycles are continuous. Unlike airplane jet engines where the exhaust gases provide the thrust, the exhaust gases for the helicopter turbo shaft engine are intercepted by a turbine that transfers the energy from these gases to a gearbox that provides power for the helicopter.

It is vital that you learn not to over-torque or over-temp the engine as this can have disastrous consequences for someone flying it, weeks or even months later. Misusing the engine will not necessarily cause a problem right away.

Starting the helicopter engine causes more stress to the components than just about any other operation due to thermal shock and wear. For this reason, the start cycles are counted and recorded in the technical log after each flight. As helicopters tend to fly for short periods compared to airplanes, it is possible that the engine could reach its start count limit before it reaches its useful life hour limit. Start-ups are expensive and so it is better to leave the engine running for ten minutes while waiting for a passenger rather than shutting the engine down and then starting up again.

The engine relies totally on the battery or Auxiliary Power Unit (APU)  for startup. The engine has to be turning fast enough to keep enough cold air flowing through the turbine otherwise it will become very hot and damage the internal components. A weak battery could run out of power before the combustion process becomes self sustaining and this would be disastrous.

A “Hung Start” can happen if the engine fails to accelerate and the RPM stays constantly low. This uses up your battery power and if the battery runs out of power then the engine slows down, less and less cold air is drawn through the engine and subsequently the temperature in the combustion chamber becomes really hot within a few seconds causing a lot of damage.

A “Wet Start” is the equivalent of a flooded engine and the igniter has failed to light the fuel. Wait for at least 5 minutes and then vent the engine. Because the battery has already been partially drained by the failed start and the venting, it is probably a good idea to get an external start using an APU to make sure you do not run out of power on your next start attempt.

If you follow the check list exactly then you should have no problem starting the helicopter engine every time. The most common reason I have seen for hot starts is low time pilots starting the engine with the throttle already partially (or fully) open.

Therefore it is vital for you to double check that the throttle is fully closed before pressing the starter button.

Turbine engines take time to “Spool up” or “Spool down” when you make power changes. For this reason you should operate the collective or throttle very smoothly to prevent engine “surging”. Surging can happen when the airflow over the compressor blades becomes disturbed causing them to stall. This leads to loud banging noises (similar to a shotgun going off) and a very noticeable vibration from the engine. Shut down immediately if this happens during startup.

The Air Inlet

The air inlet is designed to stabilize the air before it enters the compressor. A particle separator may be fitted at this point to remove any foreign matter such as dust or sand which could cause erosion of the compressor blades.

The Compressor

The compressor is made up of a series of blades and an impellor (or centrifugal) compressor. It is designed to take large quantities of air and compress it before directing it to the combustion section.

The engine in the JetRanger has a six stage axial flow compressor and a single impellor compressor.

In the axial compressor, each stage is separated by stator vanes to make sure the air hits the following blades at the correct angle. As the air flows past these blades it becomes compressed more and more. The pressure rises and its velocity decreases.

The centrifugal compressor diverts the air outwards into channels that lead to the combustion section while compressing the air further.

During startup there is a bleed valve that opens and allows some of the air to escape from the compressor. This makes it easier to get the engine up to speed and takes less power from the battery. When the engine reaches a sustainable speed the valve closes automatically.

Because of the high temperatures of the compressed air in the compressor section (up to 250 degrees celsius), this air is used to heat the cabin and for anti-icing. Anti-icing uses air from the rear of the compressor and directs it through the compressor casing and the inlet guide vanes to prevent ice forming there. When anti-icing is used there will be a small rise in Turbine Outlet Temperature (TOT).

The Combustion Section

In this section the fuel is mixed with the air and ignited. The air ducts are shaped in such a way that the flame never comes into contact with the metal casing but instead is contained within a shroud of cooler air. A large proportion of the air is used for cooling. Once the fuel is lit and the engine is up to running speed, the combustion is self sustaining. The engine is turned initially by a starter. When there is enough airflow through the engine to keep everything cool, the fuel is ignited by the igniter plug. The fuel enters the combustion chamber through a fuel nozzle that atomizes the fuel. On ignition the gases expand and flow to the turbine section at an increased velocity. Approximately 60 to 80% of the air entering the combustion chamber is used to keep the liners cool. The fuel nozzle is highly polished and engineers have to handle it very carefully as the tiniest scratch will disturb the spray pattern and cause hot spots which will eventually damage the turbine blades. It has holes for delivering fuel.

The Turbine Section

This section creates the power. The turbine inlet is the hottest part of the helicopter engine and it is too hot for temperature sensors to survive here. Temperature is therefore measured between the turbines by thermocouples (the readings are averaged and displayed in the cockpit instrument display) and is called the Turbine Outlet Temperature (TOT). The heat is kept to a manageable level by cold air extracted from the compressor which is driven through a connection by the turbine. The gases are directed through the compressor turbine blades (N1) thus ensuring that the compressor is continually powered. From there the gases pass through a two stage “Free Turbine” (N2). As the free turbine is not directly connected to the compressor, the engine is easier to turn during startup. The free turbine is connected to the Accessory gearbox which reduces the high speed of the turbine to a more manageable level. When more power is required, the compressor speed (N1) increases to supply more air. At the same time more fuel enters the combustion chamber and therefore N2 is maintained at a constant speed. The turbine blades are operating in a very hostile environment. As the temperatures are so high and the blades are spinning so very fast, centrifugal force causes the blades to stretch (blade creep). This is normal, however if the engine has had a hot start, the blade creep becomes much larger than normal and becomes permanent. The blades can make contact with the sides and expensive repairs will be required. Higher TOT temperatures are permitted during startup as the turbine is spinning relatively slowly. The N1 turbine is doing more work than the N2 turbine. It is also exposed to hotter gases. For these reasons the N1 turbine has only half of the service life of the N2 turbine.

The Accessory Gearbox

The accessory gearbox converts the high speed of the free turbine (N2) to a more manageable level. It has a drive-shaft powering the main rotor gearbox, a rear drive-shaft powering the tail rotor, a freewheel unit and attachment points for all the accessories such as fuel pump, tachometers, generator etc.

Compressor Stall

Compressor stall can occur on any turbine engine if the conditions present themselves. In order to meet the design requirements, the engine must have a relatively high power output, good fuel consumption and fast acceleration characteristics. For these reasons it is beneficial to operate as closely as possible to the stall angle of the compressor blades. Operating close to the stall angle has the following benefits:

• The volume of air passing through the engine is increased.
• The pressure ratio of the engine is increased thus increasing the power output.
• Turbine temperatures can be increased because of the greater airflow.
• The efficiency of the compressor and turbine sections are increased.

To reduce the risk of the compressor stalling during startup or acceleration, the fuel flow is carefully regulated. So what is compressor stall? Many pages could be devoted to explaining this but the following explanation should help answer the question. Compressor blades and vanes are aerofoils. The airflow over an aerofoil will separate and become turbulent if either of the following occurs:

• The velocity of the air passing over the aerofoil is too low.
• The angle of attack is too high.

If the airflow over an aerofoil separates then the aerofoil stalls. Approximately 80% of the air entering the engine is used for cooling. This means that much more air has to enter the engine than what is needed for cooling. The cooling air is used to control the length of the flame in the combustion chamber and prevent it from touching the sides of the container. The hot combustion gases are cooled by the cooling air and the cooling air is also heated by the combustion gases. This keeps the gases at an acceptable temperature as they mix and enter the turbine section. If too much fuel is supplied to the burner, there will be more than enough air to allow proper combustion. However as extra air is used during this combustion, there will be less air available for cooling and therefore the temperature inside the combustion chamber will rise. As the temperature rises, there will be more gases to be exhausted. It is possible that the volume of gases to be exhausted may exceed the capacity of the turbine and the turbine will “choke”. When this happens, the pressure inside the combustion chamber will rise rapidly and may equal or exceed the pressure that the compressor is producing. If the pressure in the combustion chamber is equal to the pressure of the compressor discharge air, then the compressor will stall. If the pressure in the combustion chamber exceeds the pressure of the compressor discharge air, then not only will the compressor stall but also the hot gases will flow from the combustion chamber into the compressor section. Both of these conditions will result in a loss of air into the combustion chamber. The flame will not have enough oxygen and will die, resulting in a rapid drop in temperature. As the temperature drops, the expansion is stopped (or greatly reduced). The turbine is no longer choked and the combustion chamber pressure drops to a very low value. The low pressure in the combustion chamber means that air can flow in the proper direction again. The compressor is no longer stalled and a “Surge” of air flows back into the combustion chamber. This extremely fast movement of air elongates the flame downstream and through the turbine causing another rapid expansion of the gases. The cycle repeats itself at approximately 120 times per second. Compressor stalls may or may not have an audible sound but there will often be a vibration. If the stall is severe a flame may emanate from the exhaust or a very loud backfire may be heard. Smoke may also be seen. If the proper corrective action is made immediately then it is unlikely that any damage will occur. The actions to be taken are:

• Reduce the throttle to flight idle.
• If the stall remains then close the throttle completely and shut down the engine.

Rapid throttle movements may induce stalling therefore it is a good idea to make smooth, slow throttle movements.

Summary

Helicopter engines have become very sophisticated. Turboshaft engines are extremely reliable as long as they are maintained and operated correctly. This post has only just touched on the subject of turboshafts but if you would like more information then please let me know.

Common Helicopter Training Mistakes

R44 Cockpit

(Updated on 2nd April 2021)

Introduction

When you first start learning to fly a helicopter, you soon realize that it is not as easy as it looks. Students make common training mistakes. Your instructor seems to do everything so effortlessly and moves the helicopter sometimes as if by mental control, as the flight controls do not appear to move.

Flying is not a natural thing for us to do. We are land animals. Being in the air takes a little getting used to. Everything looks different. It is noisy and sometimes it is hot and uncomfortable. It is very easy for us to become distracted from flying the aircraft.

Two of the biggest helicopter training mistakes that new students make are:

1. Not looking outside enough
2. Over-controlling on the flight controls

Technique

It is vital that you look well ahead of the helicopter during flight. If you start to stare at the instruments, the helicopter attitude will change very quickly and you will not notice it until quite late. In forward flight you should try to look at the horizon and keep it lined up on the same relative position on the windscreen. The horizon will appear to be a certain distance below the rotor disc or it may appear to line up with a dead bug on the windscreen. If you can keep the horizon in this same relative position by moving the cyclic control, then the helicopter will stay in level flight.

We normally spend about 75% of our time looking outside. The remaining time is spent checking our flight instruments and our engine instruments. Looking outside has the added advantage of letting us spot other aircraft in the vicinity and thereby reducing the risk of an in flight collision.

When we are hovering, it is very important not to look at the ground just in front of the helicopter. As in forward flight, we should be looking well ahead. I tend to look at something at least 30m away and even at the horizon and I try to keep the horizon (or whatever I am looking at) at the same relative position on the windscreen – just as we do in forward flight.

If you focus your vision too close to the helicopter, it is extremely difficult to determine if the helicopter is drifting or yawing and this leads to incorrect control inputs. By looking well ahead, you will see any movement of the helicopter much earlier and therefore you will be able to correct this movement earlier – which makes flying the helicopter easier.

The other common mistake is over-controlling. There is a slight delay between moving the cyclic and the aircraft subsequently reacting to your input. This delay is normally less than one second but it is enough to make life difficult for you. When you move the cyclic, always pause for a second before you move it again otherwise you will be over-controlling. Move and hold.

Also remember that the cyclic is an extremely sensitive control. A small movement on the cyclic has a very large effect on the helicopter. Therefore it is vital that you never make a large input to this control during forward flight. Larger movements may be acceptable during hovering but your instructor will advise you on that.

If you can manage to overcome the urge to look inside and stare at the instruments (or look to close to the helicopter during hovering), and you can move the cyclic without over-controlling, you are well on the way to flying the helicopter and have made a big step forward in your training.

Frontology – Weather Charts for Pilots

(Reviewed on 6th February 2021)

Introduction

Just today, one of my students asked me to post some information about fronts. Meteorology is such a large subject, it is sometimes difficult to know where to begin learning. For this post, I am going to concentrate on “Frontology” – the study of weather fronts.

As helicopter pilots, we are very aware of what an important role weather plays on the safety and comfort of our flights. It is important that we can interpret weather charts and decipher coded Meteorological Reports (METARs) and Terminal Area Forecasts (TAFs).

Synoptic charts (pressure charts) are one of the most commonly used charts that we use to try and predict the weather. We are going to look at the fronts on these charts in detail.

The Cold Front       

The best way to think of a cold front is to think of it as a parcel of cold, dense air moving along the surface of the earth. As it moves, it undercuts the less dense, warmer air ahead of it and this causes the warmer air to rise. As the air rises, it creates lower pressure and if there is enough moisture in the air, Cumulus (Cu) clouds will form. This type of cloud can produce showery rain but there will be good visibility in between the showers. Cold fronts can move up to two times faster than a warm front and because of this they can produce sharper changes in the weather.

As the cold front passes, the following occurs:

• A sudden drop in temperature.
• Atmospheric pressure starts to increase.
• The wind becomes gusty.
• Thunderstorms (with sufficient moisture).
• Cumulonimbus clouds .
• A drop in dew point temperature.
• Poor visibility starts to improve.

After the cold front passes, the following occurs:

• Temperature drops steadily.
• Atmospheric pressure increases steadily.
• Wind veers (changes direction clockwise in the northern hemisphere).
• Rain showers start to clear.
• Good visibility
• Falling dew point.

If there is not enough moisture in the air, a cold front may pass without any visible signs as there will not be enough moisture to form clouds.

The Warm Front    

Think of a warm front as a parcel of warm air moving across the surface of the earth. It will have colder, more dense air ahead of it. The warm air is forced over the top of the colder, denser air. This happens at a shallow angle. As a warm front approaches, clouds associated with the front may be seen up to 500km before the front arrives. The first clouds to be seen are the high altitude clouds (Cirrus and Cirrostratus) This is followed by the middle altitude clouds (Altostratus). Close to the front there will be Nimbostratus clouds which produce continuous rain (or snow). When the front passes there is normally a layer of Stratocumulus clouds. As the warm front passes, the following occurs:

• Sudden temperature rise.
• Atmospheric pressure levels off.
• Variable winds.
• Nimbostratus clouds with stratus and sometimes cumulonimbus clouds
• Continuous rain, snow or drizzle stops as the front passes.
• Visibility is poor but improving.
• Dew point rises steadily.

After the warm front passes, the following occurs:

• Temperature rises and then levels off.
• There is a slight rise in atmospheric pressure then a decrease.
• The wind veers (in the northern hemisphere).
• Usually no precipitation but sometimes light rain or showers.
• Clouds clearing to scattered stratus.
• Visibility is fair in haze.
• Dew point rises and then remains steady.

The Occluded Front       

An occluded front is formed when the faster moving cold front overtakes the warm front ahead of it. There are two types of occluded front.

1. Cold Occluded Front. This occurs when the air behind the front is colder than the air ahead of the front.
2. Warm Occluded front.This occurs when the air behind the front is warmer than the air ahead of the front.

A wide variety of weather may be found along an occluded front. They usually form around mature low pressure systems.

Synoptic Charts

Now that you know what the different types of front are, you are capable of making better decisions during your flights or during your flight planning. You can make reasonable predictions about the kind of weather you are likely to encounter. There are other symbols on the charts and lots more information can be derived. I will cover this in another post. One of the Meteorological websites I commonly use to get this information is Propilots.