Tail Rotor Failure
(Updated on 14th February 2021)
Different Types of Tail Rotor Failure
While training for an EASA PPL(H) licence, you will be told about tail rotor failures. There is no flight exercise for tail rotor failures under the EASA system however I like to demonstrate the different types of tail rotor failure that can occur during flight. They are as follows:
- Tail rotor failure in the hover
- Tail rotor failure in forward flight
- Stuck left pedal
- Stuck right pedal
Although tail rotor failures are extremely rare, I have been unfortunate enough to have experienced one. Because of my training, I was able to identify it and land safely. If you have a sufficiently large, flat area, there is no reason why you should not be able to do the same should this ever happen to you.
Tail Rotor Failure in the Hover
When the tail rotor fails while hovering, the nose of the helicopter will yaw rapidly to the right (in helicopters that have a rotor that turns anti-clockwise when viewed from above). You will automatically apply more and more left pedal to try and compensate but this will have no effect and you will eventually reach a point where you have full left pedal applied but the helicopter is still yawing to the right. You should now have realised that the tail rotor is not working.
You must immediately roll off the throttle and perform an engine off landing from the hover. By rolling off the throttle, you will have eliminated the torque and the helicopter will stop yawing immediately. Keep the helicopter level and let it settle. Just before the skids touch the ground, start raising the collective to reduce the rate of descent.
If your helicopter type uses a correlator to adjust the throttle (e.g. R22, R44, H269) make sure that you roll the throttle past the detent to prevent the correlator from opening the throttle up again as you raise the collective.
Tail Rotor Failure in Forward Flight
In forward flight, a tail rotor failure may not be noticed right away if you are traveling at high speed. At lower speeds it is noticeable by a nose right yaw (in helicopters that have a rotor that turns anti-clockwise when viewed from above). If airspeed is reduced, the helicopter will yaw further to the right.
The only option here is to eliminate the torque by lowering the collective and closing the throttle. Obviously you will have to perform an engine off landing but this should not be a problem to you if you have flat ground below.
On touch down, try to run on slightly. The helicopter will try to turn to the left as it slides along the ground. This could cause the helicopter to roll over but there is a way that you can prevent this from happening. If you open the throttle slightly, the increased torque will turn the nose to the right and if you close the throttle, the decrease in torque will turn the nose to the left.
I use a simple trick for this. Stick your left index finger straight out as you grip the collective. When you open the throttle, your index finger points right. This is the direction the nose will yaw and it will yaw by approximately the same number of degrees as the movement your index finger made. (Vice verse for closing throttle). So, by adjusting the throttle, you can prevent the aircraft from rolling over.
Stuck Left Pedal
It is possible that you may find yourself in the position that the left pedal is stuck in position (to the left) during flight. This could occur due to different reasons and it has been known to happen. The pedals will be locked in position. How do you land? Do NOT enter autorotation as you would for a normal tail rotor failure. Instead, experiment with different speed and power settings to get a feel for how much the nose yaws at different speeds. Set yourself up for a shallow approach (preferably to a concrete or tarmac runway where the helicopter can slide easily and the skids will not dig in). As you lower the collective for the descent, the nose will yaw even further to the left. Don’t worry about this. Gradually wash off the speed and be prepared for the helicopter to be yawing up to 30 degrees to the left. As the aircraft loses speed it will eventually lose translational lift and start to sink. You should only be a few feet above the ground at this point. As you raise the collective up to reduce the sink rate, the increase in torque will make the nose yaw right and the helicopter starts to straighten up. when the nose is almost straight, accept the speed and by using a combination of slightly forward cyclic to pitch the nose slightly down and also adjusting the throttle and collective; squeeze the helicopter down onto the runway. With a little practice, this is quite easy to do – but it does take practice.
Stuck Right Pedal
If it is the right pedal that is stuck, again, you must not enter autorotation. As with stuck left pedal, set yourself up for a shallow approach. Lowering the collective to start a descent will automatically make the nose yaw left due to the reduction in torque. You will have time to experiment with different speed and power settings to see what keeps the nose straight. Find a speed and power setting that gives a slight rate of descent at 10 – 30 knots while also keeping the nose straight. Do your approach with the intention of setting up these parameters before touching down. If it does not feel right on the approach, feel free to go around and try again. At a few feet above the ground and with the nose slightly yawing to the right, gently lower the collective. This brings the nose straight again and also makes the helicopter descend. As you touch down, use the throttle to control heading. Simple.
Instruction
Whether you are a student or a qualified pilot, it is beneficial to get an instructor to show you these tail rotor failure procedures every now and again. You never know when it might come in handy to be able to handle tail rotor failures.
The Helicopter Engine (Piston)
(Reviewed on 13th March 2021)
The Piston Engine
Piston engines are the most common type of engine to be found in modern, light helicopters. Normally there will be 4 or 6 cylinders in a horizontally opposed configuration. For information about turbine engines, visit the Turbine Engine post on this website.
Pistons move back and fourth inside the cylinders. Inside each cylinder, fuel is mixed with air and ignited. The energy produced by the combustion of the fuel air mixture causes the gases to expand and drive the piston down into the cylinder.
The piston is connected by a connecting rod “con-rod” to a drive-shaft which is forced to turn due to the movement of the piston.
Piston engines may operate on either a two stroke cycle or a four stroke engine cycle.
The Four Stroke Engine Cycle
A complete cycle of the four stroke engine comprises four strokes of the piston moving within the cylinder. This cycle is also known as the “Otto cycle” after its inventor in 1876.
The four strokes are:
- Induction
- Compression
- Combustion (or Expansion) (or Power)
- Exhaust
Induction
The Intake Stroke
During induction, the fuel and air mixture is sucked into the cylinder through an open intake valve (on the right) as the piston moves from the top of the cylinder to the bottom of the cylinder.
The exhaust valve (on the left) is closed.
Compression
The Compression Stroke
Early in the compression stroke, the inlet valve closes and the fuel/air mixture is trapped in the cylinder. The piston then moves back up to the top of the cylinder.
This compresses the mixture and causes the temperature and pressure of the fuel/air mixture to rise.
As the piston reaches the top of the cylinder and completes its compression stroke, the fuel/air mixture is ignited by a spark from the spark plug. This causes combustion which causes the gases in the cylinder to expand.
Power
The Power Stroke
As the piston has passed the top of its stroke, the expanding gases force it back down the cylinder.
This is called the power stroke as the heat energy provided by the combustion process is now converted into mechanical energy.
Just before the piston reaches the bottom of its stroke, the exhaust valve will open.
Exhaust
The Exhaust Stroke
As the piston returns to the top of the cylinder again, the burned gases are forced out of the cylinder and into the atmosphere through the exhaust manifold.
As the piston nears the top of its stroke and while the last of the burned gases are being expelled, the inlet valve opens in preparation for the next induction stroke.
In one complete Otto cycle, only one of the four strokes provides power – but the crank-shaft has rotated two times.
Engine manufacturers increase the power of the engine by adding more cylinders. This has the added bonus of making the engine run smoother. Each cylinder will have the power stroke occurring at different positions during the rotation of the crankshaft to try to even out the power impulses.
Compression Ratio
This is the ratio of the total cylinder volume when the piston is at the bottom of its stroke (bottom dead centre BDC) compared to the top volume of the cylinder when the piston is at the top of its stroke (top dead centre TDC).
The compression ratio is designed to suit the type of fuel used. If the compression ratio is too high, the fuel may ignite early and excessive wear will occur.
Valves
Piston Engine Valve Timing
Both the inlet valves and the outlet valves must open and close at the correct times in relation to the movement of the piston. The timing of the valve operation is controlled by a camshaft. The camshaft only rotates at half the speed of the crankshaft. The camshaft operates rocker arms and pushrods that push to relevant valve open. When the camshaft releases the pressure, a spring returns the valve to the closed position. A typical helicopter piston engine speed in flight is 2700 revolutions per minute (RPM). Each inlet valve and exhaust valve opens once during the four strokes of the Otto cycle. I.e. once in every two revolutions of the crankshaft. This means that at 2700 RPM, each valve will open and close 1350 times per minute = 22 times per second. This is a very short time to get the fuel/air mixture into the cylinder and exhaust the burnt gases again. To increase the efficiency of the fuel/air mixture induction, the inlet valve opens before the piston reaches top dead centre (TDC). This allows maximum time to induce the fuel/air mixture into the cylinder. It is referred to as valve lead. Similarly the exhaust valve opens before the piston reaches bottom dead centre (BDC) on the power stroke. It is worth noting that for a very short time at the start of the induction stroke, the exhaust gases are still exiting through the open exhaust valve while the fuel/air mixture is being forced into the cylinder through the open inlet valve. This period of overlap when both the inlet valve and the exhaust valve are open at the same time is known as valve overlap.
Radiotelephony – Numbers
(Reviewed on 9th March 2021)
Pronouncing Numbers
During a recent flight I was cleared by ATC to climb to “fifteen hundred” feet. If you are an aviator, you should be aware that the number “fifteen” is never used in standard radiotelephony. I decided to make this post to let you know how to transmit numbers when using the radio.
Transmission of numbers is frequently done incorrectly and this post should make you confident in how to do it.
Transmission of Numbers
Numeral Pronunciation
0 Zero Bold Text shows correct pronunciation
1 One
2 Two
3 Tree
4 Four
5 Fife
6 Six
7 Seven
8 Eight
9 Niner
Decimal Decimal
Hundred Hundred
Thousand Tousand
Almost every number transmitted to you by ATC must be repeated back (with some exceptions).
All numbers (with a few exceptions listed below) must be transmitted by pronouncing each digit separately.
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General Numbers
When transmitting messages containing aircraft call-signs, altimeter settings, flight levels (with the exception of FL 100, 200, 300 etc. which are expressed at ‘Flight Level (number) HUNDRED’), headings, wind speeds/directions, pressure settings, transponder codes and frequencies, each digit shall be transmitted separately; examples of this are as follows:
Type Number Transmitted as
Aircraft Reg. N3456S November Tree Four Fife Six Sierra
Flight Level FL 100 Flight Level One Hundred
Flight Level FL 130 Flight Level One Tree Zero
Heading 130 Degrees One Tree Zero Degrees
Speed 85 Knots Eight Fife Knots
Frequency 121.85 One Two One Decimal Eight Fife
Squawk 0234 Zero Two Tree Four
Altitude, Height and Visibility
All numbers used in the transmission of altitude, height, cloud height, visibility and runway visual range which contain whole hundreds and whole thousands should be transmitted by pronouncing each digit in the number of hundreds or thousands followed by the word HUNDRED or TOUSAND as appropriate. Combinations of thousands and whole hundreds shall be transmitted by pronouncing each digit in the number of thousands followed by the word THOUSAND and the number of hundreds followed by the word HUNDRED; examples of this convention are as follows:
Number Transmitted as
10 One Zero
100 One Hundred
2,300 Two Tousand Tree Hundred
12,000 One Two Tousand
22,000 Two Two Tousand
NOTE: Decimal points are NEVER pronounced as “point”. ALWAYS as “decimal”.
Time
When transmitting time, only the minutes of the hour are normally required. However, the hour should be included if there is any possibility of confusion. Time checks must be given to the nearest minute. Co-ordinated Universal Time (UTC) MUST be used at all times, unless specified. 2400 hours designates midnight, the end of the day, and 0000 hours the beginning of the day.
Number Transmitted as
0923 Hours Time Two Tree OR Time Zero Niner Two Tree
1400 Hours Time One Four Zero Zero
1743 Hours Time Four Tree OR Time One Seven Four Tree
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Summary
If you study this post carefully, you should have no problem pronouncing numbers correctly on the radio. You will occasionally hear other pilots and Air Traffic Controllers pronounce numbers incorrectly. They are wrong. You will know that they are wrong. You will make mistakes – just like me. Put your mistakes behind you and strive to keep a high standard. Remember – “Practice makes perfect”.
How to Determine Wind Direction in-flight
(Reviewed on 3rd September 2020)
Wind Direction
Student pilots can sometimes become bewildered and confused about how to work out where the wind is coming from when approaching to land at an uncontrolled area. Quite often they can feel the helicopter drifting and buffeting due to the wind but the direction is not obvious.
Knowing the wind direction is extremely important for any helicopter pilot. We need to be planning ahead. Knowing what direction the wind is coming from will save valuable seconds in the event of an engine failure and thus allow us to perform a safe autorotation that terminates into wind. When we carry out confined area approaches, we need to be able to assess the wind direction to be able to plan an approach. Departures from confined areas also require us to know the wind direction as quite often this will determine our departure route.
- Look for visual signals of wind direction on the ground or water. Water is a great way of finding wind direction. Look for the calm area near a shoreline. The wind is blowing over this calm area into the rougher water beyond. White streamers on the water blowing parallel to the wind mean the wind is blowing at least 20 knots.
- Flag poles on golf courses or hotels are a great way of telling the wind direction and strength although you need to be reasonably close to them to see them accurately.
- Smoke from chimneys or fires is also an accurate indication of wind direction and strength.
- Strong wind will create waves over forests and long grass or crops. You can see the speed and direction of the surface wind by studying these waves.
- Birds are clever – they always take off into wind however I have seen them approach with a tailwind before turning into wind.
- Wind turbines. If these are turning, they will be pointing into wind.
- Fly one, two or three orbits at a constant speed and constant angle of bank. Start the turn(s) over an easily identifiable feature on the ground. When you have completed 360 degrees (or any number of complete turns) note your position over the ground. You will have been blown downwind from the starting position and you now have the wind direction at the altitude you performed the turns.
I use numbers one to six (above) regularly during every flight to assess wind speed and direction. Water and smoke are my favourites. When no other suitable method is available, I use item 7 above (orbits) and this works very well but takes a minute longer.
