I wondered for a while whether this topic is suitable for a blog post at all. We primarily want to encourage customers to book a sightseeing flight. If we now speak of errors and defects, this could rather counteract this goal. On the other hand, we want to be transparent to our customers and also impart knowledge of what is absolutely essential for helicopter flying and must be done before the flight can even be carried out. There will be a separate article dealing with the various elements of the very complex flight preparation. Overall, I think that a blog post about what we pilots want and need to check in order to discover errors and how we can deal with them tends to increase trust. Now for the story I actually want to tell.
A few weeks ago we unfortunately had to turn off the running helicopter a few moments before take off, let the disappointed passengers disembark and cancel this flight and all other flights of the day in glorious weather. What happened? The magnetic check - one of the many checks that we go through before taking off - showed us a possible problem with the drive. The zero tolerance guideline forced us to cancel the flight before take-off, to send the passengers back home and to make a new appointment after the error had been analyzed, found and of course also eliminated by certified specialists. Now what the hell is this magnetic check? To do this, we have to go back a little, because the magnet has to do with the ignition, which is used in a 4-stroke gasoline engine of the helicopter.
The internal combustion engine
In order to understand that well, we have to take this opportunity to take a look at the internal combustion engine itself. It's getting a bit technical now - I'll try to explain everything very simply. The Robinson R44 helicopter has an internal combustion engine that works on the 4-stroke principle. In other words, just like the normal petrol / gasoline motor vehicles that drive millions of times on the streets in Germany. (A diesel engine is a bit different, we will leave it out at this point.) Essentially, a gasoline-air mixture is brought into a closed container, compressed, ignited and thus made to explode. The force of this explosion is used to turn the wheels of the car or, in our case, the main rotor of the helicopter, using several levers and shafts. Why is this engine now called 4-stroke? Because there are four different processes that happen one after the other and which are now briefly explained. Four isn't that much - so stay tuned and read on.
1st stroke: inlet of the gasoline-air mixture
First of all, the room in which the combustion takes place is filled with a combustible mixture of gasoline and air. This room is not square, but round and long, shaped like a pipe. The term “cylinder” is somewhat more scientifically correct. So in a car with a 4-cylinder engine, we have four of these tubular combustion chambers. In some engines, the liquid gasoline is first converted from a liquid to a gaseous state in the carburettor because it burns better in gaseous form. Hence the name “carburetor”. Other engines inject the gasoline directly into the cylinder, the spray mist evaporates and then also forms the combustible gas mixture in the cylinder.
2nd stroke: compression
The cylinder is then closed so that the later impact of the explosion does not go anywhere where it is of no use to us. For this purpose, the pipe through which the cylinder is filled with air and gasoline gas is closed by a valve. Because this valve fills the cylinder, it is called the intake valve. After combustion, the exhaust gas then flows through the exhaust valve in the direction of the exhaust. Both valves are closed. The gas-air mixture is then compressed. Imagine squeezing an air pump and holding the valve shut with your thumb. As with such a locked air pump, a piston is pushed into the cylinder, which increases the pressure on the gas.
The 4 cycles of a regular petrol gasoline engine
3rd stroke: ignition
Now the mixture is ready for the explosion. At the top of the cylinder is a spark plug that ignites the gas at the right moment with a spark. To generate this spark, it needs a certain amount of electricity. Basically, this is comparable to a lightning bolt: if the electrical charge is strong enough, we generate a sudden discharge at the right moment. Due to the explosion, the piston in the picture is pushed down again and this movement is translated into a rotating shaft via various mechanical elements such as rods and shafts. This rotation is then ultimately transferred to the wheels of the car or the main rotor of the helicopter.
4th measure: expulsion
Now there is almost no pressure in the cylinder, but there is a lot of hot exhaust gas. The exhaust valve opens and as the piston moves up again, it pushes the exhaust gas through the open exhaust valve towards the exhaust. When this is done, the exhaust valve closes and the intake valve opens to refill the piston with fresh gas. The first bar follows.
The running petrol gasoline engine
A key figure for the size of the engine is the displacement. This is the volume that the piston displaces from the lowest to the highest point. The more gas that can be ignited, the more power the engine will normally have. The cylinder or the entire engine could simply be made larger. Or more cylinders could simply be built. So if you see a car on the street that has 2.0l on the back (l for liters), that means that the engine has a total of 2 liters of displacement. If one assumes that the engine has 4 cylinders like most others, then a single cylinder of this engine has a displacement of half a liter. In other words, gas with the volume of half a milk carton is burned in each cylinder per stroke. And since the strength of the engines is often compared on the road, as in racing, the following joke is explained: “What is better than a lot of displacement? Even more displacement! "
The spark plug requires electricity to generate the spark. An ignition spark only jumps over when the voltage is quite high. This current comes from the battery (with a relatively low voltage) and is then brought to high voltage in the car via an ignition coil. Each spark plug is then supplied with power at the right time via a distributor. If the battery or ignition coil fails, none of the spark plugs will be supplied with power. With a car, all of this is not very life-threatening - you just “pull over”, unless that happens in the left lane at a speed of 250 km / h. With a helicopter, however, there is no “right-over” -flight, so you are with an older one, but Nevertheless, more reliable ignition technology remained: the magneto ignition.
Have you ever been flashed?
To understand magneto ignition, first a mini-digression on the topic of induction: Have you ever been flashed at a traffic light at red? How does the flash box notice that you drove red? Radar? No, radar is more likely to use the mobile speed cameras, which drive from one place to another and hide behind bushes, signs or guardrails like road users. These mobile radars are also very prone to errors and have to be re-calibrated or calibrated every time. With fixed speed cameras or flashing lights, you usually see structures in the street on which you are measuring. The same physical principles are used here as for magneto ignition. Your car is made of metal and moves over a loop of metal cable. When an ideally still magnetic metal object moves over a metal loop, the current in the metal cable is measurably influenced by magnetic induction. This influence is then used as a triggering signal. The loop in the street is therefore also called the induction loop.
If we now let a piece of magnetic metal (i.e. a magnet) circulate continuously over several cable reels, we get a current. After a few more steps, this current is used to ignite the right spark plugs at the right time. This magnet is attached directly to the motor and is rotated directly by it. As a result, the motor - as long as it is running - rotates the magnet and this generates the electricity required for the spark plugs. So there is no dependency on a battery, an ignition coil or a power generator. As long as the engine is running, the magnet generates enough electricity for its own spark plugs to keep it running. “Okay”, you could say now, “but what if the magnet fails?”. Well thought out - that's why we have two independent magnets, each with its own ignition circuit.
The magneto ignition in the Robinson R44 Raven II
An IO-44 engine from the American manufacturer Lycoming is installed in the Robinson R540 variant Raven II helicopter. This is an air-cooled boxer engine with six cylinders and a total of 9 liters of displacement. The Americans do not really know the European volume units of measure cubic centimeters or liters, they use inches instead of centimeters. This creates the number 540 in the name of the engine - it is 540 cubic inches - or the equivalent of 9 liters. See also our blog post "In portrait: The Robinson R44 helicopter". Two ignition magnets are attached to this engine, each of which independently supplies sufficient current for the respective ignition circuit. Due to the design of the motor, there is a magnet on the left and a magnet on the right. Conveniently, the ignition circuits are then also called “left” and “right”. In the next picture you can see an “R” and an “L” in addition to “both” for “both” next to the ignition key. With the ignition key, the pilot can only switch on the right, only the left or both at the same time.
Ignition key positions on the R44
That means in total that we have six cylinders in the R44 and each of these cylinders has two spark plugs. So the engine has a total of twelve spark plugs. One each belongs to the left, the other to the right ignition circuit. The ignition circuits are each supplied with power separately by their own magnet. If, for example, a spark plug in a cylinder fails, the spark plug of the other ignition circuit is still active in this one cylinder. If a complete magnet fails in flight, the other ignition circuit is still active. The combustion is better if there are two ignition points per cylinder, but the loss of power should not be significant. So we can stay in the air with only one functioning ignition circuit. So we always fly with two ignition circuits, so we always have the key in the “both” position for both magnets during the flight. Now you might ask, why are positions for only left and only right possible. After a long explanation we finally come to the mysterious magnetic check.
The magnetic check
Checking the magneto is one of the last checks before you can start. This check is precisely defined. The pilot first sets the speed manually to a certain value. The speed of the motor and rotor is displayed in a separate instrument on the instrument panel. In the next picture it is the instrument at the very top right. There are two pointers here - one on the left and one on the right. The one on the left is labeled “E” for engine and the one on the right is labeled “R” for rotor. As a pilot, we can always see whether the engine and rotor speed are within the desired green range. In the normal state during the flight, both pointers ideally always show the same percentage from around 101% to a maximum of 102%. During the magnet check, the pilot, while standing on the ground, uses rotary throttle to set the speed of the motor and the rotor so that 75% of the nominal speed is achieved. The ignition key is set to “both” - so both ignition circuits are active. If the speed is then manually set to 75%, an ignition circuit is switched off - by turning the ignition key to approximately “R”. A certain drop in speed is to be expected, since we only have one ignition point instead of two and that makes the combustion a little less optimal. Within two seconds, the speed can now drop by a maximum of 7% - i.e. to 68%. For example, by setting the ignition key to alternating “R” and “L”, one ignition circuit is switched off in order to see how the other one keeps the speed alone. Now, standing on the ground at 75% speed, we don't ask for a lot of power from the engine. If, however, even in this under-challenged state, one of the two ignition circuits shows a speed drop of significantly more than 7% within 2 seconds, then this check is deemed to have failed and the ignition must be checked. It is an unambiguous decision if the speed drops more than 7% or if the engine might want to shut down completely. Then the pilot knows for sure that something is wrong. Just under 7% is okay - but I personally keep an eye on that.
Part of the R44 instrument panel with the speed display in the upper right
This is what happened on a wonderful day of sightseeing at 10 a.m. Everything was planned and prepared, the weather was great, the passengers were in a good mood and it promised to be a wonderful day of flying with many great impressions and grinning faces. Then at some point came the magnetic check. When the ignition key switch was set to “L”, the engine speed dropped significantly almost instantaneously by 10% to 65%. With a further continuous slow decrease to 55%. The speed was stable there at 55%, but this does not correspond to what is required. The check was repeated a few times. Since the drop in speed was reproducibly higher than expected, it was clear that something was wrong with the ignition on the corresponding ignition circuit. Of course, the speed has recovered after switching on the other ignition circuit. Even if one ignition circuit has a defect - the other intact ignition circuit ran without any problems. So why then cancel the flights? First of all, it is a requirement. Point. Even if this regulation did not exist, the following considerations would lead to the same result: We generally assume the worst case scenario. Suppose we take off anyway with the left ignition circuit not really at full power and the right one completely intact. After an hour of flight or less, the right ignition circuit, which was previously intact, could also become defective and fail completely. The left ignition circuit alone cannot hold the speed required for flight. The pilot would then have to land the fully occupied helicopter on a meadow with little available power. Commercial pilots train in such a situation regularly and would probably work out well. Even so, it is an undesirable, potentially dangerous situation that must be avoided at all costs.
Neither sightseeing flights nor anything else today
So we canceled all flights planned for that day, which of course provoked different reactions. Some customers were understandably a little disappointed and one even angry. "Now I have to go from Munich to Jesenwang again on another day!" But that is the big exception. Most of the customers showed understanding and others were even very happy that we were working so conscientiously and taking good care of them. Here is the saying that I like to say to my customers in such cases: "It's better to stand down and want to go up than the other way around."