Discuss the problems and solutions to all of the situations that Pilot X finds himself in.
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By G-BLEW
#910957
The Engines' Beat


Richard Boswell tells the tale of young Pilot X and his first solo-flight with paying passengers on board – an important point in his career – but an hour’s delay and a passenger’s nervousness lead him to forget some basic rules...


The first engine kicked into life with the second rotation of the propeller. X watched the oil pressure rise before selecting 1,200rpm and bringing the generator online. He reached up above his head and flicked the magneto switches on for the second engine. He was hoping that the six passengers in the rear of the Islander would not be able to sense his nervousness, as he ran through the pre-start checks for the port engine in a slow and methodical manner.

X had not slept well the previous night. He had watched the weather report before going to bed and the smiling weather lady had promised early morning fog which was forecast to burn off before 11am, leaving a clear day. This worried X, as he knew that he was scheduled to take off at 10am. At 23 years old, X was the youngest pilot in the company. He’d left school at 18 and started a degree in physics, but from the moment he had completed fresher’s week, he knew he had made a mistake. A year and many missed lectures later, he switched courses to geology, in an effort to revive his interest levels. However, deep down he knew that it wasn’t the studying that interested him but rather his weekly flying lesson as part of the University Air Squadron.

By the end of his second year, and another round of poor attendance at lectures – but with another 50 hours in his logbook, he finally realised that he wanted to be a professional pilot and anything else that he did was merely marking time. He dropped out of university and applied to join the Air Force as a pilot. With almost 100 hours to his name and some good flying reports, he was quietly confident that he would make the grade, so it came as somewhat of a surprise when he was turned down as a trainee pilot but offered a place on the next intake as a navigator. He turned it down on the spot, took a huge loan from his father, and enrolled on a CPL course.
A bright and gifted pilot, he completed the course and found himself on the job market with all of the licences but not a lot of experience. Like the majority of his colleagues, he dreamt of flying big jets on long-haul routes, but right now any paid work as a pilot would do. Days turned into weeks, which merged into months as he supported himself pulling pints. Eventually the break came. Pilot X was offered a job as an Islander pilot on a single-pilot, scheduled service between the mainland and the nearby islands. He’d enjoyed the type rating and. although the aptly-named Islander was far from cutting-edge, it did hold a certain charm and he appreciated its iconic status.

The line training he found more challenging. Flying the aircraft was one thing, but being responsible for the entire flight was another matter altogether. The paperwork seemed endless and even though the aircraft was fitted with a capable autopilot, managing the flight as well as the passengers, while trying to stay ahead of the aircraft, was always challenging. After shared flights with company pilots, today was his first day alone in the cockpit, with six passengers on board who depended on him for a safe flight.

Intolerable

The second engine kicked into life almost as soon as the starter was engaged. X watched for the oil pressure rising and set the rpm again before completing the remainder of the start checks. The fog had appeared as forecast but was now also burning off, as forecast, so after a nervous one-hour delay waiting for the visibility to increase, it had now reached the legal limit for take-off and with his destination clear, it was all systems go.

Taxying out over the bumpy grass, X tried to settle his nerves. Never had he felt more alone in such a small space with so many people. Of the six passengers, one was clearly nervous and although sat well behind him, X could sense the apprehension as it crept forward and threatened to engulf him.

With engine run-ups and pre-take-off checks complete, X had one final glance around the cockpit before calling for departure. With the visibility at 1,000m, he could just about make out the end of the runway as he taxied onto it. Slowly he advanced both throttles and the cabin noise increased dramatically. The Islander bounced across the grass strip with the airspeed slowly increasing. At 50kt he lifted the nosewheel and the aircraft left the ground as the airspeed reached 65kt. He trimmed the aircraft in the climb and by 400ft he was already out of the thinning fog bank as he lowered the nose slightly and cleaned up the flaps while allowing the speed to build slightly.
Continuing the climb, he tried to relax a little and enjoy the view as he constantly retrimmed to keep the aircraft in a steady climb. It wasn’t until he passed through 1,000ft, climbing straight ahead, that he realised something wasn’t quite right – the noise level in the cockpit was becoming intolerable. Somehow he had forgotten to reduce the power after take-off and he still had maximum throttle and maximum rpm set.

He automatically reached forward and set 2,200rpm, with the blue-ended propeller control levers. He heard the rpm reduce and the sound level came down significantly, although there was now a distinct ‘beating’ sound. He nudged one of the rpm levers up and engines returned to the usual hum. Next, he moved his hand to the throttles; before setting the power, he glanced at the manifold pressure gauge. Both gauges were way over limit. His heart sank as he realised what he had done. He pulled both throttles rearwards and the gauges swung back down into the green range. However, even with his limited experience, X knew the damage to the engines had been done and sensibly elected to return to the airfield rather than risk the lengthy sea crossing. n


Variable-pitch propellers are not intrinsically complicated, however they do need a little thought and management to get the best efficiency out of the engine/propeller combination and avoid any damage to the engine and propeller.

1. What is the basic fail-safe rule when operating a variable-pitch propeller?
2. What causes the ‘beating’ X referred to?
3. How did X damage the engines?
By johnm
#911027
1. Increase revs then increase MP and reduce MP reduce Revs as noted elsewhere
2. It's the two props running at slightly different speeds and so generating almost the same frequency noise but not quite. Props are in sync when beating tweaked away.
3. Isn't the damage caused by high MP with low revs resulting in pre-ignition and so engine damage much like being careful with turbos that have a manual wastegate?
By AttorneyAtLaw
#911481
As regards point 3, you are trying to force the engine to produce more power than it is designed to produce at the given rpm with the potential for overstressing the engine. It is like driving a car at 70 mph and then suddenly changing down into second gear. Everything will be overstressed, so it is not just a question of premature combustion, though this is an effect due to too much fuel being forced into the chamber and combustion chamber pressure and temperature becoming too high for the rpm.

The dynamics are well illustrated on the ground when cycling the prop. Revs up means manifold pressure down as the engine delivers more power and the pressure drops in the manifold due to increased air flow. The reverse happens as you move to coarse pitch.
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By KNT754G
#911610
1 and 2 as posted above.

3 he has "overboosted" the engines, too much power at too l ow RPM. Potential for enormous damage. Bent con rods, twisted crankshafts, damaged pistons.

Even more of an issue in turbocharged engines where, without a wastegate or other mechanism to regulate the MAP it is possible to destroy an engine before reaching the far end of the runway.
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By Arrow IV
#920590
Have been looking at the "worst day" summary, especially on the 3rd question regarding the low RPM damage. Maybe it's my recent re-reading of the Deakin series on engine management, but I'm left baffled by some of the explanations in the Jan2011 Flyer; for example:

"force engine to produce more power than designed to": isn't any engine producing a more or less pre-defined amount of power given set mixture, manifold pressure, and RPM? how can one force it to produce more than that? was thinking about over-boosting but there is no reference to super- or turbo-chargers, and on a normally aspirated engine manifold pressure is suction anyway (less than ambient), and even if the engine is charged then the over-boost condition might well be there without the RPM reduction (assuming 2200RPM instead of 2500-2700 would cause 1-1.5 inches increased manifold pressure)

"70mph in 2nd gear": isn't it the other way around? reducing RPM was always explained to me as similar to higher gear, so wouldn't the equivalent be driving at 30mph and shifting into 5th gear? in a car that usually means weak acceleration but otherwise seems fairly uneventful (apart from subtle nudge by driving instructor)

"everything overstressed": I could understand over-torquing (esp. thinking of turbines producing massive full power at sea level against a large turbo prop prop set at 1500 RPM), and maybe that is the essence of this situation, but is a say 300hp piston engine running at 2200 RPM enough to create that sort of terminal damage within 1-2 minutes?

"premature combustion": fully accept that in a fixed-timing ignition system lower RPM will cause earlier timing, but is 2200RPM enough to go sufficiently early for premature combustion? also, if this was the case then lowering throttle would only aggravate the situation, as excess fuel tends to slow (rather than accelerate) the flame front in the cylinders

"too much fuel causing high combustion chamber pressure and high temperature": if you look at the mixture effect on pressure/temperature curves (e.g., in the Deakin articles), then on the significantly rich-of-peak side (like during take-off and initial climb) more fuel causes lower pressure and lower temperature, so how can this cause the damage?

I could never attempt to argue with superior minds like AttorneyAtLaw on these finer points, so assume the above is just bepuzzlement caused by my own ignorance of aircraft engines, but could someone help me link the dots?
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By Keef
#920597
Look on the combustion process as a very fast-burning fire (or a very slow explosion, if you prefer).

The design intent of the engine is that as the burn continues and the gases expand, they push the piston down and provide power. They also get very hot.

If the piston won't move down as fast as the design intent (RPM set too low) then the gases will push harder and get hotter than at the "correct" RPM. That may break things, will certainly overload things, and the extra heat may damage the piston.

I've seen a piston with a hole melted in the crown because the ignition was set too far advanced - the piston was still coming up when the mixture got to its hottest, so it exploded in one bang (rather than burning). That's "detonation", and high-speed detonation will destroy an engine in seconds.
By Tony Hirst
#923626
1 - to power up, mixture rich, props forward and throttle up. The opposite to power down. Prevents over boosting overrevving.

3 - over boosting causes detonation which is where the air/fuel mixture is ignited under pressure in addition and subsequent to spark ignition. This creates a very fast, uneven and hot burn. Then damage is due to shock and excessive and uneven heating. As I understand it, this different to pre-ignitition, which is hot particles of carbon igniting the mixture prematurely, but this process is generally due to overly weak mixture and is somewhat inefficient and doesn't produce the same destructive forces that detonation does. Detonation (or knocking) reduces with increased RPM, whereras pre-igniton increases. An excessively rich mixture also reduces detonation through cooling, hence the need for mixture rich abive 75% power for many piston engines.
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By Keef
#923856
High-speed detonation isn't easy to hear, but certainly happens, and will wreck an engine in seconds. Low-speed detonation makes a distinctive noise - it's sometimes called "pinking", which describes the sound it makes. Purist automotive engineers get angry when folks call it pinking.

In modern car engines, there is technology in the ignition control calibration that includes "knock sensing" which will keep the engine the right side of detonation. I doubt if it's reached Lycomings yet :)