Thanks JohnR81 - that explains some of the confusion. I wonder if the guy in the video was often swapping between Raven and Raven II - it would go some way to explaining the switch cock-up.
As I said, why would Robinson not keep a standard layout between models - just a HF accident waiting to happen. |
If he used a checklist he wouldn't have made the cock-up even if they had moved the switch to above his head. Models change and evolve that's why we have variance training.
The switch had to be faulty (intermittent?) and normally he would have found his error when starting up but on this occasion it caught him out. |
Originally Posted by [email protected]
(Post 11004399)
Thanks JohnR81 - that explains some of the confusion. I wonder if the guy in the video was often swapping between Raven and Raven II - it would go some way to explaining the switch cock-up.
As I said, why would Robinson not keep a standard layout between models - just a HF accident waiting to happen. On the Raven I you turn it until you hit START, get the engine running, then release the switch out of its momentary START position into the BOTH position. On the Raven II you turn it until you hit PRIME (the momentary position of the switch is used to run the electric primer of the fuel injection on the Raven II), prime for however many seconds you want, then release the switch out of its momentary PRIME position into the BOTH position. Then you use one of the two identical starter buttons, one located on the cyclic, the other on the collective, to start the engine. After that both variants are the same for the mag checks: two clicks back to R, then two clicks forward to BOTH, then one click back to L, then once click forward to BOTH. The muscle memory is identical. The real danger in switching back and forth between the two variants is that the carbureted Raven I requires you to manage carb heat. If you are coming out of the Raven II and forget to do that it can really ruin your day. BREAK In re-watching both the front and rear seat videos, it would seem that there might have been some concern about engine performance, and that would be commensurate with running on only one mag. In the rear seat video particularly, the pilot picks up and sets down a couple of times. And if you listen carefully I think I heard the word "engine" on the audio track. Also, on the front seat video, the manifold pressures look exceedingly high for the conditions they probably had in TX in January. If I had picked up on, say, a 15 or 20C day at TX altitudes and saw nearly 25" that would be most unusual and concerning. And the MP is sitting at 20" when they were on the skids, which is very high if the collective is all the way down, even with just one mag it doesn't do that, so perhaps he still had some collective pulled in, not sure why. Anyhow, it seems we'll never really know, because there does not appear to be an FAA or NTSB investigation opened up on this incident, at least not in any publicly accessible database. |
If it is N322SH ROBINSON HELICOPTER R44 II
Then NTSB Status - In Work Preliminary "On January 9, 2021, about 1010 central standard time, a Robinson R44 helicopter, N322SH, was substantially damaged when it was involved in an accident near Albany, Texas. The pilot and two passengers were not injured. The helicopter was operated as a Title 14 Code of Federal Regulations (CFR) Part 91 hog hunting flight. The pilot reported that it was the third flight of the day and the helicopter took off uneventfully. About 120 ft above the ground, the engine sputtered temporarily before it lost complete power. The pilot performed an autorotation to a field. During the descent, the helicopter impacted trees and landed hard right skid low. Subsequently, the main rotor blade contacted, and separated, the tail boom. The helicopter has been recovered to a secure location for further examination." |
Thanks, John. I see I'm looking in the wrong place. I didn't know they had switched over to this "CAROL" site.
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aa777888 - it's fine if someone always follows the correct procedures as you shouldn't get caught out - the HF element comes when a cursory glance doesn't identify that something looks wrong - if he flys both types and is used to seeing different pictures in each for normal flying configuration, he might forget which he is in and the subconscious alarm bells don't ring to get him to take a closer look.
Notwithstanding that, the engine performance should have been a big clue if it is at such variance from the norm as you describe. Whichever way you cut it, it seems the fault lays firmly with the guy in the right hand seat. |
Originally Posted by aa777888
(Post 11004496)
The real danger in switching back and forth between the two variants is that the carbureted Raven I requires you to manage carb heat. If you are coming out of the Raven II and forget to do that it can really ruin your day. As for the key, yeah muscle memory with how to check it, but up until this accident if you'd of asked me which way the key points, I'd of just stared back like a deer in the headlights,...as that's not something I've ever paid attention to,...let alone memorized. Funny though, I've complained about Robby's lack of standardization with regards to instrument placement (mainly when I get into a ten-hole) but never even noticed the inconsistent key setup,...? |
Interestingly, the R44 checklist includes a mag check on each mag to ensure the engine doesn't cut out on only one mag and that the RPM drop is insignificant, and then returning the key to the 'both' position and checking the RPM returns to normal...it's the same on the checklist of the Piper PA-32 using the Lycoming O-540/IO-540...so if this was followed the key would be in the 'both' position
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Lack of standardization seems to be an issue in many helicopter models. Look how many variants of the Squirrel/Astar there are. Single vs. dual hydraulics, different engines, FADEC/no FADEC, etc. Lot's of different panels in helicopters that have been around a long time, like the MD500, the 206, etc.
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Doesn’t seem too smart to have a big floppy key fob that significantly obscures the position of the key beneath it - particularly when that position is critical. Surely a more rigid coloured small flag on the key would be more sensible, to enable instant visual recognition of key position?
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Originally Posted by BDAttitude
(Post 11004936)
That's what you do in a SEP - watching for the RPM drop when when switching BOTH-L and BOTH-R. You don't have to watch the gauges - not being deaf is enough. I have no idea how practical this is with a rotor and its significantly higher inertia.
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Originally Posted by BDAttitude
(Post 11005135)
Switching an ignition circuit off leads to a step in power produced by the engine, not a step in engine speed. So no freewheeling.
On a prop with low inertia and no constant speed governor active, the new power level will lead to a nearly instantaneous drop in speed due to low masses and high aerodynamic resistance. I strongly suspect that I will take a little longer for the rotational masses in a R44 to settle for the new power level. I was asking how long that takes and if this magneto check is practical therefore. |
Freewheeling does occur. The sprag clutch leaves the engine free to decelerate to the extent that switching a mag off will make it decelerate without any inertial effects from the transmission or rotor system |
Originally Posted by 212man
(Post 11005228)
So you are saying there is a needle split when you do the mag check? There are some instructional videos available that would tend to suggest otherwise.
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Could it be that the clutch carries a small amount of negative torque that has to be overcome before freewheeling happens? And, if so, a mag check does not overcome this small amount, so they stay engaged? Because if we're talking 5-10 seconds to see a mag drop, there's definitely something "dragging up" the engine RPM, which, in an airplane takes more like an eighth of a second (i.e., near instantaneous) to change.
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Originally Posted by Vessbot
(Post 11005269)
Could it be that the clutch carries a small amount of negative torque that has to be overcome before freewheeling happens? And, if so, a mag check does not overcome this small amount, so they stay engaged? Because if we're talking 5-10 seconds to see a mag drop, there's definitely something "dragging up" the engine RPM, which, in an airplane takes more like an eighth of a second (i.e., near instantaneous) to change.
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Originally Posted by BDAttitude
(Post 11005340)
The engine delivers positive torque at all time during the magneto check, just slighltly less when on one ignition circuit. The entire powertain therefore decelerates until the new power setting (pi/30*rpm*torque, torque up, rpm down) is in balance with the aero forces. It‘s the same as if you decelerate your car without coasting (dragging the engine).
Thank you aa777888 for that up to 10s. I doubt the majority of operators will have the discipline. |
One thing I didn't think of is that the engine is turning that big squirrel cage fan right off the crankshaft, right where a prop would be on an airplane.
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In a normally aspirated piston engine, manifold pressure is determined by throttle position and engine RPM. Mags on / mags off; mixture full rich or idle cut-off; engine producing power or no power, fuel tanks full or empty; the air in the manifold doesn't know about any of that; only how how fast the engine is turning and what the throttle position is.
A surprising number of piston pilots will get this wrong until they think it through. |
why does the change take 5-10 seconds instead of being nearly instantaneous, as it should be freewheeling doesn't occur during a mag drop check, there is a power reduction, not a removal of power. |
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