Bell 206 Main Drive Shaft failure
Bell 206 Main Drive Shaft failure
Team,
I'm not familiar with the tech for a 206 (never had the opportunity to fly one). However, I'm reading a manual for a TH67 and it states that in the event of a main drive Shaft failure you'll get a sudden increase in engine rpm (as the load is shed) decrease in rotor rpm and a yaw as the torque disappears. All of this is pretty vanilla
However, it also says the engine must remain in operation to provide power to the tail rotor and that loss of control may occur if the engine is shut down.
My understanding is that conventional wisdom drives the TR from the MRGB - is this manual talking about a drive shaft failure between mrgb and the rotor system itself? Or is there something unusual about the 206 setup?
<Edit number typo>
I'm not familiar with the tech for a 206 (never had the opportunity to fly one). However, I'm reading a manual for a TH67 and it states that in the event of a main drive Shaft failure you'll get a sudden increase in engine rpm (as the load is shed) decrease in rotor rpm and a yaw as the torque disappears. All of this is pretty vanilla
However, it also says the engine must remain in operation to provide power to the tail rotor and that loss of control may occur if the engine is shut down.
My understanding is that conventional wisdom drives the TR from the MRGB - is this manual talking about a drive shaft failure between mrgb and the rotor system itself? Or is there something unusual about the 206 setup?
<Edit number typo>
Engine drives forward to the main TXMSN via that drive shaft, and rearwards to the tail rotor. Not driven by Txmsn, but by engine. With no load from the main rotor, who knows what the engine RPM will be, even at idle, but leave it running and you will at least have some control during the auto.
Engine drives forward to the main TXMSN via that drive shaft, and rearwards to the tail rotor. Not driven by Txmsn, but by engine. With no load from the main rotor, who knows what the engine RPM will be, even at idle, but leave it running and you will at least have some control during the auto.
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free wheel has a shaft aft to tail rotor and the same shaft drives the main rotor,
think of a life saver on a straw the life saver is the driven free wheel and the straw the shaft that drives everything.
the candy stops moving as the straw still spins
break the front end of the strawstraw and the main gear box stops and the aft straw still has power
think of a life saver on a straw the life saver is the driven free wheel and the straw the shaft that drives everything.
the candy stops moving as the straw still spins
break the front end of the strawstraw and the main gear box stops and the aft straw still has power
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There is a continuous connection from the MGB to the tail rotor but they are driven by the engine from a point some way down, so if the engine fails the TR will still be driven. If the connection from the accessory gearbox to the MGB is lost, however, the situation is as described. The arrangement is a common one for modern helicopters as it saves a drive shaft.
I once asked a Bell engineer how you can tell when the main shaft fails and he said "Don't worry, you will know....."
I once asked a Bell engineer how you can tell when the main shaft fails and he said "Don't worry, you will know....."
A friend was in a B212. Both engines running happily, but the shaft from the combining gearbox to the txmsn snapped. Massive double overspeed, rotor low warning, and a very nice auto saved the day.
The bit about "Don't worry, you will know" took a couple of seconds to process the data.
The bit about "Don't worry, you will know" took a couple of seconds to process the data.
While how the T/R is driven has been explained above, attached a rough diagram: The red line shows a normal condition where the M/R and T/R are driven through the free-wheel unit by the engine. The blue line shows a loss of the IDS where the T/R is still driven through the FWU provided the engine is still operating. The green line shows an engine drive loss where the autorotating M/R drives the T/R via the FWU.
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Down to idle
Somewhat recently at factory training, I was told they actually want you to roll the throttle down to idle in this situation. You’ll still have enough controllability and potentially save the engine from a giant over speed. Seems reasonable.
I was told they actually want you to roll the throttle down to idle in this situation. You’ll still have enough controllability and potentially save the engine from a giant over speed
Main Drive Shaft FailureThe following symptoms occur
a. N2 RPM rapidly increases while rotor RPM decreases
b. Yaw left
c. Engine noise increases
d. Low RPM audio activates
e. N2 stabilises 103% or above
Recovery
1. Collective – Adjust to maintain rotor RPM
2. Confirm failure
3. Throttle – Maintain fully open to provide tail rotor thrust
4. Select landing area
5. Call mayday
6. Transponder – EMER
7. Brief passengers
8. Harness – Locked
9. Visor – Down
10. Land – Power on autorotation
Note: Because tail rotor will maintain constant RPM throughout the autorotative landing, tail rotor effectiveness will be greater than usual. Care should be exercised in compensating for yaw on touchdown.
10. Land – Power on autorotation
People might think "Oh, power on auto, I can power terminate if I stuff it up..."
Shouldn't that be "Power OFF auto", even though the engine is running?
People might think "Oh, power on auto, I can power terminate if I stuff it up..."
theres no doubt the engine speed will climb when the shaft fails.
my question is how fast and how high?
Will the governor at least put up a good fight and keep the engine from grenading itself?
my question is how fast and how high?
Will the governor at least put up a good fight and keep the engine from grenading itself?
In the accident reports I've not seen any mention of how high the overspeed, but no mention has been made of any engine damage. I'd venture that during such an event the overspeed would be over before the pilot had a clue as to what just happened. An engineer may be able to comment on checks required on the engine, if any, following a drive shaft failure.
In the accident reports I've not seen any mention of how high the overspeed, but no mention has been made of any engine damage. I'd venture that during such an event the overspeed would be over before the pilot had a clue as to what just happened. An engineer may be able to comment on checks required on the engine, if any, following a drive shaft failure.
Remember, in a 206 with an IDS failure, you're not going from, say, 80% torque to *zero* torque. The engine is still driving the oil cooler and tail rotor. So it's not completely unloaded. However, it will be "some" time for the governor to react. I'm sure the left yaw snap of the nose will be, as the Brits say, lively. Pilot better have quick feet, quick hands and, oh yeah, a quick brain.
The best example of this was the 206 tour helicopter that crashed in Hawaii back in February of 2016. As the ship is on short-final, you can clearly hear the tail rotor increase RPM when the IDS fails. The 206 makes a very distinctive tail rotor growl. There is some left yaw as you'd expect, and it looks like the pilot arrests it, maybe, but the ship hits the water too quickly to tell. Doesn't look like he had much time to do anything but utter an expletive and try to pull the collective up out of its mount.
It should be noted that the 206/407 is unique in its goofy tail rotor drive configuration. Other aircraft that employ the RR-250 series engines do not have the same issue. I mean, think about it: If the d/s fails at the forward (transmission) coupling, the engine is still going to be driving what's left of that shaft. Can you imagine the god-awful beating and banging going on back there? If the failure happened at, say, 500 feet agl, everything will be over pretty dang quickly, no matter what you do. If it happens above 500 feet agl, you're going to desperately wish it would be over quickly - but it won't. (This is why most helicopter pilots don't like to fly high: Subconsciously they worry about one-in-a-million failures like this.) From a higher altitude, I might be tempted to stop-cock the engine just to stop the noise, and deal with the resulting tail rotor failure at the bottom. I pray to God I never have to find out just what I'd do in that situation.
The best example of this was the 206 tour helicopter that crashed in Hawaii back in February of 2016. As the ship is on short-final, you can clearly hear the tail rotor increase RPM when the IDS fails. The 206 makes a very distinctive tail rotor growl. There is some left yaw as you'd expect, and it looks like the pilot arrests it, maybe, but the ship hits the water too quickly to tell. Doesn't look like he had much time to do anything but utter an expletive and try to pull the collective up out of its mount.
It should be noted that the 206/407 is unique in its goofy tail rotor drive configuration. Other aircraft that employ the RR-250 series engines do not have the same issue. I mean, think about it: If the d/s fails at the forward (transmission) coupling, the engine is still going to be driving what's left of that shaft. Can you imagine the god-awful beating and banging going on back there? If the failure happened at, say, 500 feet agl, everything will be over pretty dang quickly, no matter what you do. If it happens above 500 feet agl, you're going to desperately wish it would be over quickly - but it won't. (This is why most helicopter pilots don't like to fly high: Subconsciously they worry about one-in-a-million failures like this.) From a higher altitude, I might be tempted to stop-cock the engine just to stop the noise, and deal with the resulting tail rotor failure at the bottom. I pray to God I never have to find out just what I'd do in that situation.
The only failure I'm personally aware of was a K-Flex failure and the pilot felt a klunk which put his eyes on the panel and he saw the needle splits. He caught the overspeed which he said was quick but slower than a high side gov failure.
FWIW: I don't have a RR manual handy, but I don't recall any specific overspeed inspection for the 250 series engines. Usually if the engine exceeds any RPM limits it doesn't stop and "self-inspects" by throwing a wheel loose. However, I believe the loss of a main driveshaft falls under the "sudden stoppage" special inspection which has several levels based on component damage with the 1st levels being inspections only, ending with an overhaul requirement if the engine or xsmn mounts are deformed/broken.
FWIW: I don't have a RR manual handy, but I don't recall any specific overspeed inspection for the 250 series engines. Usually if the engine exceeds any RPM limits it doesn't stop and "self-inspects" by throwing a wheel loose. However, I believe the loss of a main driveshaft falls under the "sudden stoppage" special inspection which has several levels based on component damage with the 1st levels being inspections only, ending with an overhaul requirement if the engine or xsmn mounts are deformed/broken.
It should be noted that the 206/407 is unique in its goofy tail rotor drive configuration.
AS 350 is pretty much the same. Forward shaft or couplings let go and no different.
It should be noted that the 206/407 is unique in its goofy tail rotor drive configuration. Other aircraft that employ the RR-250 series engines do not have the same issue