RPM Needle

[24Xray]

An L.A. County Fire Department 412 Captian that I had the honor of talking to made me think of this so I thought I'd share it: The engine needle is longer than the rotor needle. It should be the other way around because if you have a transmission failure what are you going to notice, a big need rolling right or a little needle rolling left.  If you notice the big needle you will most likely respond to this as an engine over speed and pull more pitch and reduce power which will cause more rpm decay. The point was to always look at the little needle.

[Lu Zuckerman]

Observation of needles on the tach. is secondary.  If you have a transmission failure it is instantaneous and you become aware of it immediately.  If you have a high energy rotor system it will try to break through the failure and it will be reflected in an instantaneous stoppage of the rotor system with this energy being used to effect the breakthrough. If you are lucky enough to break through the stoppage and your rotor system is still in tact you can go into autorotation.  That is if your rotor system can generate the necessary rotational speed to effect the autorotation as heat is being generated in the transmission and parts are fusing together.  Everything goes south from that point.  If you are flying a tandem rotor helicopter and experience a transmission failure the next thing you experience is going through the Pearly Gates.  The same is true for a low energy rotor system like a Robinson.  The rotorhead either departs the mast or,the blades fold up and either way you are dead.

[matador]

Sorry to desagree at some point with somoene as expert as you, but what you say is only true for a broken down Xmsn. You could have a "slippery" free wheeling and your engines would overspeed but the RRPM-Xmsn would be slowing down. That is why I agree with the 412 cap. I think there is a real good and simple rule that is allways true: You have to do with your colective the same thing your RRPM are doing and to do that you must be monitoring mainly the RRPM's.

P.D. I had years ago a real bad ENG XMSN chip det. in a CHINOOK and we got the chip light, after a few seconds a XMSN OIL HOT and after smelling like burning, went down in about 45 seconds, shut down inmediatelly and when the rotors stopped was just about a sudden stoppage. Actually we couldn't move rotors and took out of it more than 3 lbs. of metal chip. Really scary.

Merry xmas and happy new year

[Lu Zuckerman]

To: Matador

 

Slippery freewheeling unit?  An Italian AS-61 suffered an in-flight failure of the left freewheeing unit and the pilot heard it as a loud bang.  It took 22 seconds for him to autorotate to the water and in that time the rotor was backdriving the system.  The right freewheeling unit disconnected but the left one did not.  This resulted in the engine being back driven resulting in its’ failure as well.  When the gearbox was disassembled the left free wheeling unit was either worn down due to friction or welded due to the generated heat from the friction.  All of that in 22 seconds.

 

As far as your experience on the Chinook is concerned just consider yourself as being damned lucky.  It took you 45 seconds to get on the ground but if it were any longer you would have had a transmission seizure and loss of synchronization.  Several years ago a commercial Boeing 407 (possibly not the right number) had a seizure of a transmission and the rotors lost synchronization and crashed in the North Sea.

 

In Iran the pilot of an AB-205 realized he had a defective freewheeling unit during an autorotation when he tried to reengage the engine to the driveline.  After several attempts and a dangerous loss of rotor RPM he put the helicopter in a dive to build up rotor speed.  He made a successful run on landing and while the rotor spooled down he was filling out paper work.  At some rotor speed the rotor departed the helicopter struck the pylon and the stabilizer bar punched through the right side nearly hitting a student pilot.  It was attributed to the helicopter suffering more than three compressor stalls, none of which were entered into the helicopter logbook.  Now this guy had a “slippery” freewheeling unit.

 

[matador]

Sorry again but respectfully I have to desagree with you:

About the S 61 accident, don't know a thing about that helicopter but if you have a failure in the left freewheeling DON'T UNDERSTAND that you have to autorotate when you have the right engine on line. Of course if you do that and the other freewheeling failures to disengage must be a bad thing, but the problems is that you did a bad procedure to begin with( just my opinion) ???

In relation to the CH 47 must tell you that if you have a problem in one of the ENG XMSN where you have your freewheeling, DOES NOT have anything to do with the synch. machanism that is placed at the out put of the COMB. XMSN, so I don't see that we could have lost that system. We had a risk of engine having FOD after a possible ENG XMSN blow up?? :cool: . I remember the accident you talk about but in that case the broke the AFT XMSN  and of course they had a desynch. problem.

If you talk about the 205 having problems to engage engine after autorotate, the thing is to do a real ground touch down auto, which if you were training should be no problem. A different thing would be a short shaft failure for instance, that would make your engine overspeed but your RRPM would decrease, and that was the INICIAL POINT: I THINK IS BETTER TO HAVE A BIG RRPM NEEDLE THAT BIG N2RPM NEEDLE, FOR AS I SAID BEFORE, ROTOR RPM's are more important than N2 RPM's.

P.D. Sorry for the delay in the answer but I've been out for Xmas.

[Lu Zuckerman]

To: Matador

 

On the Italian HH-3 (S-61) the pilots did not know it was a freewheeling unit that failed.  This was not determined until the transmission was disassembled at Agusta.  The only thing the pilot knew was that he heard a loud bang accompanied by a serious grinding noise.  I understand what you are saying but in this case the pilot felt it was better to be safe than sorry.

 

Regarding the C-47 problems I had assumed that you were talking about the main transmissions and not the combining gearbox.

 

Regarding the 205 incident, the pilot made a successful run-on landing with no problems.  The loss of the main rotor was as a result of fatigue due to several unrecorded compressor stalls.  It was the first Bell helicopter to lose its’ main rotor without loss of life.

 

Regarding the size of the respective needles the instruments have been designed that way since helicopters started flying.  The methodology of presenting this information to pilots using a glass cockpit is most likely different.

 

[matador]

Glad to see that we agree mostly 100%.

BTW one thing been done same way during years does not mean that is right, it coud be an example of error persistance :laugh: (sometimes don't know if I make myself understandable in English)

[nardoo]

you are doing fine Matador, keep it up :: you too Lu

A most enjoyable and informative discussion! :cool:

[MightyGem]

As Lu says, the needles have always been like that, so we all know which is which. If one manufacturer decided to change...now that really would cause confusuion.

[Lu Zuckerman]

To: Mighty Gem

 

The manufacturer would not be able to make any changes as all instruments on aircraft are governed by a design spec part of which deals with the presentastion of information.  They can make them bigger or smaller but even then they must fit a standard instrument hole on the panel.

 

:unclesam:

[jimbo2181]

I dont think I would ever have any doubt of the difference between an engine overspeed or drivetrain failure at least in the r22.  From what ive heard you get a nice loud bang followed by an instanenous hideous sound of the engine trying to break through 200%  But im just a little hour guy so this may be ignorance.

[Lu Zuckerman]

To: Jimbo 2181

 

If you had a gearbox lock-up on an R-22 I doubt seriously if the engine could break through given the belt drive system.  If in fact the transmission locked up you would be dead because the low inertia rotor system could not provide the energy necessary to break through the lockup.

 

One more thing to think about when flying a Robbie.

 

:unclesam:

[jimbo2181]

I wasnt talking about a gearbox lock up.  From what I hear that is pretty much fatal in anything, had one happen in a Hiller 12 at our ramp and the blades pretty much stopped instantaneously.  I was talking about other drivetrain failures such as snapped belts, broken yoke flange, or the like.  Sorry for the confusion.

[rotorboy]

The other day we were talking about drive train failure in the 61. I would be more concerned about a uniball seizure, that wouldnt be a good thing, I have heard it is possible to break through it , but it will happen again very fast...

 

rb

[Lu Zuckerman]

To: rotorboy

 

When you say Uniball are you referring to the “gimbal bearing” on the swashplate?  Remember that you have hydraulic servos that can push through most seizures of the Uniball.  There may be a slight hesitation and then the swashplate moves, which may provide a jerky ride when collective is used.  However if the servos can’t push through the jam and stall then you have a major problem in that you can’t autorotate.  It is at this time that pilot skill comes into effect.  Even if the landing is successful there is a strong possibility of a blade strike when the rotor speed bleeds down.

 

IMHO

 

:unclesam: