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Starting Bell 407 With Rotor Brake Engaged


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#1 GMCK13

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Posted 09 September 2018 - 20:17

What kind of damage could happen if you started a Bell 407 with the rotor brake engaged? What if the start was aborted at 25% NG? Thanks.



#2 SBuzzkill

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Posted 10 September 2018 - 10:36

IIRC that was around the time you should abort the start if the blades don't turn, yes?  Talk to your maintenance personnel.  Better to let them know and ensure the aircraft is still safe than try to hide a mistake and make a problem worse.  


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#3 Nearly Retired

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Posted 10 September 2018 - 13:25

When I worked in the GOM, one of the unwritten, unapproved and untalked-about techniques was the rotor-brake-on start in a 206.  It was used on really windy/gusty days when you were on a sketchy platform that made a lot of turbulence on the heliport, but you made the mistake of parking with a blade sticking out over the windward edge of the deck and just had to, had to, had to get going.

 

You'd pump that brake up and get it on real good.  Then you initiate the start and get it lit off.  Around 30% or so, you reach up and snap the brake off.  You wanted to get the brake off *before* the engine started overpowering the brake.  Doing this would get that rotor spinning up in no time!  (It was easier in a 206B of course, but not impossible in an L-1/L-3 if you had one with a predictable start.  Not that I would know.)

 

No one taught the technique.  No one acknowledged it or even admitted to knowing about it - or using it.  In fact, it may not exist at all.  

 

Seems to me that they use a similar technique in the Sikorsky S-76 equipped with the C-30 engines.  The brake holds the N2 wheels from turning while the compressor and gas producer wheels start normally.  Doesn't seem to damage those engines.  And as long as the TOT doesn't go over the limits, why would it?  Ever seen the suddenness with which the S-76 rotor begins spinning upon brake release?


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#4 Eric Hunt

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Posted 10 September 2018 - 13:46

Any damage would be uneven heating of the turbine wheels, with the hot gases only hitting a couple of parts of the wheels instead of being spread all over it. I have seen one person start a 206 and get to full throttle before he realised the blade was still tied down.

 

Yes, in the S-76 B it was accepted practice to start one engine with the brake on when the VIP passengers arrived in their car and boarded. Once they were inside, release the brake, spin up the rotor to 85%, and start the other engine.



#5 apacheguy

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Posted 10 September 2018 - 16:17

Any damage would be uneven heating of the turbine wheels, with the hot gases only hitting a couple of parts of the wheels instead of being spread all over it. I have seen one person start a 206 and get to full throttle before he realised the blade was still tied down.

 

Yes, in the S-76 B it was accepted practice to start one engine with the brake on when the VIP passengers arrived in their car and boarded. Once they were inside, release the brake, spin up the rotor to 85%, and start the other engine.

 

Rotor brake starts are accepted practice in the AH-64, though pretty rare for various reasons.  Supposed to be a valid technique for shipboard or dusty area starts.  Never heard that it was a maintenance problem as it is in ATM and therefore a normal/accepted startup procedure. 


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#6 iChris

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Posted 11 September 2018 - 22:54

What kind of damage could happen if you started a Bell 407 with the rotor brake engaged?

 

What if the start was aborted at 25% NG? Thanks?

 

Worst case, a little ware on the friction linings (pads) If the brake disc was slipping against the friction of an engaged break prior to shutdown; otherwise, it’s not much of an event, no maintenance action is required.

 

Engine starts with rotor brake engaged to hold the drivetrain and rotors stationary, while the engine runs at idle, is more common with large twin-engine helicopters were there is an increased demand for more powerful hydraulic and electrical systems. Has been popular with Sikorsky models like the CH-53E, H-60, CH-54A, S-58T etc. 

 

As an example, the S-64E’s fully articulated main rotor head is around 7 feet in diameter and weighs approximately 1,900 pounds. This is in addition to its 6 main rotor blades totaling 2,100 pounds (350lbs./blade). Therefore, you can’t engage and control a rotor system with that amount of turning mass without hydraulic assist. The centrifugal force combined with that amount of mass creates a significant vibratory load.

 

With the S-64E (CH-54A) you get the ball rolling by starting the 72 HP turbine auxiliary power plant (APP, APU, or aka the "P"), that brings the Utility, Hoist, and Make-up hydraulics on line along with electrical; thereby, allowing you to start the 4,500 SHP Pratt & Whitney engines via the hoist pump that supplies the engine hydraulic starter motors.

 

In order to safely engage the rotors and accomplish the main rotor servo and flight control checks, you first need to power-up one of two stages of the main rotor tandem servos. In the S-64E the #1 engine drives the 2nd stage servo pump; therefore, the rotor brake is set to hold and delay rotor engagement until the #1 engine is started and the 2nd stage hydraulic pressure is up at its normal (2,000psi) allowing the preflight servo and flight control checks to be completed. Thereafter, the rotor brake is released and the drivetrain and rotors start turning and the 1st stage hydraulic system (3,000psi) and remaining systems come online.

 

You have to be specific to the helicopter type because systems vary. With the S-64F (CH-54B), there’s no need to set the rotor brake, the APP drives the 1st stage hydraulic pump from the accessory section of the main rotor transmission without the engine running or the rotors turning. In any case, you need hydraulic assist prior to rotor engagement.

 

We also covered a bit on starts with the rotor brake engaged back in 2015 at the following link:

 

Free Wheeling unit question that I can't figure out


Edited by iChris, 12 September 2018 - 00:39.

Regards,

Chris

#7 Fred0311

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Posted 12 September 2018 - 10:28

Nearly Retired, you'll be happy to know that your secret handshake is still "not" in use in the gulf.
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#8 Nearly Retired

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Posted 13 September 2018 - 11:23

LOL Fred, I don't know what you're not talking about ;-)

#9 Spike

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Posted 13 September 2018 - 12:36

What kind of damage could happen if you started a Bell 407 with the rotor brake engaged? What if the start was aborted at 25% NG? Thanks.

 

While your question doesn’t provide any context regarding the circumstance surrounding your question, I’ll ask the following; did you utilize the checklist or, miss that specific item during your start-up? Regardless, anytime a situation such as this occurs, inform your maintenance personnel and get their input. While the responses here have provided some good information, don’t think your employer will have the same opinion. Fess up, no matter how minor the error and learn from your mistake…..


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#10 WolftalonID

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Posted 18 September 2018 - 08:43

Reducing blade flap on windy days when a blade is hanging over the edge of a platform....it works..just dont let it wind up too much. 10-15% is what seems to be the unspoken we dont talk about it technique.
Just saying from a friend who I know whos second cousins fathers cats breeders neighbor knew about back in nam.
Sometimes we think we know it all....only later to discover we only knew all we had learned. Never stop learning.




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