question for 412 pilots and or mechanics
 

I am a mechanic myself working on Sikorsky helicopters for the last 6+ years.

I have worked around the world in a few different place's.

Currently I am working somewhere that requires me to take occasional flights in a bell 412ep. I am not familiar with these helicopters and something is alarming me.

On start up every morning these guys go through some checks and while doing those checks is sounds as if they are having compressor stalls. This happens on both of their 412ep's they have in camp.

one time when after picking up some passengers when rotor rpm started to increase as they were preparing to lift off the engine let out a much louder *bang* they continued on as if all was normal.

the mechanics informed me that this is normal on these helicopters to do this, and that it happens when switching from auto to manual governor :confused:
I did some reading and found and info letter directed at some 212 and 412s about engine manual fuel control operation procedures with helicopters using the PT6T engine. Not sure if that is what the 412ep has in it


What i am getting to is, is this safe? Would your ride in that machine? Is it normal? Should I bring this up with my company that the company we are contracting for is asking us to ride in a different contractors unsafe equipment?

thanks for any help
Tim

What is the field altitude where these checks are being done, and what are the engines flight idle stops set to? If the flight idles are set too high you can get a "pop" on change over. If I recall correctly a higher field altitude causes more of a drop in N1 on change over. I wouldn't say that is normal though, however I am not all that familiar with PT6T-3D's just -3B's.

Are you sure the noise you are hearing isn't just the extra noise from the hydraulic tests? When either system is switched off (especially number 2) they make a lot of noise, hydraulics off checks are part of the start-up procedure. I don't think that sounds like compressor stall though.

This is normal when going from auto to manual governor during pre-flight checks with the PT6T-3D or 3DF engines in the 412EP. There will be some differences in how much noise you hear depending on atmospheric conditions, altitude, etc.

412 EP Auto Manual Governor Check
 

The popping noise tends to be louder on the 3D series than on the 3B.

The noise is also more likely to occur on reselecting Auto after gently winding up to 70% N1 in Manual to check normal response then closing the throttle prior to reselecting Auto.

The popping can be minimised by reselecting Auto 2% - 3% N1 above the normal Idle setting - i.e. around 64% N1 decreasing (normal Idle 61% N1 + or - 1%)


Trog

I used to fly as crew on the B412EP in the UK and yes the noise is quite normal when completing the governor manual checks during the start up functional checks. I understand that it was not a problem as when completing these checks as the power loading was light (other engine driving the rotors and the engine being checked is at idle at the time of changing from manual to auto or vice-versa)

The only time I've heard the noise in flight ( and only on a couple of rare occasions) whilst the engine is in auto governor mode was at the bottom of a PFL (autorotation) when recovering to S&L flight whilst pulling in power (going from power on auto to power on flight). We were informed that this was "pop surge" and the a/c were to be landed on and shut down (I believe this was an instruction from Bell) and then subject to engineering inspections. Exactly what inspections, I dont know as I'm not an engineer but I suspect it is something to do with transient torque spikes as both engines are under load at this time.

Our a/c were equipped with the PT6T twin packs.

Hope this helps

thanks for the help guys. Helps me out a lot with decision making, on the helicopter i work on this wouldn't be considered normal so I was a bit nervous.

cbox chip- the altitude is 1000 ft or so. of course the DA is like 1500-5000 depending on the temp.

If the Auto / manual changeover is rigged correctly, and the bleed valve serviceable loud stall like sounds shouldn't be heard at all through the flight.Also on pick up -if you hear a more mechanical repetitive knocking -its the main rotor droop restraints catching.

Bell 412 Governor failure
 

1) Can a governor fail (low side) during a normal cruise flight?

2) What happens if it failed?

3) Would the aircraft start an uncontrolled yaw?

4) How low can the rotor RPM decay?

Bell 412 Questions
 

1) What are the colors on the main rotor blades and hub, the dots that are red, white, blue...

2) What is 2per rev, 4 per rev.....

3) Can a Governor failure (low side) lead to a Bell 412 crash?

4) During Uncontrolled yaw and descent at low Rotor RPM, can the mast bump, or can the main rotor hit the tail boom ??

Not a 412 driver, but your questions seem to be generic.
1) Yes, the indications are initially the same as of an engine failure, except for some alerts and the affected engine stabilizing at a low Ng.
2) It depends on the phase of the flight and power demand. Until malfunction identification and action taken, consider you are OEI.
3) Rare, but at very low airspeeds, if your power demand drops the NR, you may don't have enough TR authority...
4) As low as your collective is pulled up.

1. The colored dots just identify the blades for maintenance purposes.
2. 2/rev and 4/rev are vibration modes
3. Governor low side can lead to a crash, which is an almost meaningless thing to say because crashes can happen with two perfectly good engines.
4. B412 has a flex beam yoke attaching the blades to the mast. So it won't bump in the way a teetering rotor can bump the mast. Sure, you can chop the tail boom with the main rotor if you do something whacky, but that would be very rare.

1. Engine malfunctions can happen any time the engine is operating.
2. Engine governor fail to low side means it won't produce much power, or none. Revert to manual mode to restore power.
3. No.
4. As @Jimmy says, as low as your collective is pulled up. RRPM will always decay if the power available is less than the power required. Once you reach that point, the more you pull up on the collective the RRPM will continue to decrease until you reach full up.

Good engine at Max.Power and collective going up = NR going down. Constant power and lower NR = higher torque. Higher torque, higher TR thrust required, but less available, due to NR decay.
So, maybe.

His question was low side governor failure in cruise flight. This will not cause an uncontrolled yaw.

Yes, you are right. In cruise flight there's no way, sorry.

Have any of you ever experienced this phenomenon with a 412EP:

A particular aircraft will be VERY difficult to control in the yaw axis during hover. Pilots who have flown the 412 series for years will have difficulty smoothly hovering without dramatic yaw oscillations. Most noticeable when returning to this airframe after flying others for a while, so it seems to be something that you eventually learn to compensate for, but the first few days flying this one after being in other aircraft for a few months can be very difficult.

This is an AHRS equipped model that has had T/R rigging gone through completely, and Control Motion Transducers replaced also. This issue has proven very difficult to diagnose for the last two years.

Any suggestions would be greatly appreciated.
Thanks in advance.

Yes, they are crap in crosswinds - the AHRS version just makes it worse.

It's not a dangerous failure it's just crap design.

We had AHRS and non AHRS aircraft and the AHRs was definitely worse.

Thanks Crab, its proving to be very frustrating for the crew. I've flown 6 different 412 airframes, 5 EPs and 1 SP, but this is the only one that takes a few days to get a feel for. Very strange....

GS pilot - I guess you have also experienced the unexpected yaw at about 30 KIAS as you reduce speed to an OGE hover with any form of crosswind but particularly from the left?

We spent a lot of time in OGE hover for SAR training and unless you are perfectly into steady wind, yaw control is much harder in AHRS aircraft.