Buckling and Shearing of Tail Rotor Drive Shaft after engine compressor stall

[Greenvalley285]

Is it possible, that in Helicopters, Tail Rotor Drive Shaft (driven from Main Transmission, over Tailboom), buckles or shear off, at any part all along its length, after an Engine Compressor Stall.

 

The condition may occur, that, at the instance of Compressor stall, drive is taken off suddenly from Tail Drive Shaft, and it spins under momentum. But then, in post compressor stall condition, when engine regains its full RPM, drive is again provided to Tail Rotor Shaft, But since now it is at lower speed, sudden torsional load, cause it to buckle, or shear off (in worst cases). Is this explaination right?

[Gomer Pylot]

Yes, the sudden loads can damage the driveshaft. It's very thin and light, and can't handle sudden forces well.

[iChris]

Is it possible, that in Helicopters, Tail Rotor Drive Shaft (driven from Main Transmission, over Tailboom), buckles or shear off, at any part all along its length, after an Engine Compressor Stall.

 

 

In the broad context of your question, yes, compressor surges or stalls cause transient torsional loads that can cause damage to the engine, the drive train and associated airframe components. However, if we narrow it to a more practical sense to a particular aircraft and the work that it’s doing, transient surges or stalls would not normally be the cause of such buckling or shearing-off. Nothing like the buckling or shearing-off you see below, you need to relay more information:

 

Don't be so quick to blame it on compressor stall.....

 

 

Despite the overall serviceability of the tailrotor system and lack of any apparent prior damage to the tail shaft, the factors contributing to the overload event could not be conclusively established. It is likely that some time prior to the failure the component was exposed to a localized transient loading condition above that of its intended design. To produce the overload failure, the tailrotor system must have been exposed to conditions or events capable of producing a significant increase in the rotational resistance of the assembly. Possible events that could have contributed to the initial overload failure may include: a tailrotor impact, a bird strike, failure of the drive shaft bearings or a gross malfunction within the tail/main rotor gearbox system. REF: ATSB TRANSPORT SAFETY INVESTIGATION REPORT

Edited March 14, 2016 by iChris

[AS350 pilot]

Interesting question. I can honestly say, I have only had compressor stall one time. In a 500 while doing recurrent training after hover auto's, we spooled it up too quickly (on the ground) and it happened. Not sure what your experience is, but if you're worried about this and it's happening a lot….it shouldn't be.

[Greenvalley285]

OK, I understand. But consider this...

The complete drive train is normal, no binding of drive shaft at any point. There was no sudden stoppage of Main or Tail rotor. The bearings holding the drive shaft are ok (Post Incident inspection). No bird strike to any part involved.

 

However, there are at least two known compressor stalls in the flight before tail drive shaft breaks off. There may be some such events (Compressor Stalls) in the past for the helo.

 

So, in these circumstances, two possibilities....

Either imperfection in drive shaft material/ its process treatment during manufacturing, or weakening of shaft due to subject to a couple of compressor stalls (although over a prolong time).

Your Comments.........