Crack in the tail pipe

[peeush]

This is about a 'tail pipe' damage that occurred in a medium lift twin engined helicopter. This component is not the exhaust pipe of the engine, but a part of helicopter airframe and is positioned at the end of engine exhaust to facilitate smooth outflow of the exhaust gases. In fact it is a rolled, welded metal sheet for the purpose.

Now the meat part.

There was a one inch crack detected at the end of this pipe during a turn around servicing (the welding had given away). The discussion was "How bad things would get if the helicopter was flown back to the repair depot with the known fault still existing". Though legally I agree that it cannot be a good decision, nevertheless following implications were considered, had the decision to fly back to repair depot was accepted:-

1. Owing to in-flight vibrations and pulsating aerodynamics this crack may have propagated resulting in exhaust gases contacting the helicopter structure and thereby leading to a possible fire hazard.

2. Turbulent air flow so developed in the downstream of the tail pipe may result in upstream instability leading to an unstable operation of the engine.

That's how far I could see.

Any more possibilities, comments on the implications please.

Thanks for the read anyway.

[vs69]

Without knowing specifics of a/c type its hard to say but there could be scope within the SRM to stop drill the crack if it was within set dimensions and carry out a periodic inspection after every flight / every day. I guess it depends on a/c type, operating environment and the size of your crack!

[Alber Ratman]

1/. Read your SRM, is it classed as allowable damage? Stuff the areodynamics, you are not paid or skilled enough in that department even to bother and anyway the turbulence produced by your little crack is nothing compared to the big wirly thing above the roof of your aircraft. If it doesn't get your tech services to talk to the manufacturer (although I doubt yopu will need to.)

2/. Your crack will be acoustic vibration damage.. Does your SRM allow it to be stop drilled as a line repair? If it does, do so.

3/. record on the aircrafts damage chart that such a fault exists, even as allowable damage, so it is obvious that a fix is required before it goes into the hangar..:ok:

[Alber Ratman]

vs 69 has cleared up everthing else, such as periodic inspections to confirm it isn't getting worse!

[MX Trainer]

Peeush

1. - If you fly the aircraft with a known defect - that is out of limits - and not entered into the log book you will be punished.

2. If you fly the aircraft with a known defect - that is out of limits - and NOT entered into the logbook you will be punished.

3. If you fly the aircraft with a known defect - that is within limits - and entered as such into the logbook you may fly the aircraft - subject to a maintenance release stating it is fit to fly - and the flight crew acceptance of the defective aircraft. For this you will not be punished.

4. If said defect is out of limits - then your CofA is no longer valid and any flight will be cause for punishment.

5. If it is possible for said defect to create a hazard in flight that has the slightest possibility of liberating helicopter structure that structure may pass through the tail rotor with disastrous consequences for the crew. In addition if the defect causes a fire or damage of any kind it may be disastrous for the flight crew.

6. If aircraft is flown as per item 1. or 2. - your insurance will most likely be invalid.

7. If aircraft is flown as per item 1. and 2. - your reputation as an engineer or pilot will be less than desirable. You will be punished - maybe not right away but at some point down the road it may restrict you in that search for the best job in aviation.

Look here for an example
Cougar Helicopters Flight 91 - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Cougar_Helicopters_Flight_91)

and here
2009 Bond Helicopters Eurocopter AS332 crash - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/2009_Bond_Helicopters_Eurocopter_AS332_crash)

Quote from page - Bold Italics are mine!!

On 24 November 2011 the AAIB published its Formal Report 20/2011 into the accident. The cause of the accident was attributed to the catastrophic failure of the Main Rotor Gearbox as a result of a fatigue fracture of a second stage planet gear in the epicyclic module.


In addition the investigation identified three contributory factors:
1. The actions taken following the discovery of a magnetic particle on the epicyclic module chip detector on 25 March 2009, 36 flying hours prior to the accident, resulted in the particle not being recognised as an indication of degradation of the second stage planet gear, which subsequently failed.
2. After 25 March 2009, the existing detection methods did not provide any further indication of the degradation of the second stage planet gear.
3. The ring of magnets installed on the AS332 L2 and EC225 main rotor gearboxes reduced the probability of detecting released debris from the epicyclic module.
Seventeen Safety Recommendations were made as a result of the investigation.

End of page quote

Quick Summary

Go by the book - because if you don't - then the regulatory body and the legal bodies will throw it at you!!!

Hope this helps.

Mx