Stop Drill on Engine Exhaust

[sph33r]

Good morning,
On a cracked engine exhaust, is it acceptable to perform a stop drill to halt the progress of the crack? The technique is not given in the manual, so this is a matter of opinions and performing it is not an option. Id just like your opinions and reasoning on it for the sake of "discussion" with a very insisting "party".... thanks!

[Webby737]

You don't say where the crack the is located or the engine type.
There are some cases where you can stop drill a crack (on airframe and engine), normally with a 1/4" diameter hole. This would then be subject to a regular repeat inspection to ensure the crack has not propagated any further.
Unfortunately, in your case if there's nothing in the manuals there's not much you can do.
Is there a repair scheme in the manual ?
Or can your engineering dept. contact the OEM requesting a repair scheme ?

[Tom Sawyer]

As Webby says. I've stopped drilled exhaust sleeves and cones in the past (A340/CFM56 from memory). Can't remember if that was an approved temp repair in the manual or a "dispensation" with repeat inspections from the technical department based on length & depth of crack, proximity to any frames or fasteners etc.
Would be helpful if we knew if it was piston or jet engine, and actual component with the crack..

[sph33r]

Thanks for the replies, and sorry for not being detailed enough, initially the question was more of a general nature...ie is stop drilling accepted on hot parts? does the material have anything to do etc. The missing info being : helicopter, turboshaft engine, titanium exhaust with cracks always beginning at the aft side. Im basically trying to filter fact from fiction from a discussion regarding the topic that was held infront of me...
Webby, there is no mention of repair in the manual, and replacement is the given corrective action for cracks beyond a few cm...
Tom I hope above info helps...

[Tom Sawyer]

If there is no repair scheme covering a weld or stop drilling in the manual, but cracks are allowable to a stated limit (and quantity?), then I would raise a DD with reference to the AMM for the crack/s being there and for a re-inspection for propagation and additional cracks every Transit/Daily/Hours/Calendar with the overall time limit being to the cracks reach limit and/or next maint opportunity where exhaust can be replaced.

Alternatively, the operators engineering department could approve a stop drill temp repair (if they have the necessary approval scope), or approach the manufacturers for a temp repair scheme, which a DD could be raised against with the necessary requirements and limits stated in the repair scheme.

If the crack/s have reached the stated limit and/or quantity and no repair scheme available..........replace the exhaust before next flight.

[sph33r]

There is a limit to the length of the cracks and re-inspection is called for till they reach said limit, at which point they are replaced, that's what the manual calls for. Maybe i pitched the question wrong, are there standard practices for performing stop drilling, ie limitations to certain areas (hot, frames etc) or materials...

Thanks again, by the way, what is a DD?

[Tom Sawyer]

If there was a standard procedure for stop drilling on the type it would be in Chp 20 for the airframe, and Chp 70 for the engine but they would detail "how" to do it, and not where. The manual may give a reference to a repair under the section for the exhaust inspection along the lines of "if cracks are found ...............carry out repair with reference to xyz or AMM 78-10-xxxxx". The repair would give the specific areas where you can/cannot stop drill. As an example, look up fan/compressor blade inspection and there could be a reference to the blending/cropping procedure and it will usually provide detail on which parts of the blade can be blended within certain parameters, and which areas cannot be touched. If there is no specific repair procedure quoted for the cracks in the component, I would not stop drill it unless the technical department gave me a dispensation with specific detail.

DD = Deferred Defect.

[sph33r]

OK, thanks for the clear up, the phrase i heard that got me going was "stop drilling is not allowed in hot areas, on any aircraft, anywhere". But being kinda cautious about info obtained like that, I always look into it....
p.s. No I didn't find much on google...

[Webby737]

Hi sph33r,
It looks like Tom has pretty much given you all the info you need 😊
I've certainly stop drilled cracks in hot areas, exhaust sleeves on 1 of the engine types on the A330 (I can't remember which one) and turbine rear frames on CF6-50s.
I know very little about helicopters so I can't really be of much help, but airframe and engine standard practices don't differ too much, I've no idea where you heard "stop drilling is not allowed in hot areas, on any aircraft, anywhere" but it's not correct.

[wrench1]

Originally Posted by sph33r

.ie is stop drilling accepted on hot parts? ... The missing info being : helicopter, turboshaft engine,

To add to the above comments, on the helicopter side, any stop-drilling or crack limitations will be listed in the MM and any crack/stop drill is more location specific than material specific. In most cases an engine can be operated with existing cracks as there is usually not a mandatory requirement to stop-drill the crack due to not all cracks can be accessible with a drill. But it's something I always recommend when possible. The attached PDF is an example from a RR C30 MM with stop-drill note highlighted.

[sph33r]

Thanks for the intel guys, got what i was looking for. Its odd when one of your better mechanics says something and it just doesn't sound correct, so I look around before putting my foot in my mouth...