enginehistory.org

[rcore]

But rather about failure modes.

I've seen the results of all sorts of failures in automotive (i.e. inline and vee type) engines over the years. Thrown rods, broken crankshafts, blown-out main bearing webs and swallowed valves to name a few.

But I've wondered about radial engines - what sort of damage patterns result when big parts break inside? Most are probably easy to imagine, but did they tend to destroy themselves in ways that would seem unusual to those more familiar with automotive hardware?

I would guess that in operation, the signs of engine failure manifest themselves in ways that result in their being shut down before catastrophic damage can occur. But engines will always reserve the right not to give off a whole lot of advanced notice before they die.

[gwhite]

As you can imagine, an internal failure of a radial is not a pretty sight. A master rod bearing failure usually results in the bottom oil control ring “snapping” below the cylinder skirt and then tearing off the bottom of the piston. Ultimately, the M/R bearing will suffer a complete melt down that may lead to a M/R failure. This is about as bad as is can get. The result is the proverbial “wrap-up” whereby the link rods now have their geometry ruined and the rods literally wrap around each other. If it’s a 2 row radial it’s quite conceivable that the engine will continue to run as the “bad” row continually gets more ground up. It’s hard to detect a massive failure such as described above by ear, particularly on a multi engined aircraft. Instead, the primary indicator is the torquemeter or BMEP meter.

[jjuutinen]

BTW, a fellow told me that experience in Finnish service showed that e.g. one non-firing cylinder in the Twin Wasp could go unnoticed for a long time (dozens of operating hours).

[klankenau]

Graham, so you're saying that master rod bearing failure generally results in a non-rebuildable engine? To someone more familiar with automotive or marine applications, that may sound a little unusual but most aircraft engines are removed from service and rebuilt before such catastrophic failure occurs.

The most common indicators resulting in pulling an aircraft engine out of service are; excessive oil consumption, low compression, oil analysis, time in service (reaching or exceeding recommended TBO). In my flying experience (both actual and hangar flying discussions), any engine experiencing catastrophic failure (bearing, crankshaft, rod, etc) results in a non-rebuildable unit. That's not to say it's impossible to put such an engine back together and make it a great runner, but who would want to bet their life on such a beast (especially a single engined aircraft)? How would anyone feel about flying behind an engine that's suffered a prop strike?

[rcore]

Come to think of it, I did get to peer inside what was left of a Warner that came off a Cessna C-37 Airmaster after having a piston break. It parted right at the wristpin center line; the debris inside the crankcase neatly swaged a few cylinders in place permanently.

BTW, the pilot and passenger pulled off a nice, clean emergency landing on a state highway, except they angered a woman motorist when they slipped it in over her car (..."why didn't you at least honk so you could warn me!").

[jjuutinen]

Kip, there must have been hundreds if not thousands of wartime pilots who flew behind engines that had had prop strikes. But they were pilots first, not bunch of GA sissies.

[klankenau]

Yep, alot happens in the contingencies of wartime, but it's the FAA and original manufacturers who determine specifications and acceptable repairs. I recall a couple years ago the FAA mandated that any engine experiencing a prop strike required a full teardown and inspection before returning to service. Some of those A&P/A&I types out there whould be more familiar with that issue.

[gwhite]

Regarding prop strikes, the FAA mandated that if the crank run-out (direct drive) or prop shaft run-out was within prescribed limits the engine could be returned to service.

If a master rod bearing failure is caught in time the engine is quite savable. Most of the “Big Iron” flying around today has suffered some kind of major failure in its life. You can look at some of the nicks and scars on cranks and crankcases for proof. However, this surface damage needs to be buffed and polished out, it’s not as though you just leave the dents and nicks in place as a stress riser.

[kmccutcheon]

Regarding prop strikes, what gwhite said, with caveats.

Aircraft operating for hire (such as under Part 135 rules, among others) must follow the maintenance procedures outlined in their FAA-approved operating manual. These manuals typically require compliance with manufacturer’s service bulletins. Both Teledyne Continental and Textron-Lycoming service bulletins require more than just the run-out check.

Part 91 operators are not required to comply with service bulletins unless those service bulletins have been made mandatory by regulation, usually by Airworthiness Directive (AD). The AD quoted in part below is an example that applies to most Lycoming direct-drive reciprocating engines in service today.


Airworthiness Directive 2004-10-14
(e) Compliance with this AD is required as indicated before further flight if the engine experiences a propeller strike after the effective date of this AD, as defined in paragraphs (i) and (j)
of this AD.
(f) Inspect, and if necessary repair, the crankshaft counter bored recess, the alignment dowel, the bolt hole threads, and the crankshaft gear for wear, galling, corrosion, and fretting in accordance with steps 1 through 5 of Lycoming Mandatory Service Bulletin (MSB) No. 475C, dated January 30, 2003

Definition of Propeller Strike
(i) For the purposes of this AD, a propeller strike is defined as follows:
(1) Any incident, whether or not the engine is operating, that requires repair to the propeller other than minor dressing of the blades.
(2) Any incident during engine operation in which the propeller impacts a solid object that causes a drop in revolutions per minute (RPM) and also requires structural repair of the propeller (incidents requiring only paint touch-up are not included). This is not restricted to propeller strikes against the ground.
(3) A sudden RPM drop while impacting water, tall grass, or similar yielding medium, where propeller damage is not normally incurred.
(j) The preceding definitions include situations where an aircraft is stationary and the landing gear collapses causing one or more blades to be substantially bent, or where a hanger door (or other object) strikes the propeller blade. These cases should be handled as sudden stoppages because of potentially severe side loading on the crankshaft flange, front bearing, and seal.


Textron-Lycoming Mandatory Service Bulletin No. 475C:
http://www.lycoming.textron.com/support/publications/service-bulletins/pdfs/SB475C.pdf
Depending on the specific engine model, it is sometimes possible to comply with SB 475C without even removing the engine from the airframe.

Teledyne Continental Service Bulletin on the same subject:
http://www.tcmlink.com/pdf2/SB96-11A.pdf

As far as I know, there is no Airworthiness Directive requiring teardown and inspection of TCM Engines after a prop strike. In my experience with prop strikes, the weak point in Lycoming engines is the crankshaft gear that drives the cam, magnetos, and pumps, which is attached to the crankshaft with a single 5/16 x 24 bolt. TCM engines seem to break at the alternator drive, especially on engine with front-mounted alternators. The IO-550 series, for example, is only capable of a continuous driving torque of 100 lb-in, and has a static limit of 800 lb-in. The momentum of an alternator rotor running at three times crankshaft speed often cracks the thin flange to which the alternator drive bevel gear is attached.
_________________
Kimble D. McCutcheon

[klankenau]

I think what this all gets down to is that the FAA wants to see an engine removed from service BEFORE it fails, and they tend to err on the conservative side. This only becomes more conservative as the number of bodies involved increases; ie from part 91 to 121 to 135.

Lee Hodgson's discussion at the convention about running engines to destruction in a test cell is not the preferred end of life for an aircraft engine!

[wbareham]

Back in '53 we had a twin Wasp in a DC3 which was reported as being down on power crossing the Belgian coast on its way to London. Apart from the loss of power and horrendous vibration it still ran.
We found two broken slave rods,(luckily in the same bank) with their pistons jammed at the top of their strokes. Could have been worse.

[jschauer]

We had an R-975 that failed internally. We could only pull the cylinders off about 1/2 inch before it wouldn't pull any more, indicating the cylider skirts were peened over. We never did get the engine apart. We ended up giving it to a museum for display.

[apascarella]

Please excuse my ignorance here on aircraft engine maintenance. On an automotive type engine ,especially cast iron types,if a blow up occurs,spun bearing,thrown rods,the engine is usually replaced.Cast iron is difficult to repair .Aluminum automotive engines are generally ok to weld up depending on it's intended use and if it's something rare.
What about an engine like a Merlin or maybe an Allison? if such and engine has a block or head damaged ,can or will it be welded and machined to put it back into actual flying service?

[rwahlgren]

With any type of failure and the resulting damage to the engines parts.
The can or can not they be repaired for service would be the manufactures call. I think the exception to that is with the old engines who's manufacture no longer exists or no longer supports them.
Then it becomes the overhaul facility's discretion to determine the repairability of the parts, since it is thier neck that is on the line when they sign the engine off as serviceable for flight.
In the automotive and industrial engine world huge chunks can be broken off and welded back on or stiched back on in the case of cast iron, since there is no government oversight except possibly with marine engines, such repairs can be done with or without sucess.
If the question is can a radial engine crankcase be welded back together if its broken, the answer is yes, especially if it is just to be a ground running engine. I don't think it has been done at any licensed overhaul facility though. It would be better to get the PMA and make a new part than try to fix some stressed out piece of junk no matter what kind of part it is.

[jschauer]

A radial engine case is an alunimum forging that would loose it's strength if it was welded.