Crankcase breather length

[brianm]

Over the last few years I've had three different people, unprompted, mention that my breather looked "too short". As far as I know none of these people are 170 owners or A&Ps. On my plane the breather extends about 1/2" below the belly. I suspect that the length isn't critical and as long as the oil ends up on the belly instead of the firewall I'm fine. But it's got me thinking and now I'm curious to see what a "normal" breather looks like. Does anybody have any pictures or measurements they could share?

[GAHorn]

<This post edited 1/22/22 to correct error, Thx to Gary>

. Brian, you’ve got the right idea…as long as it extends below the lower edge of the firewall it’s fine….<>

AD 87-07-02 addresses this as it describes how to confirm a “whistle slot” is also installed in the breather to prevent any icing in the tube from blocking the necessary ventilation that tube provides. This is not to imply this airplane can be or should be flown in icing conditions …certainly it is NOT approved for flight in icing conditons…. But the breather tube carries considerable moisture away/out-of the engine crankcase and the lower end being in the slip-stream…the moisture can freeze and block the tube. This can create excessive crankcase pressures and may cause the prop-shaft seal to fail etc. and blow the engine oil overboard. Engine failure can result.

Here’s an illustration from the AD 87-07-02 that addresses the whistle-slot as well as the total tube length.** (And the rest of the 170 fleet is 70+ years old and unlikely to be an authoritative resource for determining original length. It might be the case but I’d be surprised if anyone can prove their tubes are original. LOL )

Click on pic to ENLARGE

PS: The AD 87-07-02 is not sufficiently clear in my opinion in that it does not actually mention Lycoming engines, but the Preamble does address STCs that apply to that engine mfr’r. Regardless, the breather-tube of any engine installed in these airplanes should be modified per the AD, in my opinion.

Here’s the full AD 87-07-02 https://rgl.faa.gov/Regulatory_and_Guid ... enDocument

** Being arbitrary I personally disagree with the FAA depiction of how the whistle-slot should be shaped. I prefer it to be reversed, i.e. upside-down to the illustration….so that the upper portion of the cut or hole is indented rather than the lower portion of the slot/hole…..in order to allow the tube to be vented, but to discourage oil from misting out of the hole and onto the firewall. Fig 1 isn’t particularly clear on this but if the whistle-slot is not already present… the instructions are to drill a hole…but no mention is made of indenting it in any way. That should also work just fine. <> FAAs’ illustration <> instruct that the lower end of the tube should <NOT>be below the cowl lip….and they refer to that lip as Item 8 in their illustration…. that portion of the illustration shows the tube lower end ABOVE that lip. <>

[n2582d]

George,
I'm missing where you find that the AD says the breather should extend below the cowl lip. It actually says the opposite.

Breather AD.png (26.57 KiB) Viewed 1373 times

This concurs with Cessna Service Letter 62-50 which calls for the breather to be flush with the bottom of the firewall on the C-172. The problem I have with it being this short is that oil from the breather will be migrating to the seam between the lower boot cowl and the firewall. A better problem to have than the tube getting frozen with ice, I suppose.

Service Letter 62-50.pdf

Service Letter 62-50

(41.26 KiB) Downloaded 108 times

[GAHorn]

George,
I'm missing where you find that the AD says the breather should extend below the cowl lip. It actually says the opposite.

Breather AD.png

This concurs with Cessna Service Letter 62-50 which calls for the breather to be flush with the bottom of the firewall on the C-172. The problem I have with it being this short is that oil from the breather will be migrating to the seam between the lower boot cowl and the firewall. A better problem to have than the tube getting frozen with ice, I suppose.

Service Letter 62-50.pdf

That’s the result of a 2:34 AM posting error I made. Thx for the catch, Gary! THE ERROR HAS BEEN CORRECTED IN THE EARLIER POST.

I agree with you Gary with regard to your implication that flush with the lower firewall might have a problem with oil migration into the seam. I am of the opinion that since the tube is in a downward/exit airflow from the warm engine compartment that any oil will hopefully be driven down/away from the firewall. (The tube should not be directly in contact with the firewall, but several inches forward of it.)

In any case, this certainly gives us all a good resource upon which to rely when deciding how long/short the tube should be. Hopefully this answers your critics Brian…??