Text Box: WARBIRD NOTES #34  8 Jul 99   (14)   
AIRFLOW WITH A NACELLE FUEL LEAK
During the last several weeks we've spent thinking about which of various procedures should be included in the B-17, DC-3, B-25, etc. emergency checklists, the subject of fuel leaks has reared its head more than once.  If I remember correctly, it seems that it was one of the first really serious discussions brought up w-a-a-y back at Reese AFB when we were buck USAF B-25 instructors sitting around the flight shack on a bad weather day, involved in another of our endless bull sessions about "what would you do if ".

 

A consensus developed back then and nothing's really changed in the decades since that would cause me to think it was invalid.  If you haven't caught on fire by the time you notice the problem, then DON'T CHANGE NUTH'IN!  To be very precise in describing the problem, if (and that's admittedly a big if) you have a bona-fide fuel leak out in the nacelle, then you'd seem to be far better off to let the existing airflow remain unchanged.  Your exhaust flame, (look at it during the night) exits the collector ring or exhaust stacks, and is visible for several inches to several feet aft of that point.  If the fuel leak is somewhere back in the accessory section, the fact that it hasn't ignited so far seems "prima facie" evidence that the exhaust flame hasn't touched it.  It should stand to reason then, that some measure of equilibrium exists in that area and will continue to exist provided the existing airflow balance is not altered.  Right?  I think so.  The fuel is under some pressure and most likely is being atomized (to some extent) as it exits to the atmosphere just outside the nacelle.  Now, anything that you do that would alter that balance, MIGHT just cause the exhaust flame to impinge upon that fuel leak and - P-F-F-F-F-T!  Maybe, anyhow!  Me, I think I'd "dance with the one who brung me"!  In other words, I wouldn't take a chance of altering that airflow in the slightest, figuring I've been lucky so far!  I'm reminded here of what one of my good friends, Connie Edwards, always says. "I'd ten times rather be born lucky than smart!" 

 

One of the things that invariably arises during a discussion on this subject is night flight.  Lets say you notice that this problem manifests itself as a partial loss of fuel pressure on the cockpit gauge but the engine continues to run fine.  This is just one of the possible scenarios you could be presented with.  Turning the booster pump off could eliminate pressurizing some fraction of the fuel line.  If the leak occurs in daylight, there is some chance that you or another crew member will be able to visually scan the nacelle and area and possibly determine whether the gauge indication is correct.  If it's during the nighttime, I'd think your chances of successfully eyeballing it would be almost nil.  That's just one of the several reasons that I think we should seriously think of prohibiting night flight in the EAA's B-17.  Another is smoke in the cockpit, but that's an entirely different matter for another time.

 

One contributor did offer the suggestion of pulling the throttle back to control the potential propeller overspeed in the event the fuel supply became erratic or interrupted.  My thought is that that's the one single thing that we've long insisted was the most important thing not to do.  Any change in power affects the airflow and again, I'd submit that this is the one thing Id want to absolutely not alter!

 

Another thought here is your location and flight condition when you first become aware of the problem.  If you're on top of an overcast or have some distance remaining to your destination, you might decide to proceed to that destination without feathering.  I'd probably buy into that decision as long as you made no change in your configuration or airspeed, etc.  Then, approaching your destination (before you changed any flight condition) manage the situation as we've discussed elsewhere here.  I personally think I'd be ill at ease (at the best) for the entire trip under those conditions, but every situation's different and that's the meaning of judgment, I guess.   

 

In the best of all worlds, once I'd decided to feather it, I think I'd rather manage it by turning the fuel supply off at the tank.  As the fuel is consumed it relieves the pressure in the fuel line, preventing any tendency for the combustible fuel to be sprayed.  Then, as soon as the fuel pressure indication falls off, move the mixture to IDLE CUT OFF and feather.  A slightly less perfect solution to me would be to just move the mixture to IDLE CUT OFF, which would immediately remove the exhaust flame as a source of combustion.  After that, feather it.                                                              R. L. Sohn       1999


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