Jay,

I found your post to be very interesting. 

Sunday afternoon I was returning to Logan Utah from Halfway Oregon, in a Cessna 150.  Was cruising along at 9,500 MSL, leaned and throttled back slightly.  I attempted to climb higher to clear some terrain and at about 10k’ my engine stumbled and began to run rough.  I pulled the carb heat on and it cleaned up momentarily before it continued to run rough, so full rich mixture added kept it stumbling along.  I have experienced this before when running Mogas, and thought I had cured the problem with a replacement fuel line between my gascolator and the carb, and installing firesleeve over that fuel line.  I’ve been able to fly to 10,500 without issues, and this time I had a mix of Mogas and Avgas (about a 5:8 ratio), and was surprised to once again experience symptoms of vapor locking.  I was almost over Idaho City (U98) and debated landing but opted to turn south to lower terrain.  Full carb heat, full rich carb mix, and occasionally having to pump the throttle to keep it running, I descended to about 6700’, skirting the east corner of Boise’s airspace, before it started to run reasonably.  I was able to adjust the mixture, but still had to maintain the carb heat setting as I made my way to Gooding for a fuel stop.   As I closed in on Gooding, I was able to adjust the carb heat off.  After I landed at Gooding and fueled up, the rest of the flight home was uneventful, even when I climbed back up to 9,500’. 

I’ve never had an issue when I’ve run straight Avgas, and thought I had enough Avgas that I couldn’t have any issues with the mixture of Mogas I was running, but maybe somebody’s chemistry changed, or some other reason that I haven’t worked out yet.  I’ve wondered about installing a fuel pump somewhere in the system, to turn on when this event happens.  I don’t suppose most of the Cessna 150’s are operating at these altitudes.  Perhaps something similar happened to the Q2 that went down in Wyoming…

Thanks for sharing your insights.

Robert Kidd

Logan, UT

Not to get too far over my skis here, but when I did the comprehensive summary of Q accident statistics (http://n8wq.scheevel.com/documents/Quickie Type Aircraft Accident Analysis.pdf ), I found that the number one cause of engine failure was actually a failure in fuel delivery (Fuel starvation due to fuel delivery problem or fuel exhaustion). This is not unlike the statistics in  homebuilts in general, so is important even if not building a Q.

I have tested my fuel delivery system’s reliability from near sea level to 18,000 feet. I find that my system does behave differently depending on altitude. By this, I mean not just mixture and carburetion, but flow through fuel lines, pumps, induction air and regulator all behave differently.  If you have not tested your fuel system for robust reliability at all altitudes within your altitude envelope, then caution is warranted as you expand the envelope, because the fuel flow may suddenly become unreliable.

So indirectly, going to your original question, your engine-altitude performance may be a complicating issue, because in high terrain, you may not be able to maintain sufficient AGL to glide to a safe landing area if an engine problem develops.

Cheers,

Jay