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Last summer climbing out of San Jose on a warm day as I passed through Fl
190 I experienced a sudden large drop in engine RPM with accompanying large
yaw to the right. I leaned the mixture by pulling out the knob about an
inch and the engine immediately came back to life. This occurred as I was
just over the Sierras! I have been asked questions like what was your fuel
flow and fuel pressure etc. etc. but at a moment like that you don't calmly
scan the panel so I wasn't able to glean that information, but I do recall
that my manifold pressure was dropping from my climb setting of 30" at 2500
RPM to where it had fallen to about 27" to 28" just before the event.
I had a second occurance climbing out of El Paso last summer with almost
identical results. This time passing through FL190 I heard a backfire sound
that sounded like it was to the left of the pilot side of the plane. Once
again the MP dropped to about 27" inspite of advancing the throttle all the
way to the firewall. As before the RPMs suddenly dropped with a large right
yaw corrected by immediately leaning the mixture. I'm not 100% sure but I
believe I looked at the fuel flow and saw a surge, but I am not aware of the
actual value. One opinion was that the aneroid on the fuel pump was bad, so
the fuel pump was replaced. The weekend after replacement I departed
Colorado Springs Aug. 1st en route to Osh Kosh by way of Dallas for some
avionics work. I climbed from our field on a pleasant temperature (65F?)
morning departing at field elevation of 6800ft up to 17,500 with no
problems and had an uneventful flight to Dallas. I thought the problem was
solved until I departed Dallas at 3:30pm with 98F temp. As I climbed
through 13,000 the problem recurred and I corrected as I had before. I
leveled off and made sure that everything was OK. I again climbed and as I
passed through 17,000 ft the engine hesitated again with a large drop in RPM
and was again corrected by leaning.
The common factors in this problem have been warm ambient temperatures
combined with a prolonged climb. One expert I had inspect the airplane
noticed that we did not have a cooling shroud around the fuel pump. We were
not aware that there was supposed to be one. After all when you spend
$52,000 on an engine you make the assumption that all necessary attachments
are included. A shroud was ordered from Lancair for about $30 and
installed. My A&P informs me that this is standard procedure on all of the
Bonanza's they work on. Since that time I have not had a recurrence of the
problem, but I have not flown in as warm temperatures as previously, so the
real test will come this summer. My A&P theorizes that warm fuel taken on
at locations on a warm day was a contributing factor keeping in mind that
the boiling point of the fuel lowers as altitude is gained. There is not
much cooling behind the aft baffling without the cooling shroud and the fuel
pump may have also been heating up. The combination of the two may have
caused fuel vaporization leading to the engine hesitation and subsequent
improvement by leaning.
I hope that the above information will save someone else the potential
problems that I had. Previously I would typically climb at about 160kts.,
but now I like to climb at about 170kts to keep things cooler. John Wright
N338L Colorado Springs
LML website: http://www.olsusa.com/Users/Mkaye/maillist.html
Builders' Bookstore: http://www.buildersbooks.com/lancair
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